metallic ion are not feasible. Further work on the generation of acidic and basic species in acetone are discussed elsewhere ( 3 ) .
monium perchlorate used in this work, John McCormick who obtained some of the conductivity dat’a,and W. D. Cooke for helpful discussions. LITERATURE CITED
ACKNOWLEDGMENT
The authors thank Walt,er Jura who prepared some of the tetrabutylam-
( 1 ) Bjerrum, J., “Stability Constants,”
Part I, The Chemical Society, London, 1957.
( 2 ) Brandt, P., Acta Chem. Scand. 15, 1639 (lg5’). ( 3 ) Streuli, C. h., Cincotta, J. J., Naricle, D. L., lIead, K. K,, A ~ cHEM, ~ 3 ~6 , 1371 (1964). (4) “Titrimetric LIethods,” D. Jackson, ed., pp. 97-120, Plenum Press, New York, 1961.
RECEIVEDfor review August, 21, 1964. Accepted November 17, 1964.
Spectrophotometric Determination of Thallium by HeteropoIy Chemistry L. G.
HARGIS and D. F. BOLTZ
Department o f Chemistry, Wayne State University, Detroit, Mich.
b A spectrophotometric study of thalIium(l) using molybdophosphoric acid as a reagent has resulted in the development of two new procedures for determining thallium. A near infrared spectrophotometric method has been developed based on the precipitation of thallium(l) molybdophosphate and the subsequent reduction of the molybdophosphate obtained on dissolution of the precipitate to give a soluble heteropoly blue with an absorbance maximum at 808 mp. The optimum concentration range for the near infrared method is 2 to 16 p.p.m. of thallium. Beer’s law is obeyed from 0 to 20 p.p.m. of thallium. A more sensitive method involves dissolution of the thallium(1) molybdophosphate precipitate is a borate buffer and measurement of the ultraviolet absorbance of the molybdate a t 227 or 209 mp. The optimum concentration range for the ultraviolet spectrophotometric method is 1 to 5 p.p.m, and 0.5 to 3 p.p.m. a t 227 mp and 209 mp, respectively. Beer’s law is obeyed from 0.5 to 6 p.p,m. of thallium.
I
. 4 INVESTIGATION ~ of the analytical applications of the heteropoly acids, the precipitation of thallium(1) using molybdophosphoric acid as the precipitant wa5 examined. This paper reports the results of this study in which two different indirect methods were uwd to measure the thallium. Thallium previously has been determined spectrophotometrically by using hydrochloric acid (11), p-aminophenol (R), p-phenetidine ( 9 ) , rhodamine B (12)) brilliant green (16), methyl violet (6), dithizone (fO), bis(dimethylaminophenyl) antipyrylcarbinol ( d ) , dibenzyldithiocarbamate (7’)) and thionalide ( 2 ) . Only the methods using hj-drochloric acid or thionalide as reagent involve the reaction of thallium(1). The thionalide pi
240
ANALYTICAL CHEMISTRY
method is an indirect method of poor precision. The use of molybdophosphoric acid as a reagent for the determination of thallium was first reported by Pavelka and Morth ( I S ) , who developed a turbidimetric method. EXPERIMENTAL
Apparatus. The absorbance measurements were made in 1.000-cm. silica cells using either a Cary 14 recording spectrophotometer or a Beckman DU spectrophotometer. The p H measurements were made on a Leeds and Xorthrup No. 7664 p H meter. Solutions. Standard Thallium Solution. Dissolve 0.3260 gram of thallium(1) nitrate, T I N 0 8 ( K & K Labs, Inc., Plainview, N. Y.), in distilled water and dilute to 250 ml. One milliliter of this solution contains 1.0 mg. of thallium. Molybdophosghoric Acid Solution. Dissolve 10.0 grama of molybdophosphoric acid (;\.lallinckrodt, analytical reagent grade) in water, filter through medium porosity sintered glass, and dilute to 100 ml. with water. Molybdate Solution. Dissolve 14.7 grams of sodium molybdate dihydrate, Na2111oO4.2H2O(lIallinckrodt, analytical