Quantitative Microdetermination of Magnesium in Plant Tissue and Soil Extracts A Rapid Colorimetric Method MATTHEW DROSDOFF AND D. CHARLES SEARP-ISS Bureau of Plant I n d u s t r y , Soils, a n d Agricultural Engineering, C'. S . D e p a r t m e n t of Agriczdture, Gainesrille, Fla.
-4rapid quantitative colorimetric method is proposed for the determination of magnesium in plant tissue and soil extracts. It is based on the use of thiazole )-ellow and a compensating starch solution to give a reproducible color that can be measured in a photoelectric colorimeter. It is not necessary to remove iron, aluminum, manganese, phosphorus, or calcium. Both the precision and accuracy are well within accepted standards for colorimetric methods.
Transfer the aliquot to a 50-nil. volumetric flask and add enough n-ater to bring the volume to about 25 ml. ildd 1 ml. of the hydroxylamine hydrochloride solution and 5 ml. of the starch-compensating solution and mix the solution. -4dd exactly 1 ml. of thiazole yellow solution and mix the solution, and then add 5 ml. of the sodium hydroxide solution. Bring the flask to volume with water, mix thoroughly the contents, and allow them to stand for 10 minutes before reading on a photoelectric colorimeter using a green filter. The color is stable for several hours. The concentration of magnesium is obtained from a sta,ndard curve as described belox.. A Cenco photelometer v i t h the green filter ,525 p Tyas used in this work. The colorimeter is set to read 100% transmittance m-ith a blank solution that is run by the same procedure used rvith the samples and standards.
QUANTITATIVE procedure for determining small amounts of magnesium in plant material by using titan yellow has been described by Kidson ( 2 ) . Gillam ( 1 ) described a titan yellow method for determining magnesium in fertilizers and soil extract,s. I n both methods iron, aluminum, phosphorus, and calcium have to be removed and ammonium salts destroyed by time-consuming processes. Peech and English (4)examined the titan yellow procedure carefully and developed a direct, rapid semiquantitative met,hod for the determination of magrirsium in soil extracts in the presence of calcium, iron, aluminum, and mznganese. By using their procedure as a basis, and thiazole yellow as recommended by Mikkelsen and Toth (3) instead of tit'an yellow, a rapid quantitative colorimetric method has been developed for determining magne4um in tung leaves, and probably can be used for other plant tissue.
STANDARD CURVES
REAGESTS
Thiazole Yellow, 0.10%. Dissolve 0.10 gram of thiazole yellow (obtained from General Dyestuffs Corporation, Yew York, S . Y.) in 100 ml. of water and store in a dark bottle. This reagent will keep a t least 2 months under ordinary conditions. Hydroxylamine Hydrochloride, 5%. DiPsolve 25 grams of hydroxylamine hydrochloride in water and dilute to 500 ml. Store in a dark-colored bottle. Sodium Hydroxide, 2-5 N . Dissolve 100 grams of sodium hydroxide in r a t e r and dilute to 1 liter. Starch Solution, 2 % . To 2 grams of C.P. soluble qtarch add enough lvater t o make a paste and add slowly, lvhile stirring, the remainder of 100 ml. of hot water. Filter if nwesmry. This reagent should be prepared freshly as peeded. Best results were obtained with Baker and Adamson's reagent grade. Compensating Solution. Dissolve 3.7 grams of calcium chloride (CaC1?.2H?O),0.74 gram of aluminum sulfate (A41a(SOc)a.18H,O), 0.36 gram of manganous chloride (MnC12.4H2O),and 0.60 gram of sodium phosphate (SaaPOd) in about 500 ml. of water containing 10 ml. of concentrated hydrochloric acid. Dilute to 1 liter. Starch Compensating Reagent. Mix equal volumes of the starch solution and compensating solution. Prepare daily as needed. Standard Magnesium Solution (25 p.p.m. of magnesium). Dissolve 250 mg. of reagent grade magnesium metal in dilute hydrochloric acid solution (150 ml. of water plus 10 ml. of concentrated hydrochloric acid) and bring to volume in a 250-ml. volumetric flask. Dilute this solution 1 to 40 for the !Torking standard of 25 p.p.m. of magnesium. PROCEDURE
Take an aliquot of the hydrochloric acid solution of plant ash that should contain 0.025 to 0.15 mg. of magnesium. IVith tung leaves a convenient procedure has been to ash 2 grams of dry ground material, dissolve the ash in dilute hydrochloric acid, and make up the solution to 100 ml. A 1-ml. aliquot of this solution is satisfactory to cover the range 0.10 to 0.707c of magnesium in the leaves on a dry basis. For smaller or larger concentrations than this the aliquots should be adjusted accordingly.
673
U-hen 0-, I-,2-, and 5-nil, aliquots of the Ftandard solution o f 25 p.p.m. of magnesium \\-ere analyzed exactly as described above and the readings plotted on semilog paper, a straight-line relationship n-as obtained. Because of temperature variations and possibly other factors, good reproduribility of the standard curve was not obtained. Therefore for the highest accuracy and precision it is necessary to run a 5-ml. aliquot of the standard solution in duplicate nit,h each set of determinations. -4curve for the set is then const,ructed by drax-ing a straight line from 0 eoncentration and 1 0 0 ~ ctransmittance through the point 01)tained in the reading of the standard. It is ronvenicnt to prcpare in advance a number of curves, one or two on each sheet of graph paper, and t o use the appropriate curve for earh set of determinations. The colorimeter reading for the 5-ml. aliquot of the standard usually is in the range 75.0 to 79.0 viith the Cenco photelometer used in this laboratory. ANALYTICAL RESULTS
The precision of the method was tested by analyzing 13 samples of tung leaves from widely different localities, ranging in magnesium content from 0.11 to 0.88% on an oven-dry basis. Each sample was ashed in triplicate, and triplicate determinations were made on each of the ashings. Thus nine determinations were made on each sample. A statistical analysis of the data obtained showed that the standard deviation of a single determination is 0.019%. Hence the standard deviation of the difference between two duplicate determinations is 0.027. This means that about two thirds of duplicate determinations should agree within 0.027'% and 95W of them should agree 15-ithin O . O N % . The standard error is 4y0 of the mean of all determinations, which is well within accepted standards for colorimetric methods. The accuracy of the method vias tested by comparison with the standard volumetric procedure of titrating the magnesiumammonium phosphate precipitate with standardized 0.1 lV hydrochloric acid. Ten samples of t,ung leaves from widely
ANALYTICAL CHEMISTRY
674 Table I.
Magnesiiini Determimations on T u n g Leaf Samples (70 Magnesium. D r y Weight BaalsUVolumetric Colorimetric
Sa inpic
0.14 0.29 0.59 0.33 0.33 0.37 0.39 0.46 0.54 0.61 0,376 a
0.15 0.27 0.31 0.33 0.34 0 34 0.33 0.45 0.52 0.58 0 364
Mean of triplicnte detrrminationz.
different localities and ranging in magnesium content irom 0.14 to 0.61% by the standard method were used for the comparison. Triplicate determinations were run on each sample by the two methods. The data are given in Table I. The values obtained by the two methods were in close agreement; the mean of the colorimetric determinations was lower than that of the volumetric determinations, but the difference was not statistically significant (Table I). It was noted that the differences between volumetric and colorimetric readings were greatest for high percentages of magnesium. The coefficient of correlation 1% as found to be 1-0.992 and the coefficient of regression of volumetric on colorimetric readings was 1.069 * 0.048 ( 5 ) . As the regression coefficient does not differ significantly from unity, the trend obser;ed in the ten samples of Table I is probably due to chance and one would not be justified in using values calculated from the regression equation. The method was found applicable to the determination of exchangeable magnesium in soils, using a 0.1 S acid solution of the ammonium acetate extract. Eight soil samples from different parts of the tung belt were run for exchangeable magnesium by the method described above and by the standard gravimetric procedure, using 8-hydroxyquinoline. Duplicate determinations were made on each sample by the two method3 (Table 11). The values obtained by the two methods were in good agreement. The coefficient of correlation was found to be 0.9985; and the coc4firient of regression of gravimetric on colorimetric valrir. I\ a c 1.007 * 0 025. n-hich has high statistical qgrific-airct~
+
/
DISCUSSION In addition to the calcium and aluminum in the compensating solution suggested by Peech and English ( 4 ,it was found necessary to add manganese and phosphorus as well. Although hydroxylamine was used to prevent manganese interference a s recommended by Peech and English, some samples high in manganese gave results that were too high. Addition of manganese chloride to the compensating solution corrected this error satisfactorily. Because there was some intensification of color in the presence of phosphate ions, it is necessary to include phosphate in the compensating solution. The amount of phosphate as well as calcium, aluminum, and manganese added in the compensating solution is on the average about five times as much as is present in the aliquot of the ash solution being analyzed, so that the effects of these ions present in the sample are negligible.
