Colorimetric Methods for the Determination of Phosphorus

ratio of sulfuric acid to molybdenum blue is very important. Since sulfuric acid affects also the sensitivity of the reagent to phosphorus pentoxide w...
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Colorimetric Methods for the Determination of Phosphorus In the Presence of Silica, Arsenic, Iron, and Nitrates CH. ZINZADZE,' New Jersey Experiment Station, New Brunswick, N. J.

The upper limit of concentration Two methods are outlined for the molybwhich gives a readable color by a new molybdenum blue dimetric determination of phosphate with the two methods outlined should method for the determinabe k e d at 0.3 mg. of phosphorus improvements in the preparation of the pentoxide in a 50-cc. determination of ]phosphorus was described reagents leading to greater stability of the tion. The quantity of molybdediffering from older methods by num blue necessary to give the color and the elimination of the influence of the use of a solution of pure maximum color with this amount silicates, arsenates, nitrates, and ferric molybdenum blue in sulfuric of phosphorus pentoxide was found t o be about 17.6 mg. of Mooz.acid as reagent. (The first pubiron. 4Mo03. A 20 per cent excess of lication of this series was on The molybdenum blue reagent method molybdenum trioxide above that colorimetric determination of required for this formula was found produces the most stable color, but is potassium, 20. Lewis and Marto give somewhat better results. slower than the reduction method using moy, 9, compared this method stannous chloride. with others.) The blue color Elimination of Influence disappears upon addition to a Gum arabic is offered as a protective of Silicates, Arsenates, Nitest solution but reappears, upon colloid to retard the formation of turbidity trates, and Ferric Iron heating in proportion to the which results from the use of stannous amount, of phosphorus pentoxide Silicates give with molybdechloride. present. The blue color thus num blue the same color as Several other reducing agents for color formed is stable for several days. phosphates. Addition of inAlten, Weiland, and Loofmann creasing quantities of sulfuric development are listed. (1) compared this method with acid decreases the sensitivity of molybdenum blue to silica others aLd it has been employed successfully by many other investigators (3, 4,6, 7 , 8, 12, 14, by repression of the ionization of the weak silicic acid to a point where it does not develop a blue color, whereas the 16, 17, 18). Since the earlier publications, slight changes stronger phosphoric acid remains ionized and reacts with in reagent preparation and analytical procedure have been molybdate to produce a blue color. The author found made to eliminate the influence of silicates, arsenates, ni(Table I) that besides the sulfuric acid concentration the trates, and ferric iron. ratio of sulfuric acid to molybdenum blue is very important. Colorless molybdenum trioxide, upon partial reduction in acid solution, yields a blue complex to which most investigaSince sulfuric acid affects also the sensitivity of the reagent to phosphorus pentoxide when used in concentrations less than tors assign the formula MoOz.4MoOa. This molybdenum blue complex is extremely unstable under conditions of 0.18 N and more than 0.36 N , it was necessary to choose a varying; acidity, so that simple fivefold aqueous dilution ratio which would not affect the sensitivity to phosphorus of the 25 N sulfuric acid solution is sufficient to void the pentoxide and eliminate the effect of about 150 mg. of SiOn. color. On the other hand, phosphoric and arsenic acids The ratio of sulfuric acid to molybdenum blue equal to about 35 was found to be best. form stable blue salts-(MoO~.4MoO~)2H~P0~. These blue salts may be formed (1) by the addition of molybdenum blue reagent to phosphate or arsenate solutions, (2) by TABLEI. INFLUENCE OF SULFURIC ACID-MOLYBDENUM BLUE RATIO(BY WEIGHT)ON SENBITIVITY TO SILICA the reduction of molybdenum trioxide, or (3) by theoxidation of mo1,ybdenum dioxide, in the presence of phosphate or [Optimum quantity of molybdenum blue (17.6 mg.) j, used] Silica (Sios) Sulfuric Acid-Molybdenum Blue Ratio arsenate. Mg./60 cc. 20 25 30 35 40 1. The molybdenum blue reagent method cited above (28) -0 was developed by the author and is extended in this paper to-60 gether with a modification of the reduction method. 100 2. The reduction method was developed in 1887 by Osmond 160 (11) who used stannous chloride as the reducing agent. Subse+- blue colored. quent investigators proposed the use of other reducing agents. not colored. Bell and Doisy, Briggs and Benedict (3) used hydroquinone and bisulfite. Fiske and Subbarow (6) used aminonaphtholsulfonic acid and bisulfite; Tschopp and Tschopp (17)used p-methylSodium bisulfite was used to eliminate the influence of araminoplienol sulfate and bisulfite, etc. But most investigators senates, nitrates, and ferric iron. (DenigBs, Truog, 28,etc.) are still using stannous chloride recomARGENATES.Arsenic pentoxide like phosphorus pentoxide mended by Osmond. gives a blue color with molybdenum blue, b u t arsenic and 3. The oxidation method was tried first by the writer using potassium permanganate as oxidant. It was hoped that in phosphorus trioxides do not. Tschopp and Tschopp (17) this case the molybdenum blue formed would be reactive enough and Tananieff and Potschinok (16) used sodium bisulfite for to give the blue phosphomolybdate in the cold, but the reaction the reduction of arsenic pentoxide to trioxide. P e t t (13) did not proceed at ordinary temperatures, and it was necessary studied the influence of temperature and concentrations of to heat the solution. Therefore the oxidation method has no advantage over the previous two methods and was abandoned. sulfuric acid and sodium bisulfite on the reduction of arsenic -_ pentoxide. The author's experiments partly c o n k e d the 1 From the University of Tiflis; Republic of Georgia (Caucasus).