reagent), in distilled water, add 43 nil. of concentrated perchloric acid, and dilute to 500 ml. with distilled water. Hydrazine Sulfate Solution. Dissolve 0.30 gram of hydrazine sulfate, NH2SH2H2S04(Eastman No. 575) in distilled water, add 43 nil. of concentrated perchloric acid, and dilute to 500 ml. with distilled water. This solution contains 3.0 nig. of S2H,SO4 per 5 ml. and is 1.01V in perchloric acid. Tin( 11) Chloride Solution. Dissolve 2.38 grams of tin(I1) chloride dihydrate, SnCl2,2H2O(Baker, reagent), in 40 ml. of concentrated hydrochloric acid and dilute to 1 liter with distilled water. Add several tin pellets to the solution. This solution is 0.2y0in tin(I1) chloride and 0.5S in hydrochloric acid. Buffer Solution. Dissolve 48 grams of boric acid, H3I303 (Mallinckrodt, analytical reagent), in 1 liter of distilled
water containing 11.5 grams of sodium hydroxide. The pH of this solution is about 9. Recommended General Procedure for Near Infrared Method. Weigh,
or measure by volume, a sample containing 0.1 to 0.8 mg. of thallium and treat it so that the thalliurn(1) is present. Either sulfurous acid or hydrogen sulfide in acidic solution will reduce thallium(II1) to the univalent state. Any excess reducing agent should be destroyed by heating and the final volume should be only a few milliliters. Transfer the sample t o a 40-ml. centrifuge tube, add 3.0 ml. of 2.O2V perchloric acid solution, and dilute to 7 ml. with water. To precipitate the thallium, add 5.0 ml. of the molybdophosphoric acid reagent solution, mix well, and allow to stand for 10 minutes. The acidity of the solution should be 0 . 5 5 in perchloric acid. Centrifuge the solution and discard the supernate. Wash the precipitate once with 15 ml. of 0.5N perchloric acid solution. The volume of the wash solution must be larger than the sample volume. to prevent small amounts of the precipitate from remaining on the surface of the solution and not being centrifuged down. hfter discarding the wash solution, mix the precipitate with about 10 ml. of 1.0N perchloric acid solution. Add 3.0 ml. of the sodium molybdate reagent to supply the excess molybdate necessary for the formation of a st,able heteropoly blue system and then 5 ml. of the hydrazine sulfate solution. Mix well and place in a water bath a t 100” C. for 10 minutes. Cool rapidly in ice water until solution is a t rooin temperature and transfer the solution to a 50-ml. volumetric flask. Dilute to volume with 1.ON perchloric acid solution to maintain the desired acidity and measure the absorbance in 1.000-cm. cells a t 808 nip against a reagent blank solution. The amount of thallium is determined by reference to a calibration plot of absorbance us. p.p.m. prepared by following this recommended procedure using aliquots of the standard thallium(1) solution.
,
Recommended General Procedure for Ultraviolet Method. Use a sample containing 0.5 to 6 1i.p.m. of thallium and carry out the precipitation as described in the heteropoly blue procedure. Mix the washed precipitate in the centrifuge tube with about 20 ml. of the buffer solution to dissolve the precipitate and allow to stand until the precipitate is completely dissolved (about 3-4 minutes). Transfer the solution to a 100-ml. volumetric flask and dilute to the mark with the buffer solution. Measure the absorbance in 1.000-cm. cells at 227 or 209 mp against the buffer solution as a reference. The concentration of thallium is determined by reference to a calibration graph of absorbance us. p.1i.m. of thallium. This calibration graph is prepared using aliquots of the standard thallium solution and following this recommended procedure.
