Colorimetric Microdetermination of Boron JAMES A. NAFTEL, Alabama Agricultural Experiment Station, Auburn, Ala.
R
ECOGNITION of the practical importance of boron in relation to agriculture is one of the recent interesting contributions of science. The failure to realize the full importance of boron earlier is due in part to the lack of a method of analysis sensitive enough to determine the small amounts usually present in soils or plants. Furthermore, the boron requirements of plants are confined to a narrow range of concentrations to avoid toxicity on one hand and deficiency of boron on the other. For crop plants in general, the concentration of soluble boron in soils should not exceed approximately one part per million of soil. A micromethod for the determination of boron in soils and plants is evidently desirable. The early methods of analysis for boron were chiefly designed for macroamounts by separation and estimation by gravimetric or volumetric procedures (3, 6). Later, colorimetric methods for microamounts of boron were proposed (1, 8, 4, 6 ) ,but these methods were not satisfactory from the standpoint of rapidity or accuracy. Accordingly, this investigation was begun for the purpose of designing a method of analysis which would be sufficiently sensitive, accurate, and rapid for routine analysis of soluble boron in soils and in plants. The need of a method for determining 1 to 10 micrograms of boron, and the desirability of employing small samples, 10 grams of soil or 1 gram of plant material, led t o an investigation of colorimetric methods. The method proposed by Cassal and Gerrans (2) and the colorimetric method of Bertrand and Agulhon ( I ) offered sufficient promise in their sensitivity but seemed undesirable in the details of procedure and accuracy. The modification of the latter method proposed by Scott and Webb (6) is not entirely satisfactory, especially because it is necessary to employ extremely small final volumes of solution. Apparently, no further study has been made of the method of Cassal and Gerrans, which involves the distinctive use of oxalic acid instead of acetic acid in the boric acid-curcumin test, since it was reported in 1903. It was decided to follow the method of Cassal and Gerrans, in using colored solutions rather than test papers. The method of Cassal and Gerrans is tedious and long. The solutions are evaporated to dryness several times; the volatilized boron is recovered in potash bulbs and finally added to the original boron not volatilized. It appeared that the above objection could be eliminated by utilizing certain steps in the method of Gooch (5) whereby solutions containing boron might be evaporated to dryness and the residue ignited without the loss of boron if an excess of calcium is present. Other details of the method investigated were concentration of reagents, time and temperature of drying residue, effect of volume, and method of measuring colored solutions.
residue is taken up in 95 per cent ethyl alcohol, clarified by filtering or centrifuging, and compared with standards. REAGENTS REQUIRED. A 0.10 N suspension of calcium hydroxide. Solution containing 20 ml. of concentrated hydrochloric acid and 80 ml. of a saturated solution of oxalic acid prepared each day. A 0.10 per cent curcumin or 1.0 per cent turmeric extract in 95 per cent ethyl alcohol. Shake occasionally for 4 to 6 hours and filter; this extract should be prepared daily. Ethyl alcohol, 95 per cent. Standard solution of boric acid.
