Redox Determination of Tervalent and Total Cobalt in Presence of Excess Tungstate LOUIS C. W. B A K E R ’ and T H O M A S P. M C C U T C H E O N Department
of Chemistry, University of Pennsylvania, Philadelphia, Pa.
Total cobalt can be determined readily in the presence of excess tungstate, molybdate, and iron by-a volumetric redox method. The end point (dichromate us. ferrous) can be observed potentiometrically in the presence of precipitated tungstic acid by employing platinum electrodes and a polarizing voltage. The method can be adapted to determination of tervalent cobalt in the presence of bivalent cobalt. The latter may be found from the difference between the total and tervalent cobalt determinations.’
T
H I S paper describes how total cobalt or, alternatively, tervalent cobalt can be determined easily in the presence of excess tungstate. Bivalent cobalt may be found from the difference between the two determinations. 3lolybdate and iron do not, interfere. Amnionia or amines may cause the results to be slightly lovr. for tervalent cobalt Ivhen bivalent cobalt is also present. The procedures are modifications of Sarver’s quick volumetric method (Q), which utilized a redox indicator. It is well known that diphenylamine and diphenylbenzidine indicators are precipitated by traces of tungstate, and t h a t diphenylaminesulfonic acid may be employed when moderate amounts of tungstate are present (8, 10, 13, f4). I n the present study satisfactory results xere obtained with Sarver’s original method (Q), using a diphenylamine sulfonate and solutions of the concentrations cited below, when the gram-atom ratio of tungsten to cobalt was less than 4 to 1 (weight ratio less than 12.5 t o 1). When the proportion of uncomplexed tungstate was higher, the indicator was unsatisfactory. It appears that in such circumstances the indicator is rapidly adsorbrd on the colloidal tungstic acids formed in acidic solutions. (If several drops of indicator were added just before the final end poiiit, a very vague color change could sometimes be observed : but the considerable indicator blank was then very variable.) Under these conditions a sharp end point can be observed potentiometrically by using platinum electrodes and a polarizing voltage. The end point, which can be conveniently determined with a Serfass electron ray titrimetcr, must be approached by addition of reducing agent, rather than from the other direction. This technique, involving no determination of an indicator blank (14),gives an excellent end point even wheii the final solution contains large amounts of precipitated tungstic acid. I n certain procedures, such as the estimation of chromium and vanadium (12, f4, f5), diphenylamine sulfonate indicator can be used in the presence of excess tungstate, provided the l a b ter is held in solution as a complex phosphate or fluoride. It was evident in the authors’ experiments t h a t phosphatawas unable, under the conditions of the reaction, t o hold the tungsten ill solution ( 1 4 ) . The methods which have been recommended for the formation of the tungstate-fluoride complex are not appropriate in the cobalt determinations. It has been shown ( 1 2 ) t h a t the tungsten-fluoride complex contains some tungsten in a n oxidation state lon-er than +6 when the complex is formed by oxidation of solutions containing tungsten in lower states of oxidation. It \vas evident t h a t such lo7,Yer states exist in the cobalt cleterniination. It was evident that such lower states exist in the cobalt determination because intense tungsten-blue 1
Present address, Department of Chemistry, Boston Unirersity, Boston
1 5 , hIas,-.
colors formed in the basic solution upon addition of the standard ferrous solution. Treatments, as with persulfate (12,f4),which are necessary to complete the oxidation of tungstate-fluoride complex, cannot be used once the standard ferrous solution has been added. T h a t all of the uncomplexed tungstate was reoxidized in the procedure described below, was evident from the bleaching of the color upon acidification and from the fact t h a t the results were not high. EXPERIMENTAL
Chloropentamniinecobalt( I1 I ) d i e h l o r i d e , [CoCl(r\“&]C12 (theoretical cobalt = 23.53y0), was chosen as a source of cobalt ( 5 ) because it is a good primary standard for that element ( 4 , 5 ) . Cobalt was determined in three samples, each weighing approximately 0.6 gram, by evaporating to dryness with a little sulfuric acid, igniting, and xveighing the cobalt sulfate ( CoSOa) formed ( 4 , 1 1 ) . Found, per cent cobalt: 23.50, 23.52, 23.54; average, 53.52. Three samples, each containing enough cobalt to oxidize 15 to 20 ml. of 0 . 