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results of Pett. It was found that a t ordinary temperatures (20" to 30" C.) at least 10 hours are required forthereduction; at 50" to 100" C. it takes only one hour. In the author's experiments the upper admissible limit of concentration was found to be 20 mg. of arsenic pentoxide in 50 cc. It is necessary to reduce arsenic pentoxide before the addition of molybdenum blue, since blue arsenomolybdate once obtained cannot be reduced by the addition of sodium bisulfite. NITRATES. When the solution contains nitrates, the sulfuric acid of molybdenum blue solution liberates nitric acid, which oxidizes the blue phosphomolybdate, causing the color to fade. Sodium bisulfite eliminates the influence of about 50 mg. of nitric acid in 50 cc. FERRIC IRON even in very small amounts (2 mg. of Fez03 and more) produces a greenish tint, Ferrous iron does not have such an undesirable effect. Sodium bisulfite eliminates the influence of about 50 mg. of ferric oxide in 50 cc. Upon the addition of sodium bisulfite the solution becomes red in the presence of ferric oxide but the color disappears gradually and in a few minutes the solution is clear again. It should be emphasized that sodium bisulfite gives a blue color even in the absence of phosphorus pentoxide, unless sulfuric acid is previously added.

Description of Methods The following reagents are used in both methods: 1. 0.1 N pure, carbon dioxide-free sodium (or potassium) hydroxide solution and phenolphthalein indicator. 2. 25 N sulfuric acid solution (c. P., A. C. S., phosphorus-and arsenic-free.) 2. 1 N sulfuric acid solution: 40 cc. of 25 N sulfuric acid diluted to 1liter with distilled water. 4. Acid molybdic (anhydride), c. P., A. C. S., very fine powder. Extract quantity of Mooa in it should be known; it must be not less than 99.5 per cent, (Specify these requirements when ordering.) 5. Sodium bicarbonate solution, 2 per cent. Prepare fresh solution every month. 6. a-Dinitrophenol (2,4-dinitrophenol). Prepare a saturated aqueous solution by heating to about 50" C.; allow to stand overnight and decant the clear solution. 7. Anhydrous sodium bisulfite (NaHSOs) solution, 8 per cent. This solution keeps only a week. 8. Standard phosphate solution. Dissolve 0.0767 gram of c. P. dry monobasic potassium phosphate in about 200 cc. of, distilled water; add 10 cc. of 1 N sulfuric acid and 6 drops of 0.1 N potassium permanganate as preservatives; make up to exactly 2 liters. This is 20 p. p. m. of phosphorus pentoxide solution; 1 cc. = 0.02 mg. of Pz06. This solution keeps indefinitely in a well-stoppered Pyrex bottle, and from it the standard solutions are prepared by taking appropriate aliquots in 50-cc. volumetric flasks.