Procedure Place an aliquot of a soil extract or plant ash extract, containing from 0.5 to 8.0 micrograms of boron in a porcelain evaporating dish. Render the extract alkaline by adding 5 ml. or more of a 0.10 N calcium hydroxide suspension and evaporate to dryness at full heat on a water bath. Remove the dish and allow to cool t o :oom temperature, at the same time cooling the water bath t o 55 * 3' C. To the cooled residue add 1ml. of the solution containing 80 ml. of a saturated solution of oxalic acid and 20 per cent hydrochloric acid, and 2 ml. of a 0.10 per cent extract of curcumin or l per cent turmeric. Rotate the dish so that the reagents come into contact with all the residue and evaporate to dryness on the water bath at 55" C. Continue heating for 30 minutes at this temperature, then remove the dishes and allow t o cool. Extract the residue with 95 per cent ethyl alcohol and transfer with a policeman t o a filter or to a 15-ml. centrifuge tube. Filter and wash thoroughly with ethyl alcohol or throw down the solid phase with the centrifuge (about 10 minutes at 1500 r. p. m.) and dilute the liquid phase to constant volume; 25-ml. volume is convenient when using a 20-ml. cell with a colorimeter. Compare these solutions with standard solutions similarly prepared. The range of concentrations for the standards which have been used
Method The color reaction occurs when a solution of boric acid and oxalic acid is evaporated to dryness with curcumin; the solution is obtained when the dried residue is extracted with ethyl alcohol. An aliquot of the solution containing boron is rendered alkaline with calcium hydroxide, evaporated a t full heat on the water bath, and then cooled to room temperature. Then oxalic acid and either curcumin or an extract of turmeric are added and evaporated to dryness at 55' C. and further heated a t this temperature for 30 minutes. The
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FIGURE 1. CALIBRATION CURVEOF BORON STANDARDS OBTAINED WITH ELECTROPHOTOMETER 407
INDUSTRIAL AND ENGl[NEERING CHEMISTRY
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OF OXALICACIDON INTENSITY OF COLOR OF TABLEI. EFFECT STANDARD BORON SOLUTIONS
VOL. 11, NO. 7
DIFFERENT VOLUMESAT SAME CONCENTRATIONS OF BORON.Two concentrations of a standard boron solution
and blanks a t volumes of 25,50, and 100 ml. were analyzed to determine the effect of dilution on the results. As may be Boron seen in Table 111, there is little variation in the observed Solutions No. readings a t the same concentration. P . p. m. BORON IN SOILEXTRACTS FROM GREENHOUSE SOILCULTURES. In order to test the proposed method on soil extracts and to determine whether differences in soluble boron resulting from treatment in the greenhouse could be determined by 20.0 1 52.1 0.16 5 21.2 11-12 the method, 10 grams of soil were extracted with 50 ml. of 38.0 2 49.5 52:5 0.16 10 40.0 13-14 solution by three different methods, These results are sum48.2 3 45.7 48.0 0.16 20 48.4 15-16 marized in Table IV. There were no difficulties encountered in the procedure, and with the exception of the two lowest amounts of boron in the extracts, satisfactory agreement between duplicate determinations was obtained. BORONIN PLANTMATERIAL.Soybean hay from greenOF CURCUMIN AND TURMERIC ON INTENSITY house liming experiments was analyzed by the proposed TABLE11. EFFECT OF COLOR OF STANDARD BORON SOLUTIONS method for boron. Duplicate 1.0-gram samples of dried and 1.0 Per Cent Turmeric 0.10 Per Cent Curcumin ground plant material were ashed and dissolved with 1 cc. of CurcuScale Tu:Scale No. Boron min readings Boron meric readings N hydrochloric acid. The extracts were filtered, the residue P.p.m. M1. P. p . m. MI. was washed with hot water, and the filtrate was diluted to 0 2.0 0 0 0 0 0 1-2 2.0 50 ml. Duplicate aliquots of the ash extract were analyzed 2.0 2-4 0.08 33.3 33.4 2.0 0.04 17.6 18.0 _ 2.0 43.7 44.3 0.12 51.0 51.0 2.0 0.16 5-6 and the results are shown in Table V. Very satisfactory 2.0 0.16 54.5 51.2 66.6 66.5 2.0 11.32 7-8 results were obtained by this procedure on plant material. 2.0 0.20 57.6 57.5 0 0.5 3.0 9-10 1 M1. of Oxalic Acid, of Different Concentrations Per cent solution Scale readings
11-12 13-14 15-16 17-18 19-20 21-22 23-24
6.04 0.16 0.32
3.0 3.0 3.0 4.0 4.0 4.0 4.0
18.0 53.0 67.5 0 19.5 52.2 66.7
18.8 52.5 67.6 0 18.4 53.5 66.7
Increasing Amounts of 20 Per Cent Oxalic Acid Scale readings MI.
n 6.08 0.16 0.20
4.0 4.0 4.0 4.0
0 0 32.3 33.2 52.0 51.2 57.5 57.5
CONTAMINATION FROM ELEMENTS OTHER THANBORON.
