Semimicroanalysis of Saline Soil Solutions R. F. REITERIEIER U. S. Regional Salinity Laboratory, U. S. Department of Agriculture, Riverside, Calif.
A system of photometric and volumetric analytical semimicromethods for ions that contribute to soil salinity is described. These methods involve a considerable reduction in the quantity of soil solution required, which is an important consideration in the extraction of such solutions. I n addition, they involve a saving of reagents and time. The precision and accuracy of the methods are considered adequate for most soil analyses.
T
HE detailed analysis of soil solutions is rendered difficult b y the large volume of sample required b y the standard analytical methods. The method of extraction, size of apparatus, size of soil sample, necessity for repetition of the extraction, and length of time required for evtractioii have been influenced b y the necessity of securing sufficient solution for complete analysis. (In this paper, the term “soil solution” refers to the aqueous solution occurring in the soil a t field moisture; the term “soil extract” refers to the solution obtained from a soil that has been mixed n i t h an artificially high quantity of water-e. g., a t soil-water weight ratios of 1 to 2 and 1 to 5 . ) I n a discussion of these factors, Eaton and Sokoloff (IO) pointed out that “a material reduction in the quantity of solution required in the laboratory would minimize some of the difficulties”. Anderson, Keyes, and Cromer ( 4 ) recently mentioned the necessity of altering analytical conditions in the direction of microchemistry. The staff of this laboratory has been engaged in the examination and development of methods for the extraction of soil solutions, particularly of saline and irrigated soils. The pressure-membrane method, described b y Richards (28),is a n effective means of obtaining solutions from soils covering wide ranges of moisture content, texture, structure, and salt content. This method appears to be especially well adapted to soils at lorn moisture contents-e. g., near the wilting range ( 2 7 ) . The advantages of the pressure-membrane method would be largely lost if i t were necessary to apply the conventional analytical methods to the limited volumes of solution obtainable from comparatively dry soils. Consequently, the development of the pressure-membrane method has emphasized the need for semimicroanalytical
methods applicable to small samples of soil solution. I n addition to the small amount of sample required, the methods outlined here involve a saving of time and reagent, a consideration which might be of even greater importance to some analysts. There is a n expanding interest in the application of microanalytical met’hods to problems of agricultural chemistry. Peech (24) recently published a scheme for the microdetermination of exchangeable bases in soils. K a l l (36) has developed a set of microprocedures for the determination of some inorganic constit’uentsof plant ash. This article presents photometric and volumetric methods for the semimicrodetermination of calcium, magnesium, sodium, potassium, ammonium, carbonate, bicarbonate, chloride, sulfate, and nitrate ions. These methods generally represent adaptations of other methods previously published for the analysis of soils, waters, plants, and clinical specimens. The methods necessarily vary as to convenience and accuracy. The aim has been to develop simple procedures whose precision would not be seriously less than that of corresponding macromethotis. The methods described apply primarily to saline alkaline soils in which salts of alkali and alkaline earth metals pretlominste. For use under other conditions, where additional interfering substances might occur, appropriate modifications might be necessary. If the soil solution is not analyzed immediately, the concentration of some ions may be appreciably altered by the activity of microorganisms. Treatments to minimize the direct effect of such processes on the nutrient ions and indirect effects on other ions are usually not reliable. I n addition, calcium Carbonate and calcium sulfate precipitate from some solut’ionsafter extraction. For these reasons, soil solutions should be analyzed as soon as possible. The centrifuge procedures involve the use of an 8-place centrifuge head rotating at 3000 r. p. m. in a S o . 2 International centrifuge. Heavy-duty 12-ml. conical centrifuge tubes are necessary at this high speed in place of the ordinary 15-ml. tubes. -416-place head rotating at 2000 r. p. m. also provides acceptable results, and may even be more practical for a large number of samples. An angle head rotating at 3000 r. p. m. was tested, but in general the results were inferior with respect to precipitate compaction and over-all accuracy. A photoelectric photometer is very satisfactory for the colorimetric measurements because of its speed and relative precision. An Aminco Type F double photocell photometer (manufactured by the American Instrument Company, Silver Spring, Md.) was used in this work. It has a permanent mounting of six pairs of matched color filters and permits the use of both optical cells and photometer test tubes. A constant-voltage transformer in the 115-volt power line eliminates fluctuations in light transmission due to voltage variations. Other color-measuring instruments,
393
Vol. 1.5, No. 6
INDUSTRIAL AND ENGINEERING CHEMISTRY
394
OF SOILSAKD THEIREXTRACTS TABLE I. DESCRIPTION r
Accession NO. 56”
57 62 635 685
79 84 85 86
248“ 314 a
Soil T y p e Location Meloland, Calif. Imperial clay Imperial, Calif. Imperial clay Delta, Utah Oasis clay loam Delta, Utah Oasis clay subsoil Vale, Ore. Vale silty clay loam Glendale, Aric. Cajon silty clay loam Las Cruces, 1.Alex Gila adobe clav Roswell, N. Mex. Regan clay loam Fort Collins loam Laramie, Wyo. Indio very fine sandy Coachella, Calif. loam Buttonwillow, Calif. hlerced clay loam
PH 7.8 7.2 7.7
C,
%
8.0 10.0
7.7 8.0 7.8 8.0
0.34 0.33 1.07 0.58 0.27 1.10 0.92 0.93 0.99
9.1 7.8
0.49 2.34
Soils used only for organic matter investigation.
b Values corrected for medium chloride contents and probably less accurate t h a n
such as spectrophotometers, gradation photometers, neutral wedge photometers, and visual color comparators, can also be used. For accurate photometric work it is usually hazardous to rely on permanent photometer calibration curves, because of the variable conditions that affect colorimetric procedures. In this laboratory it is a matter of routine to take a series of standards through the analytical procedure each time a group of samples is analyzed. The calibration of microburets and small pipets is recommended. A 2-liter beaker covered by a 20-hole perforated brass plate, which holds the centrifuge tubes vertical, makes an adequate water bath. Ionic concentrations are calculated in terms of milliequivalents per liter (m. e., 1.). Attention is called to the increasing use among nater chemists of the term “equivalent per million”, e. p. m . ( 3 ) . This unit of concentration is numerically the same as milliequivalents per liter if the specific gravity of the solution is unity. I n addition to the determination of ionic concentrations, the analysis of soil solutions usually includes the p H value and the electrical conductiyity as a measure of the total electrolyte concentration. For conductivity measurements on small samples, a micromodification of the common pipet type of conductivity cell, 11-hich holds approximately 5 ml., is very convenient. For pH measurements a Beckman “one-drop” glass electrode (manufactured by Sational Technical Laboratories, South Pasadena, Calif.) is satisfactory. Capillary glass microelectrodes, which require eren less sample, are also available.
Description of Soils and Extracts The systematic investigations of organic matter and precision and accuracy reported here were made on extracts of eleven soil samples of different soil types from various localities. Pertinent characteristics of these soils and extracts are presented in Table I. Two of these soils, 68 and 248, are “black alkali” soils. The organic carbon contents of the soils and extracts were determined by the chromic acid oxidation method of Schollenberger (30), involving the modified phosphoric acid reagent of Purvis and Higson (26). The experimental values were multiplied by the factor 1.15, according to Allison ( I ) , which corrects for incomplete oxidation of the organic matter. The carbon contents of the extracts were determined on the evaporation residue of 25-ml. aliquots. Chloride reduces chromic acid, and appropriate corrections are included in the eight extract values reported. The soil carbon contents of the more saline soils also include corrections for chloride, which are very slight compared to those for the corresponding extracts. The p H values were determined by a glass electrode assembly. The soil p H measurements iyere made on saturated soil pastes.