WAVELENGTH,
rnp
RESULTS Figure 1 . Absorption spectrum of heteropoly blue corresponding to 10 Effect of Solution Variables for p.p.m. of thallium(1) Near Infrared Method. THALLIUM CONCENTRATION. Figure 1 shows the absorption spectrum of the recoefficients for the thallium(1) and duced molybdophosphate anion when using hydrazine sulfate as the reductant molybdophosphate ions. A concentration corresponding to 5 ml. of the (3). different spectrum is obtained reagent was chosen as optimum and the when hexachlorostannite(I1) is used as the reductant. The latter reductant amount of reagent added was carefully controlled. In using 5 ml. of the was used initially to study the effect of several of the solution variables rereagent solution, a 0.5-ml. volume error results in an 0.003 absorbance error. lated to the precipitation process. I t DIGESTIONTIME;.Five milliliters of was later concluded that this reductant had several disadvantages and the final molybdophosphoric acid were added to each of the five solutions which conprocedure was developed using hydrazine sulfate as reductant. With hydratained 0.7 mg. of thallium and had acidities of 0.5X in perchloric acid. zine sulfate as the reductant, Beer's law The solutions were allowed to stand 5, is obeyed from 0 to 20 p.1i.m. of thallium. The optimum concentration 10, 20, 30, and 50 minutes, respectively, range is 2 to 16 p.1i.m. of thallium based before centrifuging. The corresponding absorbances, using tin(I1) chloride as on a Ringbom plot ( 1 , 1 4 ) . reductant, were 0.524, 0.527, 0.522 ACIDITYPRIORTo PRECIPITATION. Table I shows the effect of acidity on the 0.522, and 0.524. A digestion time of 10 minutes was select'ed as sufficient to precipitation of thallium(1) molybdoensure complete preripitation. phosphate based on the absorbance of the heteropoly blue obtained with MOLYBDATE CONCENTRATION I N REtin(I1) chloride as reductant. Per13y using 8 p.1i.m. of thalDUCTANT. chloric acid was used and 1 hour was lium, 5 ml. of the hydrazine sulfate solution, and 1.0, 2.0, 3.0, 4.0, and 5.0 allowed to ensure complete precipitation. An acidity of 0.5N in perchloric ml. of the sodium molybdate reagent acid was chosen as optimum. solution, absorbances of 0.472, 0.498 MOLYBDOPHOSPHORIC ACID CON0.497, 0.501, and 0.501 were obtained. CENTRATION. The effect of molybdoThe corresponding amounts of the molybdate solution were used in prephosphoric acid concentration on the quantitative precipitation of thallium (I) paring the reference solutions. The was studied by comparing the absorbvolume of 3.0 ml. of molybdate solution was chosen as sufficient to Iirovide the ance obtained when the resulting precipitate was dissolved and the color concentration of molybdate ions neressary in this reduction process to give a developed using tin(I1) chloride as the reduct'ant. Beyond a lO-3JP constable heteropoly blue system with centration of molybdophosphoric acid, maximum absorbance. the absorbance continues to increase HYI)RAZINE SULFATE CONCENTRAslightly with a n increase in the molybTION. With the use of 8 p.p.m. of dophosphoric acid concentration even thallium, solutions of the resulting a t high acid concentrations. This slow precipitates were reduced with 1.0, 3.0, rise is attributed to the increase in ionic 5.0, 7.0, and 9.0 ml. of the hydrazine strength of the solution. A slight insulfate solution. The absorbances obtained were 0.460, 0.504, 0.503, 0.502, crease in solubility results from the concomitant decrease in the activity and 0.502, respectively. Five millili-
ters of this reagent was considered sufficient. FIKALACIDITY. With the use of 8 p.1i.m. of thallium, solutions of the resulting precipitates were reduced with hydrazine sulfate as acidities of 0.5, 0.7, 1.0, 1.2, and 1 . 5 5 in perchloric acid. Reference solutions were prepared corresponding to the same final acidities. The corresponding absorbances were 0.577, 0.505, 0.503, 0.502, and 0.502. The reference (blank) solution with a final acidity of 0.5N is highly colored because of formation of some reduced molybdate. An acidity of l.OAV was selected as optimum. TIMEOF HEATING.With 10 p.p.m. of thallium, the effect of heating times of 5, 7, 10, and 15 minutes on the development of the heteropoly blue color was studied. Absorbances of 0.621, 0.620, 0.620, and 0.622 were obtained. Ten minutes of heating was selected as being sufficient for obtaining maximum absorbance. STABILITY. The final colored solutions are stable for at least 12 hours when the recommended procedure is used. DIVERSEIONS. A study was made to determine the amounts of various ions which can be present without interfering with the determination of 8 1j.p.m. of thallium. Errors less than + 3% were considered negligible. The cations tested were added as chlorides, nitrates, or perchlorates and the anions were added as sodium salts. Table I1 summarizes the results of this study. Effect of Solution Variables for CONUltraviolet Method. THALLIUM CENTRAnoN. Figure 2 shows t h e ultraviolet absorption spectrum of a solution containing molybdate resulting from the decomposition of t h e thallium(1) molybdophosphate precipitate. Thallium(1) ions also absorb a t both 227 and 209 mp; the absorptivity is about one tenth that of the corresponding amount of molybdate. Consequently, thallium( I) tends only to make the shoulder a t 227 mp less defined and does not alter appreciably the overall appearance of the absorption spectrum caused by molybdate. Conformity to Beer's law was observed for solutions corresponding to 0.5 to 6 p.1i.m. of thallium. The optimum concentration range is 1 to 5 p.1i.m. of
Table I.
