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 t h e requirenients for t h e 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.
ANALYTICAL CHEMISTRY
448 FIGURE I C O M P A R I S O N OF ULTRAVIOLET S P E C T R A OF M E R C U R I C C O M P L E X E S I. A C E T A T E (IO FIRM) 2. C H L O R I D E (6F1PMl 3 T H I O C Y A N A T E IIOPPM) 4. IODIDE I10 PPMI
5 THIOCYANATE LlOPPMl , ( I N B U T A N O L )
WAVELENGTH IMUI
The thiocyanate reagent was prepared by dissolving 50 grams of otassium thiocyanate (Baker and Adamson, reagent grade) ancfdiluting to 1 liter with redistilled water. The solutions used in the study of the effect of diverse ions were prepared from reagent grade chemicals. The 1-butanol used in the extraction of the mercuric thiocyanate complex was Matheson No. 2272. General Procedure. The required amount of standard mercuric solution was transferred to a 50-ml. volumetric flask. Any acid or solution containing a diverse ion was added, followed by the addition of the thiocyanate reagent. The solution was diluted to the graduation mark with redistilled water. In extracting the mercuric thiocyanate complex with 1-butanol, the aqueous solution was transferred to a 100-ml. pear-shaped separatory funnel, The flask was rinsed with butanol which was added to the separatory funnel. The alcoholic extracts were transferred to 50-ml. flasks and diluted to the mark with the extractant. RESULTS
Nature of Ultraviolet Absorption Spectra. The insolubility of most mercurous compounds limits their usefulness in ultraviolet spectrophotometry. Mercurous nitrate in I N nitric acid gave an absorbancy maximum a t about 300 mp when water was used as reference solution but the absorbancy maximum disappeared when 1iV nitric acid was used in the reference cell. This indicated that the absorbancy maximum was due to nitric acid rather than to a mercurous complex. Mercurous nitrate exhibits general ultraviolet absorptivity below 270 mp. Xercuric acetate solutions, when treated with an excess of acetic acid, did not give an absorbancy maximum, only general absorptivity a t the lower wave lengths. No absorbancy maximum is obtained for a mercuric sulfate solution containing an excess of sulfuric acid. As previously reported ( 7 ) , mercuric chloride, mercuric bromide, and mercuric iodide complexes have absorbancy maxima a t 230, 245, and 322 mp, respectively. The colorless mercuric thiocyanate complex was found to have a characteristic absorbancy maximum a t 281 mp. A comparison of the ultraviolet absorption spectra of the chloride, iodide, and
thiocyanate complexes shows that the thiocyanate complex is slightly less sensitive. Inasmuch as this investigation was concerned primarily with the study of thiocyanate complexes, further discussion shall be limited to the utilization of the mercuric thiocyanate complex in the spectrophotometric determination of mercury. Nature of Mercuric Thiocyanate Complex. Presumably, there are a number of mercuric thiocyanate complexes which may exist -e.g., Hg(SCK)+, Hg(SCN)*, Hg(SCN)-a, and Hg(SCS)a- -. Because of the high concentration of thiocyanate ions, it is believed that the Hg(SCN)a-- complex is the predominating ion in aqueous solutions considered in the present study, An experiment was designed to determine the charge of the mercuric thiocyanate complex by using cationic and anionic exchange resins (Permutit Q and Permutit 5-2). When a s o h tion containing the mercuric thiocyanate complex was passed through the cationic exchanger, the effluent upon dilution to a fixed volume had the same absorbancy as a mercuric thiocyanate solution of the same concentration (10 p.p.m. of mercury) which had not passed through the ion exchange column. The blank solution used in the reference cell was also passed through the exchange column. When this experiment was repeated using an anionic exchanger, the absorbancy of the resulting effluent was negligible, indicating removal of the complex. This observation supports the assumption that mercury( 11) is in the form of one or more negatively charged complexes. Effect of Solution Variables. MERCLRYCONCEKTRATION. Conformity to Beer’s law was observed a t 281 mp for 0 to 15 p.p.m. of mercury. The optimum concentration range is 1 t o 12 p.p.m. of mercury. Khen the mercuric thiocyanate complex is extracted with 1-butanol, conformity to Beer’s law was observed for 0 to 12 p.p.m. of mercury when absorbancy measurements were made a t 286 mp. THIOCYANATE CO~CENTRATIOS. The effect of various concentrations of thiocyanate was determined using 5 p.p.m. of mercury. The absorbancy increased until a concentration of 0.01 gram of potassium thiocyanate per milliliter of solution \$as
Table I.