In order to determine whether other elements would affect the method for boron, an aqueous extract from a fertile soil .. 0.04 .. .. 0.16 was added to known amounts of standard boron solutions. .. .. 0.32 The solutions were analyzed and are reported in Table VI. The boron found in the soil extract and that in the standard solution are additive, and it appears that the ions other than EFFECT OF DIFFERENT VOLUMES AT SAME CONCENTABLE111. boron did not contaminate the solutions as far as this method TRATIONS ON INTENSITY OF COLOR OF STANDARD BORON SOLUTIONS is concerned. Solutions Scale Readings I n the event that solutions containing excessive amounts No. Boron Volume Of duplicates Average of soluble salts are to be analyzed for boron and found t o P . p . m. M1. interfere with the determination, boron may be separated by 0 0 25 0 1-2 0 39.0 39.65 25 40.3 0.10 3-4 volatilization with methyl alcohol in the usual way. The 58.7 57.80 0.20 25 56.9 5-6 boron content may then be determined colorimetrically as 7-8 0 50 0 0 outlined above. 9-10 0.10 50 39.2 46'4 39.80 11-12 13-14 15-16 17-18
n
0.20 0 0.10 0.20
50
100
100 100
59.0 0 38.2 56.3
59.8
38'0 54.7
59.40 0 38.10 55.50
is 0.02 to 0.32 p. p. m. of boron in a 25-ml. volume or 0.5 to 8.0 micrograms of boron diluted from a standard containing 1p. p. m. of boron. A calibration curve for this range is shown graphically in Figure 1, as obtained with a Fisher electrophotometer.
Tests of Method The proposed colorimetric micromethod for boron has been subjected to varied tests of changes in concentration of reagents as well as on different boron-containing substances. Representative results of some of these trials are given below. OXALICACID REQUIRED.A standard solution of boric acid containing 1p. p. m. of boron was analyzed with increasing amounts of oxalic acid. The results, shown in Table I, indicate that a t least 1 ml. of a 20 per cent solution of oxalic acid is required while higher concentrations have little effect. CURCUMINOR TURMERIC EXTRACTREQUIRED. The standard solution of boric acid and blanks were analyzed with 2, 3, or 4 ml. of a 0.10 per cent solution of curcumin or with 2 and 4 ml. of a 1 per cent solution of turmeric. The results are shown in Table 11. When the boron solutions containing different amounts of either curcumin or turmeric are read with their corresponding blanks the results are almost identical.
TABLEIV. DETERMINATION OF SOLUBLE BORON (Extracted by different methods from Ruston sandy loam obtained from greenhouse liming experiment) -Boron ExtractedSoil Treatment Test 0 10 Boiling Refluxing No. N HC1 water with water Lime Borax Lb./acre P.p.m. P.p.m. P.p.m. 1 0,090 0.120 0.888 None 0 2 0.090 0.096 0.864 Av. 0.090 0.108 0.876 1 0,030 0.048 0.360 150% Ca saturated 0 2 0.060 0.072 0.420 Av. 0.045 0.060 0.390 1 0.300 0.336 1.280 2 0.256 0.360 1.100 150% Ca saturated 15 Av. 0,278 0.332 1.190
TABLEV. BORON CONTENT OF SOYBEAN HAY (Grown in greenhouse with varying lime and borax treatments) Soil Treatment Lime, per cent Ca No. saturation Borax Scale Readings Dry Boron Plants in Lb./acre P. p . m. 1-2 m 0 34.7 33.5 10.5 - _. 150 0 24.5 3-4 7.0 0 24.0 5-6 150 7-8 50 15 55 7 25.0 9-10 50 15 58.0 57.3 11-12 150 50.0 15 7 0 . 98 13-14 I50 15 71 15-16 Blank .. 0 0 ..
r;y:ii4
JULY 15, 1939
ANALYTICAL EDITION
TABLEVI. EFFECTOF IONSOTHERTHANBORONON DETERMINATION OF BORON No.
@
Soil Extraeta Added M1.
60.0 grams of
Standard Boron Solutions Boron ~1.p.p.m.
Boron Found P.p.m.
Total Boron Present P.P.~.
Decatur clay extracted for 24 hours with 400
OD.
Error
%
of water.