C,
% 0.00094
pH 7.8 7.0 7.6
0 0032b 0.0044 0 0034 0 0032
10.2
... ...
8.1
0 0053
7.3 8.1 7.6 7.6
0 0047 0.0047b
9.0 7.6
Extract Characteriatics Electrical conductivity, Soil: water K X 106 @ 25’ C. ratio Color 125 1:2 Verypaleyellow 1 :5 Colorless 803 1:5 Yellow 998 557 1:s Yellow 1:5 Brown 555 75 1:2 Yellow 85 1 :2 Pale yellow 1:2 Pale yellow 353 1: 2 Pale yellow 544 /2
625
1 :2 I:5
Reddish brown Dark yellow
those for other s o ~ l s .
Removal of Organic Matter The possible interference of organic matter in the analysis of soil solutions often raises questions concerning the necessity for its removal. It may interfere in such mays as color masking, reducing action, mechanical contamination of precipitates, and in other direct and indirect ways. The magnitude of these effects is usually unknon-n. I n some systems of analysis, all samples are treated to remove organic matter, regardless of the amount and composition. I n other cases the solutions are analyzed without prior separation of the organic fraction. As the time involved in the preliminary removal of organic matter represents an appreciable fraction of the total time required for analysis, information as to the feasibility of omitting this operation is important, especially in the routine analysis of a large number of soil samples. It is also possible that some or all of the methods for removing organic matter may actually introduce errors into the analytical results. These considerations may affect both macro- and microanalytical methods. The eleven water extracts of Table I ryere subjected to four different treatments: ignition, oxidation by hydrogen peroxide, oxidation by bromine, and adsorption by carbon. Other possible treatments were not investigated systematically because they would definitely introduce various kinds of interference. The treated and untreated samples were analyzed for the ions mentioned except nitrate and ammonium. Sodium was determined by a gravimetric uranyl zinc acetate procedure [39, Sect. 70 (b), p. 421 instead of the colorimetric method. Bromine removed the color from all samples, but the analytical values agreed with those of the untreated solutions. Consequently, there would be no advantage in the use of this oxidant in the scheme of analysis described here. Carbon not only adsorbed the colored constituents but significantly reduced the concentrations of most of the ions, especially calcium and magnesium, and lowered the bicarbonate-carbonate value of every sample. However, i t did not affect the chloride values, which indicates that carbon treatment may be useful in removing color that interferes with the chloride titration. Hydrogen peroxide treatment a t a temperature not exceeding 100” C. caused a general decrease of ions in most samples, particularly of sodium and chloride. The loss of chloride is probably a result of oxidation to chlorine. The cause of the losses of the other ions remains somewhat obscure. These results, coupled with the resistance of some organic matter to oxidation by peroxide and the possibility of the catalytic decomposition of peroxide by soil constituents, make this treatment unsatisfactory. The ignition treatments were made in porcelain casseroles
June 15, 1943
ANALYTICAL EDITION
at 600” C. The more resistant organic constituents did not decompose completely over extended periods at lower temperatures. This has been observed also on base-exchange residue ignitions. The results of ignition mere variable. Sodium and chloride were lost from every sample, to about the same extent. Decreases in magnesium and sulfate occurred in several samples. Calcium showed no significant change except a n increase in one “black alkali” sample, 68. Potassium Tvas extremely high in ten ignited samples, upwards to 500 per cent of the correct value. Fresh samples of four solutions that were extremely erratic in this respect were ignited in platinum dishes, and these yielded the correct values for potassium. The excess potassium evidently originated in the material of the casseroles; the results indicate a n exchange of sodium for potassium. The two black alkali samples were also treated n ith nitric acid and boiled, to precipitate the colored humates. Analysis of the filtrates showed no appreciable deviation from the untreated samples. I n this investigation, carbonate determinations were made only on the untreated and the carbon-treated samples, as the other treatments precluded this determination. Chlorides could not be determined on the bromine-treated sample. The results of this investigation suggest the following recommendations and possible conclusions. For titration of carbonate species in dark solutions, a potentiometric titration can be substituted for the indicator procedure. Purified decolorizing carbon can safely be used to treat dark solutions prior to the chloride titration. Carbon can evidently be used to remove the color of solutions prior to the determination of sodium and potassium. Ignition may cause appreciable loss of many ions common to saline soils, especially sodium and chloride, Ignitions should not be made in porcelain ware. Oxidation of organic matter b y bromine and hydrogen peroxide accomplishes no apparent beneficial results. With especial regard to the inorganic composition of black alkali solutions, the inclusions of ions such as calcium and magnesium that may be combined with the humates may not always be desirable.
Precision and Accuracy To demonstrate the possible ranges of precision and accuracy that can be expected from the various methods, mater extracts of seT-en soils of Table I were systematically analyzed in duplicate by the semimicromethods and b y the corresponding macro- or standard methods in use at this laboratory. The results for each particular ion are presented in the section devoted to the d scussion of that method. Organic matter n-as not removed from these extracts prior to their analysis by either the macro- or microprocedures. 44s the extracts vary considerably in composition, some reported determinations may involve quantities of ions that do not represent favorable conditions for the evaluation of the accuracy of a method. This applies also to the macromethods. I n addition to the comparisons reported here, many other similar studies have been made on soil solutions and soil extracts, n aters, plant nutrient culture solutions, and plant ash extracts. These studies have yielded results as satisfactory as those presented in this paper. In the succeeding tables, several symbols and terms are used that possibly require brief explanations. The letters A and B indicate duplicate determinations. The mean IS the average of the duplicate value?, reported t o the same decimal point. The per cent deviation represents the average deviation of the duplicates from the mean divided by the mean value, and times 100; t h s figure is an index of preclsion or reproducibility. The average per cent deviation is the arithmetical average of the per cent deviation values for the entire group of extracts; comparison of the two values obtained for two methods provides a measure of the
395
relative precision of the two methods. The per cent error represents the algebraic percentile deviation of the mean semimicro value from the mean macro value; this calculation assumes that the macromethod usually provides the more correct result. The average per cent error is the arithmetical average of the percentile errors for the entire group of semimicrodeterminations; this figure provides a general index of the over-all accuracy of the method. VOLUMETRIC DETERMINATION OF CALCIUM
