have not been used for determination of this substance. During the course of the measurements, quenching factors for saturation with air were determined. These are only approximate because they depend on the solubility of oxygen in ethyl alcohol and are thus critically dependent on temperature which was not accurately controlled for these measurements. The fluorescence of naphthalene, normally regarded as weak, is increased by a factor of six by deaeration of the solution. This emphasizes again the neressity to test for
oxygen quenching when the fluorescence of a new compound is being investigated. CONCLUSIONS
The attachment of a fluorescent screen monitor to a spectrofluorometer allows the direct recording of true fluorescence excitation spectra, compensates for fluctuations in the intensity of the exciting light when a fluorescence emission spectrum is recorded, and can be used to calibrate the fluorescence spectrometer/multiplier phototube in the visible and ultraviolet regions.
LITERATURE CITED
(1) Benford, F., Lloyd, G. P., Schwarz, S.,J. Opt. SOC.Amer. 38, 445,964 (1948). (2) Hatchard, C. G., Parker, C. 4.,Proc. Roy. SOC.A235, 518-36 (1956). (3) hlelhuish, W. H., J . Phys. Chem. 65,
229-35 (1961). (4) Parker, C. A., Analyst 84, 446-53 (1959). (5) Parker, C. A , , iVature 182, 1002-4 (1958). (6) Parker, C. A., Proc. Boy. SOC.A220, 104-16 (1953). (. 7,) Parker. C. A , . Rees. W. T.. Analust 85, 587-600 (1960). ‘
RECEIVED for review October 20, 1961. Accepted Januarx 9, 1962.
A Comparison of Methods for Spot Test Detection and Spectrophotometric Determination of Glyoxal EUGENE SAWICKI, THOMAS R. HAUSER, and RONALD WILSON Robert A. Taft Sanitary Engineering Center, Cincinnati 26, Ohio
b The 1,2-dianiIinoethane, 2,3-diaminophenazine, 2-aminobenzenethiol, 2,4-dinitrophenylhydrazine, and 4nitrophenylhydrazine procedures for the determination of glyoxal are compared. The last three are new methods for the determination of glyoxal. The 2-aminobenzenethiol and the 1,2-dianiIinoethane procedures are the most highly selective for the determination of glyoxal, while the 2,3diaminophenazine procedure is more selective than the 4-nitrophenylhydrazine method. O f all the methods, the 3-methyl-2-benzothiazolone hydrazone and the 2-hydrazinobenzothiazole plus p-nitrobenzenediazonium fluoborate procedures have the poorest selectivity for glyoxal. The Dechary, Kun, and Pitot diaminophenazine method is about 2 0 times more sensitive than the 2-aminobenzenethiol and 1,2dianilinoethane methods. The 4-nitrophenylhydrazine procedure is, b y far, the most sensitive method of all; however, pyruvaldehyde and biacetyl also react with this reagent.
G-
can be readily detected (6,Ia) or determined ( I S ) with 1,2 dianilinoethane. 2,3 - Diaminophenazine can also be used for the determination of glyoxal ( 2 ) . Other reagents that hare been used with fairly good selectivity in the detection of glyoxal are 2,4-dinitrophenylhydrazine ( d ) ,4-nitrophenyl hydrazine (4,5),2-aminobenzenethiol, 2,3-diaminonaphthalene, and 2-hydrazinobenzothiazole ( 6 ) , salicylalhydrazone, and 2-hydroxy-lLYOXAL
naphthalhydrazone (9). The last three methods are fluorescence spot tests. EXPERIMENTAL
1,2-Dianilinoethane dihydrochloride was obtained from Aldrich Chemical Co., Inc., MilTI aukee, Vis. 2-Aminobenzenethiol was obtained from Laboratory Services Inc., Cincinnati 9, Ohio. 2,4Dinitrophenylhydrazine, 4-nitrophenylhydrazine, 10% aqueous tetraethylammonium hydrouide, and 1,2phenylenediamine dihydrochloride were obtained from Distillation Products Industries, Rochester, K. Y.; 29y0 methanolic tetraethylammonium hydroxide was obtained from Southwestern Analytical Chemicals, Austin, Tex. Glyoxal sodium bisulfite (K & K Laboratories, Jamaica, N. Y.) was crystallized from 40% aqueous ethyl alcohol and then dried over phosphorus pentoxide in a vacuum for 3 days. dnalysis by the Galbraith Laboratories, Inc., Knoxville, Tenn., showed the molecular structure to be (CHO)*.2 NaHS03.H 2 0 . Preparation of 2,d-Diaminophenazine. Steigman’s procedure (11) was simplified in the following manner. One hundred milliliters of 4oyO aqueous ferric chloride solution was poured into a stirred solution containing 33.5 grams of 1,2-phenylenediamine dihydrochloride in 100 ml. of water a t room temperature. A red-brown precipitate was quickly formed. The mixture was then brought to the boiling point, and approximately 200 ml. of hot water was added to bring the precipitate into solution. The hot solution was filtered and allowed to cool. The blue-black needles were collected and washed with ether. Nine Reagents.
