Ultraviolet Spectrophotometric Determination of Bismuth by Iodide and Thiourea Methods N. M. LlSlCKl'
and
D. F. BOLTZ
W a y n e University, Detroit, M i c h .
A spectrophotometric study was undertaken to investigate the feasibility of increasing the sensitivity of iodide and thiourea methods for the determination of bismuth by making spectrophotometric measurements in the ultraviolet region of the spectrum. The absorption spectra of both complexes have characteristic ultraviolet absorbance maxima. By using ultraviolet absorbance maxima instead of the absorbance maxima found in the visible region, a large increase in sensitivity is achieved for both methods. The effect of diverse ions and the optimum conditions for the formation of each complex were studied. The optimum concentration range for both ultraviolet spectrophotometric methods is 0.6 to 6 p.p.m. of bismuth when 1.000-cm. absorption cells are used.
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XE of the classical colorimetric methods for the detennina-
tion of bismuth is based on the yellow color of the tetraiodobismuthate(II1) complex (6, 7 , 10, 11). A colorimetric method for determining antimony is based on the tetraiodoantimonate(II1) complex. Elkind, Gayer, and Boltz (2) showed that the tetraiodoantimonate(111) complex has a characteristic ultraviolet absorbance maximum, which could be used to increase the sensitivity of the iodide method by approvimately 350%, and that bismuth(II1) interfered in the ultraviolet spectrophotometric determination of antimony. This spectrophotometric investigation was made to ascertain whether increased sensitivity could be obtained for bismuth(II1) complexes by performing spectrophotometric measurements in the ultraviolet region of the spectrum. A survey of the literature revealed that Merritt, Hershenson, and Rogers (8) recommended the use of hydrochloric acid to form the tetrachlorobismuthate(II1) complex. This complev eshibitrd maximum absorbance at 327 mp and served as a suitable basis for the ultraviolet spectrophotometric determination of bismuth. They also showed the ultraviolet spectrophotometric curves for the tetraiodo- and tetrabromobismuthate(II1) compleves, which have characteristic absorbance mavima at approyimately 340 and 375 mp, respectively. Kingerly and Hume ( 4 ) showed that the hexathiocyanoI03 bismuthate(II1) complex has a characteristic absorbance maximum a t 330 mp. I n addition to the tetraiodobismuthate(111)complex it was decided to study the bismuth( 111)-thiourea complex, which has been used in colorimetric analysis although its sensitivity is much less than the dithiaone method and slightly less sensitive than the iodide method (6). Recently, Nielsch and Boltz (9) investigated the use of thiourea as a reagent in the colorimetric determination of bismuth in nitric, hydrochloric, and hydrobromic acid solutions. GENERAL EXPERIMENTAL WORK
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cells lvith a reagent blank solution in reference cell, unless otherwise stated. A standard bismuth solution was prepared by dissolving 0.2321 gram of hydrated bismuth nitrate, Bi(N03)3.5H20, in 5 ml. of perchloric acid, or in 5 ml. of concentrated nitric acid, and diluting t o 1 liter in a volumetric flask. The use of perchloric acid is preferable for ultraviolet measurements. The standard solution has a titer of 0.100 mg. of bismuth per ml., this value being confirmed by a gravimetric determination as bismuthyl chloride. The potassium iodide-ascorbic acid reagent was prepared by dissolving 140 grams of reagent grade potassium iodide and 10 grams of ascorbic acid in distilled water and diluting to 1 liter. A 1O.V sulfuric solution was prepared by adding carefully 280 ml. of concentrated reagent grade sulfuric acid t o distilled water and diluting to 1 liter. The thiourea solution was prepared by dissolving 60 grams of reagent grade thiourea in approximately 500 nil of distilled water and warming to effect dissolution. The solution \vas filtered through a sintered-glass filter crucible of medium porosity and diluted to 500 mi. Each milliliter of the thiourea solution contained 0.12 gram of thiourea. Reagent grade (70%) perchloric acid was used. The solutions used in studying the effect of diverse ions w r e prepared using reagent grade chemicals. General Procedure. The required volume of the standard bismuth(II1) solution is transferred to a 50-ml. volumetric flask. After the desired volume of acid and any diverse ion solution is added, the color-forming reagent is added. The solution is then diluted to the graduation mark with distilled n-ater. The reference absorption cell contains a solution, which has the same concentration of acid mid color-forming reagent. IODIDE METHOD
Effect O f Solution Variables. BISMUTH C O S C E K T R A T I O S . Figure 1 shows the absorption spectrum for the tetraiodobismuthate(II1) complex, m-hich exhibits absorbance maxima a t 337 and 465 mp. This figure also indicates the increase in sensitivity obtainable by making the spectrophotometric measurements in the ultraviolet region a t 337 mp. The ultraviolet epectrophotometric curves obtained with 0.5, 1, 2, 4, and 10 p.p.m. of bismuth are shown in Figure 1. Conformity to Beer's law was found a t 337 mp. The optimum concentration is 0.6 to 6 p.p.m. of bismuth for spectrophotometric measurements a t 337 mfi, based on the virtual linear portion of a Ringbom plot.
