ANALYTICAL CHEMISTRY
258 and the contents shaken. The flark should be kept stoppered to avoid loss of hydrogen cyanide. The solution is then diluted t,o the mark with 8.11 sulfuric acid and the absorbanre of the solution measured a t 500 nip in a spectrophotometer.
The temperature at which the color was measured did not have any effect on the absorbance obtained. Most measurements were made with the solutions a t room temperature, but even a 20" rise did not affect the dading.
DISCUSSION
The spectrum of t'he manganic sulfate complex is given in Figure 3. Beer's law is obeyed over the range of 2 to 70 mg. of manganese per 100 ml. of solution. and the absorbance index is about. two times that obtained in the Tomula and Aho pyrophosphate method. The standard deviation of the method was estimated to be 0.006 absorbance units from 12 replicate 15-mg. samples run over a period of a month. In the method bromine is produced both by the oxidation of manganese and by the decomposition of bromate in strongly acid medium. The bromine causes an interfering color which is discharged by the addition of cyanide to form cyanogen bromide. The colored coniplex wts stable for a week without loss of intensity. Three solutions, containing the same amount of manganic sulfate complex, were prepared using lead dioxide as the osidizing agent. After excess oxidizing agent Tvas removed, cyanogen bromide was added to one of the solutions, bromate and cyanide to the second solution, and nothing to the third. The solution containing bromate and cyanide was stable for a week, whereas the other two solutions rapidly lost intensity. These findings agree wit,h those of Belcher and West ( 1 ) and indicate that the stability of the complex is the result of excess bromate in the solution rather than the increased acidity or the presence of cyanogen bromide. The manganic pyrophosphabe complex was stable for 12 hours. The order of reagent addition had no effect on the absorbance of the complex with onc notable exception. The manganese must he added to the solution before t.he bromat,e, for the cyanogen bromide is not a strong enough osidizing agent to oxidize manganese to the plus three state.
INTERFERENCES
Of the interfering ions bromide. chromic, and cobalt in bhe original pyrophosphate procedure, only chromic ion is a sourcp of difficulty in the sulfate method. When present at concentrations equal to that of manganese (15 mg. per 100 ml. of solution) arsenous, ferrous, chromic, and stannous ions show a 4y0 int'ci.ference. Ions forming insoluble sulfates cause difficulty. but this can be prevented by filtmt,ion through a sintered-glass filter funnel, Ions which have been investigated and show no int,ciference at the same concentration as the manganese are robalt, nickel, zinc, ferric, aluminum. copper, stannic, arsenic, cadniiLini, bismuth, cerous, ceric, nitrate, fluoride, bromide, and dichromate. For applying the determination in the measurement of recovcwd carrier, this extent of freedom from interference is more than adequate. ACKNOWLEDGMENT
The authors are indebted t o t8he;\tomic Energy CommiPsion for partial financial support,. LITER 4TURE CITED
(3)
(4) (5) (6)
Belcher, R., and West, T. S.. Aual. Chim. Acta, 6 , 322 (195'13. Bell, K. N., Ind. Eng. Chevr., 40, 1464 (1948). Fiske, C. H., and Subbarow, T., J . B i d . Chem., 66, 375 (19'15). Jones, L. T.,TKD. ESG.CHEY.. - 4 s . i ~ED., . 14, 536 (1942). Tomula, E . S., and .4ho, V.,A u a . -4cad. Sei. Fennieae, A52, T o . 4 (1939). I b i d . , A55, S o . 1 (1940)
R E C E I V E for D review M a y 14, 1'334. .\wepted Sovernber 2 , 1954.
Molybdenum Blue Reaction and Determination of Phosphorus in Waters Containing Arsenic, Silicon, and Germanium HARRY LEVINE, J. J. ROWE, and F. S. GRlMALDl U. S. Geological Survey, Washington 25, D. C.
,
hlicrograiii amounts of phosphate are usuall~ cletermined by the molybdenum blue reaction, but this reaction is nqt specific for phosphorus. The research established the range of conditions under which phosphate, arsenate, silicate, and germanate give the molybdenum blue reaction for differentiating these elements, and developed a method for the determination of phosphate in waters containing up to 10 p.p.m. of the oxides of germanium, arsenic(V), and silicon. With stannous chloride or l-amino-2-naphthol-4-sulfonicacid as the reducing agent no conditions were found for distinguishing silicate from germanate and phosphate from arsenate. In the recommended procedure the phosphate is concentrated by coprecipitation on aluminum h>droxide, and coprecipitated arsenic, germanium, and silicon are volatilized by a mixture of hydrofluoric, hydrochloric, and hydrobromic acids prior to the determination of phosphate. The authors are able to report that the total phosphorus content of several samples of sea water from the Gulf of \lexica ranged from 0.018 to 0.059 mg. of phosphorus pentoxide per liter of water.
