V O L U M E 25, NO. 1 2 , D E C E M B E R 1 9 5 3 Table 11. Weight Ratios of Elements Causing 5% Error Ratio (M/xb)
Element
Element
Ratio (M/Xb)
1809 stannous chloride, but measurements must be made almost immediately, since the iron is rapidly reoxidized by air. Titanium may be determined independently by the peroxide method and its contribution to the absorbance a t 281 mp subtracted from the total absorbance. The remainder is due t o niobium. Comparatively simple means of separation are available to reduce the concentrations of the other ions below the interfering ratio. The one exception is found in tantalum. The permissible weight ratio of tantalum to niobium was increased to 10 by the addition of sufficient hydrofluoric acid to make the solution about 0.01M in fluoride. Sulfate does not interfere up to 0.1M concentration, introduced as potassium bisulfate. Sulfuric acid tends to decrease the solubility of the hydrogen chloride. SUMMARY
Figure 4. Absorption Spectra of Interfering Cations i n Concentrated Hydrochloric Acid
The chloride complex which occurs in concentrated hydrochloric acid forms the basis for a simple, accurate, and rapid spectrophotometric method for the determination of niobium, in which absorbance is measured a t 281 mp. The method may be applied to any material that may be obtained in a hydrochloric acid solution. Interfering ions are vanadium(V), chromium(III), lead(II), iron(III), copper(II), molybdenum, and titanium(1V). The interferences from moderate amounts of iron(II1) and copper(I1) may be eliminated by reduction to the lower valence with tin(I1). Tantalum up to ten times the amount of niobium measured can be tolerated.
5 Y of metal per ml.
LITERATURE CITED
O.OC
240
” 260
280
WAVELENGTH
300
up
from the absorbance of several different concentrations of each ion a t 281 mp. These calculated weight ratios, given in Table 11, are presented as guides to the relative errors caused by the presence of the various ions. The absorption curves of several interfering elements are shown in Figure 4. The interference due to the presence of copper may be eliminated by addition of a small measured excess of stannous rhloride dissolved in concentrated hydrochloric acid to both the sample and blank solutions. The interference of iron can be reduced temporarily by the addition of a five- to tenfold excess of
(1) Desesa, AT. A., and Rogers, L. B., Anal. Chim. Acta. 6, 534-41 (1952). (2) Freund, H., and Levitt, A, ANAL,CHEM.,23, 1813-16 (1951). (3) Geld, I , and Carroll, R., Zbid., 21, 1098-1101 (1949). (4) Lauw-Zecha, A. B. H., Lord, S. S., and Hume, D. N., Ibid., 24, 1169-73 (1952). (5) Telep, G., and Boltz, D. F., Zbid., 24, 163-5 (1952). (6) Wernet, J., Z . anorg. u. allgem. Chem., 267, 213-37 (1952). RECEIVEDfor review June 24, 1953. Accepted September 28, 1953. Presented in part before the Division of Analytical Chemistry a t the 123rd Meeting of the AMERICAX CxiEhircaL SOCIETY, LOBh g e l e s , Calif. Published with the approval of the Oregon State College Monographs Publications Committeeas Research Paper No. 232, Department of Chemistry, School of Science.
Spectrophotometric Determination of Arsenic and Tungsten as Mixed Heteropoly Acids DELORA K. GULLSTROM WITH M. G. -MIELLON Department of Chemistry, Purdue University, Lafayette, Znd.
I
T HSS been known for years that a yellow color is formed on
adding an excess of molybdate and vanadate solutions to one containing orthophosphate. The colored component is assumed to be molybdovanadophosphoric acid, a mixed heteropoly complex. Misson (4, 6) first proposed this reaction as a colorimetric method for the determination of phosphorus in steel. The color reaction and its analytical application Fere investigated spectrophotometrically by Kitson and Mellon ( 2 ) . Wright and Mellon (11) studied the analogous mixed complex, tungstovanadophosphoric acid. I t s use for the determination of vanadium was proposed by Sandell (7). It seems reasonable that other mixed acids might have useful analytical applications. More specifically, since the addition of vanadate enhances the color of molybdophosphoric acid, it was of
interest to study such a possibility with the colorless molybdoarsenic acid. At the same time tungstovanadophosphoric acid was restudied with the aim of using this complex for the determination of tungsten.