Molybdenum Blue Reagent Method Reagents used are: 1. 0.1 N potassium permanganate. 2. Molybdenum metal, c. P., A. C. S. Very fine powder, containing not less than 99.5 per cent of molybdenum. (Specify these re uirements when ordering.) 3. dolybdenum blue reagent (Solution I) Take, in a 3-liter Erlenmeyer flask, 1010cc. of 25 N sulfuric acid; add acid molybdic (anhydride) containing exactly 40.11 grams of MOO!; boil very gently, with occasional shaking, just until solution is complete, avoiding the evolution of white fumes; cool to room tempersture; dilute with distilled water to about 998 cc., and cool again. Finally make up to exactly 1 liter with distilled water and mix well. (The solution has a bluish color.) Solution 11. Place 500 cc. of Solution I in a 3-liter Erlenmeyer flask; add 1.78 grams of molybdenum powder and boil very gently (with precautions as before) for exactly 15 minutes from incipient boiling, shaking from time to time. Allow to cool t o room temperature; decant the solution from the small residue which may be present into a 500-cc. volumetric flask; dilute with distilled water to about 498 cc., and cool again. Finally make up

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to exactly 500 cc. with distilled water and mix well. (The solution has a greenish blue color.) A 5-cc. aliquot of Solution I1 is diluted to about 50 cc. with distilled water and titrated with 0.1 N potassium permanganate. (It is necessary to use a pipet previously wet inside with water and washed down afterward with a few cubic centimeters of water in order to deliver 5 cc. of the viscous reagent accurately.) The molybdenum blue reagent is finally prepared by mixing certain quantities of Solutions I and 11, so that 5 cc. of the resulting mixture correspond to 5 cc. of 0.1 N potassium permanganate. I t will keep a t least 4 years, and probably indefinitely, provided it is of sufficient purity and is kept free from dust, vapors, and other contamination in a glass-stoppered Pyrex bottle.

A number of chemical houses have been selling the reagent which was suggested in a previous publication, but it was found by the author that some of the manufactured reagent samples were not pure enough and therefore did not keep long. Now several chemical houses propose t o sell pure reagent with the guarantee that it will always correspond exactly to the description given here. I n spite of t h a t it is necessary to check the reagent, whether prepared in the laboratory or purchased, b y making the following two tests: 1. TESTOF ACIDITY. Dilute 5 cc. of the reagent to 500 cc. and titrate 10 cc. of this diluted solution with 0.1 N sodium hydroxide (phenolphthalein indicator). If the reagent is exactly 25 N , 24.9 to 25.1 cc. of hydroxide must neutralize it. 2. TESTOF MOLYBDENUM CONCENTRATION. Titrate with 0.1 N potassium permanganate as indicated above; 5 cc. should oxidize an equal amount of the molybdenum blue reagent. This test must be repeated from time to time (once a month) in order to be sure that the quantity of molybdenum blue in the reagent does not change. If it changes-i. e., less than 4.9 cc. of 0.1 N potassium permanganate oxidizes 5 cc. of the reagent-discard the reagent. Only 0.5 cc. of the molybdenum blue reagent is necessary for each phosphorus determination, but it is difficult to measure the reagent exactly, because of its viscosity. The difference of a few drops of the reagent can cause noticeable error, since solutions with different quantities are not comparable. It was found convenient to dilute the reagent ten times with distilled water and then to take 5 cc. of this diluted reagent each determination. The diluted reagent does not keep more than a day.