Calcium is determined by a method involving precipitation as the oxalate, centrifugal washing, and direct titration in perchloric acid solution with ammonium hexanitrato cerate, with nitro-ferroin as indicator. (The amhonium hexanitrato cerate and nitro-ferroin can be obtained from the G. Frederick Smith Chemical Company, Columbus, Ohio.) The precipitation and washing technique represents a combination of modified Clark and Collip (9) and Blasdale ( 8 ) procedures, while the titration technique follows the procedure of Smith and Get2 (32). The use of cerate and nitro-ferroin permits a direct titration at room temperature with a very sharp endpoint change from red to pale blue, and a low blank correction. REAGESTS. (Keep reagents B, C, D, and E in Pyrex bottles.) A. Methyl orange, 0.01 per cent in water. B. 1 to 1 hydrochloric acid. C. 1 K oxalic acid. D. 1 to 1 ammonium hydroxide. E. 1 to 50 ammonium hydroxide. F. 4 S perchloric acid. Dilute 340 ml. of 70 per cent perchloric acid or 430 rnl. of 60 Der cent perchloric acid to 1liter. G. 0.01 N ammonium hexanitrat0 cerate in 1 N perchloric acid. Dissolve 5.76 grams of “standard or reference purity” ammonium hexanitrato cerate in 250 ml. of 4 iV perchloric acid and dilute to 1 liter. The reagent should be standardized in the following manner: Pipet 5 or 10 ml. of fresh standard 0.01 N sodium oxalate into a small beaker containing 5 ml. of 4 N perchloric acid, add 0.2 ml. of nitro-ferroin indicator and titrate with the cerate solution to the pale blue end point. Determine a blank titration correction on a similar sample minus t,he oxalate solution. The milliliters of oxalate used divided by the corrected milliliters of cerate and times 0.01 provide the normality of the cerate. Do not attempt to adjust the solution to exactly 0.01 -V, and restandardize whenever the reagent is used several days or more apart. Keep in a dark bottle away from light. H. Sitro-ferroin indicator (nitro-orthophenanthroline ferrous sulfate). Dilute the stock 0.025 Jf indicator solution 1 to 20. Use 0.1 ml. in analyses and 0.2 ml. in standardizations. PROCEDURE. Pipet an aliquot containing 0.005 to 0.08m. e. of calcium into a clean 12-ml. conical centrifuge tube, dilute or evaporate to 5 ml., and add 1 drop of (A), 2 drops of (B), and 1 ml. of (C). Heat to the boiling point in a water bath. While twirling the tube, add (D) dropwise until the solution just turns yellow. Replace in the bath, and after 30 minutes cool the tube n air or in water. If necessary add more (D) to keep the solution just yellow. Centrifuge at 3000 r. p. m. for 10 minutes. Carefully decant the supernatant liquid into a 2 5 , 50-, or 100-ml. volumetric flask. Stir the precipitate, and rinse the sides of the tube with a stream of 5 ml. of (E) blown from a pipet. Centrifuge at 3000 r. p. m. for 10 minutes. Decant the washings into the same flask. Drain the tube by inversion on filter paper for 10 minutes. Wipe the mouth of the tube with a clean towel or lintless filter paper. Blow into the tube 3 ml. of (F) from a pipet. When the precipitate is dissolved, add 0.1 ml. of (H). Titrate with (G) from a 10-mi. microburet to the pale blue end point. If more than 5 ml. of (G) is required, transfer the sample to a small beaker and complete the titration. Determine the blank correction in the same manner; it usually is about 0.03 ml. Dilute the supernatant liquids in the volumetric flask to volume and save for the magnesium determination. CALCULATION. $1. e. of Ca per liter = (corrected ml. of cerate solution x normality of cerate X 1000) + ml. in sample aliquot. Precision a n d Accuracy T h e calcium concentrations of the eeven soil extracts indicated in Table I were determined by this procedure and by a calcium oxalabe-potassium permanganate volumetric macromethod outlined b y Kilcox (39, pp. 3&9) and based on the calcium-magnesium separation technique of Blasdale ( 8 ) .
Vol. 15, No. 6
INDUSTRIAL AND ENGINEERING CHEMISTRY
396
is best prepared by dilution of a more concentrated solution of ma nesium sulfate that has been standarfized by ___Lfacromethod-----Semimicromethod-gravimetric determination of magneso11 _ Calcium _ ___ ~ DeviCalcium Devisium. ho. Aliquot A R .\lean ation Aliquot A B Mean ation Error G. 1 N sulfuric acid. Ml. M . e./liter .If I . .W.e . ' l i f e r R % 5 H. Ammonium molybdate reagent. 57 .iO 30 05 3 9 . 0 9 3 9 . 0 7 0.05 1 39.9 39.9 39.9 0.00 +"I Dissolve 40 grams of ammonium 10 3.57 3.58 3.58 0.14 ~ 1 . 1 3 53 3.54 3.54 0.14 62 200 molybdate in 400 ml. of water at 60" C., -1.4 79 200 2.19 2.20 2.20 0.23 10 2.16 2.18 2 . 1 7 0.46 add 456 ml. of arsenic-free concentrated 2 34 2.84 2.34 0.00 -0.4 10 2.32 2.33 0.43 84 200 2.31 0.18 2 28.6 28.9 28.8 28.3; 28.45 28.40 0.52 +1.4 85 50 sulfuric acid to 1000 ml. of water, and 0.33 2 2 4 . 8 2 4 . 9 24.9 0.20 +1.9 86 50 24 30 2 4 . 5 1 2 4 . 4 3 cool both solutions. Stir the molyb314 200 7 71 7 80 7.76 0.58 5 7.75 7.80 7.78 0.32 +0.3 - -date solution into the acid solution AV. 0.22 0.30 1.2 and dilute to 2000 ml. when cool. The reagent is a 2 per cent solution of ammonium molybdate in 8 N sulfuric acid. It keeps indefinitely in a brown bottle. I. Stannous chloride reagent. Place 0.300 gram of c. P. The results, presented in Table 11, indicate highly satisstannous chloride dihydrate in a 100-ml. volumetric flask. Disfactory precision and accuracy for the semimicromethod. solve rapidly in water, dilute to the mark, and mix. Any turbidity The reproducibility data demonstrate that it usually is unwill be removed on mixing with reagent H. Prepare fresh daily. necessary to replicate analytical samples. PROCEDURE. From the volumetric flask containing the calcium-free sample pipet an aliquot containing 0.0005 t o 0.003 m. e. It has been known that the clinical calcium methods inof magnesium into a 12-ml. conical centrifuge tube and dilute or volve a negative error due to loss of calcium oxalate on decantevaporate to 5 ml. Add 1 ml. each of (A) and (B) and 1 drop of ing and a positive error resulting from incomplete washing of (C). Heat to 90" C. in a water bath and while twirling the tube add (D) dropwise until pink. Cool, add 2 ml. of (D), and stir the precipitate; Wang (3'7)indicates that these two errors are with a thin glass rod. Withdraw the rod, stopper the tube, and very evenly balanced in most analyses. The present results let stand overnight. support this view and show that the net resultant error is of Centrifuge at 3000 r. p. m. for 10 minutes, decant carefully, slight magnitude. drain on filter paper for 10 minutes, and wipe the mouth of the tube with a clean towel or lintless filter paper. Wash the precipitate and sides of the tube with a stream of 5 ml. of (E) from a COLORIMETRIC DETERMIRATION OF MAGNESIUM pipet equipped with a rubber aspirator bulb or by a similar arrangement. Centrifuge at 3000 r. p. m. for 5 minutes, decant, Magnesium is determined on calcium-free solutions by drain for 5 minutes, and wipe the mouth of the tube. Repeat this washing procedure once. precipitation as magnesium ammonium phosphate hexaPipet 2 ml. of (G) into the tube and dilute t o about 10 ml. hydrate, centrifugal washing, and colorimetric estimation of After 5 minutes, wash the contents into a 100-ml. volumetric flask the phosphate content by the ceruleomolybdate reaction. to which exactly 5 ml. of (H) have previously been added. Dilute This standard clinical procedure, recently described for plant to about 60 ml. and pipet in 1 ml. of (I) while rapidly twirling the flask. Dilute to the mark and mix. At exactly 10 minutes ash by Wall (36), has been modified in some details. No after adding (I) measure the light transmission of the blue solumethod for the precise determination of small amounts of tion in a photometer test tube through the 650-millimicron filter magnesium in soils appears to have been advanced. Although versus that of water in a similar tube. Previously, the phothe method described does not involve a high degree of pretometer is balanced at 100 per cent transmission with water in both tubes The accuracy is increased somewhat by the use of the cision, the use of duplicate analytical samples usually prosame test tube for all samples and standards. vides satisfactory accuracy. Prepare a photometer calibration curve on semilogarithmic Because of the sensitivity of the colorimetric phosphate graph paper by taking a series of 0, 0.5, 1, 2, and 3 ml. of (F) measurement, usually only a fraction of the filtrate from the through the same entire procedure. A typical calibration is shown in Figure 1. The amount of magnesium in the sample is calcium determination is used for the magnesium determinaobtained by simple interpolation on the curve. tion. This practice is also influenced by the inhibition of Because of the effect of oxalate, the magnesium sample should precipitation of magnesium by high concentrations of oxalate not represent more than one fifth of the calcium sample. If the ion (14,24), which must be reduced to a safe value. magnesium concentration is so low that it cannot be accurately T 4BLE 11.