EXPERIMENTAL WORK Apparatus and Solutions. Absorbance measurements were made in 1-cm. cells with a Beckman DU spectrophotometer. Incidental reagents were prepared as follows: 5% sodium molybdate dihydrate, NanMoOc.2H20, in water; 1 % sodium metavanadate, NaV03, in dilute sodium hydroxide, subsequently neutralized with hydrochloric acid; 1.7N hydrochloric acid (1 t o 6). A standard solution of arsenic was prepared by dissolving, with heating, arsenic pentoxide in water and diluting to a concentra-
ANALYTICAL CHEMISTRY
1810 tion of 100 p.p.ni. of arsenic. The standard solution of tungsten, containing 200 p.p.m., was tnade by dissolving sodium tungstate dihydrate in wat,er. All solutions were stored i n polyethylene bot,tles to avoid contamination by silicate. Color Reactions. Although molybdovanadophosphoric acid has been known for some years, the exact nature of the complex is not clear. It, seems probable that, other mixed heteropoly acids are of the same t,ype. Kitson and Mellon ( 2 ; spplied the method of Vosburgh and Cooper (9) to determine the ratio of phosphorus to vanadium in molybdovanadophosphoric acid. The results indicated t h a t the ratio is 1 to 1. Their data were redetermined in this work, with the same result. U ~ i n gthe same procedure on molybdovanadoarsenic acid indicated a ratio of 1 to 1 of vanadium to arsenic. Analogous determinations with tungstovanadophosphoric acid also gave a 1 t.0 1 ratio of vanadium to phosphorus. These latter results are less certain became the solutions were measured immediately after mixing to prevent precipitation of tungsten t,rioxide. I n this case it is possible that measurement was not made under equilibrium conditions. It has been postulated that. niolybdovanadophosphoric acid is formed by the substitution of both molybdenum and vanadium oxide radicals for the oxygen in the phosphate. By analogy, other mixed heteropoly compounds have been assumed t,o be similar. However, if this were the case, the ratio of vanadium to phosphorus should be at least 2 to 1. The theoretical interpretation of the observed evidence i j not apparent from present knowledge of heteropoly compounds. Fortunately, the obscure nature of the mixed heteropoly wid' does not impair their analytical applica tions.
Table 1. Absorbance Readings for Reagent for Arsenic Determination z's. Distilled Water W a r e Length, mr
460 440 420 400 380
Absorbance 0.015 0.055 0.I16 0.226 0,370
DETER3IINATIOS OF ARSENIC
The most sensitive colorimetric method for determining arsenic is based on the reduction of molybdoarsenic acid to molybdenum blue. This method usually requires preliminary distillation of the arsenic from interfering constituents. -4procedure avoiding this separative operation would be preferable. Since molybdoarsenic a a d solution is colorless, i t Tas hoped t h a t the addition of vanadate to form molybdovanadoarsenic acid would shift. the absorption to the visible region. Under suitable conditions a miature of arsenic, molybdic, and vanadic acids does have a yellow hue. The analytical possibilities of this system, assumed to be a miled heteropoly acid, are reported herein. Effect of Variables on the Color Development. In order to study possible effects of variation in the several reactions concerned in the color reaction, preliminary experiments were made to select conditions for a stai ting system. The following mixture of reactants seemed appropriate: 10 ml. of arsenate solution (100 p.p.m. arsenic), 7.5 ml. of vanadate solution ( l % ) ,6 ml. of hydrochloric acid (1.7N),and 10 ml. of sodium molybdate solution ( 5 % ) were mixed in that order, The solution was diluted to 50 ml. in a volumetric flask and measured a t 400, 420, 440, and 460 mp. A reagent blank containing the same concentration of reagents except the arsenate was used as the reference system. Considerable color develops in the blank solution, as shown in Table I. Since a spectrophotometric curve between 340 and 500 mw shows no maximum absorption, the wave lengths were chosen arbitrarily.