grams (%yoyield) of the hydrochloride salt was obtained and used in the qualitative and quantitative procedures. Calculated for C12H10N4. HC1. H 2 0: K, 21.2; C1, 13.2. Found: N, 21.5; C1, 13.4. If necessary, the compound can be purified by Fischer and Hepp’s procedure (3). Reagent Solutions. 2-.4n11Ko~EKZEKETHIOL. d 1% aqueous solution containing 6 ml. of concentrated hydrochloric acid per 100 ml. of solution. The solution is stable for at least 24 hours. 1,2-DIANILISOETHANE DIHYDROCHLORIDE. A 1% aqueous solution. 2,4-DIKITROPHENYLHYDRAZIKE. A O.lyo solution in dimethylformamide containing 1% concentrated sulfuric acid. The solution is stable for a t least 24 hours. 4-NITROPHENYLHYDRAZINE. A 0.10/, solution in dimethylformamide containing lyGconcentrated sulfuric acid. The solution is stable for a t least 2 days. 2,3-DIAMINOPHEXAZINE HYDROCHLORIDE. A 0.02y0solution in 50% acetic acid. The solution is stable for a t least 1 week. Apparatus. Cary recording spectrophotometers Models 11 and 14 with 1-cm. cells rvere used. Spectrophotometric Procedures. 2ANIKOBENZENETHIOL. A mixture of 3 ml. of aqueous test solution, 3 ml. of reagent, and 6 ml. of concentrated hydrochloric acid is heated in 3 boiling n-ater bath for 2 minutes. Then 1 ml. of 1.65% sodium nitrite solution is added. After the mixture is cooled to room temperature under the tap, it is diluted to 25 ml. with concentrated hydrochloric acid. The absorbance of the blue solution is read a t 600 mp within 10 minutes. VOL. 34, NO. 4 , APRIL 1962
505
100' C. and then cooled. A blue color indicates glyoxal. Identification limit is 0.1 fig; the concentration limit is approximately 1 to 300,000. 2,3-DIAMINOPHENAZINE SPOT PAPER TEST. Five microliters of reagent and 5 11. of test solution are spotted on paper. After the spot is heated 5 minutes with a jet of steam, it is treated with 5 ~ 1 of . o.01570 aqueous sodium nitrite solution and then with 5 pl. of 25y0 aqueous hypophosphorous acid. The spot is again heated for 5 minutes in a jet of steam. A blue color indicates glyoxal; the identification limit is 1 pg.
20-
1 MECHANISM
L---_L 400
~
500
'30
600
Lw Figure 1 .