Effect of Acidity on Precipitation of Thallium(1) Molybdophosphate HC104, N 0 0 0 0 1 1 2
001 01 10
50 00 50 00
Absorbance 0 499 0 ,510 0 59F, 0 600 0 600 0 377 0 .356
VOL. 37, NO. 2, FEBRUARY 1965
241
thallium a t 227 mp and 0.5 to 3 p.p.m. a t 209 mp based on Ringbom plots (1, 1 4 ) . This indirect ultraviolet method is 2.5 to 5.0 times as sensitive when absorbance measurements are made a t 227 and 209 mp as the heteropoly blue procedure described previously. The absorbance values for the smaller amounts of thallium can be increased by using a smaller final volume. NATUREOF BUFFER. Several buffers were tried to determine which might be the most suitable. A basic solution is necessary to dissolve the precipitate, but a high hydroxyl ion concentration is undesirable because of the absorptivity in the lower wavelength region of the ultraviolet. The borate buffer ultimately was chosen because it exhibits no appreciable absorption in the ultraviolet. The lower wavelength cut-off, which is caused by absorption by hydroxyl ions, is about 195 mr. A smaller slit width is obtainable a t 227 mp than a t 209 mp. An ammonium hydroxide-ammonium chloride buffer can also be used but gives a wider slit width and a higher wavelength cut-off. Borate buffers of pH 8, 9, and 10 gave the same final absorbances when used in a determination. A buffer of about pH 9 was chosen as satisfactory. STABILITY.The final solutions are stable for at least 20 hours if the recommended procedure is used: DIVERSEIONS. A study was made to determine the permissible amounts of various ions that may be present without interfering in the determination of 4 p.p.m. of thallium. Errors of less than
DISCUSSION
WAVELENGTH, mu
Figure 2. Ultraviolet absorption spectrum for solution resulting from dissolution of thallium(1) molybdophosphate precipitate (4 p.p.m of thallium) in buffer solution
*3% were considered negligible. The cations tested were added as chlorides, nitrates, or perchlorates and the anions were added as sodium salts. Table I1 summarizes the results of this study. The interference of many of the heavy metal ions probably results from coprecipitation and the enhanced ultraviolet absorptivity caused by these additional ions. This type of interference can probably be decreased by including a reprecipitation step in the procedure.
Table II.