The study reported concerns the formation of two heteropoly complexes, molybdovanadoarsenic acid and tungstovanadophosphoricacid, and their wlorimetric applications to the determination of arsenic and tungsten. The effects of the following variables were determined for each method: acidity, concentrations of reagents and of the desired constituents, order of adding reagents, stability, and 52 diverse ions. A procedure for the determination of arsenic is recommended, together with its application to the analysis of Paris green. Likewise, a procedure is recommended for the determination of tungsten, with application to tungsten steels from the National Bureau of Standards. The method for arsenic is rapid, convenient, and reliable for the range of 1 to 30 p.p.m. Determination of arsenic in Paris green proved feasible without separating the element from the copper. The method for tungsten is useful for the range of 10 to 120 p.p.m.
Using these experimental conditions, each of the several variable factors was then studied separately to find optimum conditions. Acidity. The optimum amount of acid appears to be 6 ml. Additional acid decreases the color of the heteropoly acid. Less acid causes an extremely high blank and necessitates a prohibitively wide slit. Sulfuric and nitric acids of the same normality give the same absorbances. Vanadate Concentration. The color steadily increases as vanadate concentration increases up to 7 ml. of 1% sodium vanadate, even when a reagent blank containing the same amount of vanadate and molybdate is employed. The absorbance values then level out, regardless of the vanadate concentration. \'anadate concentration is critical, however, and carefully controlled concentrations should be used in the blank and sample solutions. Molybdate Concentration. hlolybdate must be in exceas in order to have complete color development. The color increases up to 8 ml. of 5% sodium molybdate. Ten milliliters (kl'iare recommended to ensure complete color development. Order of Adding Reagents. The reagents should be added to the arsenic solution only in one order: acid, vanadate, and molybdate. If the acid is added last, a positive error results because of the deeper yellox color formed between vanadate and molybdate a t higher p H values. This color does not disappear readily upon addition of acid. If the arsenate and molybdate are mixed before the addition of vanadate, molybdoarsenic acid forms. I t is not readily converted to the mixed acid. A composite reagent can be added directly to the arsenate solution. The absorbance readings are the same as if the recommended amounts, previously given, are added separately. The order of addition of the components of this reagent seems to be unimportant. Twenty-five grams of sodium molybdate dihydrate are dissolved in 70 to 80 ml. of distilled water. Sodium metavanadate (3.75 grams) is dissolved in 50 ml. of dilute sodium hydroxide and then neutralized with hydrochloric acid. These two solutions are mixed with 50 ml. of concentrated hydrochloric acid, cooled, and diluted to 250 ml. Five milliliters of this reagent will give optimum color in the arsenic determination. The blank contains 5 ml. of this reagent diluted to 50 ml. The reagent is stable for a t least a week. Arsenic Concentration, Beer's law applies from 1 to 30 p.p.m. for all wave lengths used. Greatest sensitivity is obtained a t 400 mp. Table I1 shows representative absorbance values. The molar absorptivity a t 400 mp is 2540 liters per mole per centimeter.
1811
V O L U M E 2 5 , NO. 12, D E C E M B E R 1 9 5 3 Stability of Color. The color is stable for a t least 24 hours. Diverse Ions. To study the effect of diverse ions, 10 ml. of a solution containing 100 p.p.m. of arsenic was added to a 50-nil. volumetric flask. To it was added 5 ml. of a solution containing 10 grams per liter of the diverse ion, followed by 5 ml. of reagent. The solution was diluted to 50 ml. and measured a t the desipated wave lengths. In cases of interference, a more dilute solution of the diverse ion was tested until the relative error in measurement was 2% or less of the amount being measured. The cation constituents were added as chlorides, nitrates, or sulfates. The anionic constituents were added as their sodium or potassium salts. The error does not exceed 2% for amounts up to 1000 p.p.m. of any of the following cations: aluminum, ammonium, barium, beryllium, cadmium, calcium, cobalt(II), copper(II), lithium, magnesium, manganese(II), mercury(II), potassium, strontium, uranium(VI), and zinc. The error is 2% or less for amounts of the folloning anions in concentrations up to 1000 p.p.m. : acetate, bromide, chloride, fluoride, formate, iodate, nitrate, nitrite, oxalate, perchlorate, periodate, selenate, and sulfate. Bismuth(TII), lead, thorium, and zirconium(V1) interfere because basic salts precipitate a t the pH of the determination. Silver interferes because of precipitation of the chloride. Ions such as dichromate, nickel, and permanganate interfere because of their color. Reducing agents such as thiosulfate and thiocyanate reduce the heteropoly acid to heteropoly blue. Other ions, such as borate, citrate, and tartrate, give a negative error because they prevent formation of the mixed heteropoly acid through complexation of the reagent. Ions such as germanate, phosphate, silicate, tungstate, and vanadate give positive errors because of the formation of additional heteropoly acids. ;1summary of the effect of interfering ions is given in Table 111.