cOMP.ARISOK
O F ?vlACRO-
AKD
SEJII>IICROYETHODS FOR
C.ALCIC\I
7--
REAGENTS.The concentrations of molybdate and sulfuric acid used in the development of the blue color are those recommended b;; Truog and Meyer (541,but the strength of the reagent has been modified slightly. The stannous chloride reagent is prepared daily and not acidified, according to Zinzadze (41). Because of the effect of time on the color, especially of darker solutions, photometer readings are made at exactly 10 minutes after addition of the stannous chloride. The ammoniacal wash liquid is similar to that recommended by Wang (37) for the washing of calclum oxalate precipitates. Reagents A, B, D, E, and F should be kept in Pyrex bottles and replaced if the precipitation blank color becomes too intense. A. 30 per cent ammonium chloride solution. Dissolve 30 grams of recrystallized ammonium chloride in water and dilute to 100 ml. Filter before use. B. 5 per cent ammonium dihydrogen phosphate solution. Dissolve 25 grams of ammonium dihydrogen phosphate in water and dilute to 500 ml. Filter before use. C. Phenolphthalein, 1 per cent in 60 per cent ethanol. D. Concentrated ammonium hydroxide. E. Ammoniacal wash liquid. Mix 20 ml. of concentrated ammonium hydroxide with 80 ml. of water, 100 ml. of ethanol, and 100 ml. of ether. F. Standard 0.001 N magnesium sulfate. This
3
R
0 X
$ 2
m W z a a
2 W
z 1
0 10
20
30
40
50
60
70
80
90 100
PERCENT LIGHT TRANSMISSION
FIGURE 1. PHOTOMETER CALIBRATION CURVEFOR MAGNESIUM
June 15, 1943
ANALYTICAL EDITION
397
determined in this manner, a larger fraction of the calcium filtrate or even all of it can be used, provided an equivalent amount of oxalic acid is added to each standard sample before precipitation of the magnesium. Results obtained under these conditions should not be expected to be as satisfactory as those obtained by the routine procedure. M. e. of Mg per liter = (m. e. of Mg as found CALCULATION. by interpolation x 1000) + (ml. in Ca aliquot X fraction of Ca aliquot used for Mg determination).
porcelain crucible. Wash with several portions of glacial acetic acid, then likewise with ether. Dry in a desiccator over calcium chloride for one hour. B. Uranyl zinc acetate reagent. Solution 1. Stir 80 grams of uranyl acetate dihydrate into a mixture of 14 ml. of glacial acetic acid and 427 ml. of water. Solution 2 . Stir 220 grams of zinc acetate dihydrate into a mixture of 7 ml. of glacial acetic acid and 294 ml. of water. Heat the two solutions separately on a water bath and stir until the salts are dissolved. Mix while hot, and when cool add 0.2 gram of (A). Let stand overnight. Keep in a TABLE 111. COMPARISON O F MACRO-AND SEMIMICROMETHODS FOR kIAGNESIUM dark bottle and filter before Macromethod Semimicromethod use. [For preparation of (A), Soil Magnesium DeviMagnesium this reagent does not have to DeviNO. Aliquot A B Mean ation Aliquot A B C D Mean ation Error besaturatedwith (A).] M1. M . e./liter 70 lM1. M . e./litet % % C. Acetic a c i d - e t h a n o l -1.9 wash liquid. Mix 75 ml. of 13.1 13.2 13.4 0.2 12.5 13.1 1.91 57 50 1 3 . 3 4 1 3 . 3 6 1 3 . 3 5 0.08 62 200 3.33 3.35 3.34 0.30 0.8 3.41 3.46 3.20 3.38 3.36 2.46 $0.6 glacial acetic acid with425 ml. 79 200 1.08 1 . 0 9 1 . 0 9 0 . 4 6 1.6 1.11 1.11 1.12 1.11 1.11 0.23 +1.8 of 95 per centethanol. Shake 84 200 0.58 0.58 0.58 0.00 2 0.58 0.60 0.56 0.57 0.58 2 . 1 6 0.0 +i.s with an excess of (A). Keep 0.2 12.9 12.9 12.3 12.7 12.7 1.57 85 50 1 2 . 4 6 12.50 12.48 0.16 32.1 32.7 32.1 32.5 32.4 0.1 0.77 3 2 . 4 8 32.82 32.65 0.52 86 50 -0.8 in a dark bottle and filter be314 200 6 33 6.38 6.36 0.39 0.4 6.10 6 . 4 8 6 . 5 0 6.45 6.38 2.24 +0.3 foreuse. - _ _ Av. 0.27 1.62 1.0 D. 0.1 N ammonium thiocyanate. Dissolve 3.81 grams of c . P . a m m o n i u m t h i o cyanate in water and dilute to 500 ml. Prepare a sufficient quantity fresh each time. Precision and Accuracy E. Ether, c. P., anhydrous. F. Standard 0.005 N sodium chloride. Dissolve 0.2923 T h e magnesium concentrations of the seven soil extracts grams of dry recrystallized sodium chloride in xater and dilute to were determined b y this procedure and b y a gravimetric exactly 1 liter in a volumetric flask. method described b y Wilcox (39, pp. 39-40), except t h a t the magnesium ammonium phosphate hexahydrate precipitates were collected on porous-bottomed ceramic Gooch crucibles, washed with ammonium hydroxide, ethanol, and ether, and weighed as the hexahydrate. Colorimetric magnesium determinations were made on duplicate fractions of each of the two calcium-free semimicroanalytical samples. This arrangement provides an opportunity to decide whether errors possibly arising in the separation of magnesium from calcium contribute significantly to the over-all errors. I n Table 111, colorimetric samples A and B are from one calcium-free aliquot, and C and D from the other. The precision of the colorimetric method for most samples as shown b y variations among four replicates is not very high. However, the use of average values from duplicate aliquots usually results in acceptable accuracy. Four ex0 I tracts indicate that errors in the separation of calcium may influence the results, but this is hardly significant and is not upheld b y previous data. The interference of other ions, such as sodium and potassium, is always possible when a double precipitation is not made. Hillebrand and Lundell (14) discuss these and other interferences in detail. Aside from the various theoretical aspects of the method, i t provides satisPipet an aliquot containing 0.002 to 0.012 m. e. PROCEDURE. factory results if too much faith is not placed in a single of sodium into a clean 12-ml. conical centrifuge tube. Evaporate in a water bath to 0.2 ml. Cool, add 8 ml. of (B), stopper, and determination. mix immediately by repeated inversions for one minute. Let stand one hour. Remove the stopper and centrifuge at 3000 COLORIMETRIC DETERMINATION OF SODIUM r. p. m. for 10 minutes. Drain on filter paper for 10 minutes. Wipe the mouth of the tube with a clean towel or lintless filter Sodium is determined b y a procedure which follows closely paper. Stir the precipitate and wash the sides of the tube with 4 the clinical method of Hoffman and Osgood (17). Sodium ml. of (C) in a stream from a pipet equipped with a rubber aspirator bulb. Centrifuge at 3000 r. p. m. for 10 minutes, decant, uranyl zinc acetate is precipitated, centrifugally washed, and and drain for 10 minutes. Wipe the mouth of the tube. Wash dissolved, and the yellow color is compared with those of with 5 ml. of ether, but centrifuge for only 5 minutes and decant sodium standards. Dissolving the precipitates in ammonium carefully without draining. (If the tube is drained after the thiocyanate solution helps t o stabilize the color against temether wash, portions of a precipitate may drop from the tube.) Repeat the ether washing and decanting once. perature changes. Because of the great sensitivity of the When the ether is completely evaporated, pipet into the tube gravimetric uranyl zinc acetate method, the colorimetric exactly 10 ml. of (D), mix by inversion until the precipitate is distechnique sometimes may not extend the analytical range solved, and centrifuge at 3000 r. p. m. for 5 minutes to remove any t o the same extent as for the determination of some other phosphate precipitate. Pour into a 1-inch optical cell and measure the light transmission of the solution through the 420ions. Nevertheless, the photometric method does increase millimicron filter versus that of (D) in a similar cell. Previously, the precision of estimation of small quantities of sodium. balance the photometer a t 100 per cent transmission with (D) in both cells. A calibration curve is prepared by taking a series of REAGENTS. A. Sodium uranyl zinc acetate crystals. Add 0, 0.5, 1, 1.5, 2, and 2.5 ml. of (F) through the same procedure 125 ml. of (B) to 5 ml. of 2 per cent sodium chloride solution, stir, and plotting the results on ordinary graph paper. Figure 2 and after 15 minutes collect the precipitate in a porous-bottomed
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1
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Vol. 15, No. 6
shows a typical curve. The amount of sodium in the sample is obtained by simple interpolation on the curve. M. e. of Xa per liter = (m. e. of Ka as found by CALCULATION. interpolation X 1000) t ml. in aliquot.