Near Infrared Method. An indication of the precision of this method was ascertained from the results of 12 samples, each containing 8 p.p.m. of thallium. The mean absorbance value was 0.503; the range was 0.495 to 0.511. The standard deviation was 0.005 absorbance unit, or a relative standard deviation of 1.0%. &Ibout 40 minutes are required for a series of four determinations. Although a blue hue was obtained using tin(I1) chloride as the reductant, the absorption band near 810-830 m,u, usually attributed to heteropoly blue, was absent under the conditions used in the reduction of solutions obtained from dissolution of the thallium(1) molybdophosphate precipitate. Although the results are reproducible with tin(I1) chloride as reductant, slightly less sensitivity than with the hydrazine sulfate method, color stability over a limited pH range, and difficulties with plating of absorption cells with molybdenum blue indicated the superiority of the hydrazine sulfate reduction method. The instability and plating properties of the material obtained using tin(I1) chloride are characteristic of molybdenum blue, a reduced molybdate complex containing no hetero atom (3). Ultraviolet Method. An indication of the precision of this procedure was ascertained from the results of 12 samples, each containing 4 p.p.m. of thallium. The mean absorbance values were 0.650 and 1.226 a t 227 mp and 209 mp, respectively. The
Effect of Diverse Ions
(0.4 mg. of thallium) Near infrared method Amount % R.E., Tolerance, Amount added, mg. 808 mp mg. added, mg. 10 30 7.8 30 30 15 30 6.4 20 30 -5.0 30 60 100 100 5.4 15 30 30 6.4 10 30 100 7.6 20 30 30 4.8 30 400 3.6 25 20 100 4 . 0 30 20 4.0 30 30 30 30 15 5.4 4 3 4 -3.8 30 15 100 5.8 30 7 2 10 30 30 8 3 10 30 30 6 0 15 30 >loo 0 0 4 0 4 400 1 4 400 400 400 400 400 2 8 400 400 1 0 400 2 8 400 400 400 -9 2 125 400 400 30 -7 0 15 30 -6 8 40 100 100 3 6 10 100 30
Ion Added as Lead( 11) Mercury( 11) Bismuth(II1) Silver Nickel Zinc Cobalt Cadmium Copper( 11) Iron( 11) Iron( 111) Arsenic(V) Arsenic(111) 1Iagnesium Calcium Aluminum Ammonium Stilfate Chloride Nitrate Nitrite Acetate Fliioride Citrate EDTAb h (Ethylenedinitri1o)tetraacetate ion a Larger amounts interfere.
242
0
ANALYTICAL CHEMISTRY
Ultraviolet method % R.E., 227 mp 209 mp 5.7 6.6 8.2 8.2 3.8 4.6 7.9 8.5 4.0 5.1 4.5 5.6 3.1 3.2 0.9 0.6 3.8 5.1 3.2 3.1 5.8 4.4 -2.3 -1.3 -2.9 -1.8 6 2 7 2 12 6 17 0 7 7 9 1
% R.E.,
>loo
1 4 -1 7 2 6 4 5 -7 7 -8 5 -8 0
-3 7
>IO0
2 5
-1 1
4 7 -7 -8 -9 -3
5 0 5 8 1
6
Tolerance, mg. 15 10 25 40 20
70 30 400 80 25 15 4a 100 15 5 10 0 400 400 400 250 150 10 40 80
standard deviation was 0.005 and 0.012 absorbance units at, 227 mp and 209 mp or a relative btandard deviation of 0.8% and 1 .0%, respectively. .$bout 30 minutes are required for a series of four determinations. The utmost care should be used in measurement a t the 209 mp wavelength because the reagent blank solution is absorbing and small differences in operational procedure or spectrophotometer settings often result in significant deviations. Composition of the precipitate was determined by relating the final ultraviolet absorbance to the amount of molybdate and comparing this with the amount of thallium taken initially. Standard absorption spectra were prepared using known amounts of molybdate and thallium(1) dissolved in the buffer solution. The absorbance obtained for a solut,ion of a dissolved precipitate of thallium(1) molybdophosphate was corrected for the absorbance due to thallium and then the
corresponding amount of molybdate was obtained by referring to the absorbance us. molybdate concentration graph. Molar ratios of molybdate to thallium of 6.2 to 1 when the absorbance was measured a t 227 mp and of 6.1 to 1 when the absorbance was measured a t 209 mp were obtained. These values correspond closely to a formula of T12 H P ( M O ~ Ofor ~~ the ) ~precipitation. This formula corresponds to that obtained for the ammonium salt of molybdophosphoric acid in other investigations ( 5 , 1 5 ,f7). LITERATURE CITED
(1) Ayres, C. H., ANAL.CHEM.21, 652 (1949). (2) Berg, R., Fahrenkamp, E. S., Roebling, W., Mikrochemie (Molisch Festschrzft) 44, p. 44 (1936). ( 3 ) Boltz, I>. F., Mellon, M. G., IND. ENG.CHEM.,ANAL.En. 19, 873 (1947). (4)'Bnsev, A. I., Tiptsova, 1.. G., 'Yauchn. Doh. Vysshei Shkoly, Khim. i Khim. Technol. 1959, No. 1, p. 105. (5) Clarens, J., C o m p . Rend. 166, 259 (1918).