Interfering Ions in Aqueous Solution
Anit. Permissible .4dded, Error, Amt., on Added as P.P.M. % P.P .ILI Nitrate KaNOs 400 7 300” NaNOx Nitrite 50 4 25 KeCraOi 1 10 Dichromate 0 Vanadate NaxV04 1 20 0 Cu(C104)z 1 7 0 Cupric Co(C1Odz Cobaltous 1 10 0 Ferrous Fe(C104)z 5 7 0 Ferric Fe(ClO4)a 1 4 0 Plumbous Pb(CzHa0z)z 20 13 10“ Stannic SnCl4 10 7 3 Bismutbous Bi(NO3)s 1 8 0 a The interference was not a linear function of concentration. Permissible amount was determined by experiment.
.
Table 11.
Interfering Ions in Extraction of Mercuric Thiocyanate Complex with 1-Butanol
Ion Ammonium Aluminum Antimonous Bismuthous Cadmium Cobaltous Cupric Dichromate Ferrous Ferric Iodide hlaneanous XI agiiesium Molybdate Plumbous Nitrate Stannic Sulfate Vanadate Zinc
Added as
Amt. .4dded, P.P.M. 100 50 5 10 500 10 5 5
3 1 500 25
200 1000 10 500 5 500 10
500
Error,
% 4
- 105
3 17 12.5 10 4 4 2 -8 4
Permissible Amt., P.P.M. 50 25
1
5
100 2 1
5
2 1 1 100 10 100
4
500
7 7 11 2 6 2.5
2 206
1 500
2 5001
449
V O L U M E 26, NO. 3, M A R C H 1 9 5 4 obtained. Increasing the thiocyanate concentration beyond this value has no effect on the absorbancy. I n order to ensure sufficient thiocyanate when larger amounts of mercury and other ions were present, it was decided that 20 ml. of a 5 7 , potassium thiocyanate solution would be used to prepare 50 ml. of solution in subsequent determinations. ACIDITY. The effect of making the solutions 0.lX in respect to sulfuric, nitric, hydrochloric, and perchloric acid was investigated using 5 p.p.m. of mercury. When the perchloric acid was used, there was a precipitate of potassium perchlorate but the other acids did not affect the absorption spectra appreciably. The acidity of the solution was increased until it was 1.h’ in sulfuric acid without affecting the absorbancy readings a t 281 mp, Khen no acid was added, the solutions had a p H of 5 to 6 and had absorption spectra identical to those shown in Figure 1. Higher concentrations of hydrochloric acid should be avoided because there is a tendency to form the mercuric chloride complex when an appreciable amount of chloride ion is present in highly acidic solution. DIVERSEIONS.The effect of certain diverse ions was studied using 5 p.p.m. of mercury. An error of less than 2.5y0 was considered negligible. One thousand parts per million of the following parts did not interfere: ammonium, acetate, cadmium, chloride, magnesium, manganese(II), molybdate, perchlorate, phosphate, potassium, sodium, sulfate, tungstate, and zinc. Those ions which interfered are listed in Table I. Extractability of Mercuric Thiocyanate Complex. I t was found that 1-butanol was a satisfactory extractant for the mercuric thiocyanate complex. Preliminary experiments indicated that the distribution constant for butanol-water ratio was about 15. Thus, two extractions with 20- to 23-ml. portions of butanol would be sufficient to remove all but a negligible amount of the mercuric thiocyanate complex. The effect of acidifying the solution prior to extraction v,-as investigated using 5 p.p.m. of mercury. Making the solution 0.1N in respect to sulfuric, hydrochloric, or nitric acid did not change the absorbancy values. The addition of perchloric acid caused potassium perchlorate to precipitate, The absorbancy index a t 286 mp for the mercuric thiocyanate complex in 1-butanol is smaller than the absorbancy index a t 281 nip for the mercuric thiocyanate in water, as indicated by comparison of curves 3 and 5 in Figure I. The mercuric thiocyanate complex in butanol was stable for over a 24-hour period. A study of the effect of diverse ions on the extractability of
mercuric thiocyanate was made using 10 p.p.m. of mercury A negligible error (leps than 2.5%) was found for 1000 p.p.m. of the following ions: acetate, chloride, perchlorate, phosphate, potassium, sodium, and tungstate. Those ions causing interference are listed in Table 11. RECOMMENDED GENERAL PROCEDURE