Discussion The proposed method is most satisfactory when used with a photoelectric colorimeter in routine determinations. A calibration curve is readily formed from the readings obtained with a series of standards. From this curve, a table may be made to facilitate calculations of concentrations of boron in the samples. Obviously, the amount of curcumin or turmeric per unit volume must be kept identical with that of the blank; when this is done, dilutions of deeply colored solutions may be employed. Where a photoelectric colorimeter is not available, series of standards or balancing methods may be used but these methods require more time and are less accurate. The fact that the colors are reduced on standing after approximately 2 to 3 hours renders the series of standards method laborious unless artificial standards are used. The substitution of 50 per cent ethyl alcohol, by volume, for the 95 per cent ethyl alcohol used for extracting the colored
409
residue gave satisfactory results when the solutions were read within an hour; on standing the aqueous extracts faded more rapidly than the alcohol extracts. The substitution of 1 per cent turmeric extract for the 0.10 per cent curcumin is advisable, since the former is inexpensive and does not “crawl” in the evaporating dishes as much as the curcumin. These extracts deteriorate on standing in light and it is recommended that the extracts of turmeric be prepared daily. All reagent flasks should be boron-free, since either acids or bases might extract boron from glass containing this element; Kavalier Bohemian glass is satisfactory. Calcium hydroxide was used to prevent the volatilization of boron and the 0.10 N suspensions served the purpose of obtaining a more intimate contact of the reacting substances a t the drying point, the point a t which the color is developed.
Summary
A microgram procedure for a colorimetric microdetermination of boron involving the reaction between boric acid in the presence of oxalic acid and curcumin is outlined. It is accurate for the extremely low amounts of boron generally found in soil extracts and in plants. Literature Cited (1) Bertrand, G., and Agulhon, H., Bull. SOC. chim., 7, 90, 125 (1910). (2) Cassal, C. E., and Gerrans, H., Chem. News, 87, 27 (1903). (3) Chapin, W. H., J . Am. Chem. SOC.,30, 1691 (1908). (4) Holmes, W. C., J . Assoc. Oficiul Aor. Chem., 10, 522 (1927). (5) Gooch, F. A., Am. Chem. J., 9, 23 (1887). (6) Scott, W. W., and Webb, S. K., IND.ENQ.CHEM.,Anal. Ed., 4, 180 (1932).
Microviscometer JOHN R. BOWMAN, Mellon Institute of Industrial Research, Pittsburgh, Penna.
A microviscometer is described having absolute accuracy better than 4 per cent, and precision within 0.1 per cent in the range from 2 to 10,000 centistokes. The method is simple and rapid, and requires only one drop (about 0.03 gram) of sample.
continuous below the meniscus. The method is capable of good precision but is slow, because experimentally determined corrections must be made for surface tension and drainage. Levin’s method (1) is rapid, but not capable of great accuracy. A short capillary dipping into a minute reservoir is again employed, but the transit of the meniscus rising with surface tension is timed. Many inherent errors exist in this method which cannot be easily corrected.
N CONNECTION with some exhaustive fractionation of oils a t this laboratory, a new viscometer has been developed. The instrument was designed primarily for examining extremely small samples, less than 0.1 gram, but is so simple, rapid, and accurate that it is believed to be as satisfactory in general viscometry as any of the popular macro types. The apparatus is entirely self-contained, and stands about 60 cm. (24 inches) high from a base 25 X 30 cm. (10 x 12 inches). While not intended for use as an absolute method, accuracy better than 4 per cent is obtainable in this sense; the precision is much closer, within 0.1 per cent. Even in the hands of an unskilled operator, the complete cycle of a determination, including sampling, charging, timing, and cleaning, requires less than 10 minutes. The microviscometer discussed here is of the capillary type, but differs from the numerous modifications of the Ostwald pipet in that it has no bulb. Two such instruments have been described. That of Lidstone (2) depends in principle on the fall, under gravity, of a liquid column previously drawn up into the capillary from a small reservoir, the column being
Principle
1
The present method depends on the rate of fall, under gravity, of a short segment of liquid contained in a longer capillary. The tube is vertical, straight, and of uniform bore; a length of 25 cm. has been found convenient. It bears three etched marks, two near the bottom and one near the top, and is shown with its vapor jacket in Figure 1. In making a determination, a minute drop of the liquid is placed on the open lower end of the tube, and, with the aid of suction, a column is drawn up t o the first mark, C. The lower end is then wiped clean with filter paper or a lintless cloth, and suction again applied until the upper meniscus of the column segment is drawn above the top mark, A. Finally, the upper end of the capillary is opened t o the air, and the transit of the upper meniscus between marks A and B is measured with a timing device.
Theoretical Discussion The classical law of Poiseuille for the flow, u, through a tube of radius r and length 1, under a pressure p is as follows:
