V O L U M E 26, NO. 2, F E B R U A R Y 1 9 5 4
411
The method of analysis using aluminum powder was more rapid and required fewer reagents and less equipment than the reductor procedures.
interfere with reduction by aluminum. They would have to be removed prior to reduction or corrected for by a determination using some other method.
PROCEDURE FOR A S S A Y O F AMMONIUM MOLYBDATE
4(:h?OWLEUC:hlENT
A weight of sample chosen to yield from 1 to 2 meq. of molybdenum was transferred to a 125-ml. Erlenmeyer flask and dissolved in 25 ml. of water. Then 14 meq. of No. 400 aluminum povder were added. The sides of the flask were washed down with 25 ml. of 2.50M sulfuric acid. A supply of nitrogen was passed into the flask through a tapered glass tube. The sample was then heated on a small electric hot plate turned to low heat. After the aluminum had dissolved, the sample was heated for 5 more minutes with nitrogen bubbling through the mixture. Then the sample was removed from the hot plate and was immediately titrated with standardized potassium permanganate solution. The nitrogen stream was kept on during the titration and provided the necessary stirring. DISCUSSION
The silver reductor procedure ( 1 ) for the determination of niolvbdenum is somewhat inconvenient, since i t is necessary to preheat the sample, the reductor, and the wash solutions. The Jones reductor procedure (3) is more convenient, but requires the use of iron(II1) alum solution. Rahm (5) haq rcwntly mrntioned some of the disadvantag6q inherent in t h r use of the Jones reductor. The use of finelv divided aluminum powder as the reducing agent requires the use of an inrrt atmosphere to prevent oxidation of molybdenum(II1) by air. This was acromplished by passing nitrogen gas from a cylinder into the Erlenmeyer flask used for the reduction and titration. T h r nitrogen also provided thr necessary stirring. .4ny substances that arc reducible with the Jones ieductor
The authors wish to thank the ,%luminum Co. of America for providing samples of foil, and thc Reynolds Metals Co. for providing samples of powder. LITERATURE CITED
( I ) Rirnbaum, N., and Walden, G. H., Jr., J. Am. Chem. Soc., 60,
64 (1938). (2) Boulanger, C., Compt. rend., 191, 56 (1930). (3) Hillebrand, W. F., and Lundell, G. E. F., “Applied Inorganic Analysis,” p. 249, New York, John Wiley & Sons, 1929. (4) Hiskey, C. F., J . A m . Chem. Sac., 61, 3125 (1939). (5) Rahm, J. A , , ANAL.CHEM.,24, 1832 (1952). (6) Riepel, E. R . , and Schwarta, R. D., Ibid., 24, 1803 (1952). (7) Furman, li. €I., “Scott’s Standard Methods of Chemical Analysis,” 5th ed., Vol. I, p. 593, New York, D. Van S o s trand Co., 1939. ( 8 ) Sidgwick, N. V., “The Chemical Elements and Their Compounds,” Vol. I, p. 1055, London, Oxford University Press, 1950. (9) Willard, H. H., and Diehl, €I., “Advanced Quantitative Analysis,’’p. 227, Kew Tork, D. Yan Nostrand Co., 1943. (10) Willard, H. H., and Furman. S . H., “Elementary Quantitative Aiialysis,” 3rd rd., 1). 203, Yew York, D. Van Nostrand Co., 1940. RECEIVED for review May 2 , l958.
Accei>ted September 29, 1QZ3. PEwnted before the Division of Analyticul Cheinistry a t the 123rd Meeting of SocxE,rY, Lo.? Angdes, Calif., 1953. Taken in part the AYEHICANCHEMICAL from a thesis submitted by I?. D . Schwarta t o the Graduate School of the University of Buffalo in partial inlfill~nrntof the requirements for the df-grre of doctor of philosophy.
Spectrophotometric Determination of Sodium Hypophosphite as a Molybdenum Blue Complex ANTHONY P. SCANZILLO Thomson Laboratory, General Electric Co., Lynn, Mass.
T
HEIU are only a few quaiititative chemical mcthods for drtermining sodium hypophosphitc or hypophosphorous acid. In the gravimetric methods, hypophosphorous acid is converted into phosphoric acid by adding nitric acid to an aqueous solution, evaporating to a small volume, and completing the oxidation according to Treadwell and Hall (9) or according to the Xnierican Society for Testing Materials method ( 1 ) . The phosphoric acid or phosphate may then be deterininrd hy conventional methods ( 4 , 11). In the volumetric detrrmiiiation, hypophosphorus acid and hypophosphites are oxidized with potassium permanganate (5) or bromide-bromate solution (4).anti thr rscess is determined iodometrically. Treadwell and Hall (IO) include a procedure for dctcrmining phosphorous and hypophosphorous acids in mistures by an indirect analysis. One portion of the samplc is osidized to phosphoric acid, and the phosphate is drtermined as magnesium pyrophosphate. A second portion is treated with mercuric chloride. and the mercurous chloride formed is neighed. From these data the amount of each acid present may be determined by use of simultaneous equations. In this laboratory, however, the results wrre very low when the mercuric chloride method was used on solutions containing known amounts of sodium hypophosphite. It is possible that some of the mercuric chloride is reduced to metallic mercury (8). The method developed is simple and applicable in the presence of sodium phosphitp. According t o Millard (6, 7), hypophos-
phorous acid or hypophosphites give a blue coloratiori 151th ammonium molybdate and sulfurous acid. Ebaugh and Smith (3, 6) claimed that molybdic ncid is reduced t o the pentoside and that the method is not reliable for quantitative mark. However, it KRS found that thc color produced obeys Beer’s I:rn METHOD
Reagents. Ammonium lirpta molybdate, loyo. Dissolve 100 grams of C.P. ammonium molyldate in 600 ml. of hot water Digest on 3, steam bath, cool, and dilute to 1 liter with distilled water. Sulfurous Acid, assay 6% sulfur dioxide by volume. Merck’s reagent grade gave the lowest blank and most consistent result*. Apparatus. Transmittancy measurements were made with a Coleman Model 14 Universal spectrophotometer, using matc~hed round cuvettes, 19 mm. in diameter. Procedure. For solutions of high concentration, take an a-ml. aliquot containing not more than 0.5 gram of sodium hypophosphite (NaH2P02.H20) and dilute to 1000 ml. in a volumetric flask and mix well. Transfer a b-ml. aliquot containing not more than 2.5 mg. of sodium hypophosphite to a 100-ml. volumetric flask and add water t o bring the volume t o 10 ml. Add 10 ml. of ammonium molybdate solution and 10 ml. of sulfurous acid and heat in a boiling water bath for 3 minutes. Let stand a t room temperature for 1 minute, then cool immediatelv in running water. Prepare a blank by taking 10 ml. of distilled water through the same steps as the sample, including the addition of the reagents. Read the percentage transmittancy of the sample a t 470 mp against the prepared blank. Then read the
ANALYTICAL CHEMISTRY
412 milligrams of sodium hypophosphite present in 100 ml. of the solution from a previously prepared standard curve.
R.lg’ Of NaH2P02’H20 a x 6
‘Oo0
=
grams of S a H 2 P 0 2 . H 2per 0 liter
of original sample Preparation of Standard Curve. Prepare a solution containing 0.5 gram of sodium hypophosphite per liter. By means of a microburet, transfer 1-, 2-, 3-, 4-, and 5-ml. aliquots to 100-ml. volumetric flasks and add water to bring each volume to 10 ml. Treat each aliquot as the 6-ml. aliquot in the procedure. REPRODUCIBILITY OF RESULTS
Several determinations on a sodium hypophosphite solution, prepared to contain 0.5 mg. of sodium hypophosphite per ml., were run on different days over a 4-week period. Measurements were made with a Coleman Model 14 Cniversal spectrophotometer (Table I).
was not thoroughly studied. However, aliquots taken from solutions buffered to a pH of 5 gave very good results. The transmittancy of the colored solutions did not show a noticeable change over a 10-minute period. However, the solutions should be kept stoppered to prevent loss of sulfur dioxide. DISCUSSION
The method presented in this paper may be used in conjunction with the molybdivanadophosphoric acid method for the determination of phosphorous and hypophosphorous acids in mixtures. An aliquot is taken and treated by the method given in this paper, and the sodium hypophosphite is determined. A second aliquot is taken, the phosphorous and hypophosphorous acids are oxidized to phosphoric acid (1, 9), and the phosphate is determined photometrically by the molybdivanadophosphoric acid method (2) and reported as total sodium hypophosphite.
WAVE LENGTH
A spectral transmittance curve obtained with a General Electric recording spectrophotometer is shown in Figure 1. hlaximum (rather than minimum) transmittancy is indicated a t 470 I n order to obtain a suitable range, a working curve was constructed a t this wave length. Measurement in the 680-mp range gives greater sensitivity. mG.
VARIABLES
Effect of Molybdate Concentration. The molybdate concentration is not critical. From 5 to 10 ml. of molybdate reagent may be added to the unknown solution, but the same quantity should be added to the sample and the blank. Effect of Sulfurous Acid Concentration. The sulfurous acid additions are not critical. From 10 to 15 ml. of sulfurous acid may be added to the unknown solution, but the same quantity should be used as was used in preparing the calibration curve. Each bottle of sulfurous acid should be tested by carrying a standard through all the steps of the method in order to check the calibration curve. Effect of Diverse Ions. Phosphites give no color reaction with ammonium molybdate and sulfurous acid, as shown by Table 11, but phosphates interfere seriously. The only metallic ion tested was nickel in the form of nickel chloride. Tests sho\T that this compound does not interfere. Effect of Heating. Heating in boiling water for over 4 minutes greatly increases the color density of the blank. Other Variables. The effect of acidity on this determination
400 20 40 60 Bo 500 20 40 60 8 0 600 20 4 0 60 80 700
WAVELENGTH IN MILLIMICRONS
Figure 1. Typical Spectral Transmittance Curve of Color Produced by adding ammonium molybdate and sulfurous acid to a sodium hypophosphite solution
Subtracting the sodium hypophosphite from the total sodium hypophosphite and multiplying by the appropriate factor gives the sodium phosphite. LITERATURE CITED
Table I.
Analysis of Sodium Hypophosphite Solutions Standard Deviation, Mg. 0.015 0.027 0.028 0.034 0.048
NaHzP0n.H10, Mg./100 M1. Present Found 0.5 0.496 1 .o 1.003 1.5 1.518 2.0 2.02 2.5 2.51
No. of Detns. 5 17 52 9 6
Table 11. Determination of Sodium Hypophosphite (Sodium phosphite present) Sodium Phosphite Sodium Hypophosphite (NaHzPOz.HIO), (KazHPOs.5HzO) Added, hIg./100 MI. , ~ g . 1 1 0 0nil. Added Found 5.0 2.5 2.0 1.5 1.5 1.0 1 .o
0
0 0.5 1.5 1.5 2.5 2.5
S i1 Xi1 0.48 1.48 1.53 2.55 2.55
(1) Am. SOC.Testing Materials, “Methods of Chemical Analysis of Metals,” p. 84, 1950. (2) Ibid., p. 293. (3) Ebaugh, C., and Smith, E. F., J . Am. Chem. SOC.,21, 384 (1899). (4) Jenkins; G. L., and Bruening, C. F., J . Am. Pharm. Assoc., 25, 19-27 (1936). (5) Kolthoff, I. M., Mensel, I. H., and Furman, S . H., “Volumetric Analysis,” Vol. 11, p. 303, S e w York, John Wiley & Sons, 1929. (6) Melior, J. W., “Comprehensive Treatise on Inorganic and Theoretical Chemistry,” Vol. VIII, p. 877, Xew York. Longmans, Green & Co., 1948. (7) Millard, E. J., Pharm. J . , (3) 19, 585 (1889). (8) Roscoe, H. E., and Schorlemmer, C., “A Treatise on Chemistry,” Vol. I, p. 647, London, RIacmillan Co., 1911. (9) Treadwell, F. P., and Hall, W. T., “Analytical Chemistry,” 9th ed., Vol. 11, p. 321, Kew York, John Wiley & Sons, 1949. (10) Ibid., p. 323. (11) Ibid., p. 371. RECEIVED for review November 29, 1951. Accepted October 21, 1953