PROCEDURE FOR DETERMINATION OF PHOSPHORUS. (In phosphate solutions containing no arsenic pentoxide, not more than about 2 mg. of ferric oxide, and about 10 mg. of nitrate, the determination of phosphorus can be made more quickly, 21.) Take 0.5 to 15 cc. of the standard or unknown solution (containing 0.01 to 0.3 mg. of phosphorus pentoxide) in a 50-cc. volumetric flask (with a mark at 30 cc.); add 5 drops of a-dinitrophenol and neutralize drop by dro with 2 per cent sodium bicarbonate when acid, or with 1 $sulfuric acid when alkaline, to a very faint yellow; then add separately 5 cc. of 1 N sulfuric acid and 5 cc. of 8 per cent sodium bisulfite; make up to about 30 cc. with distilled water and shake well; allow to stand overnight or heat on a steam bath for at least 1hour; add 5 cc. of ten times diluted molybdenum blue reagent, and continue to heat on the steam bath a t least 30 minutes more (the velocity of color development is a function of the temperature: At 20' to 30' C. the maximum color is attained in 3 days; at 50" C. in 10 hours; at 70' C. in 3 hours; at 95" to 100" C. in 30 minutes; and by direct boiling 4 to 5 minutes); allow to cool or cool to room temperature; make up to 50 cc. with distilled water; shake thoroughly and measure the color in an ordinary or photoelectric colorimeter (22). The blue color of the solutions remains stable for 2 to 3 days, provided they are stored in the dark in well-stoppered flasks. If the reagents as prepared are pure, no blue color will appear in the phosphorus-free check; it remains slightly yellowish.

It is necessary to follow exactly the directions given for unknown as well as for standard solutions, because only those determinations having the same p H reagents are comparable. For the same reason the blue color once obtained cannot be diluted.

JULY 15, 1933

ANALYTICAL EDITION

When the color of the unknown solution is as deep as the the decolor of standard solution containing 0.3 mg. of PzO~, termination must be repeated with a smaller quantity. The diffweiice in concentration between standard and unknown solution should not be more than 30 per cent. The standard solutions can be used for about 2 to 3 days, but to prevent errors it is best to prepare them with each set of determinations.

Reduction Method The principle of this method has been outlined above. If stannous chloride is used as the reducing agent, this method has the following advantages over the molybdenum blue reagent method: It does not require heating and is therefore quicker, and i t is slightly more sensitive to small quantities of phosphorus pentoxide (0.005 to 0.05 mg.). However, it has a very serious disadvantage: The blue color so developed is not stable. The following modification of the reduction method obviates the difficulties of previous investigators (8, 6, 17) by the use of pure molybdenum trioxide instead of ammonium molybdate, by the choice of the proper sulfuric acid-molybdenum blue ratio, and b y careful standardization of reagent and procedure. Furthermore the influence of certain interterferiiig elements, such as silicates, arsenates, nitrates, and ferric iron, is eliminated and the stability of the blue color increaEed. The following reagents are used: 1. i'vlolybdenum trioxide reagent. Take, in a 500-cc. Erlenmeyer flask, 101 cc. of 25 N sulfuric acid; add acid molybdic (anhydride) containing exactly 4.01 grams of Mooa; dissolve as in the preparation of Solution I of the molybdenum blue reagent; cool to room temperature; pour gradually into a 1-liter volumetric flask containing about 900 cc. of distilled water; dilute with distilled water to about 998 cc.; cool again; finally make up to exactly 1liter with distilled water, and shake well. To determine that the acidity is exactly 2.5 N , dilute 10 cc. of the reagent to 100 cc., observing the precautions for pipetting a viscous solution. Titrate 10 cc. of this diluted solution with 0.1 N sodium hydroxide (phenolphthalein indicator) ; 24.9 to 25.1 cc. of hydroxide must neutralize it. The molybdenum trioxide reagent keeps indefinitely, provided it is kept free from dust, reducing vapor, and other contamination in a glass-stoppered Pyrex bottle. 2. Gum arabic solution. Add 10 grams of pure gum arabic (white powder, U. S. P.) to 1 liter of warm water (about 50" C.) and allow to stand for about 0.5 hour, shaking from time to time until the solution is complete. Cool to room temperature and add 1 cc. of toluene (c. P., A. C. S.) which preserves the solution for 3 t o 4 weeks. Keep in a glass-stoppered Pyrex bottle. It does not matter if a precipitate forms after a few days; siphon off the clear supernatant solution for use. Prepare a fresh solution every month. 3. Stannous chloride solution. Use fresh C. P., A. C. S., SnC12.2Hz0 preserved in well-stoppered bottles and soluble without residue. Add 0.16 gram of stannous chloride to 200 cc. of 1 per cent gum arabic solution and shake thoroughly. This solution has a slight turbidity which disappears during the determination. The solution does not keep more than about 12 hours.