Table 11. Determinations of Exchangeable Magnesium in Soils Exchangeable hlagnesium per 100 Grams of Snaa
Location hlorriston, Fla.
Soil Type Lakeland fine sand
Hague, Fla.
Ft. hIeade loamy fine sand Orangeburg sandy loam Ora fine sandy loam Pheba fine sandy loam Gainesville fine sandy loam Magnolia, Miss. Ruston fine sandy loam a X e a n of duplicate determinations.
Fairhope, Ala, Folsom, La. Sunnyhill, La. Hague, Fla.
Depth, Inches
Colorimetric, thiazole yellow, m.e.
Gravimetric, S-hydroxyquinoline,
7-14 0-6 0-6
0.05 0.16 0.25
0.05 0.18 0.21
0-6 0-6
0.29 0.32 0.64 1.04
0.27 0.30 0.62 1.22
12-36
2.73
2.96
0-5 0-6
m.e.
LITERATURE CITED
(1) Gillam, W. S., IND.EXG.CHEM.,ANAL.E ~ . , 1 3499-501 , (1941). (2) Kidson, E. B., New Zealand J. Sci. Technol., 27, 411-13 (1946). (3) Mikkelsen, D.S., and Toth, S. J., J. Am. SOC.Agron., 39, 165-6 (1947). (4) Peech, Michael, and English, Leah, Soil Sci., 57,167-95 (1944). (5) Snedecor, G. W., “Statistical Methods,” 4th ed., Amm, Iowa, Iowa State College Press, 1946. R ?:cmvt:n Septeiiihrr 2 , 194i.
Microdetermination of Carbon Monoxide in Air A Portable Instrument .4RSOLD 0. BECKMAN, JAMES D. JIcCULLOUGH, AND ROBERT A. CRANE Laboratories of Arnold 0 . Beckman, Pasadena 2 , C a l v . , a n d Cniversity of California,LOS .Ingeles 24,
T
H E instrumelit described was developed in response to a request from the Office of Scientific Research and Development. Three models of the instrument were built; two were intended for laboratory use only, and the third mas designed for field use by inclusion of an internal battery. The instrument makes use of two chemical reactions, both of which go to completion a t 180” C.
CO (gas)
+ HgO (s, red) = C o n (gas) + Hg (gas, + SeSz (s) = PHgS is) + HgSe (s’l
3Hg (gas)
11)
(2) Although neither of these reactions is new, the conditions and method of application are radically different from those used by previous investigators. Both reactions have been evtensively
calv.
studied during the present investigation. In the case of Reaction 1 this study led to an accurate gravimetric method for the determination of carbon monoxide in air, based on the loss in weight of the mercuric oxide. The analytical procedure as well as the study of Reaction 1 has been described in an earlier communication (4). Sordlander (5) using Reaction 2 developed an instrument for the detection and semiquantitative determination of mercur) vapor in air by use of a selenium sulfide paper prepared by coating the precipitated material onto white bond paper. The air stream being tested for mercury was heated to 70” C. and caused to impinge normally on the coated surface. The intensity of blackening as measured by photometry of the exposed paper gave an approximation of thP mercury concentration in the air.
