INDUSTRIAL AND ENGINEERING CHEMISTRY
636
TABLE111. COPPERMETHOD 1 Product
2
Dry Wt. of 25 Cc.
Blank
3
4
5
Sample
Blank minus Sample
1.OM Copper Solution
6 Valence of Cop e r X cc. o f I..O M Solution
7
MiEliequiva lents pm Gram Cc. cc. cc. 100 gram8 4.55 25.00 20.45 I 0.353 0.0428 0.0856 24.2 2.42 25.15 22.73 0.360 0.0228 0.0456 I1 12.6 0.45 25.15 24.70 0.362 0.00428 0.00856 2.4 I11 1. Dry weight of starch represented b 25 cc. of co per solution. 2. Cc. of standard thjosulfate requge8for 25 cc. orcopper solution. 3. Co. of standard thiosulfate required for 25 00. of copper solution, from sample. 4. Column 2 minus Column 3. 5 . Column 4 times normality of thiosulfate. 6. Column 5 times valence of copper. 7. Column 6 divided by Column 1 and multiplied by 100.
TABLEIv. COhlPARISON Product
I
I1
111
O F VALUES BY
METHODS
COPPER
AKD
SILVER
Copper Silver Milliequivalents/lOO grams 24.2 23.3 11.2 12.6 2.4 3.8 5.9 6.0 9.2 9.3 11.6 10.4 19.6 18.7 21.0 17.6 27.8 23.8 30.4 28.9 2.4 3.5 2.4 3.7
dissolve the cuprous iodide formed. Add about 1 cc. of starch solution and titrate with standard thiosulfate. Express as milliequivalents per 100 grams of starch (Table 111). The values obtained by the copper method are slightly higher wherever oxidation is moderate or high (Table IV).
Vol. 14, No. 8
This may be due to the higher concentration of copper ions a t equilibrium-e. g., twice as many monovalent spver ions as divalent copper ions are taken out of solution for a given number of COOH groups. Products A, B, C, D, E, F, and G are commercial oxidized cornstarches; H is an unmodified cornstarch; J is a hydrolyzed, thinboiling cornstarch.
Colorimetric Determination of COOH Groups in Modified Starch
SOLUTIONS.Copper acetate, 1 per cent, and potassium ferrocyanide, 1 er cent. PROCEDURE. Transfer agout 1gram of unknown to a 50-cc. beaker, add 25 cc. of copper solution, stir well, and let stand for 10 minutes. Filter at the pump on a small Buchner funnel. Wash with distilled water until wash water gives no test for copper with potassium ferrocyanide solution. Transfer to a 50-cc. beaker, add 25 cc. of potassium ferrocyanide solution, stir well, and let stand 2 minutes. Filter and wash until wash water is colorless. Dry and compare with standards. Standards may be prepared by treating known oxidized products as described above.
Acknowledgment The author is very grateful to Dr. Frieden, chief of the Stein-Hall Technical Department, New York, N. Y., for his helpful suggestions and also for permission to publish this article.
Literature Cited Felton, Farley, and Hixon, Cereal Chem., 15, No. 5, 678 (1938). “Scott’s Standard Methods of Chemical Analysis”, 5th ed., Vol. I, page 824, New York, D. Van Nostrand Co., 1939. Ibid., p. 1211. Sookne and Harris, Am. Dyestuf Reptr., 30, No. 5, 107 (1941).
Colorimetric Method for Phosphates Modification of A. 0. A. C. Molybdenum Blue Method L. S. STOLOFF’, Naval Clothing Depot, Brooklyn, N. Y.
A
GOOD review of colorimetric methods for phosphates and a discussion of their relative values are given by Woods and Mellon (3). All the methods depend on formation of the complex phosphomolybdic acid and a selective reduction of this acid to form a blue compound of unknown composition, generally referred to as molybdenum blue. The color developed is not stable and the results are so variable as to require the use of standards run parallel with the unknown. I n spite of the vast amount of work reported, there seems to be no basic study of the reactions involved. The procedure of the Association of OfficiaI Agricultural Chemists (1) with slight modification was used for study because of the extreme simplicity of the reagents and method. An excess of an acidic solution of ammonium molybdate is added to the sample to form phosphomolybdic acid, which is then reduced with 0.5 per cent solution of hydroquinone ac1 Present
address, 10 Kearny St., Newark, N. J.
cording to the method of Bell and Doisy (2). A 20 per cent solution of sodium sulfite is next added before diluting to volume. The modifications were a proportionate increase in the volumes of sample and reagents used and color measurements by means of a Coleman Universal spectrophotometer. I n carrying through this procedure it was observed that the color first developed was green, which became blue on addition of the sodium sulfite. Since the sodium sulfite would alter the p H of the solution, the effect’ of pH on the color was studied, The percentage transmission-wave length curve of the colored solution was found to be smooth, having but one maximum in the visible range. The maximum point was used to identify the color, as it would correspond to visual identification. The readings for stability of color density were made a t the point of maximum transmission a t intervals of 3, 10, and 15 minutes after the addition of the reducing agent. A solution of c. P. potassium dihydrogen phosphate was used as the source of phosphate for all tests.