Reagents. ~ I I X ECOLORAST. D Dissolve 25 grams of sodium molybdate dihydrate, Ka?MoOc.H20, in 70 to 80 ml. of distilled water. Dissolve 3.75 grams of sodium metavanadate, KaVOa, in dilute sodium hydroxide and neutralize with hydrochloric acid. Mix these tu-o solutions with 50 ml. of concentrated hydrochloric acid, cool, and dilute to 250 nil. Store the solution in a polyethylene bottle. STAXDARD ARSESICSOLUTIOS. DiF;l;oive0,0750 gram of arsenic pentoxide, Asnos, in 300 ml. of water. Heat until dissolution is complete, cool, and dilute to 500 ml. This solution contains approximately 1007 of arsenic per nil. Store in a polyethylene bottle. Preparation of Calibration Curve. With a pipet transfer 0, 2.00, 4.00, 6.00, 8.00, and 10.00 mi. of the standard arsenic solution into each of six 50-ml. volumetric flasks. To each flask add 5 ml. of the molybdate-vanadate-hvdrochloric acid reagent, dilute to 50 ml., and measure at t.he dw-ired wave length. Use the sample containing 0 ml. of arsenic a~ a hlank. Plot a curve of absorbance against weight of zrsenic.
Table IV.
Analysis of Paris Green Samples for Arsenic Using Molybdovanadoarsenic Acid Arsenic Found, % Colorimetric Av. Range Detns
Sample So.
Detns.
1 2 3
8 3 3
7 95 11 45 15 11
1 0 25 1 0 13 1 0 14
4
3
17 39
1 0 19
3 3 3 3
Titrimetric Ar. Range
7 92 1 1 30 15 32 17 71
1 0 05 1 0 05 fO 08 1 0 0%
Procedure. Select a sample containing up to 30 p.p,m. of arsenic(V). If the concentrations of the other ions exceed the amounts permissible, as shown in Table 111. separate the arsenic as the trichloride by the method of Scherrer (8). Adjust the pH to approximate neutrality> using dilute hydrochloric acid or sodium hydroxide. Add 5 ml. of reagent, dilute t o 50 ml. in a volumetric flask, and determine the absorbance a t the same wave length used for the calibration curve. Use a reagent blank for this measurement. Refer to the calibration curve to find the amount of arsenic in the sample. AYALYSIS OF PARIS GREEN SAMPLES
RECOMMENDED METHOD
Apparatus. The spectrophotometer must be capable of measurement a t 400 mp. The band width should be narrow for most accurate readings in this region of the spectrum.
Table 11. Absorbance Readings for 20 P.P.M. of Arsenic at Various Wave Lengths Using a Reagent Blank Wave Length, mu 480 ~ .
.
440 420 400 380
Bbsorbance 0.205 0.293 0.435 0.645
A X 3.80 0.100
0,901
(Each solution contained 20 p.p.rn. of arsenic; measurement uas made at 400 420, 440, and 460 rnp in I-rin. cells using a reagent blank) Maximum Permissible Ion Amounta, P.P.M. f:r + + + 200 25 .4u+++ 0 25 Citrate--200 0 CrzOi-GeOs-0
E::+ T-b
0
0 0 0
200 0 25 0
For an error (relative) not exceeding 2%. b Oxidizes to iodine.