Experimental Work with Stannous Chloride Solution Stannous chloride produces maximum color in a few minutes a t room temperature, but its use is accompanied by rapid fading of the blue color. T o retard the formation of turbidity, the protective colloids (10, 19)gum arabic, gelatin, agaragar, soluble starch, and egg albumin were studied. Only gum arabic gave good results, Experiments showed that the optimum concentrations were: 4 mg. of stannous chloride plus 50 mg. of gum arabic for each determination. The quantity of gum arabic can be varied without any considerable effect on the determination; but any slight change in the quantiiy of stannous chloride has a great influence. The

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limits between which the optimum amounts can vary were found to be 3 to 6 mg. of stannous chloride per determination (50 cc.). The narrowness of these limits is the weakest point in the use of stannous chloride. Therefore, it is absolutely necessary t o have a stannous chloride salt which corresponds as nearly as possible to SnC12.2H20. The concentration of stannous chloride solution is very important. Most investigators use a few drops of concentrated solution for each determination, but the author found that slight variations in size or quantity of drops cause appreciable differences. By the use of larger volume of diluted solution, accurate measurements of the solution are assured and the results are comparable. T h e addition of acid, although temporarily removing the slight initial turbidity of stannous chloride solution, accelerated its later formation and the ultimate oxidation of the salt-that is, the reducing power of the solution not containing acid lasts longer.

Procedure for Determination of Phosphorus Prepare the aliquot for analysis as in the molybdenum blue reagent method up to the addition of the molybdenum blue reagent. If stannous chloride is used as reducing agent, allow to cool (using other reducing agents, as described below, this cooling i s not necessary) or cool to room temperature (in warm ,solution the color fades in a few minutes), add to each flask 5 cc. of the molybdenum trioxide reagent, and mix well; prepare all standards and unknown solutions exactly in this manner; then add rapidly in turn to each flask, while shaking, 5 cc. of stannous chloride solution; make up to 50 cc. with distilled water and shake thorough1 ; after 20 minutes make comparisons. The determinations slould not stand longer than 6 hours; after this time the solution becomes faintly turbid. Standards and unknowns must be prepared rapidly and simultaneously. Other precautions are the same as for the molybdenum blue reagent method.

Reducing Agents Other than Stannous Chloride The following reducing agents may be used instead of stannous chloride: 1. p-Methylaminophenol sulfate, 0.4 per cent solution. Dissolve 30 grams of sodium bisulfite, anhydrous c. P., in about 200 cc. of distilled water; add 2 grams of pure dry p-methylaminophenol sulfate (17) and shake well; when dissolved make up to 500 cc. 2. Hydroquinone, 0.5 per cent solution. Dissolve 30 grams of sodium bisulfite, anhydrous c. P., and 2.5 grams of pure hydroquinone in 500 cc. of distilled water. 3. Aminonaphtholsulfonic acid, 0.05 per cent solution. Dissolve 15 grams of sodium bisulfite, anhydrous c. P., in about 100 cc. of distilled water; add 0.5 gram of dry pure l-amino-2naphthol-4-sulfonic acid in powdered form (6) and 1.5 grams of sodium sulfite, anhydrous c. P., and shake well. Warm if necessary and when dissolved make up to 500 cc. Use 5 cc. of either and develop color at 95" to 100' C. for 30 minutes.