Visible absorption spectra
G l y o x a l bir-4-nitrophenylhydrazone (2 X 1 O-aM) in dimethylformamide containing a drop o f concentrated hydrochloric acid (-..-..) Methanol containing 10% o f dimethylformamide and 10% o f 29% methanolic tetraethylammonium hydroxide (. .) Methyl cellosolve containing 10% o f dimethylformamide and 10% of 29% methanolic
.. . . . ..
tetraethylammonium hydroxide (---I Dimethylformamide containing 10% o f 29% methanolic tetraethylammonium hydroxide (-)
The types of chromogens believed to have been formed in the 1,2-dianilinoethane (6,12), 2-aminobenzenethiol ( 6 ) ,and 2,3-diaminophenazine (2) procedures have been reported. The essential reactions for the determination of glyoxal with 4-nitrophenylhydrazine are shown in Structure I. O ~ N ~ N H N H o, H c - GHO
H,NHN~NO,
i O ~ N ~ N H - -C NH - C H = N - N H ~ N O ~
1,2-DISNILINOETHANEDIHYDROCHLOA mixture of 1 ml. of aqueous test solution and 1 ml. of 1% aqueous reagent solution is heated for 3 minutes T a t the boiling point. The mixture is cooled to room temperature and diluted The following evidence for the mechato 10 ml. with methyl cellosolve. The nism should be invaluable in any further absorbance is read a t 555 mp after 10 modifications of the procedure. The minutes. The color is stable for 20 spectrum of pure glyoxal bis-4-nitrominutes. 4-NITROPHENYLHYDRAZIXE. A mixphenylhydrazone in dimethylformamide ture of 1 ml. of aqueous test solution containing 10% of 10% aqueous tetraand 1 ml. of reagent is heated in a ethylammonium hydroxide solution and boiling water bath for 10 minutes. 10% additional water contained a band It is then cooled under the tap to room a t a maximum wavelength of 702 mp temperature. After the addition of 1 with E equal to 106,000. In the deml. of 10% aqueous tetraethylammotermination of glyoxal by the recomnium hydroxide, the mixture is diluted mended 4-nitrophenylhydrazine proto 10 ml. with dimethylformamide. cedure, a band was also obtained a t 702 The absorbance is read a t 702 mp within 40 minutes after final dilution. mp with E of 97,000, representing a 92% An alternative procedure consists of yield of the final chromogen. substituting a reagent containing 0.05% Further valuable data and evidence 4-nitrophenylhydrazine and 0.5% sulfor the reaction sequence can be obfuric acid in dimethylformarnide for the tained from an examination of the abrecommended reagent and 29y0 methsorption spectra of the pure bis-4anolic tetraethylammonium hydroxide nitrophenylhydrazone in solvents of for the other alkali and finally diluting varying basicity (Figure 1). I n TI eakly to 5 ml. with dimethylformarnide. The acidic dimethylformamide, the absorpsame data are obtained for glyoxal. Spot Test Procedures. 2,3-Dition spectrum of the neutral compound AMINOPHENAZIKE MICROTUBETEST. is obtained ( A max. 456 mp, E 72,000). A drop (0.03 cc.) of the test solution I n a fairly strong basic solvent, such as and a drop of the reagent are heated alkaline methanol, the absorption speca t 100' C. for 2 minutes. The microtrum indicates a large proportion of tube is cooled in running t a p water. monoanion (X Max. 595 mp, E 50,000) T h e mixture is treated with a drop and a small amount of the dianion of 0.015y0 aqueous sodium nitrite (shoulders a t 675 and 725 mp). I n the solution and a drop of 25% aqueous more strongly basic methyl cellosolve hypophosphorous acid. The tube and its contents are heated 2 minutes a t and dimethylformamide solutions, the RIDE.