%As=---
’
Table 111. Effect of Interfering Ions
a
To test the method, samples of Paris green were analyzed for arsenic content. -4sample of 0 1 gram was used. Ten milliliters of a 5% solution of ammonium peroxydisulfate was added. The solution was boiled for 5 minute6 to ensure arsenic(V). This also brings about complete sample dissolution. Heating destroys the excess oxidizing agent. The solutions were cooled and made up to 50 ml. These solutions were diluted 10 to 25 times depending on the arsenic concentration. Measurement was made a t 400 mp. The arsenic concentration was calculated from the following equation:
This equation is valid for an initial dilution t o 50 mI. which is then diluted 2 ml. to 50 ml. The value of 3.80 was calculated from the best straight line for Beer’s law plot. Table IV shows the results obtained on four samples. CONCLUSIONS
The procedure is a rapid, convenient method for the determina* tion of arsenic in a concentration range of 1t o 30 p.p.m. The sensitivity is less than that of the heteropoly blue method or the extraction method recommended by Wadelin and hlellon ( I O ) . The absorptivity for the heteropoly blue procedure given by Boltz and Mellon ( 1 ) is 25,400 liters per mole per centimeter a t 840 mG and 5100 liters per mole per centimeter a t 370 mp in the extraction procedure recommended by Tadelin and Mellon. The absorptivity of this procedure is 2540 at 400 mp. The procedure is reproducible and accurate for mixtures containing arsenic as Paris green. Measurement can be made a t any desired lvave length between
1812
ANALYTICAL CHEMISTRY
370 and 460 mp. The lower wave lengths may sometimes be desirable to avoid interference from colored ions which absorb in the visible.
DETERMINATION OF TUNGSTEN
Sandell ( 6 ) mentions the use of tungstovanadophosphoric acid for the determination of tungsten. Although Lennard (3) used this means for the determination of tungsten, he did not study the optimum conditions. Thus, acidity is important in affecting the extent of color development, but he does not specify the amount of acid used in the determination. Hls reported sensitivity is much less than that which can be obtained under optimum conditions. The general objective of this work, then, was to investigate the practicability of the use of tungstovanadophosphoric acid for the determination of tungsten. It was hoped that this procedure might eliminate or minimize the interferences inherent in other methods for tungsten. Effects of Variables on the Color Development. In order to study possible effects of variation in the several variables concerned in the color reaction, preliminary experiments \-,-eremade to select conditions for the starting system. The following mixture of reactants was used; 40 p.p.m. of tungsten as tungstate, 1 ml. of phosphoric acid (Baker’s 85%), and 3 ml. of neutral solution of 1 % ’ sodium vanadate were mixed. The solution was diluted to 50 ml. in a volumetric flask and measured a t 400, 420, 440, and 460 mp. The reagent blank contained the same concentrations of reagents except tungstate. Table V shows typical blank readings. Since a spectrophotometric curve between 260 and 500 mp shows no maximum absorption, these wave lengths were chosen arbitrarily.
Table V. Absorbance Readings for the Reagent for Tungsten Determination us. Distilled Water Wave Length, mr
460 440 420 400
380 360
Absorbance 0.030 0.036 0.057 0.077 0.100 0.130
Using these experimental conditions, each of the several variable factors was then studied separately to find optimum conditions. Acid and Vanadate Concentrations. Varying amounts of phosphoric acid and sodium vanadate were added to a solution containing 40 p.p.m. of tungsten(V1). The optimum concentrations of vanadate and phosphoric acid are not independent variables. It is advisable to use enough acid to give a colorless blank with the amount of vanadate used. If 0.5 ml. of phosphoric m-as used with 2 ml. of a neutral solution of 1% sodium vanadate, optimum results were obtained. Although the p H was important, excess phosphate had no effect on the color. The common mineral acids, hydrochloric, nitric. and sulfuric, can be used to adjust the acidity. The optimum pH is 1.8as measured on a Beckman p H meter. If less acid is used, a highly colored yellow blank results. If more acid is used, there is a ‘decrease in the sensitivity of the method. The ratio of the two reagents is very critical. It was decided to prepare a mixed vanadate-phosphate reagent to add to the solutions containing tungsten. This reagent is made up as follows: 20 ml. of phosphoric acid and 80 d.of 1% sodium vanadate solution were mixed in a 100-ml. volumetric flask. The reagent was then made up to 100 ml. n-ith distilled water. The reagent is stable for about three days; but if it is kept longer, a precipitate develops and low results are obtained. Two and one half milliliters of reagent are used in 50 ml. of solution. Tungsten Concentration. Beer’s law applies from 10 to 120
p.p.m. for all wave lengths used. The molar absorptivity a t 400 mp is 621 liters per mole per centimeter. Absorbances for several wave lengths are given in Table VI. A spectrophotometric curve was run in the ultraviolet down to 260 mp and no absorption maximum occurs a t any wave length. The wave lengths used for measurement were chosen arbitrarily. It might be desirable to shift the wave length for measurement in order to eliminate interferences which do not absorb at another wave length. Stability of Color. The color is stable for at least 24 hours.