Sample. Weigh, or measure by volume, a sample containing a sufficient amount of mercury so that the resultant solution obtained following the necessary preparative treatment contains 0.1 to 3.0 mg. of mercury per 100 ml. Desired Constituent. Transfer a 25-ml. aliquot to a 50-ml. flask. Add 20 ml. of a 5y0 potassium thiocyanate reagent, dilute to the mark, and mix thoroughly. Measure the absorbancy at 281 mp using 1-cm. silica cells and a reagent blank solution. DISCUSSION
The ultraviolet spectrophotometric determination method for determining mercury has been proposed using the colorless mercuric thiocyanate complex which exhibits a characteristic absorbancy maximum a t 281 mp in aqueous solutions and a t 286 mp in 1-butanol extracts. An indication of the precision of the proposed method can be obtained from the following data. Using 5 p.p.m. of mercury in aqueous solutions of pH 6 a mean absorbancy value of 0.391 was obtained for 12 determinations. The standard deviation was 0.0057, or 1.5%. Using 10 p.p.m. of mercury in 1-butanol extracts a mean absorbancy value of 0.441 was obtained for 15 determinations. The standard deviation in this case was 0.009, or 2.0%. LITERATURE CITED
(1)
Cholak, J., and Hubbard, D. 31.,IND.ENG.CHEM.,ANAL.ED.,
18. 149 - - - f1946) (2) Hibbard, D. &I., Ibid., 12, 768 (1940). I
\ - - - - ,
(3) Laird, F. W., and Smith, Sister Alonaa, Ibid., 10, 576 (1938). (4) . . Law, E. P..and Nelson, K. W., J . Assoc. Offic. Agr. Chemists, 25, 399 (1942). (5) Kolthoff, I. M., and Sandell, E. B., “Textbook of Quantitative Inorganic Analysis,” 3rd ed., p. 548, New York, Macmillan Co., 1952. (6) hfarkle, G. E., and Boltz, D. F., AXAL.CHEM.,25, 1261 (1953). (7) Merritt, C . , Hershenson, H. h l . , and Rogers, L. B., Ibid., 25, 572 (1953). (8) Richmann, H. J., IND. Esc. CHEM.,ASAL. ED., 11, 66 (1939). RECEIVED for review August 20, 1953. Accepted November 27, 1953. Presented before the Division of Analytical Chemistry at the 124th Meeting of the . k M P R I C A S C H E U I C A L SOCIETY, Chicago, 111.
Spectrophotometric Determination of Ethanedial J. M. DECHARY, ERNEST KUN’, and H. C . PITOT Departments o f M e d i c i n e and Biochemistry, Tulane University, N e w Orleans, La.
Specific and sensitive analytical procedures were required for the determination of ethanedial and 2-oxopropanal in biochemical materials, particularly animal tissues. The present colorimetric methods for the determination of the a-dicarbonyl compounds as derivatives of 1,2-benzenediamine (the quinoxalines) do not meet the above requirements. A highly sensitive color reaction was obtained when ethanedial was heated with 2,3-diaminophenazine in 15.8N sulfuric acid solution. This reaction was made suitable for spectrophotometry by tetrazotizing the excess color reagent and reducing the tetrazonium salt thus formed with hypophosphorous acid. Based on this procedure, ethanedial was determined in amounts of 0.01 to 0.2 1
Present address, University of Wisconsin, Madison, Wis.
micromole. A possible mechanism, based on the formation of pyrazino[b]phenazine,is proposed for the color reaction. The method has been adapted for biochemical analysis and made specific for ethanedial and 2-oxopropanal by the use of 4.3N acetic acid instead of sulfuric acid.
T
HE continuation of biochemical studies on the metabolism of ethanedial and 2-oxopropanal in animal tissues (4, 5 ) is partly dependent on sensitive and specific analytical procedures. The procedures should permit the determination of the a-dicarbony1 compounds in the presence of other substances containing the carbonyl function which occur in biological material. The derivatives of a-dicarbonyl compounds with 1,a-benzenediamine, the quinoxalines, are reasonably specific. In acid so-