T
HE literature on thc detci,iiiitiatiori of phosphorus hy tlic
molybdenum blue react,ion is voluminous. The method5 are based on the formation of mol!hdophosphoric acid and its subsequent reduction to a blue ronipound. The original Denigba method ( 1 ) has been riiodified for the determination of phosphorus in sea water. Important papers on water analysis include those of Zinzadze ( 8 ) ,Kalle (4))Redfield et al. ( 5 ) , Woods et nl. ( 7 ) , Harvey (3),and Robinson et a!.( 6 ) . The molybdenum blue reaction is not specific for phorphoruF because arsenic (V), germanium, and silicon also form heteropoly acids with molybdenum, which also yield blue compound5 on reduction. Some selectivity for phosphorus may be obtained by control of acidity. For example, at' high acidity the heteropoly acids of phosphorus and arsenic may be reduced without iriterference from small amounts of silicon and germanium. The interference of arsenate in sea water may be eliminated by reducing the arsenate to arsenite before t,he addition of ammonium molybdate. Various agents-such as sodium hydrogen sulfite--have been proposed for the reduction of arsenic, but the literature rontains conflicting statements as to their effectiveness. It ie also reported that arsenite may enhance the intensity of the blue color of the reduced molybdophosphoric acid.
V O L U M E 2 7 , NO. 2, F E B R U A R Y 1 9 5 5 Various reducing agents have been employed for the reductioii of molybdophosphoric acid, so as to increase the stability of the resulting molybdenum blue complex and t o make t,he reaction less sensitive to disturbing ioIis. The first aim of this research sought to establish the conditions under which phoPphorus, arsenic, silicon, and germanium give the molybdenum blue rewtion. The data should increase the knodedgc of the chemistry of the heteropoly acids of these elements and should reveal differences in behavior that may be of analytical significance. The second aim was to develop it method for the determin:ttion of microgram amounts of phosphorus applicable t,o witers containing arsenate, silic,ate, and germanate.
STUDIES OF MOLYBDENUII BLUE REACTION The variables studied ivere the acidity, molybdate coricentration, and concentrstioii of the reducing agent. Two series of experiments \yere made. Stannous chloride was ui;ed its the reducing agent in one seritxs anti I-:t1nino-2-naphthol-i-eulfonic
.
259
acid, introduced by F i s k : i i i t i Sul)liarrow ( 2 ) , in the other. on the basis of their p r e ~ e n t These reducing agents iver(3 c.liosc~~i popularity and are reprcseiitiitivc~of a11 inorganic and a11 organic reducing agent. procedure was used in t81iis Procedure. The folloiviq gc~tic~tA study. Five milliliters of oiie solution (10 y 3f phosphorus pentoside, 25 y of arsenic pentoside; 25 y of silicon dioxide, or 25 y of germanium dioxide) wri'e atidrcl to a 100-m!. beaker. Yest a known amount of hydrochlot,ic- :ic-id \vas added, and the solution diluted to 25 ml. v i t h distillrti iv:itc>i.. The molybdate solution in knoFvn amounts was thrii :idtletl, and the volume adjusted to 45 ml. with wxter. Finally, :I liiio~vriamount of reducing agent was added and the volume adjustcd to 50 ml. In the experiment where stannous chloride was usrd a~ the reducing agent, the absorbance of the resulting solution n.ns measured a t 7 3 5 mp after 3 minutes a t room teniper;Iture using I-cm. cells, slit, at 0.06 mm. (effective band n-idth 3.3 mp), and \vat,er as reference. The same procedure was used for 1-amino-2-naphthol-4-sulforiic. acid n-hen silicon and germaniuni t olutions were used. Test solutions containing phosphorus arsenic, however, w ~ r eheated just to boiling and cooled bcfore measuring the absor1)nnce. This heating was necessary iii the cases of phosphorus anti arsenic because the reduction of the het,eropoly acids procredcd too s l o ~ l ya t room temperaturcx with thc organic' reductant.
IO
MI. Conc. HCI Figure 1. Molybdenum blue fields with 1amino-2-naphthol-4-sulfonic acid as reducin agent I. Silicon or germanium 11. Phosphorus o r arsenic
I
2
3
4
5
MI. C onc. HC I Figure 2. AIolgbdenum blue fields w-ith stannous chloride as reducing agent 1. 11.