Table VI. Absorbance Readings for 40 P.P.M. of Tungsten at Various Wave Lengths Using a Reagent Blank Wave Length, mlr
Absorbance
4R0 _. .
0.049
440 420 400 3 80
0.070 0.104
0.136 0.179
Diverse Ions. To study the effect of diverse ions, 10 ml. of a solution containing 200 p.p.m. of tungsten were added to a 50-ml. volumetric flask. To it was added 5 ml. of a solution containing 10 grams per liter of the diverse ion and 2.5 ml. of reagent. The solution was diluted to 50 ml. and measured a t the designated wave lengths. If a given ion interfered, a more dilute solution of diverse ion was tested until the relative error in the measurement Tvas 2% or less. The cations rYere added as chlorides, nitrates, or sulfates, The anions were added as their sodium or potassium mlts. The error does not exceed 2% for amounts up to 1000 p.p.m. of any of the following cations: aluminum, ammonium, barium, beryllium, cadmium, calcium, cobalt(II), copper(II), lithium, magnesium, manganese( II), mercury(II), potassium, strontium, uranium(VI), and zinc. The error is 2% or less for amounts of the following anions in concentrations up to 1000 p.p.m.: acetate, borate, bromide, formate, iodate, nitrate, perchlorate, phosphate, selenate, silicate, sulfate, and thiocyanate. Bismuth, lead; thorium, and zirconium(V1) interfere because basic salts precipitate a t the pH of the determination. Silver interferes because of the precipitation of the chloride. Ions such as dichromate and permanganate interfere because they themselves are colored. Thiosulfate interferes because it reduces the heteropoly acid. Oxidizing agents interfere because of the formation of the peroxide of vanadium. Ions such as oxalate and citrate cause a negative error because they destroy the heteropoly acid formed. Ions such as arsenate and germanate give negative errors because of the formation of less colored heteropoly compounds. A summary of the effect of the interfering ions is given in Table VII. RECOMMENDED PROCEDURE
Apparatus. The spectrophotometer must be capable of measurement a t 400 mp. The band width should be narrow for the most accurate readings in this region of the spectrum. Reagents. PHOSPHORIC ACID-SODIUMVANADATE SOLUTION. Dissolve 0.8 gram of sodium metavanadate, NaVOa, in 50 ml. of dilute sodium hydroxide, neutralize, and dilute to 80 ml. in a graduate cylinder. Mix with 20 ml. of sirupy phosphoric acid (Baker’s 85%) in a 100-ml. volumetric flask. Cool and dilute to 100 ml. Store in a polyethylene bottle. STAKDARD TUNGSTATE SOLUTION.Dissolve 0.3587 gram of sodium tungstate dihydrate, Na2WO(.2H2OJin a liter of water. This solution contains 200 p.p.m. of tungsten. Store in a polyethylene bottIe.
V O L U M E 25, NO. 1 2 , D E C E M B E R 1 9 5 3 Preparation of Calibration Curve. With a pipet transfer 5, 10, 15, 20, and 25 ml. of the standard tungstate solution into five 50ml. volumetric flasks. Add 2.5 ml. of the vanadate-phosphoric acid reagent, dilute to 50 ml., mix, and measure a t the desired wave length. Use a sample containing 0 ml. of tungstate as a blank. Plot a curve of absorbance against weight of arsenic. Procedure. Select a sample containing from 10 to 120 p.p.m. of tungsten. If the concentrations of other ions exceed the amounts permissible (see Table VII), separate the tungsten as tungsten trioxide. Filter this off in a sintered-glass crucible, discard the filtrate, redissolve the precipitate in sodium hydroxide, and neutralize the solution. Add 2.5 ml. of mixed reagent t o the neutral solution containing the tungsten, dilute to 50 ml. in a volumetric flask, and determine the absorbance a t the same wave length used for the calibration curve. Use a reagent blank for this measurement. Refer to the calibration curve to find the amount of tungsten in the sample.