ANALYTICAL EDITION
August 15, 1942
A preliminary experiment was run using varying amounts of sodium hydroxide in place of the sodium sulfite (Table I). There was a decided change in the position of the transmission maximum, but the density of both colors was found to be variable. The relation of the two colors and their maxima are shown in Figure 1. A gradual shift in the pH was also observed. The decomposition noted was a decided fading in the intensity of the color and the formation of a grayish turbidity. To cover the p H range in which the color change was observed, the acidity of the ammonium molybdate reagent was changed from 3 N to 1 N sulfuric acid and varying amounts of 20 per cent sodium succinate were used instead of sodium sulfite, in order to rule out any effect of the sulfite ion. The results are shown in Table 11.
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TlBLE 111. EFFECTO F pH
ON
BLANKS
Sodium Succinate
Transmission Oriaina
Transmission after 4 Hours
M1.
“0 98.3 98.3 98.3 98.3 98.3 98.3 98.3
88.5 92.9 94.2 97.8 98.0 98.3 98.3
0 0.25 0.50 1 .oo 1.50 2.00 2.50
70
To determine the reason for the increase in color density a t the lower pH values, blanks were run in the same manner as the preceding experiment (Table 111). Per cent transmission was referred to distilled water. Since the only source of molybdenum blue in the experiment is the molybdate, it may be concluded that there is a slow reduction of molybdate in acid solution and that the rate of reaction is reduced to zero when the color having a transmission maximum a t 460 mp is formed.
Modified Procedure These findings led to the development of a modified procedure.
roo
460
520
580
640
700
VraVelen&th, m p
FIGURE1.
PER CENT TRANSMITTANCY-WAVE LENGTH OF MOLYBDENUM BLUE CURVES
At p H 1.3 (broken line) and 4.7 (solid line). for both
Concentration same
These findings show that there is a range of color density stability corresponding to a color having a maximum transmission a t 460 mp, The decrease in color density a t the highest pH may be explained by the “decomposition” observed a t higher pH values. The recorded decrease in color density would therefore be due to a slow decomposition. As the pH is increased, the decrease in color density become,9 even more rapid until a point is reached where there is total decomposition, as recorded in Table I.
TABLEI. PRELIMINARY EXPERIMENT 1.0 A: NaOH M1. 0 1.0 2.0 3.0
Transmission Maximum PH Mp 1.3 485 1.6 485 4.4 460 Decomposition
TABLE11. EFFECT OF pH Sodium Succinate M1. 0 0.20 0.50 0.75 1.00 1.50 2.00 2.25 2.75
PH
Transmission Maximum
Change in Color Density
‘+f#
1.3 1.9 2.5 3.2 4.0 4.3 4.7 4.8 4.9
485 485 475 465 460 460 460
460 460
Increase Increase Increase Increase None None None None Decrease
REAGENTS.Ammonium molybdate solution, 5 grams of ammonium molybdate per 100 ml. of 1 N sulfuric acid. Hydroquinone solution, 0.5 gram of hydroquinone per 100 mI. of distilled water made slightly acid with one drop of concentrated sulfuric acid per 100 ml. Sodium succinate solution, 20 grams (anhydrous basis) of sodium succinate per 100 ml. of distilled water. SAMPLE.All the hosphorus should be in the ortho form of the acid and in clear, coyorless solution made neutral or slightly acid t o litmus (pH 5.0). To a 25-ml. volumetric flask add an aliquot of PROCEDURE. sample up t o 15ml.containing not more than0.3mg. of phosphorus. If less than 10 ml. is used, make the volume up t o 10 ml. with distilled water to prevent precipitation of phosphomolybdic acid. Then add the following in order, mixing well after each addition (Mohr pipets may be used): 2 ml. of ammonium molybdate solution, 2 ml. of hydroquinone solution, and 2.5 ml. of sodium succinate solution. Make up t o 25 ml. with distilled water and measure the color at 460 mp within 4 hours, as succinate may crystallize out since the pH is near its isoelectric point. Each reaction seems to be instantaneous and complete, since delays as long as 15 minutes in the addition of any of the reagents have no effect on the final result, outside of the slow reduction of molybdate before the buffer is added. There is no appreciable change due to this cause for periods up to 5 minutes. Buffers other than sodium succinate may be used. If a 20 per cent sodium sulfite solution is used, the amount needed in the above procedure is 3 ml. The author selected the succinate buffer since it is more stable than sulfite over long periods of storage. The effective range of this method is 0 to 0.35 mg. of phosphorus, in which the system follows Beer’s law very closely.
Discussion The author suggests that the reason for the observed color stability is that it occurs in the isoelectric range of both the phosphomolybdate and molybdate where reactivity is a t a minimum. If the hydroquinone is added after the buffer, there is no reaction.
Literature Cited (1) Assoc. Official Agr. Chem., O5cial and Tentative Methods of Analysis, Section XII,31, 32, and 33 (1935). (2) Bell and Doisy, J.Bid. Chem., 44, 55 (1920). (3) Woods, J. T., and Mellon, M. G., ISD. ENG.CHEM.,ANAL.ED., 13,760 (1941).