These reducing agents offer the advantage of greater keeping qualities (6 weeks) and stability of the blue color produced (2 days) b u t require heat to develop maximum color. Although more sensitive than in the original methods (S,6, l?'), color development is less rapid. At 20" to 30" C. the maximum color is attained in 5 hours, a t 40" C. in 2 hours, at 60" C. in 1 hour, and 95" to 100' C. in 30 minutes.

Detection of Phosphorus with Drop Method To a few drops of the unknown solution in a small test tube add one drop of 1 N sulfuric acid and one drop of 8 per cent sodium bisulfite, and boil very gently in an open flame for about 0.5 minute. After that it is possible to proceed in two different ways: Add to the boiling solution one dro of molybdenum blue reagent diluted ten times and boil another $alf minute, or cool it and add one drop of molybdenum oxide reagent and one drop of stannous chloride solution.

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In this way it is possible to get a preliminary estimate of the approximate concentration of phosphorus in solution. Literature Cited Alten, F., Weiland, H., and Loofmann, H., 2. PjZanzenernilhr. Dtingung Bodenk., 32 (A), 33-50 (1933).

Benedict, S . R., and Theis, R. C., J. Biol. Chem., 61,63-6 (1924). Deemer, R. B., and Schricker, J. A., J. Assoc. oficial Agr, Chem,, 16, 226-32 (1933).

Dreyspring, C., and Heinz, W., 2. PjZanzenernahr Dangung Bodenk., 35 (A), 362-74 (1934).

Feher, D., Phosphorsilure, 3, 429-61; 4, 508-29 (1933); 2. Pflanrenerntthr. Dtingung Bodenk., 33 (A), 320-35 (1934). Fiske, C. H., and Subbarow, Y., J. Biol. Chem., 66, 376-400 (1926).

Gerritz, H. W., IND. ENG.C H ~ MAnal. . , Ed., 7,116-18 (1935). Guyer, A,, and Likiernik, A,, Helu. Chim. Acta, 16, 1033-44 (1933).

Lewis, A. H., and Marmoy, F. B., J. SOC.Chem. Ind., 52, 17782T (1933). Liebknecht, O., and Gerb, L., Z. angew. Chem., 45, 744-5 (1932).

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(11) Osmond, F., Bull. 8oc. chim., 47,745-8 (1887). (12) Petrie, A. H. K., Australian J. EzptZ. Biol. Med. Sci., 11, 26-34 (1933). (13) Pett, L.’B., Biochem. J., 27, 1672-6 (1933). (14) Robinson, W. O., Dudley, H. C., Williams, K. T., and Byers, H. C., IND. ENG.CHEM.,Anal. Ed., 6,274-6 (1934). (15) Tananieff, N. A., and Potschinok, Ch. N., 2. anal. Chem., 88, 271-8 (1932). (16) Treadwell, W. G., and Hartnagel, I., Helv. Chirn. Acta, 15, 1023-9 (1932). (17) Tschopp, E., and Tschopp, E., Ibid., 15, 793-809 (1932). (18) Ward, R. R., Soil Sci., 24,85-97 (1933). (19) Yoe, I. H., “Photometric Chemical Analysis,” Vol. 1, pp. 91-3, New York, John Wiley & Sons, 1928. (20) Zinzadze, Ch., Chimie & Industrie, 27 (3-bis),841-3 (1932). (21) Zinradze, Ch., IND. ENQ.CHEM.,Anal. Ed., 7,230 (1935). (22) Ibid., p. 280. (23) Zinzadze, Ch., 2. PjZanzenernilhr. Ditngung Bodenk., 16 (A), 129-84 (1930): Ann. agron., 1,321-36 (1931). RECEIVED October 8, 1934. Read before the annual meeting of the Association of Official Agricultural Chemists, Washington, D. C., October 30,1934.

Colorimetric Methods for Determination of Arsenic in Phosphorus-Free Solutions CH. ZINZADZE, New Jersey Experiment Station, New Brunswick, N. J.