Table VII. Effect of Interfering Ions (Each solution contained 40 p.p.m. of tungsten: measurements were made at 400, 420, 440, and 460 mw in 1-cm. cells using a reagent blank) Maximum Permissible Ion -4mounta, P.P .\I. Cr+++ 0 Au + 25 Fe+T+ 0 Nl++ 25 Citrate--0 CrlO: - 0 F25 GeOl-200 I -b 0 KO2 10 2d Oxalate-1040 Mn040 0 SOa-Tartrate-0 s203 - 0 As04 - - 0 voa0 a For a n error (relative) not exceeding 2%. b Oxidizes t o iodine. +
1813
Table VIII. Analysis of Two National Bureau of Standards Samples for Tungsten Using Tungstovanadophosphoric Acid Tungsten, % ’
Sample NO.
50 A
Found 17 6 18 5 17 8 18 2
SBSG
18 2
..
75
75.4 75.2 74.0 .. 76.8 .. 5 -4verage of several determinations carried out b y bureau and contributing analysts.
purities, these quantities are too small to interfere with the determination. If no previous separation is made, results are 3 to 5% high. Table VI11 shows results obtained on two samples.
concLusIoNs The procedure is a rapid convenient method for the determination of tungsten in the concentration range of 10 to 120 p.p.m. The sensitivity of the method, although not high, compares favorably with that of colorimetric methods for tungsten. The absorptivity is 621 liters per mole centimeter a t 400 mp. The procedure is reproducible and accurate. It has been applied to samples of known tungsten content of the Sational Bureau of Standards. RIeasurement can be made a t any desired wave length between 360 and 4GO mp. K a v e lengths other than 400 mp may sometimes be desirable to eliminate interference from diverse ions. ACKNOWLEDGRIENT
The authors gratefully acknowledge the support of this work through a grant by Eli Lilly and Co. Analysis of Standard Samples. Samples of steel from the National Bureau of Standards were treated in the following manner: to 0.1-gram samples add 10 ml. of water and 25 ml. of concentrated hydrochloric acid. Heat the solutions on the hot plate until hydrogen is no longer evolved. Add concentrated nitric acid dropwise to the hot solutions until dissolution is complete. Cool the samples and add 25 ml. of distilled water. Let the samples stand for 2 hours and then filter through a fine-pore, sintered-glass crucible, without trying to remove quantitatively the precipitate of tungsten trioxide. Wash the beaker and precipitate with 2 N nitric acid. Discard the filtrate and dissolve the precipitate in the beaker in dilute sodium hydroxide solution. Treat the precipitate in the crucible with three portions of IN sodium hydroxide and collect the filtrate. Neutralize the solution with hydrochloric acid and make up to 100 m]. in a volumetric flask. Make a suitable dilution in a 50-ml. volumetric flask to bring the tungsten concentration within the range of the method. lldd 2.5 ml. of the vanadate-phosphoric acid reagent, dilute the solution to 50 ml., and measure a t 400 mp.
(1) Boltz, D. F., and Mellon, M. G., AXAL.CHEM.,19, 873 (1947). (2) Kitson, R. E., and Mellon, 11.G., IND.ENG.CHEM.,Asar.. ED., 16, 379 (1944). (3) Lennard, G. J., AnaZyst, 74, 263 (1949). (4) Misson, G., Ann. chim. anal. et chim. appl., 4 , 267 (1922). (5) illisson, G., Chem. Ztg., 32, 633 (1908). (6) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” 2nd ed., p. 588, K e w York, Interscience Publishers, 1950. (7) Sandell, E. B., IND.ESG. C ” d x , =INAL. ED.,8, 336 (1936). (8) Scherrer, J. A , , J . ReseaTch S a t l . B U T .Standards, 21, 95 (1938). (9) T-osburgh, W. C., and Cooper, G. R., J . Am. Chem. Soc., 63,437 (1941). (10) Wadelin, Coe, and Rfellon, 11. G., A n a l y s t , 77, 708 (1952). (11) Wright, E . R., and Ifellon, 11.G., ISD. ENG.CHEM.,A s ~ L ED., . 9, 251 (1937).
-4lthough i t is known that the tungsten trioxide precipitate obtained in this manner is contaminated xith small amounts of im-
RECEIVED for review April 20, 1953. Accepted September 25, 1953. Abstracted from a portion of t h e thesis presented by Delora K . Gullstrom in partial fulfillment of the requirements for the degree of doctor of philosophy.
LITERATURE CITED