PROCEDURE. Pipet an aliquot containing 0.0003 to 0.005 m. e. of potassium into a clean 12-ml. giaduated conical centrifuge tube, dilute or evaporate to 1 ml., blow in 2 ml. of (A) from a pipet, and twirl the tube for a few seconds. Let stand 3 hours in a refrigerator at about 5" C., and mix by twirling several times during this interval. Wash the upper walls of the tube with a stream of 0.5 Precision and Accuracy ml. of water, but do not mix it with the precipitation liquid. Centrifuge at 3000 r. p. m. for 10 minutes and drain on filter The macromethod used for comparison was the original paper for 5 minutes. \vipe the mouth of the tube with a ,,lean gravimetric uranyl zinc acetate method of Barber and Kolttowel or lintlese filter paper. Stir the precipitate and wash the tube walls with a stream of 3 ml. of (B). Centrifuge for 5 minutes and drain. Repeat this washing once. (Ammonia should be absent from the atmosphere TABLE Iv. COMPARISOS O F M h C R O - A S D SE1\IIblICROMETHODSFOR s O D I C M during the preceding operations.) Macromethod-----------Semimicromethod---Vigorously stir the precipitate with a Soil Sodium DeviSodium Devistream of 5 ml. of water from a pipet. KO. Aliquot A B Mean ation Aliquot h B Mean ation Error Immediately place the tube in a boiling MI. M . e./liter 70 .M1. M. e./liter 70 70 water bath and keep it there until the 57 5 2 9 . 7 1 2 9 . 8 3 29.77 0.20 0.3 30.0 30.3 30.2 0.50 +1.4 2 86.15 86.35 8 6 . 2 5 0.12 0.1 86.5 87.5 87.0 0.58 +0.9 p r e c i p i t a t e is completely 62 3.47 3.57 3.52 1.42 +2.9 Cool, add exactly 2 ml. of (C), and then 79 50 3.40 3.44 3.42 0.59 2 84 50 5.16 5.16 5.16 0.00 2 5.05 5.15 5.10 0.98 -1.2 blowinexactly2ml.of (D). Dilute to 85 50 3.48 3.48 3.48 0.00 2 3.53 3.63 3.58 1.40 +2.9 the 12-ml. mark and mix by inver86 10 21.71 21.71 21.71 0.00 0.4 21.3 21.3 21.3 0.00 -1.9 5 52.8453.0252.93 0 . 1 7 0.2 52.5 53.0 52.8 0.47 -0.2 sion. If turbid, centrifuge for 5 minutes. 314 - __ After an interval of 15 to 30 minutes, Av. 0.15 0.76 1.6 compare the light transmission in a 1-inch optical cell through the 580millimicron filter with that of water in a similar cell. Previously, balance the photometer at 100 per cent transmission hoff (6) as modified by Wilcox [59,Sect. 70 (b), p. 421. The with water in both cells. results are presented in Table 11'. Prepare a calibration curve for each set of samples by carrying a The colorimetric results are similar to those found for magseries of 0, 0.1, 0.2, 0.3, 0.4, and 0.5 ml. of 0.01 N potassium nesium in that while the precision of the method is not great chloride through the same operations. The amount of Otassium in the sample is found by interpolation on this curve. straightthe accuracy obtained b y using duplicate samples is satisline semilogarithmic calibration curve is obtained for the range-0 factory. Since the precipitation procedures of the colorito 0.005 m. e. of potassium, as shown in Figure 3. metric and gravimetric methods are very similar, the source of any errors must lie mainly in the centrifuge or colormeasuring technique. Hoffman and Osgood (17) point out ' I ' 1 I that the color intensity is affected appreciably by acid, which must be carefully washed out of the centrifuge tube by the ether. Also, the low sensitivity of photometers and colorimeters to yellon- colors tends to reduce the attainable precision. 7 -
if
-
COLORIMETRIC DETERMIYATION OF POTASSIUM
Potassium is determined by a photometric method in which the cobalt content of the centrifuged cobaltinitrite precipitate is estimated by treatment with ferrocyanide and choline hydrochloride, according to a method of Jacobs and Hoffman ( I @ , later modified for the photoelectric colorimeter by Hoffman (16). Morris and Gerdel (29) describe a similar method for plant samples. The technique of precipitation of the potassium sodium cobaltinitrite involves some suggestions of Volk (35) applied to the clinical method of Kramer and Tisdall (21). Provision is included for the rolatilization of ammonia from samples containing amounts that would cause positive errors in the potassium value. REAGENTS.A. 30 per cent sodium cobaltinitrite reagent. Dissolve 30 grams of c. P. sodium cobaltinitrite in water, add 2 ml. of glacial acetic acid, and dilute to 100 ml. Prepare a sufficient quantity fresh daily. Filter before use. B. 70 per cent ethanol. C. 0.4 per cent choline hydrochloride. Recrystallize choline hvdrochloride as follows: Dissolve in a minimum quantity of akolute ethanol, filter, and precipitate by addition of excess ether. Collect on a suction funnel, wash with ether, and dry in a desiccator. Dissolve 0.2 gram in 50 ml. of water. Prepare a sufficient quantity of the solution fresh daily. D. 0.8 per cent potassium ferrocyanide. Recrystallize potassium ferrocyanide from a boiling saturated aqueous solution by cooling. Collect on a suction funnel and pull air through until dry. Dissolve 0.4 gram in 50 ml. of water. Prepare a sufficient fresh quantity of the solution daily. E. Standard 0.01 N potassium chloride solution. Dissolve 0.7456 gram of dry recrystallized potassium chloride in water and dilute to exactly 1 liter. F. 1 N sodium hydroxide. G. 1 N acetic acid. H. Methyl orange, 0.01 per cent in water.
PERCENT LIGHT TRANSMISSION
FIGURE3.