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V O L U M E 2 7 , N O . 11, N O V E M B E R 1 9 5 5 IODIDE COXCESTRATIOS.The effect of iodide concentrat,ion was determined, using 4 p.p.m. of bismuth, 10 ml. of 10X sulfuric acid, and amounts of potassium iodide varying from 0.25 to 3.5 grams per 50 ml. I t was found that 2.5 to 3.5 grams of potassium iodide per 50 ml. m-as sufficient to assure attainment of maximum absorptivity. Hence, in subsequent work, 20 ml. of the potassium iodide-ascorbic acid reagent were used giving a final concentration of 5.0% in respect to potassium iodide. This concent'ration is much higher than that usually used in the colorimetric determination of bismuth. ; ~ C I D COSCENTRATION. The use of sulfuric acid is recommended, because hydrochloric acid tends to decrease the absorlinnce through formation of tetrachlorobismut'hate( 111) ion, nit'ric acid has a characteristic absorbance in the ultraviolet region, and perchloric acid precipitates potassium perchlorate. The concentrat,ion of sulfuric acid was varied from 1.5 to 2 . 5 S with no appreciable effect on absorptivity. Trim. The tetraiodobismuthate(II1) complex is stable for a t least 24 hours, using the original reference blank solution in the reference cell. The absorbances of the blank and the sample change slowly, but, at the same rate; thus, for maximum precision and accuracy, both should be the same age. d difference of 5% has been observed on substituting a fresh blank for an aged blank TThen measuring a solution t,hat has been standing for 24 hours. DIVERSE10s COWEXTRATION. The effect of certain diverse ions was determined using 4 p.p.m. of bismuth, 20 ml. of the potassium iodide-ascorbic acid reagent per 50 ml., and a final ncidity of 2 S in respect to sulfuric acid. The spectrophotometric measurements were made a t 337 mp. An error of less than 2.tjyOwas obtained with 1000 p.p.m. of the following ions: acetate. ammonium, bromide, cadmium, calcium, chloride, ferrous, fluoride, manganous, magnesium, nitrate, perchlorate, potassium, sodium, sulfate, Versenate [(ethylenedinitrilo)tetraacetate], and zinc. Table I lists those ions which TTere found to interfere. Although a preliminarj- separation of bismuth( 111) from copper(I1) is possible by extracting with dithizone ( S ) , the effect of cupric ion n-as investigated inasmuch as it was expected that iodine mould be liberated by cupric ions. Figure 2 shows that 50 p.p.m. of cupric ion can be tolerated, as the iodine liberated does not affect the spectrophotometric measurement at 337 mp, Larger amounts of cupric ion interfere because of the development of a turbidity due to cuprous iodide. Ferric ions which were expected to interfere seriously in the iodide method did not liberate sufficient iodine to introduce a serious interference. T h e ascorbic acid in reagent is presumably effective in reducing the oxidation potential of the ferric ion below the value necessary for oxidation of iodide ions to iodine. The fact that ferric fluoride complex does not liberate iodine from an iodide solution
1723 and the i n c t that fluoride ions do not interfere in the determination of bismuth, also, suggests another method of increasing t,he tolerance to ferric ions. An attempt to eliminate the interference due to antimony(II1) by complexing the antimony(II1) lvith fluoride ions was not very successful. Using 3 p.p.m. of bismuth and 0.S p.p.m. of antimony an error of 45y0 (+1.35 p.p.m. bismuth) was obtained. A solution of the same concentration in respect to bismuth and antimony, but also 500 p.p.m. in fluoride ions, gave an error of 3375, ( + 1 p.p.ni. bismuth).
