445
V O L U M E 26, N O . 3, M A R C H 1 9 5 4 Table I.
Indirect Thorium Titrationsa
Thorium, Mg. Taken Found 44.89 22.44 22.44 11.22 11.22 11.22 2.244 1,122
44.95 22.43 22.43 11.23 11.22 11.23 2.242 1.122
Error hIg.
%
0.06 -0.01 -0.01 0.01 0.00
0.15 -0.05 -0.05 0.10 0.00 0.10 -0.10 0.00
0.01 -0.002 0.000
a A measured excess of 0.009962.11 or 0.00096?M Versenate was added to aliquot8 of 0.00967M or 0.000967.M thorium solutions. The excess Versenate was automatically titrated with 0 01233.V copper solution. The wave length was 290 or 320 mp depending o n whether a relatively small or large excess of Versenate was added.
Figure 6 shows the automatic spectrophotometric titration curve of a small excess of Versenate with standard copper solution a t a wave length of 290 mp. I n the titration, 11.00 ml. of 0 . 0 0 9 6 2 i ~Versenate were added to 10.00 ml. of 0.00967M thorium nitrate. The solution was diluted to about 100 to 150 ml. in the titration cell buffered with 1 ml. of sodium acetateacetic acid buffer, and the excess Versenate automatically titrated with 0.01233M copper solution. This curve shows a continual rise in absorbance after the end point, caused by the absorbance of copper acetate a t this wave length. This rise is not found a t somewhat higher wave lengths (Figure 3, wave length 320 mp). Experimental data for indirect thorium titrations are given in Table I. The results are precise and accurate to within 1 part per thousand PROCEDURE FOR INDIRECT TITRATION
An aliquot of 0.00967M thorium is pipetted into the titration cell, and a known excess of standard disodium dihydrogen Versenate is added. One milliliter of sodium acetate-acetic acid buffer is added and the solution diluted to about 100 to 150 ml. in the titration cell. The titration cell is set in the sample compartment of the spectrophotometer, and the cover, stirrer, and buret are moved into position. The spectrophotometer is balanced to zero absorbance against distilled water in a 5.00-em. cell. The buret motor and chart are turned on simultaneously. The time required to reach the end point is determined from the recorded titration curve. The time necessary to reach the end point is easily converted to milliliters, because the syringe buret is calibrated in milliliters per second. T o determine when a small excess of reagent has been added, a small known amount (about 1.00 ml.) of O.OlI1.f copper solution is added to the unknown thorium solution. The solution is diluted to about 100 ml., the titration cell is set in the sample cell compartment of the spectrophotometer, and the cover and stirrer are moved into position. The 11-ave length is set a t 320 mp. The tip of a suitable stopcock buret is inserted in the hole in place of the tip from the syringe microburet. The standard Versenate solution is added from this buret, while stirring, to the unknown thorium solution containing the known amount of copper. The addition of Versenate is stopped
when the absorbance of the copper-Versenate complex is indicated. Approximately 0.5-ml. additions of Versenate are then continued until the absorbance does not increase after a n addition, indicating that all the copper is the form of copper-Versenate complex and all the thorium is in the form of thoriumVersenate complex. The quantity of Versenate is determined from the milliliters added from the stopcock buret. The buret is removed, 1 ml. of sodium acetate-acetic acid buffer is added, and the excess Versenate is accurately determined by automatic spectrophotometric titration with standard copper solution delivered from the syringe microburet. It is, of course, necessary to add the millimoles of copper originally added to the thorium solution to the millimoles of copper required in the back titration before subtracting from the millimoles of Versenate added to the solution: Total ml. of standard Cu = ml. of Cu originally added of Cu used in titration
+ ml.
Mg. of T h = 232.1 (ml. of Versenate X molarity of Versenate total ml. of Cu X molarity of Cu)
-
INTERFERENCES
According to Fritz and Ford ( 2 ) no detectable interferences were caused by the following ions: K f , Na+, Li+, B a + +, Mg + +, Sr+-, Mn++, Co++, Cd++, Zn++, AI++1 Ag+9 Cr+++, La+++, and TrOZ++. S o work by the authors has been done in this direction. As the conditions used in this procedure were similar to those reported by Fritz and Ford, it is believed that practically no interference will be caused by these ions. Unknown concentrations of the following ions interfere: Pb++, Cu++, Xi++, B i t + + , Fe+++, ZrO++, Sn++, SII+~,and Ce+++. Levine and Grimaldi ( 6 ) have shown that a single extraction with mesityl oxide separates thorium from all metals except zirconium, uranium, and vanadium. A Versene titration, therefore, results in an almost specific analytical method for thorium. LITERATURE CITED
Blaedel, W. J., and M a l m s t a d t , H. V., ANAL. CHEM.,2 3 , 471 (1951).