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N ANOTHER paper (6)citations of the use of molybdenum blue methods for the determination of phosphorus i n different materials have been given. Deemer and Schricker ( I ) used this method for the determination of arsenic in plants. Slight changes and improvements which were found useful in connection with the determination of phosphorus are also applicable to the determination of arsenic. Only pentoxides (As205 and P206)give the blue-colored compounds with molybdenum blue. Arsenic pentoxide is much more easily reduced than phosphorus pentoxide and thus i t is possible to eliminate the influence of arsenic in the determination of phosphorus, but not vice versa. Consequently, the methods given in the previous paper can be used for the determination of arsenic only in solutions free from phosphorus pentoxide and containing not more than 2 mg. of ferric oxide and 10 mg. of nitrates, because sodium bisulfite reduces arsenic pentoxide and cannot be used in this determination. These methods areparticularlysuitable for arsenic distillates; distillation is described in detail by Deemer and Schricker (1). REAGENTS. With the exception of sodium bisulfite, the reagents used are exactly the same as for the determination of phosphorus (8). STANDARD ARSENICPENTOXIDI SOLUTION.Dissolve 0.0200 gram of pure anhydrous arsenic pentoxide in about 5 cc. of 2 per cent sodium bicarbonate and 100 cc. of distilled water; add 6 CC. of 1 N sulfuric acid and 3 drops of 0.1 N potassium permanganate and make up to a liter. This is 20 p. p. m. of arsenic pentoxide; 1 cc. = 0.02 mg. of AszOr. From this stock solution the standard solutions are prepared by taking appropriate aliquots in 50-cc. volumetric flasks.

perature; make up to 50 cc. with distilled water; mix well and measure the color in an ordinary or photoelectric colorimeter (a). The precautions here are the same as for the determination of phosphorus.

Reduction Method This method may be used when the molybdenum blue reagent method is inconvenient. For the determination of arsenic only stannous chloride can be used as a reducing agent; the three reducing agents mentioned in the previous paper (2) contain sodium bisulfite and are therefore not suitable. Prepare the standard or unknown as in the PROCEDURE. molybdenum blue reagent method up to the addition of the molybdenum blue reagent; add instead 5 cc. of molybdenum trioxide reagent; make up to 40 cc. with distilled water and mix; prepare all standard and unknown solutions exactly in this manner; then add rapidly in turn to each flask 5 cc. of stannous chloride solution while shaking; make up to 50 cc. with distilled water and mix thoroughly. After 20 minutes make comarisons. The determination should not stand longer than 6 Rours.

Detection of Arsenic by Drop Method To a few drops of solution add 1 drop of ten times diluted molybdenum blue reagent and boil very gently about 0.5 minute; or add one drop of molybdenum trioxide reagent and one drop of stannous chloride solution.

Aclrnowledgment The author wishes to express his sincere thanks t o John A. Schricker of the U. S. Department of Agriculture for helpful literary suggestions and criticism of the manuscript.

Molybdenum Blue Reagent Method

Literature Cited

Pipet 0.5 t o 30 cc. of the standard or unknown PROCEDURE. solution, corresponding to 0.005 to 0.3 mg. of As206, into a 50-cc. volumetric flask having a mark at 40 cc.; add 5 drops of alphadinitrophenol and neutralize with a 2 per cent sodium bicarbonate solution when acid, or with 1 N sulfuric acid when alkaline, t o a faint yellow; add 5 cc. of ten times diluted molybdenum blue reagent; make up to 40 cc. with distilled water and mix; heat on a steam bath for 30 minutes; allow to cool or cool to room tem-

(1) . . Deemer, R. B., and Schricker, J. A., J . Assoc. Oficial Agr. Chem., 16, 226-32 (1933). (2) Zinzadze. Ch.. IND. ENG.CHEM.,Anal. Ed., 7, 227 (1935). (3) Ibid., p. 280. R ~ C E I Y EOctober D 8, 1934. Read before the annual meeting of the ABsociation of 0 5 c i a l Agricultural Chemists, Washington, D. C., October 30, 1934.