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which K i t h the TTas iodide adoptedconcentration, for the ultraviolet spectrophotometric method. a tolerance larger than 20 0.1 p.p.m. of antimony cannot be e e I0 achieved by utilizing a fluoride concentration of 500 to 1000 0 290 320 362 p.p.m. A preliminary separation WAVE LENGTH,mg of antimony from bismuth b y Figure 2. Effect of distillation of antiniony(II1) cupric ion halides was not investigated. 1. 4 p . p . m . Bi Likewise, a preliminary separa2. 4 p . p . m . R ; + 5 0 p . p . r n . C ~ tion of bismuth from lead bv electrolysis v a s not investigated as a means of circumventing the interference due to the plumbous ion ( 1 ) . Recommended General Procedure. PREPARATIOS OF S.wPLE. Weigh, or measure by volume, a sample containing sufficient bismuth, so that t,he resulting solution contains 0.3 to 3 mg. of bismuth per 100 ml. of prepared solution. The solution should be 1 to 2 N in sulfuric acid. bIE.4SUREMENT O F DESIREDCONSTITUEST. Transfer a 10-nil. aliquot of the prepared solution to a 50-ml. volumetric flask. Add 10 ml:of the 1 0 N sulfuric acid and 20 ml. of the potassium iodide-ascorbic acid reagent. Dilute to the mark with distilled water and mix thoroughly. Measure the transmittance, or absorbance a t 337 mp, using 1.000-em. silica cells. A reagent h1:mk solution i p used in the reference ahsorption cell. e c 4 40 c
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THIOUREA 3IETHOD
Effect of Solution Variables. BISMUTH COSCENTRATIOS. Figure 3 shows the characteristic absorpt'ion spectrum for the bismuth(II1) t,hiourea complex, which has absorbance maxima a t 322 and 470 mp. The effect of 0.5, 1, 2, 4, and 6 p.p.m. of bismuth on the ultraviolet absorption spectrum are shown in Figure 3. Conformity to Beer's law was found a t 322 mp. The optimum concentration range is 0.6 to 6 p.p.m. of bismuth n-hen measurements are made in the ultraviolet region on the basis of a Ringhoni plot. ~ C I DCOSCESTRATIOS.The effect of perchloric acid concentration was studied using 5 p.p.m. of bismuth. The final acidity was varied from 0.1 to Table I. Interfering Diverse Ions 2AVusing variable amounts of 6 S Iodide Method Thiourea Method perchloric acid. I t was found Amount Error, Amount Error, added, in p,p.m. Tolerand. added. i n p.p.tn. Tolerance", that with final acid concentraIon Added As p.p.m. Bi p.p.m. p.p.m. Bi p.p.n1. tions below 1-V the maximum Antimony 0.8 1.35 0 0.8 0 4 0 0 5 0.6 2 1 Lead(I1) absorptivity was not obtained. 0 18 0 -1 5 n 17 3 0.3 RIercury(I1) S o change in absorptivity was 50 Arsenic(II1) 0 35 50 30 10 10 Tin(I1) ... ... ... 10 noted jn solutions which were 1 Silver 10 ... 10 ... ... 0 23 Copper(I1) p0 5 1 50 to 2 5 in perchloric acid. A final Cobalt(I1) 00 25 ... ... acidity of 1N x a s used in subseIron(II1) 1no0 500 0.25 40 10 Nickel(I1) 150 0.2 57'3 150 1000 quent testing for the effect of diVanadate 10 -0.2 5 0.18 200 300 Tungstate 10 0.2 5 3 0.24 verse ions. 10 Iron(I1) ... 300 1000 0.3 THIOUREACOXCESTRATIOZ. Chloride -0.33 500 ... 100 Bromide 500 -0.25 210 The effect of thiourea concentraFluoride 500 -0.10 230 tion was studied using 5 p.p.ni. a .Is p.p.in.. rausingless t h a n 2.:7 error u-ing & p.p.m. of bismuth. b Larger siitounts caused interference. of bismuth. The thiourea concentration was varied from 0.6
1724
ANALYTICAL CHEMISTRY