F r i t z , J. S., a n d Ford, J. J., Ibid., 25, 1640 (1953). G o d d u , R. F., a n d Hume, D. N., Ibid., 22, 1314 (1950). Laitinen, H. A., a n d Ziegler, W., University of Illinois, Ph.D. thesis, 1952. Levine, H., a n d Grimaldi, F. S., U. S. A t o m i c E n e r g y C o m m i s sion, AECD-3186 (1950). Lingane, J . J., AKAL.CHEM.,20, 285 (1948). RIuller, R. H., a n d P a r t r i d g e , H. RI., I n d . Eng. Chem., 20, 423 (1928).
IND. ENQ. CHEY.,ANAL.ED.,1 5 , 6 4 2 (1943). P l u m b , R. C., Hartell, d. E., a n d Bersworth, F. C., J . Phys. Colloid Chern., 5 4 , 1 2 0 8 (1950). S c o t t , W. W,, “ S t a n d a r d l l e t h o d s of C h e m i c a l .%nalysis,” pp. 94G-53, New I’ork. D. V a n N o s t r a n d Co., 1939. Sweetser, P. B., a n d Bricker, C. E., Aix.4~.CHEM., 24, 409 O s b u r n , R. H., Elliot, J. H., a n d M a r t i n , A. F.,
(1952).
I b i d . , 25, 253 (1953). RECEIVED for review June 13, 1953. Accepted Sovember 24, 1953.
Spectrophotometric Determination of Telluric Acid LAWRENCE W. SCOTT’ and GUY WILLIAM LEONARD, JR. Department of Chemistry, Kansas State College, Manhattan, Kan.
T
ELLURIC acid has been determined by various methods including refractometric analysis, acid-base titration, and oxidation-reduction methods (a, 4, 6, 7 ) . These methods are involved and time-consuming, and require careful control of the experimental conditions. Since telluric acid solutions absorb in the ultraviolet, the possibility of a spectrophotometric determination was investigated. 1 Present address, Department of Chemistry, Oregon State College, Corvallis, Ore.
REAGENTS AND EQUIPMENT
The telluric acid for this investigation was prepared by the method of Horner and Leonard ( 3 ) and purified by repeated recrystallization from water. The other reagents were reagent grade. The Beckman Model DU spectrophotometer, equipped with a set of thermospacers, vias used with 1-em. silica absorption cells for measuring the absorbancy of the samples. The Beckman thermospacers and lamp housing cooling coils were connected
446
ANALYTICAL CHEMISTRY
The methods suggested in the literature for the determination of telluric acid are involved and time-consuming, and require careful control of the experimental conditions. The adaptation of a spectrophotometric techniquewould increase the ease and speedof the determination. This study describes a quantitative method for the determination of telluric acid based upon its absorption in the ultraviolet region of the spectrum. Under conditions of the determination, Beer’s law is followed up to 156 mg. of telluric acid per 100 ml. of solution. For minimum relative error, the concentration range of telluric acid should be between 65 to 210 mg. A simple and accurate method is described for the spectrophotometric determination of telluric acid. Within given limits this method can be used to determine telluric acid in the presence of bases, arsenites, arsenates, tellurium dioxide, sellenium dioxide, and other acids.
Table I.
210
C.)
Impurity
>.
z
PROCEDURE
Place a sample containing 65 to 210 mg. of telluric acid in a 100-ml. volumetric flask. Add 10 ml. of 6.6X ammonium hydroxide and mix thoroughly. 4 d d enough distilled water to bring the volume to 100 ml. Place the flask in a constant temperature bath which is maintained a t a selected temperature near the average room temperature. After the sample has reached the desired temperature, measure the absorbancy a t 260 mp in a cell compartment maintained a t the same temperature as the bath. Use a 0.66W ammonium hydroxide solution as the blank. For concentrations greater than 210 mg. per 100 ml., a wave length longer than 260 mp Tyould have to be selected, while for a concentration less than 65 mg. per 100 ml; a wave length shorter than 260 mp could be used. EXPERIRIEXTAL
Change in Absorption Spectrum. Preliminary investigations indicated that a t least t n o species of telluric acid existed in an aqueous medium. The equilibrium between these species appeared to be p H dependent. Upon the addition of sodium, potassium, or ammonium hydroxide, the absorption Rpectrum of telluric acid shifts toward longer wave lengths (Figure 1). After the initial rise, each curve in Figure 1 continues straight to total absorbancy. Figure 1 shows that the change in the absorption curves is approaching a limit as the strength of the base is in-
02d
I1 polo
Figure 1. Absorption Curves All solutions contained 2 grams of telluric acid per 100 ml. Distilled water was used as blank 1.