506
ANALYTICAL CHEMISTRY
dianion is almost exclusively present. The dianion absorbs a t longer wavelength and with greater intensity in alkaline dimethylformamide (A max. 702 mp, E 106,000) than in alkaline methyl cellosolve (X max. 692 mp, E 93,000). This is because conjugated anions usually absorb a t longer wavelengths and with greater intensity in the more basic solvent ( 7 ) . I n a more weakly alkaline methyl cellosolve solution containing 10% of a 10% aqueous tetraethylammonium hydroxide solution and an additional 10% water, the n-avelength maximum was shifted to 637 mp. This absorption a t shorter wavelengths is apparently derived from the monoanion. The dianion can also be obtained in both methanol and methyl cellosolve solution by increasing the amount of alkali in the standard analytical procedure. For example, a mixture of 1 ml. of aqueous test solution and 1 ml. of the 0.1% reagent solution was heated 10 minutes a t 100" C., cooled, treated with 3 ml. of 10% aqueous tetraethylammonium hydroxide and enough solvent to dilute to 10 ml. K i t h methyl cellosolve and methyl alcohol, the following wavelength maxima and molar absorptivities were obtained: 692 mp, E 80,000 and 688 mp, E 67,000, respectively. IThen dimethylformamide was used as the test solvent and 29% niethanolic tetraethylammonium hydroxide as the base, dilution with dimethylformamide gave a solution absorbing a t 702 mp, c 102,000; dilution with methyl cellosolve gave A max. 692 mp and with methanol, h max. 595 mp. These spectra, identical with that obtained from pure glyoxal bis-p-nitrophenylhydrazone, indicate clearly that they are all derived from the bis-4-nitrophenylhydrazone and not from glyoxal 4-nitrop henylhydrazone. DISCUSSION
2-Aminobenzenethiol Procedure. Optimum intensities, E equals 4500, R ere obtained when approximately one to two equivalents of reagents nere present for every equivalent of glyoxal. Although lower results !?-ere obtained, a lyO concentration of reagent was chosen, for a nider concentration of glyoxal could thus be analyzed. 'The acid was necessary for color formation. TWOminutes of heating a t 100" C. gave best results. Optimum intensities \?-ereobtained with approximately 1.6 to l.7Y0 sodium nitrite. The sodium nitrite solution doubled the intensity, and the color intensity was stable for about 10 minutes. Beer's law was obeyed from 150 to 1000 pg. of glyoxal per 25 ml. of final solution. In the determination of diverse amounts of glyoxal, molar absorptivities of 2400 = 200 were obtained. Eighteen repli-
cate determinations were made on a test solution containing 500 pg. of glyoxal, and the standard deviation was 3.18%. Negative results were obtained with glycolaldehyde, pyruvaldehyde, biacetyl, acrolein, formaldehyde, nitromethane, acetaldehyde, propionaldehyde, chloral, and crotonaldehyde. 1,2-Dianilinoethane Procedure. Optimum results were obtained wit'h 1% reagent' a n d 3 minutes of heating at' 100" C. Reproducible results were obtained, b u t Beer's law was not obeyed. Fifteen replicate determinations were made on a test solution containing 580 pg. of glyoxal, and the standard deviation was 1.32%. The color intensity was stable for about 20 minutes. Khile the previously described method ( I S ) is more complicated and involves much more time than the present method, the final color intensity obtained in the former method is constant for a t least 2 days. Kegative results were obtained 11-ith formaldehyde, acetaldehyde, propionaldehyde, acrolein, biacetyl, glpcolaldehyde, m-glyceraldehyde, 1,3-dihydroxyacetone, and nitromethane. Formaldehyde, acetaldehyde, glucose and erythrose have been previously reported as giving negative results ( I S ) . Pyrul-aldehyde gave a band a t 653 mp with E 1600. This could result from impurity. 4-Nitrophenylhydrazine Procedure. K i t h a n increase in the volume of t h e test solution to 2 , 3, 4, or 5 ml., t h e color intensity decreased drastically. Dimethylfornianiide could be substituted for water as the test solvent with very little change in absorption masiinuiii and int'eneity. hIaximum intensities were obtained when the rcbagent concentrat'iori reached 0.1 to 0.3y0. V i t h a decrease in reagent concentration, the intensity gradually decreased until a t approximately 0.01 to 0.027;, the intensity dropped sharply. K i t h heating times of 5 to 15 minutes, niasiinurii intensities iTere obtained. With heating times of 2 or 30 minutes, the intensity lms decreased by approximately 3%. The effect of alkali on the color intensity n-as investigated. K h e n 5% aqueous tetraethylammonium hydroxide was used instead of the 10% solution, the color did not develop. Tu-entynine per cent metlianolic tetraethylammonium hydroxide also can be used. Dilution n-ith methyl cellosolve gave a band a t 637 nip wit'h e 47,000, whereas dilution with methanol gave a band a t 582 nip n-ith E 20,000. These bands are apparently derived from the monoanion. From a n initial absorbance of 0.95 a t X max. 702 mp, the absorbance had decreased to 0.94 in 35 minutes and to 0.31 in 18 hours. By this time the band a t 702 mp had changed into a shoulder, and a new band had appeared
2,3-octanedione-reacted only slightly. Other diketone-type compounds reacted in the procedure to give bands around 580 to 6lOmp (Table I). Benzil and the other compounds t h a t form chromogens absorbing near 600 mp do not interfere in glyoxal determination unless present in relatively large amounts. The results obtained for these compounds are not as reproducible nor reliable as those obtained for glyoxal, pgruvaldehyde, and biacetyl. To determine any of these miscellaneous carbonyl compounds, the variables would hare to be reinvestigated to obtain optimum results for t h a t particular compound.