1 s standard ferrous solution in the later operation, were accurately weighed into 500-ml. Erlenmeyer flasks which had standard-taper ground-glass necks. T o each flask was added over six times as many moles of sodium orthotiingstate dih?drate, Nal\T’04.2H,O, as the number of moles of complex salt already therein. Total cobalt m s determined in each mixture according to the following modification of Sarver’s method: T v e n t y milliliters of a s t e r m-ere added to dissolve the compounds. Five milliliters of 6N siilfuric acid were adckd, followed by 4 grams of solid sodium perborate, and then 20 nil. of 6-V sodium hydroxide. T h e flas!c was imniediatrly fitted with a standard-taper groundglass stopper holding a 40-ml. dropping funnel. [The lower tip of the furiiiel TWS scaled to the bottom of the stopper in such a way that no pocket of gas was above the funnel’s tip. T h e ground-glass parts were lubricated with a high vacuum silicone, because greases and waxes ( 9 ) melt and leak during the later operations.] The mixture in the flask was boiled for 5 minutes with the funnel stopcock open. After the mixture was cooled, a small additional lump of sodium perborate was added, t o reoxidize any cobalt which had been reduced b y ammonia before all of t,hat gas was boiled a m y ( 7 ) . The mixture was then boiled for another 10 minutes. A little boiled distilled water was added to the dropping funnel to prevent air from being sucked back into the flask n-hen the heating was discontinued. These operations precipitated the cobalt as cobaltic hydroxide, decomposed the excess perborate, and expelled all free oxygen from the flask. It was removed from the flame and shaken to allow the superheated water t o form steam, arid the stopcock was closed just when the flask began t o suck the water from the funnel. Excess standard 0.1N Mohr’s salt solution was at once added t o the dropping funnel from a buret, and this solution was admitted cautiously to the flask in such a ivap t h a t no air entered. The content of the flask was swirled, and 35 ml. of G N sulfuric acid were immediately admitted through the dropping funnel. The content was swirled again and 10 ml. of 25% phosphoric acid were added. The cobaltic hydroxide dissolved immediately and completely upon acidification. Air as then admitted and the funnel was rinsed into the flask. The solution had the typical pink color of cobaltous ion. The solution vas transferred to an 800-ml. beaker t o which excess standard 0.lNpotassium dichrcmate was added. The mixture was back-titrated with more 0 . 1 S Mohr’s salt to the potentiometric end point. Found, per cent cobalt: 23.52, 23.25, 23.45; ?verage, 23.41. Two more samples of the same size were weighed out and mixed with sodium orthotungstate dihydrate as before. T o one of these about 0.2 gram of cobaltous chloride hexahydrate was added. Both were analyzed for tervalent cobalt b y using exactly the same procedure, except t h a t no additions of perborate vere made. I n both cases 23.00y0 tervalent cobalt was found. That, these results are a little low is probably attributable t o the slight reduction of cobaltic hydroxide by ammonia which is known t o occur in basic solution ( 7 ) . N o such effect was noted when analyzing compounds which contained n o amine or ammonia. 1625
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ANALYTICAL CHEMISTRY
LA series of heteropoly tungstocobaltates ( 3 ) )containing both (2) Baker, L. C. W., Loev, B., and McCutcheon, T. P.. [bid.. 72, bivalent and tervalent cobalt, was analyzed by these methods. 2374 (1950). I n some cmes the proportion of tungstate was several times that (3) Baker, L. C. W.,and McCutcheon, T. P., Abstracts of Papers cited above, but the end point was always sharp. The substances Presented at . Fourth Meeting-in-Miniature. Philadelphia are not good primary standards, but the results showed excellerit Section, AMERICAX CHEMICAL SOCIETY, p. 81, 1951. consistency. Examples: percentages calculated for CS6H(4) Baker, L. C. W., and AfcCutcheon, T. P.. ANAL.CHEM.,22, 944 [ C O + ~ C O + ~ W.13H20: ~ ~ O ~ Zcesium, ] 19.8; tungsten, 54.8; water, (1950). 6.04; cobalt (tervalent), 1.47. Found: cesium, 19.9; tungsten, ( 5 ) Biltz, H., and Biltr, W., “Laboratory Methods of Inorganic 54.9; water, 6.00; cobalt (tervalent), 1.47. (Total cobalt is Chemistry;” 2nd ed. (adapted from the German by W Hall always a little high in compounds of this anion because the method and A. Blanchard), pp. 173-4, Wiley, New York, 1928. of preparation leaves them impure in that respect.) Percentages (6) Friedheim and Keller, Ber., 394, 4301 (1907). s +3W12042 1.18H20 : potassium, 4.57 : calculated for K ~ H[ Co (7) Horan, H. A., and Eppig. H. J., J . -4m.Chem. SOC.,71, 581 tungsten, 64.5; water, 10.8; cobalt (tervalent), 1.72. Found: (1949). potassium, 4.60; tungsten, 64.5; water, 10.5; cobalt (tervalent), (8) Kolthoff, I. M.. and Sandell, E. R., ”Textbook of Quant,itative 1.79. The method is applicable in the presence of molybdates Inorganic Analysis,” rev. ed., p. 493, >lacmillan, Xew York, also. Percentages calculated for (SH~)3H5[Co(OH)(MoOa)6] 1946. 3 H 2 0 (2,6): cobalt. 