CALIBR.4TION CURVE POTASSIUM
PHOTOMETER
FOR
If a qualitative or quantitative test for ammonia indicates sufficient to interfere, it can be removed by the following preliminary treatment. Pipet the potassium aliquot into a 15-ml. beaker, dilute to 5 ml., and add 0.2 ml. of (F). Boil slowly until just dry, add 5 ml. of water, and boil again until the residue is barely moist. Dissolve in a minimum volume of water, not over 0.5 ml. and decant into a graduated 12-ml. centrifuge tube. Rinse the beaker with several 0.2-ml. portions of water, and add these to the centrifuge tube. Add 1 drop of (H) and neutralize Tvith (G). Evaporate in a water bath to a volume of 1 ml. Cool and proceed in the regular manner. The series of standards should be treated in the same manner because of the possible addition of potassium by way of the extra reagents. hl. e. of K per liter = (m. e. of K in aliquot as CALCULATION. found by interpolation X 1000) t ml. in aliquot. Precision and Accuracy
Quantitative comparisons with macromethods are presented in Table V. Samples 62 8.5, and 314 were analyzed
ANALYTICAL EDITION
June 15, 1943
b y the gravimetric cobaltinitrite method of Wilcox (40). The remainder were analyzed b y a volumetric cobaltinitrite procedure, in which the potassium was precipitated according to Wilcox (40) and filtered and titrated according to Hibbard and Stout ( I S ) . Kone of the three methods shows a high degree of precision on these samples. Cobaltinitrite methods-usually-are not the most accurate methods for potassium, but are widely used in soil analysis because of their convenience and sensitivity. The precipitation in both the macro- and semimicroprocedures is probably affected b y the same factors. Interferences in saline soils may occur because of the low proportion of potassium to other ions, such as sodium and calcium. I n gypsiferous soils, the deposition of calcium sulfate sometimes occurs on evaporation, and the double salt CaS04.K2S04 may be formed. The predictable accuracy of the colorimetric method is very satisfactory when compared to the macroprocedures. COLORIMETRIC DETERMINATION OF AMMONIUM
I n samples containing appreciable quantities of ammonium ion resulting from ammonification, fertilization, or other source, i t may be desirable to determine its concentration. This can be accomplished b y the procedure outlined under the description of the colorimetric determination of nitrate.
399
other titratable anions, such as silicate, phosphate, and borate (a). I n spite of these difficulties, no other method approaches it in convenience. The indicated method of calculation is the customary one and assumes the absence of other titratable ions. REAGENTS. A. Phenolphthalein, 1 per cent in 60 per cent ethanol. B. Methyl orange, 0.01 per cent in water. C. Standard 0.01 N sulfuric acid. PROCEDURE. Pipet an aliquot containing 0.005 to 0.04 m. e. of chloride into a 15-ml. wide-mouthed porcelain crucible or a small porcelain casserole. Add one drop of (A). If the solution turns pink, add (C) from a 10-ml. microburet dropwise at 5-second jntervals until the color just disappears. Record the buret reading. Add 2 drops of (B) and titrate to the first orange color. Save the titrated sample for the chloride determination. An indicator correction blank in boiled water should be determined, and applied if it is not negligible. The lighting should be adequate for the recognition of the various colors. The use of comparison color standards at the correct end points is helpful. l f A is the milliliters of (C) to the phenolCALCULATIOX. ahthalein end Doint and B the milliliters t o the methyl orange end point, hi. e. ofCOs-- per liter = ( 2 A X 0.01 X 1000) c ml. in aliquot. hi. e. of HC0,- per liter = [ ( B - 2 9 ) X 0.01 X 10001 + ml. in aliquot. Precision and -4ccuracy
A similar method employing 0.05 S sulfuric acid was used for comparison (5, p. 535; 39, p. 18). The results on the seven extracts are TABLE T'. COMP.IRISOS O F ?vI.iCRO- AND SE1\IIVICRO>lETHODSF O R POTASSIUM presented in Table VI. As the ___- Macromethod----Semimicromethod Potassium DeviPotassium DviSoil samples were neutral to phenolNO. Aliquot ation Aliquot -4 B Mean ation Error phthalein, the listed bicarbonate llfl. M . e./liter % 'W. M. e./liter % 70 values consist of methyl orange alka1.04 +2.9 10 0.428 0.437 0,433 0.95 57 200 0.417 0.425 0,421 0.63 $2.2 3.20 3.22 0.96 1 3.18 100 3.10 3.16 3.13 62 linities. 0.30 -4.1 0.00 5 0.673 0.677 0,675 100 0.704 0.704 0.704 79 The precision of the semimicro0.87 +1.8 0.59 10 0,340 0.346 0,343 100 0.335 0.339 0,337 84 0.98 -0.6 3.57 0.70 1 3.53 3.60 3.61 3.59 100 3.56 S5 method corresponds favorably to 0 . 6 2 + 2 . 3 1.128 1.121 5 1.114 1.096 0 . 9 1 100 1.086 1.106 86 1.01 -1.0 0.99 5 0.98 1.00 1.00 2.00 1.02 100 0.98 314 that of the official method. This ~ _ _ Bv. 0.87 0.78 2.1 should be expected of a method involving merely a straightforward titration. It is somewhat more difficult to estimate the accuracy of the VOLUMETRIC DETERMINATION OF CARBOXATE AND method; although b y comparison with the official method i t BICARBONATE is satisfactorv. the various factors nreviouslv discussed affect both methods. Comparisons ma& on oiher samples by These ions are determined by a micromodification of the potentiometric and indicator procedures, not reported here, Karder (38) alkalimetric titration to the phenolphthalein provided similar accuracy. and methyl orange end points, respectively. Titration methods are rauid in oneration, but can be highly inaccurate under the ordinary conditions 'of sampling and analysis. These VOLUMETRIC DETERMINATION OF CHLORIDE factors \yere stressed b y Johnston (IO). More recently, Benedetti-Pichler et al. ( 7 ) discussed the various errors of the Chloride is determined by a micromodification of Mohr's ordinary Karder procedure. The estimation of a small volumetric silver nitrate-potassium chromate method (33). amount of carbonate in the presence of a much larger quanThe determination is made on the carbonate-bicarbonate tity of bicarbonate, a condition frequently encountered ill sample, which has been neutralized to methyl orange in t h a t alkaline soil extracts, usually involves considerable error. [Hirsch (15) recently described the construction of a slide rule for bhe calculation of these ions from the p H TABLE \:I. COXPARISON O F MACRO- .4ND SEMIMICROMETHODS FOR value and the methyl orange alkalinity. His BICARBOSATE charts, based on earlier calculations involv-Macromethod . --SemimicromethodSoil Bicarbonate ~ ~ ~ . i Bicarbonate ~~,,iing carbonate-bicarbonate equilibria, show No. Aliquot A B Mean ation Aliquot A B Mean ation Error that no titration method, potentiometric or MI. M . e./liter 70 .M1. iM.e./liter 7 0 % colorimetric, can be theoretically exact. The 57 100 0.38 0.39 0 . 3 9 1.28 10 0 . 3 7 0.37 0.37 0.00 -5.1 62 100 1.73 1.73 1.73 0.00 10 1 . 7 6 1 . 7 8 1.77 0.57 f2.3 use of such a slide rule should provide more 79 100 2.67 2.69 2 . 6 8 0.37 10 2.67 2 . 7 0 2.69 0.56 f0.4 0.00 -1.7 accurate results than those obtained from 84 10 2.27 2.27 2.27 0.65 100 2.29 2.32 2.31 0.56 -0.6 10 1 . 7 9 1 . 8 1 1.80 0.28 86 100 1.80 1.81 1 . 8 1 the usual calculation, especially where the 86 100 2 . 8 5 2.86 2.86 0.54 -2.1 0.17 10 2 . 7 8 2 . 8 1 2.80 10 1.11 1.13 1.12 0.89 4-3.7 carbonate value represents a small frac314 50 1.07 1.08 1.08 0.46 __ __ tion of the total alkalinity.] Additional Av. 0.46 0.45 2.3 errors may result from the presence of I