2.
3.
4.
Telluric Telluric Telluric Telluric
acid acid acid acid
solution in 0.02M sodium hydroxide in 0.04M sodium hydroxide in 0.08.M sodium hydroxide
Allowable Concn., Ng./lOO RIl.
0 75-
0 0.50
through a centrifugal pump to a Sargent 5-84860 constant temperature bath.
Interference Study
(Each solution was 0.66M in NH4OH and contained 92 mg.-of telluric’acid per 100 ml.; the concentration of the interfering substance uas increased until interference occurred; the measurements were made a t 280 mil and.at
-
a
m (z
$
025-
m 4
000
i‘
1 I
00
05 “,OH
Figure 2.
I
I
IO
15
MOLARITY
Saturation Curves
creased. To ascertain the optimum c o n c e n t r a t i o n of base, saturation curves were deter0 751 mined for tKo different concentrations of telluric acid.*- Since potassium hydroxide and sodium hydroxide solutions, because of the presence of impurities such as carbonate, were found to have greater a b s o r b a n c i e s than ammonium hydroxide, the latter m-as s e l e c t e d . Figure 2 s h o w that the absorbancies of the two rng. H ,Te Q / 100ml. c o n c e n t r a t i o n s of Figure 3. Calibration Curve telluric acid, with increasing concentraObtained with described analytical procedure tion of ammoniuni hydroxide, be c a m e essentially constant a t 0.66.V ammonium hydroxide, although there was a twentyfold difference in the concentrations of telluric acid. The 0.0861 telluric acid was about six times more concentrated than the highest concentration that could be measured a t 260 mp Rith 0 66M ammonium hydroxide. Beer’s Law. Figure 3 is a typical calibration plot obtained by maintaining the concentration of ammonium hydroxide at 0.66.11 and varying the concentration of the telluric acid. The straightline portion of this plot shol%sthat the absorbancy of telluric acid solutions in 0.66M ammonium hydroxide follows Beer’s law up to a concentration of 156 mg. per 100 ml. with only a slight steepening of the slope above that concentration.
V O L U M E 2 6 , NO. 3, M A R C H 1 9 5 4 Accuracy, Precision, and Range. A Ringbom plot (1, 5) shows that the optimum concentration range for the recommended conditions is from 65 to 210 mg. of telluric acid per 100 ml. In this range the relative analysis error is less than 0.8% for a precision within 0.2% measuring the transmittancy. Concentrations of telluric acid lower than 65 mg. per 100 ml. can be determined a t 260 mp, but the relative analysis error will increase as the concentration of the sample to be measured is decreased. Analyses of ten separately prepared samples, each containing 92 mg. of telluric acid, gave an average value of 91.94 mg. with an average deviation of 0.11 mg., a range of 0.52 mg., and a standard deviation of 0.16 mg. Effect of Temperature. An investigation of the temperature dependency indicated a change of 0.002 in the absorbancy per degi ee centigrade change in temperature. The temperature of the Beckman cell compartment was easily maintained within f0.5" C. With such temperature tolerances, the magnitude of error caused by temperature fluctuations is less than that of instrumental reading errors. Interferences. Table I gives the maximum amount of several contaminators which can be present without interfering There is no interference from tellurium dioxide because of its low solubility in neutral or slightly acid solutions. Modification of the Procedure. The permissible concentration of impuiities may be extended by a study of the particular ions present-for example, the absorption spectrum for the nitrate ion hm :i mauimum at 300 mp and a minimum a t 260 mp, Therefoie. I)> mpasuring the absorbancy a t 300 mp and again a t 260 mp, the abqorbancy due to the nitrate ion as well as that of the
447
telluric acid can be calculated. The usual calculations are simplified in this case, for telluric acid does not absorb a t 300 mp The use of a buffer of ammonium hydroxide and ammonium sulfate instead of only ammonium hydroxide will help to extend the permissible limits of impurities. These modifications would not be general but would apply only to the special conditions for which they were developed. CONCLUSIONS
Telluric acid can be successfully determined by a spectrophotometric method. Within given limits this technique can be applied to mixtures of telluric acid with other acids, tellurium dioxide, sellenium dioxide, arsenites, arsenates, and bases. The suggested procedure is simple, rapid, direct, and precise. LITERATURE CITED
Ayres, G. H., . ~ N A L .CHEM.,21, 652 (1949). Gooch, F. d.,and Horvland, J., A m . J . Sci., [3] 48, 375 (1894). Homer, H. J., and Leonard, G. W.,J . Am. Chem. SOC.,74, 3694 (1952).
