XOVEMBER 15, 1940
ANALYTICAL EDITION
2 atoms, the approximate formula of the complex being CaiHziKsOicVz.
Literature Cited (1)
(2)
Ephraim, F., Ber., 64B, 1215 (1931). Flagg, J. F., and Furman, K. H., IND.EXG.C H E M . , Anal. Ed., 12, 529 (1940).
C3)
Katzoff, S.,and Roseman, R., J . Am. Chem. Soc., 58, 1 7 8 5 ( 1 9 3 6 ) .
(4) Meulen,
H. ter, and Hesslinga. J., “Neue Methoden der or-
(5) (6)
(7) (8)
665
ganisch-chemischen Analyse”, Leipzig, Akademische Verlagsgesellschaft, 1927. hlontequi, R., and Gallego, >I., Anales soc. espafi, fis. quim., 32, 134 (1934). Pearson, Th. G., Z. anal. Chem., 112, 179 ( 1 9 3 8 ) . Scott, W. W., “Standard Methods of Chemical Analysis”, 5th ed., Vol. I, p. 6 3 3 , New York, D. \-an Nostrand Co., 1939. Turner, IT.-A,, 4 m .J . Sci., 41, 3 3 9 ( 1 9 1 6 ) ; 42, 109 ( 1 9 1 6 ) .
PRESESTED befrjre t h e Division uf I’h:-sical a n d Inorganic Chemistry nt the 99th 1 I e e t i n S nf t h e .inieric:in C h t ~ t u ~ c Su,,icty, al C i n c i n n a t i , Ohiu.
Colorimetric Determination of Phosphate S. R. DI(:Ii>I.iT 4 N D R . H . BR-kI-. Illinois .igricultural Experinierit Station, Urbana, Ill.
A colorimetric method for phosphates is described which employs a molybdatehydrochloric acid solution instead of a molybdate-sulfuric acid solution. This method is not affected by chlorides or by ferric ion up to 15 p. p. m. Fading is less rapid than with most methods. The method is applicable for phosphate determinations in soil fusions, hydrochloric acid extracts of soils, w-ater analyses, oceanographic analyses, plant oxidations in which the sample is taken up in hydrochloric acid, and biological determinations.
‘T
HE reduction of phosphomolybdic acid with stannous
chloride to form a blue solution was first reported by Osmond in 1887 (9). Denighs in 1920 (3) modified the conditions to make the reaction more nearly quant’itative and since that time a large number of further modifications h a r e been published. Although Deniges used hydrochloric acid as well as sulfuric acid in preliminary studies, his final procedure, as well as all modifications that have been suggest’ed subsequently, employs sulfuric acid. However, no reason for t h e nonuse of hydrochloric acid has been published. A review of the literature pert’ainingto the determination of phosphates in various substances reveals that a t the present time two methods are in general use-that of Truog and Meyer (13) and that of Kuttner (7, 8 ) as modified by Youngburg ( 6 , 1 4 ) . If the concentration of reagents in the final dilute solution for these two methods is compared (Table I), i t is seen that Youngburg’s modification differs from Truog’s in the relatively much greater quantity of ammonium molybdate i t contains. The three reagents are so interrelated in this determination t h a t the ratios of molybdate to acid and of stannous chloride t o acid perhaps exert a greater influence on the depth of color developed than the actual concentration of any of the three in the final solution. For a given quantity of phosphate the Youngburg method produces a slightly deeper color, and fading is less rapid than with the Truog procedure. Kuttner and Cohen ( 7 ) found that hydrochloric acid in 0.5 N solution caused very rapid fading. Truog and Meyer found that 2 p. p. m. of ferric ion as ferric sulfate decreased the color intensity and caused troublesome greenish tints. Dyer and Wrenshall (5), with the Truog and Meyer method, noticed that the rate of fading increased as phosphate concentration increased. More recently Smith, Dyer, Wrenshall, and DeLong ( 1 1 ) have reported on additional factors n-hich influence color development and rate of fading 51-ith the Truog and Meyer procedure. They found that solutions containing 1.0 p. p. ni. or more of ferric ion prevent nor-
mal color development at a given phosphate concentration. They suggest dilution of the solution !Then feasible, or addition of larger amount,s of stannous chloride. As is evident from their data, an appreciable loss of accuracy resulted when 2 or 3 drops instead of 1 drop of stannous chloride Tvere used. Different concentrations of iron still caused significant differences in color development, so that results obtained by this procedure could not be interpreted unless the ferric-ion concentration of each unknov-n solution was also determined. These norkrrs used ferric chloride as a source of iron, consequently, they could not vary t’he ferric-ion concentration without also varying the chloride-ion concentration. Hence the results which they considered to be due solely to changes in t,he ferric-ion concentration were actually due t o the combined effect of at least two variables. TABLE I. C~OSCESTR.~TIOSOF RE.LGEXTS (Cumparison of Youngbuig and T r u o g and \Ieyrr Final ConcenFinal tration, Concen“a tration, Molyhdate. H ~ S O I Ratio 70 Ratio l.O