5.97; nitrogen, 4.28; molybdenum, 18.6. (9) Sarver, L. A., IXD. EXG.C m x . , .&SAL. ED., 5, 276 (1933). Found: cobalt, 5.96; nitrogen, 4.42; molybdenum, 48.3. Percentages calculated for (CH3”3)6[(Co06~lo60,5)~],18&0 (1,2, (10) Sarver, L. A., and Kolthoff. I. AI., J . Am. Chem. Soc., 53, 2902 15): cobalt, 4.81; nitrogen, 3.42: molybdenum, 47.0. Found: (1931). cobalt, 4.80; nitrogen, 3.53; molybdenum, 46.7. A sample of (11) Treadwell, F. P., and Hall, W. T., “ilnalytical Chemistry,” vol. the ammonium salt of t,he last-mentioned anion (1,2,6,15) gave 11, 8th ed., p. 145, Wiley, New York, 1935. 4.74% cobalt by the 1-nitroso-2-naphthol method and 4.74% (12) Willard, H. H., and Diehl, H., “Xdvanced Quantitative h a l y cobalt by the method here described. sis,” pp. 233, 236, 243-4, 249-50, Van Nostrand, New York, 1943. ACKNOWLEDGMENT (13) Willard, H. H.. and Young, P., Ind. Eng. Chem., 2 0 , 764 (1928). Helpful advice was received from Donald Cooke, Arno Heyn, (14) Willard, H . H., and Young, P., TND. ENG.CHEM.,ANAL.ED.,4, and Wallace hlcNabb. 187 (1932); 5, 154 (1933). (15) Wolfe, C. W., Block, AI., and Baker, L. C. W,, J . A n i . C‘hem. REFERENCES Soc., 77, 2200 (1955). ( 1 ) Baker, L. C. W., Foster, G., Tan, K., Scholnick. F., and RlcR E C E I V Efor D review January 28, 1955. Arcepted M a y 3, 1955 Cutcheori, T. P., .I. A n ! . (‘hen!. Soc., 77, 2136 (1955).
Spectrophotometric Determination of Phosphorus as Molybdovanadophosphoric Acid KENNETH P. QUINLAN
and
MICHAEL A. DESESA
Raw M a t e r i a l s Development Laboratory, N a t i o n a l Lead Co., Inc., Winchester, Mass.
The molybdovanadophosphoric acid method for the spectrophotometric determination of phosphorus has been extensively reviewed. The optimum concentrations of acid, vanadium(V), and molybdenum(V1) were determined by factorial experiment. The optimum color development occurs in solutions which are 0.4M in acid, 0.02 to 0.06M molybdenum(VI), and 1.0 to 4.0mM vanadiumw). The optimum range is 3 to 20 p.p.m. of phosphorus pentoxide for 1-em. cells. Dichromate is the only serious interference, but can be eliminated by volatilization of the chromium as chromy1 chloride.
A
N E F F O R T 4s currently being made to review critically the
analytical methods employed in this laboratory, R ith the intention of replacing any which may have become outdated, by more rapid or accurate procedures. The tlvo requirements for routine control analyses are speed and a reasonable degree of accburacy. With these needs in mind, the literature was reviewed for methods of analysis for phosphorus. Various spectrophotometric procedures have been proposed and employed for the determination of phosphorus. Because spectrophotometric provedures offer the speed and accurary mentioned as requisites for a routine analysis, these procedures were examined in greater detail. I n common with arsenic, germanium, and silicon, phosphorus forms a yellow heteropoly acid with excess molybdate. These heteropoly acids may be utilized directly in spectrophotometric procedures or they may be reduced to molybdenum blue with an increase in sengtivity. While either of these methods can be used
for phosphorus determination, they are subject to serious interference if arsenic, silicon, or germanium is also present. .Is t,he 9amples treated in this laboratory are ores and ore products, all of these interferences may be encountered. Therefore, the heteropoly methods, as such, \+-erenot considered further. Various workers have attempted to make the molybdophosphorir arid method more selective by extracting the complex into an organic solvent. T h e literature on this subject has been recently reviewed by Wadeljn and Mellon ( 1 7 ) . However, the reagents which are most selective are difficult t o separate from the aqueous phase, and the other proposea reagents are not selective enough to justify the extra time involved in performing the extraction. Another popular spectrophotometric method for phosphorus is based on the formation of the yellow molybdovanadophosphoric acid as originally proposed by Misson ( 2 3 ) . This method seems to be the most sperific spectrophotometric method proposed for phosphorus, and has been used for t’he determination of phosphorus in steels and iron ores (3, 5 , 7-10! I S , 14, 16, 1 8 ) ; in uranium metal, uranium oxides, and uranium phosphate ( 1 , 1 2 ) ; and in phosphate rock ( 2 , 6 ) , limestone ( 4 , 16), and biological materials ( 1 1 ) . It was decided to investigate the possible ap plication of this method to the routine determination of phosphorus in uraniferous ores and ore products. A review of t h r literature reveals a surprising lack of conformity in the procedures for developing the color of molybdovanadophosphoric acid. Therefore, it was necessary to make a critical study of the optimum conditions for color development before the method could be employed. The results of this study and a modified procedure are reported in this paper.