Lingane, J. J., and Siedrzch, L., Ibid., 70, 1997 (1948). Ringbom, d.,Z. Anal. Chena., 115, 332 (1939). Rosenheim, h.,and Weinheber, AI., Z . anorg. Chem., 69, 266-9 (1910).
Urban, F., and Meloche, T'. W.,J . Am. Chem. Soc., 50, 3003 (1928).
RECEIVED for review June 22, 1953. Accepted December 21, 1953. Presented before the Division of Analytical Chemistry a t the 124th Meeting of CHEMICAL QocIErT, Chicago, 111. Abstracted from a thesis the AMERICAK submitted b y Lawrence W. Scott in partial fulfillment of the requirenients for the degree of master of science. Kansas State College
UItraviolet Spect ro photomet ric Det e rmination of Mercury as Mercuric Thiocyanate Complex G. E. MARKLE and D.
F. BOLTZ
W a y n e University, Detroit, M i c h .
A systematic ultraviolet spectrophotometric study of the thiocyanate complexes of various elements has been undertaken for the purpose of determining those thiocyanate complexes which exhibit characteristic ultraviolet absorption spectra, the optimum conditions for the formation of such complexes, and the possibility of using their absorption spectra in spectrophotometric analysis. It w a s found that the colorless mercuric thiocyanate complex in aqueous solution exhibits a characteristic absorbancy maximum at 281 mp. The effect of thiocyanate concentration, acidity, mercury concentration, and diverse ions w-as studied. I-Butanol can be used to extract the mercuric thiocyanate complex. Conformity to Beer's law was observed for mercuric thiocyanate complex in both aqueous solution and the butanol extracts, although the sensitivity is less for the butanol extracts. An ultraviolet spectrophotometric method for the determination of mercury is proposed which is suitable for determining 1 to 12 p.p.m. of mercury in aqueous solution and 1 to 20 p.p.m. in butanol extracts.
A
SYSTELIATIC investigation of the ultraviolet absorption spectra of thiocyanate complexes of certain elements is in progress in thislaboratory. The existence of an ultraviolet absorption spectrum for the molybdenum thiocyanate complex, which is suitable for the spectrophotometric determination of Emall amounts of molybdenum, was presented in a previous publica-
tion (6). This paper reports the results of a spectrophotometric study of the mercuric thiocyanate complex. There are several reagents which have been used in the colorimetric determination of mercuric ions. Thus, s-diphenylcarbazone (S), dithizone ( 4 , 8 ) , and di-P-naphthylthiocarbazone ( 1 , 8 ) have been shown to be sufficiently sensitive and fairly satisfactory for determining small amounts of mercuric ion. The sensitivity of the thiocyanate method is slight]! less than the sensitivity of the colorimetric methods using the above reagents. Holvever, the thioc! anate method employs a readily available reagent and the procedure is relativel3- simple. llerritt, Hershenson, and Rozers ( 7 ) have shown t h a t mercuric ions form complexes v ith chloride, bromide, and iodide ions n-hich have characteristic ultraviolet absorption spectra. The fact that mercuric ions form colorless thiocyanate complexes has been used in the titrimetric dptermination of mercury by a modified Yolhard procedure ( 5 ) . G E Y E R i L EXPERIPIEYTAL WORK
Apparatus. The absorbancy meaqurements were made in 1.000-cm. silica cells with a Beckman llodel DU spectrophotometer equipped with an ultraviolet accessory srt. The reference cell contained a reagent blank solution unless otherwise specified. All pH measurements XTere made with a Leeds and h-orthrup universal pH meter equipped with a glass electrode. Reagents. .I standard mercuric solution was prepared by dissolving 0.6767 gram of mercuric chloride (Raker and hdamson, reaeent grade) and diluting to 500 ml. with distilled water. One milliliter of this solution rontained 1 mg. of mercury.