(6) Efremov, G. V., Syiii, C., Vestnik Leningrad. Cniu. 13, Ser. Fiz i Khim. KO. 3, p. 156 (1958). (7) Foley, W. T., Pottie, R. F., ANAL. CHEM.2 8 , 1101 (1956). ( 8 ) Gladyshev, V. P., Tolstikov, G. A,, Zavodskaya Lab. 22, 1166 (1956). ( 9 ) Iijinia, S., Kamemoto, Y., J . Chem. SOC.Japan, Pure Chem. Sect. 75, 1294 i 1S.i4\ \ - -
- I
(10) Jamrog, D., Piotrowski, J., M e d . Pracy 9, 299 (1958). (11) Merritt, C., Jr., Henhenson, H., Rogers, L. B., ANAL. CHEM. 25, 572 (1953). (12) Onishi, H., Bull. Chem. SOC. Japan 29, 945 (1956). (13) Pavelka, T., Morth, H., Mikrochemze 11, 30 (1932). (14) Ringbom, A., Z. Anal. Chem. 115, 332 (1939). (15) Stockdale, D., Analyst 83, 24 (1958). (16) Voskresenskaya, N. T., Zhur. Anal. Khzm. 11, 585 (1956). (17) Wendlandt, W. W., Anal. Chzm. Acta 20, 267 (19.59). RECEIVEDfor review March 20, 1964. Accepted November 19, 1064. Presented March 3, 1964, at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy.
Spectrophotometric Determination of High Molecular Weight Quaternary Ammonium Cations with Picric Acid Application to Residual Amounts in Polysaccharides J. H. SLONEKER, J. B. MOOBERRY, P. R. SCHMIDT, J. E. PITTSLEY, P. R. WATSON, and ALLENE JEANES Northern Regional Research laboratory, Peoria, 111.
b Quaternary ammonium compounds serve as precipitants for isolation and purification of acidic polysaccharides. An assay procedure was developed to measure the residual quantities of quaternary ammonium cation remaining in the purified polysaccharide. After the polysaccharide is hydrolyzed with phosphoric acid, the quaternary ammonium cations are precipitated quantitatively with picric acid. The insoluble quaternary ammonium picrate salt is selectively extracted from the aqueous hydrolyzate with chloroform, and the absorbance of the salt is measured spectrophotometrically ( A 365 mg, e 1.5 X 1 04). The procedure is sensitive to about 8 gg. of quaternary ammonium cation in 80 mg. of the microbial polysaccharides and is not affected by the presence of amino acids or amino sugars. The procedure may be adapted also to the determination of certain secondary and tertiary amines.
Q
UATERNARY AMMONIUM COMPOUNDS
have been used frequently in research either to purify polyanionic polysaccharides or to fractionate acidic polysaccharides from neutral polysaccharides by a selective precipitation technique (16, 17). Xt this laboratory an industrially feasible method has been developed by which an acidic microbial polysaccharide is precipitated and recovered from the fermentation broth with Arquad 16-50, a commercial preparation of cetyltrimethylammonium (CTA) chloride (1, 2). Since CTh ions have no visible or ultraviolet absorption spectrum, residual amounts of this base in the polysaccharide cannot be measured direct'ly by spectrophotometry. Previous assay procedures (4, 7 , 2 1 , f2), like our method, took advant'age of the strong cationic properties of quaternary ammonium ions (QK+). i\nionic dyes were reacted with the Q N + and the resultant color complexes were measured sl)ectrol)hotometrically,
However, these methods were unsatisfactory for our purposes. Hydrolysis and neutralization of the polysaccharide solutions produced high concentrations of carbohydrate and salt. In such mistures the polyfunctional dyes developed comples equilibria with the Q N + (f2), which caused poor reproducibility of results. Att'empts to sei)arate the QS+ from carbohydrate and salt on cellulose ion-exchange columns were also unsatisfactory ( 11 ) . The determination of Q N + in the presence of unhydrolyzed polysaccharide and in the absence of salt was prohibited by the extremely viscous nature of t'hese solutions. Picric acid mas chosen as the reagent for assaying QS+because it has a strong ultraviolet absorl)tion spect um and a single strong anionic functional group that reacts railidly and quantitatively with the base. 'This reagent has been used for many years to precipitate and identify organic cations and to determine the molecular weight of organic, VOL. 37, NO. 2, FEBRUARY 1965
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