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brown bottle. Suspend 4 grams in 150 ml. of water in a 250-ml. volumetric flask, add 7 ml. of 1 N hydrochloric acid, shake until dissolved, and dilute to the mark. Filter before use. D. 95 per cent acetone. REAGEXTS.A. 5 per cent potassium chromate indicator. E. Phenol red. Prepare a 0.05 per cent aqueous solution. To reduce the titration blank caused by the indicator, add suffiDissolve 5 grams O f potassium chromate in 50 ml. Of water and add 1 N silver nitrate dropwise until a slight permanent red precient 0.05 N sodium hydroxide so that \?hen diluted in the titracipitate is produced. Filter and dilute to 100 ml. tion sample, the blank will be no greater than 0.05 ml. of 0.01 N B. Standard 0.005 N silver nitrate solution. Dissolve 0.8495 sodium hydroxide. -4s an illustration, 3 parts of phenol red gram of C. P. silver nitrate in water and dilute to exactly 1 liter. solution are treated wit11 1 part of 0.05 sodium hydroxide, !Then Keep in a brown bottle away from light. 0.2 ml. of indicator is used in the titration of unknowns and PROCEDURE. To the sample preserved from the carbonatestandard phthalate. bicarbonate determination, add 4 drops of (A). While stirring, F. Standard 0.01 N sodium hydroxide. This should be titrate under a bright light with (B) from a 10-ml..microburet to standardized against 5 ml. of 0.01 N potassium acid phthalate a t the first permanent light brown color. The titration blank corthe boiling point with phenol red. The end point is the deep rection varies with the volume of the sample at the end point, and purplish-red color that persists on boiling. The total volume of usually increases regularly from about 0.03 to 0.20 ml. as the titrated sample at the end point should be 10 ml. Do not atvolume increases from 2 to 12 ml. tempt t o make exactly 0.01 N . M. e. of C1- per liter = (ml. of AgKOs - ml. Pipet an aliquot containing 0.005 to 0.08 m. e. of CALCULATION. PROCEDURE. sulfate into a clean 12-ml. conical centrifuge tube. Dilute or of AgN03for blank) X 0.005 X 1000 i ml. in aliquot. evaporate to 5 ml. Add 2 drops of (A) and then (B) dropwise until yellow. Place in ice water, and after 5 minutes blow in 2 ml. of (C) from a TABLE VII. COMPARISON O F b f A C R 0 - AND SEMIMICROMETHODS FOR CHLORIDE pipet, and mix well by twirling. Let -Macromethod Semimicromethod stand 20 minutes in ice water, and Soil Chloride DeviChloride Devicentrifuge at 3000 r. p. m. for 10 minNo. Aliquot A B Mean ation Aliquot A B Mean ation Error utes. Decant carefully without drainMZ. M . e./liter % ML M. e./liter % % ing. Wash the tube walls and stir 57 20 63.3 63.5 6 3 4 0.16 0.5 63.4 63.6 63.5 0.16 +0.2 the reci itate with a stream of 5 ml. 0.06 0.5 80.4 80.5 80.5 80.2 80.3 80.3 0.06 $0.2 62 20 Eom a pipet. Centrifuge at of 10 1.25 1.26 1.26 0.40 1.30 1.30 1.30 0.00 -3.1 79 100 3000 r. p. m. for 5 minutes, decant, 10 0.99 1.00 1.00 0.50 1.04 1.05 1.05 0.48 -4.8 84 100 85 100 1.52 1.52 1.52 0.00 10 1.49 1.61 1.50 0.67 -1.3 and wash twice again in the same 86 100 1.77 1.78 1.78 0.28 10 1.76 1.79 1.78 0.84 0.0 manner. 314 50 17.93 17.94 17.94 0.03 2 17.93 17.98 17.96 0.14 +0.1 Wash the precipitate into a 50- or - Av. 0.14 0.40 1.4 100-ml. beaker with a 10-ml. stream of water, add 0.2 ml. of (E), and titrate boiling hot with (F) from a 10-ml. microburet. During the titration, pour the hot solution back and forth Precision and Accuracy from the beaker to the centrifuge tube, to remove any adhering precipitate. Titrate to the same permanent end-point tint used in the standardization. The total volume of titrated sample at The seven soil extracts were analyzed b y this procedure the end point should be 10 ml. Determine the titration blank by and by the official Mohr method (5, p. 528) as outlined by exactly the same procedure. Wilcox (39, p. 18). The results are presented in Table VII. M.e. of SO1 per liter = (ml. of NaOH - ml. of CALCULATION. The reproducibility of the semimicromethod is seen to be NaOH for blank) X normality of NaOH X 1000 + ml. in aliquot. very satisfactory, comparable to that of the macromethod. Both the precision and accuracy improve with increasing Precision and Accuracy quantities of chloride. A gravimetric barium sulfate method (39, p. 19) was used The end point of the chloride titration sometimes is not for comparison. Table VI11 contains the results. The semivery distinct, which is probably the main source of error. micromethod shows a high degree of precision and a preBecause of the low bicarbonate content of several extracts, dictable accuracy of about 2 per cent. the aliquots used for the bicarbonate determination were It is important that all titrations, including unknowns, larger than those for the chloride determination. blanks, and standardizations, be made to the same phenol red end point in equal final volumes of sample. Precipitation in VOLUMETRIC DETERMINATION OF SULFATE an acid solution precludes the interfering precipitation of phosphate. Sulfate is determined by precipitation as benzidine sulfate, centrifugation, and direct titration of the liberated sulfuric COLORIMETRIC DETERMINATION OF NITRATE acid with dilute standard base. The reagents and precipitaNitrate is determined b y a distillation method, because tion procedure are based on Fiske's modification (11) of the the well-known phenoldisulfonic acid method is seriously Rosenheim-Drummond method (29)- Precipitation in a n affected b y chloride. The procedure involves reduction of ice bath and centrifugal mashing appear to increase the nitrate to ammonia, distillation into dilute acid, nesslerization, accuracy. A source of error in benzidine methods has been and photometric comparison with standards similarly treated. the coprecipitation of benzidine hydrochloride, which is Any nitrite is included in the nitrate value. Ammonium ion difficult to wash out of the precipitate ( I d ) . It is believed can be determined by preliminary distillation in the absence that the method described here reduces this error. of Devarda's alloy. REAGENTS.A. Bromophenol blue, 0.04 per cent in 95 per Various nitrogenous organic compounds are hydrolyzed on cent ethanol. boiling with sodium hydroxide, sodium carbonate, and magB. 1 N hydrochloric acid, nesium oxide with the formation of ammonia (23, 25, 31). C. Benzidine hydrochloride reagent. Purify benzidine hyIf ammonia is distilled prior to the nitrate reduction, possible drochloride and prepare the reagent as follows: Dissolve 10 errors arising from the hydrolysis of such compounds in soil grams of benzidine hydrochloride in 400 ml. of 1 N hydrochloric acid by warming to 50" C. Filter, add 40 ml. of concentrated solutions or extracts are included in the ammonium value. hydrochloric acid with stirring, cool in ice water for 30 minutes, According to Nichols and Foote (25) and Shrikhande (SI), and collect crystals on a Buchner funnel. Wash with cold 1 N this error in the estimation of free ammonia can be eliminated hydrochloric acid, then with two 25-ml. portions of cold 95 per b y distilling the ammonia from a solution buffered a t pH 7.4. cent ethanol and four portions of ether, When dry, transfer to a procedure. \T7ith adequate lighting and appropriate blank corrections, satisfactory accuracy can be obtained.