Silicon or germanium Phoaphorus or arsenic
llolybdate concentratio:i ; t i i d widity were studied siniulta(~ i n n-hich the amounts of neously. Forty combinatioiis 1 1 ~ 3 1 ~used, Iiiolybdate iolution were varied from 0.5 to 10 nil. in increments of about 2 ml., and conceiit,rattd Iiydt.ochl rica acid varied from 0.02 to 5 inl. in increments of ahout I tnl. Follolying a similar rep1 nt:ttion used by Harvry (J), the fields representing the coiiclitions uiider which the mol~~litic~iium blue reactions take place arc given in Figures 1 atid 2 . The amount of hydrochloric acid plotted on the abscissa includw the hydrochloric acid derived from the stannous chloride rc:igent when this reductant is used. The data for silicon and germ:iiiiuni are plotted together because the same data are obtained for b d i elements. This is also true for arsenic and phosphoi,us. The molybdenum blue reaction is assumed to occur only if the blank shows a very small absorbance ( and tc.st,s were made with 0.1, 0.3, 1, and 3 ml. of stannous chloride rolutioii for each combination of niolybdate and hydrochloric arid coirc~~iitration. The concentration of the org:itiicx reductant is relatively unimportant in the germanium reactioti: the same molybdenum Iiluc field was obtained for the various concentrations of reducbtant, tested. The concentration of the reductant is slightly moi'e iinportant for arsenate; 0.1 nil. of reductant is more nearly optimum. The fields of reactivity for .silicate and phosphatc are spotty and the concentration of reductant is more critiral for 110th. The optimum amount of organic reductant for silicate Qhould be between 1 and 2 ml., and for phosphate bet~veeii0.5 :tiid 1 ml. The molybdenum blue reactions for silicate and gerinaiiatc were found to be especially sensitive t o changes in concentration of the stannous chloride, and for this reason stannous chloride is not an ideal reagent for these two elements. The molybdenum blue field was exceptionally spotty, and a reaction occurred under only a f e n of t h e conditions. If this reagent, is used, about 0.1 ml. of stannous chloride solution should be nearly optimum for either clement. Arsenate is least sensitive t o stannous chloride concentration, and t h e optimum airiount of the reductant should be between 0.1 and 1 ml. of reagent. Stannous chloride seems to be better than the l-ainino-2-naphthol-4-sulfonic acid for tlir re-
ANALYTICAL CHEMISTRY
260
* duction of molybdophosphoric acid. The field of reactivity is again somewhat spotty unless the amount of stannous chloride is within the range of 0.1 to 0.5 ml. The conditions for which the molybdenum blue reactions take place depend on the particular reducing agent chosen; for the reducing agents tested, silicate cannot be differentiated from germanate, nor can phosphate be differentiated from arsenate. It is unlikely that such differentiations can be made with some other
Blank , I
0.I
BEH4VIOR OF ARSENIC, GERM4NUb1, 4YD SILICON
. I
The optimum amount of stannous chloride solution was taken as 0.3 ml. The data plotted in Figure 4 illustrate the effect of varying the molybdate concentration, when the total amount of hydrochloric acid is fixed a t 2.5 ml. and stannous chloride solution is fixed a t 0.3 ml. Five milliliters of molybdate reagent were taken as optimum. I n Figure 5 ! the acidity was varied in solutions a t optimum concentrations of stannous chloride and ammonium molybdate. The optimum amount of hydrochloric acid is 2.5 ml. or a concentration of 5 % by volume. The spectral transmittance curve, Figure 6 shows an optimum wave length a t 735 mp. Under the optimum cmditions determined, a straight-line relationship is obtained between absorbance and phosphate concentration up to 80 y of phosphorus pentoxide per 50 ml., the maximum concentration tested (an absorbance of 0.35 is given by this solution). The absorbances of the standards were redetermined after allowing the solutions to stand for 1 hour; the absorbances decreased by an average of 5 % .
I
0.5 0.7 0.9 MI. o f 0 . 5 % SnCI, Solution Figure 3. Effect of stannous chloride
0.3
concentration Solutions, 50 ml., containing 5 ml. of 2 70molgbdate solution and 2.5 ml. of concd. HC1
The interference of arsenic (111), arsenic (V), germanium, and silicon was checked, under optimum conditions for phosphate, by determining the absorbance of 25 y of each of the oxides of these elements alone and in the presence of 5 y of phosphorus pentoside. T h e absorbances were additive. The estent of interference of these elements is summarized as follons: Oxides,
PI06 Equtv., y 8