to 4.8 grams of thiourea per 50 ml. The absorptivity iucreases rapidly until a concentration of approximately 2.5 grams of thiourea per 50 ml. is reached with a very slight increase in absorptivity being detected after the concentration exceeds Y grams of thiourea per 50 ml. A final concentration of 3 grams of thiourea per 50 ml. is recommended. TIME. Solutions of the bismuth(II1) thiourea cornpl,I\ were found to be stable for a t least 24 hours. DIVERSEION CONCENTRATION. The effect of diverse ions was studied using 5 p.p.m. of bismuth, 25 ml. of the thiourea reagent, and 10 ml. of 1 to 1 perchloric acid solution. An error of less than 2.5% !$as considered negligible. I t was found that 1000 p.p.m. of the following ions did not cause interference: acetate, ammonium, cadmium, cobalt( 11), manganese( 11), magnesium, nitrate, phosphate, potassium, silver, sodium, sulfate, Versenate, and zinc Those ions causing interfereuce are listed in Table I. Recommended General Procedure. PREPARATION OF SAMPLE. Weigh, or measure by volume, a sample containing sufficient bismuth, so that the resulting solution contains 0.3 to 3 mg. of bismuth per 100 ml. of prepared solution. The solution qhould be 1 to 2 5 in perchloric acid. MEASUREMENT OF DESIRED COXSTITUEST.Transfer a 10-ml. :lliquot to a 50-ml. volumetric flask. Add 10 ml. of a 1 to 1 perchloric acid and 25 ml. of the thiourea reagent. Dilute to the mark with distilled water and mix thoroughly. Use a reagent blank solution in the reference absorption cell, 1.000-em. silica cells, and measure the transmittance, or absorbance, a t 322 m p .
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method is slightly more sensitive when spectrophotometric measurements are made in the ultraviolet region. An indication of the precision of the ultraviolet spectrophotometric determination of bismuth by the thiourea method was ascertained from the results of eight samples, each containing 4 p.p.m. of bismuth. These samples gave a mean absorbance value of 0.684 with a standard deviation of 0.009, or 1.3%.
DISCUSSION
An ultraviolet spectrophotometric study has been made of the iodide method for the spectrophotometric determination of bismuth, and it has been found that greatly increased sensitivity can be obtained by making spectrophotometric measurement a t the ultraviolet absorbance maximum, 337 mp, instead of a t the absorbance maximum in the visible region, 465 mp. The recommended general procedure for the iodide method, using 4 p.p.m. of bismuth, gave a standard deviation of 0.004 absorbance unit, 0.6%, for eight samples. It was found that the bismuth( 111) thiourea complex has an absorbance maximum a t 322 mfi in addition to its known absorbance maximum a t 470 mp. Using the recommended general procedure developed, the sensitivity of the thiourea method was increased almost fourfold by making spectrophotometric measurements a t the ultraviolet absorbance maximum. The molar absorptivities are 3.6 X lo4 and 9.3 X 10s a t 322 m i and 470 mr, respectively. Thus, whereas the iodide method is more sensitive than the thiourea method for spectrophotometric measurements made in the visible region, the thiourea
LITERATURE CITED
(1)
Clarke, B. L., Wooten, L. A., and Luke, C. L., I n d . Eng. C'hem..
8. 411 (1935). ( 2 ) Elkind, -4., Gayer, K. H., and Roltz, D. F.. d s a ~ ('HEM., . 25, 1744 (1953). (3) (4)
Haddock, L. A,, Analyst, 59, 163 (1934). Kingerly, W. D., and Hume. D. S . , J . Am. Chem.
Soc.. 71,
2393 (1949).
Leonard, C. S., J . Pharmacol., 28, 81 (1926). Mahr, C., 2.anal. Chem., 94, 161 (1933); 97, 96 (1934). McChesney, E. W., IND.EKG.CHmr., ANAL. ED., 18, 146 (1946).
(8) Merritt, C., Jr., Hershenson, H. M.. and Rogers, L. H.. ASAL. (9)
CHEM.,25,572 (1953). Nielsch, W., and Bolta, G., Z. anal. Chem., 142, 321 (1954);
143.13.168 (1954). (10) S p r o u t R. and Gettler, 4. IND.ENG.CHmr.. :\NAL. ED.. 13,462 (1941). (11) Wiegand, C. S. W., Lann, G. H.. and Kalich, F. T.'., Ibid., 13, 912 (1941). ,
c.,
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RECEIVED f o r review April 7, 1955. rlccepted July 21, 1955. Pittsburgh conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh. Pa., March 1. 1955.