?
7 -
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water. C. 0.01 N sulfuric acid. D. Standard 0.01 N potassium nitrate. Dissolve 1.011 grams of dry recrystallized potassium nitrate in water and dilute to exactly 1 liter. E. Standard 0.01 N ammonium sulfate. Dissolve 0.6607 gram of pyridine-free ammonium sulfate in water and dilute to exactly 1 liter. F. Nessler reagent (19). “Dissolve 22.5 grams of iodine in 20 cc. of water containing 30 grams of potassium iodide. After the solution is completed, add 30 grams of pure metallic mercury, and shake the mixture well, keeping it from becoming hot by immersing in tap water from time t o time. Cont’inue this until the supernatant liquid has lost all of the yellow color due to iodine. Decant the supernatant aqueous solution and test a portion by adding a few drops thereof to 1 cc. of a 1 per cent soluble starch solution.
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
CILCULATIOK.M. e. of NOaper liter = (m. e. of KOa in aliquot as found by interpolation on Nos curve X 1000) -+ ml. in aliquot. M. e. of NHr per liter = (m. e. of KH4 in aliquot as found bv interpolation on NH4 curve X 1000) +- ml. in aliquot. Precision and -4ccuracy
The seven soil extracts were analyzed for nitrate by this method and by a regular Devarda procedure (39, p. 21) in which the ammonia is collected in boric acid solution and titrated with 0.05 A; sulfuric acid. For this purpose, no preliminary separation of ammonia was made, and the values in Table IX include all nitrogen that \vould be liberated under the analytical conditions. The extracts of soils 84 m d 86 contained so little nitrate that the results by the titration method are probably inaccurrtte. By excluding these from the accuracy comparisons, the average “error” of the semimicromethod is reduced from 4.0 to 1.5 per cent. As for some other ions, the srmimicromethod sometimes may actually provide more accurate results than the comparison method. The colorimetric method s h o m a satisfactory degree of precision.
Discussion The volume of sample used in the semimicroanalysis of the seven soil eytracts comprised to I l 3 6 of thgt used in the comparison methods, with an average of 1/21. This represents B considerable reduction in sample requirements. I n general, semimicroanalysis requires less time, although no quantitative comparisons have been made. The economy effected in the analytical reagents is often important. The accuracy obtainable under these conditions is not seriously reduced, and is adequate for most soil analyses. The average predictable error of the methods is about 2 per cent, based on comparisons 1%ith methods involving much larger samples. Many of the methods are sufficiently precise that replication of determinations usually is unnecessary. The quantitative reproducibility data presented for the various methods should assist in determining the desirability of replication, based on particular requirements.
Acknowledgment The assistance of Betty Mabry, Barbara Pederson, K. R. Goodwin, L. IT. Healton, L. R. Weaver, and A. F. Rendel in developing and testing these methods is gratefully acknowledged.
Literature Cited (1) Allison, L. E., Soil Sci., 40, 311 (1935). (2) Am. Pub. Health Assoc., “Standard Methods for Examination of Water and Sewage”, 8th ed., pp. 64-8, New York, 1936.
Vol. 15, No. 6
(3) Am. Soc. Testing Materials, Part 111, p. 541, 1940. (4) Anderson, M . S.,Keyes, M. G., and Cromer, G. W., U. S. Dept. Ber.. Tech. Bull. 813 (June. 1942). ( 5 ) Assoc. ‘OfficialAgr. Chem., “Official and Tentative Methods of Analysis”, 5th ed., 1940. (6) Barber, H. H., and Kolthoff, I. M., J . Am. Chem. SOC.,50, 1625 (1928). (7) Benedetti-Pichler, A. A , , Cefola, iM.,and Waldman, B., IND. ENG: CHEX.,ANAL.ED., 11, 327 (1939). (8) Blasdale, W. C . , J . Am. Chem. SOC.,31, 917 (1909). (9) Clark, E. P., and Collip, J. B., J. Biol. Chem., 63, 461 (1925). (10) Eaton, F. M., and Sokoloff, V. P., Soil Sci., 40,237 (1935). (11) Fiske, C. H., J . Biol. Chem., 47,59 (1921). (12) Hibbard, P. L., Soil Sci., 8, 61 (1919). (13) Hibbard. P. L.. and Stout. P. R.. J . Assoc. Offccial Am. Chem.. 16, 137 (1933).
Hillebrand, TV. F., and Lundell, G. E. F., “Applied Inorganic Analysis”, p. 510, New York, John Wiley & Sons, 1929. Hirsch, A. A., IND.ENG.CHEM.,ANAL. ED., 14, 943 (1942). Hoffman, W. S.,J . Bid. Chem., 120, 57 (1937). Hoffman, W. S., and Osgood, B., Ibid., 124, 347 (1938). Jacobs, H . R. D., and Hoffman, W. S.,Ibid., 93,685 (1931). Johnston, J., J . Am. Chem. SOC.,38, 947 (1916). Koch, F. C., and McKeekin, T. L., I b i d . , 46, 2066 (1924). Kramer, B., and Tisdall, F. F., J . Biol. Chem., 46, 339 (1921). Morris, V. H., and Gerdel, R. W., Plant Physiol., 8, 315 (1933). Nichols, M.S.,and Foote, M.E., IND.ENG.CHEM.,ANAL.ED., 3. 311 11931).
Peech,M‘., I&., 13, 436 (1941). Pucher, G. W., Vickery, H. B., and Leavenworth, C. S., I b i d . , 7, 152 (1935).
Purvis, E. R., and Higson, G. E., Jr., Ibid., 11, 19 (1939). Reitemeier, R. F., and Richards, L. A., unpublished manuscript. Richards, L. A , , Soil Sci., 51, 377 (1941). Rosenheim, O., and Drummond, J. C., Biochem. J., 8, 143 (1914).
Schollenberger, C. J., Soil Sci., 24, 65 (1927). Shrikhande, J. G., IND. ENG.CHEM.,ANAL.ED.,13. 187 (1941). Smith, G. F., and Gets, C . A., I b i d . , 10, 304 (1938). Treadwell, F. P., “Analytical Chemistry. Volume 11. Quantitative Analysis”, tr. and rev. by W. T. Hall, 7th ed., p. 604, New York, John Wiley & Sons, 1930. Truog, E., and Meyer, A. H., IND.ENG.CHEM.,ANAL.ED., 1, 136 (1929).
Volk, N. J., J . Am. SOC.Agron., 33, 685 (1941). Wall. >I. E., Plant Physiol., 15, 537 (1940). Wang, C. C., J . Biol.Chem., 111,443 (1935). Warder, R. B., Chem. NEWS.43,228 (1881). Wilcox, L. V., “Methods of Analysis Used in the Rubidoux Laboratory, Riverside, Calif.”, mimeographed, Division of Irrigation Agriculture, Bur. Plant Industry, U. S. Dept. Agr., 3rd ed. (January, 1941). Wilcox, L. V., IND.ENG.CHEM.,ANAL.ED., 9, 136 (1937). Zinsadre, Ch., Ibid., 7, 227 (1935). COSTRIBUTION from the U. S. Regional Salinity Laboratory, Bureaus of Plant Industry, Soils, and Agricultural Engineering, Agricultural Research Administration, U. S. Department of Agriculture, Riverside, Calif., in cooperation with t h e eleven western states and t h e Territory of Hawaii. This article is a revised edition of a mimeographed publication given limited distribution since November, 1941.