Table I. Determination of Glyoxal and Other Compounds with 4-Nitrophenylhydrazine
Molar Maximum AbsorpWave- tivity, Compound length B X 10-3 Glyoxal 702 97 Pyruvaldehy de 695 78 Biacetyl 647 25 684 25 Glycolaldehyde 702 16 1,3-Dihydroxyacetone 690 18 DL-Glyceraldehyde 3 685 2,3-0ctanedione 2 698 Benzil 4 578 a-Furil 7 580 a-pyridyl 24 575 6.6 '-Dimethvl-2.2 " , '-di' pyridyl 570 50 l-Phenyl-1,2-propanedione 580 12 1,4'-Xitrophenyl-l, 2propanedione 605 6 1.4-Diacetvlbenzene 593 25 1;4-Diphenyl-2-butene1,4dione 648 8 Acenaphthenequinone 570 28 Phenanthraquinone 565 70 Isatin 610 25 1,4-Benzoquinone 575 6
COMPARISON OF METHODS
~~
a t 612 mp. Kumerous tests over a period of a month with varying concentrations of glyoxal gave molar absorptivity values of 97,000 i 2000. Twenty replicate determinations were made on a test solution containing 10 pg. of glyoxal, and the standard deviation was 1.05%. Beer's lam was obeyed from 0.7 (for A = 0.1) to 14.5 pg. of glyoxal per 10 ml. of final solution. The absorptivity obtained for glyoxal in the procedure was 1.67 pg.-l ml. em The test was not specific for glyoxal, because pyruvaldehyde, glycolaldehyde, dihydroxy acetone, and biacetyl also reacted (Table I). Glycolaldehyde reacts to some extent because i t forms a little glyoxal under test conditions (1). Larger aliphatic oc,fi-diketones-e.g.,
Table II.
The larger the molar absorptivity and the smaller the dilution factor, the greater the sensitivity of a spectrophotometric procedure. By this criterion, the 4-nitrophenylhydraxine procedure is by far the most sensitive method for glyoxal determination. Its molar absorptivity is four times greater than that obtained with the 2,3-diaminophenazine procedure (Table 11). Its dilution factor (essentially the final volume to which a unit volume of test solution has to be diluted) is almost a s favorable as that obtained with diaminophenaxine. The 1,2-dianilinoethane procedure is least sensitive. All colors obeyed Beer's law except those in the 1,P-dianilinoethane procedure. All colors were stable enough for satisfactory measurements to be made. Results obtained in the 4nitrophenylhydraxine procedure were the most reproducible. I n this respect, the 2-aminobenzenethiol procedure was least satisfactory. The 2-aminobenxenethiol and 1,2-dianilinoethane procedures were the most highly selective for glyoxal, while the Dechary, Kun, and Pitot ( 2 ) 2,3-diaminophenaxine procedure B m s more highly selective than
Comparison of Spectrophotometric Methods for the Determination of Glyoxal
Reagent 1,2-Dianilinoethane 2-Aminobenzenethiol HBT and NOFe ( I O ) 2,3-Diaminophenazinef
3-?vlethyl-2-benzothiazolone hydrazone (8) 2,4-Dinitrophenylhydrazine
Amax. 1
mp
555 600 605 600 664 608
Molar Absorptivity E x 10-3 1.9d 2.4 15
23 28 65
Absorptivity," pg.-1 R.11. Cm.-l 0.033 0.042 0.26
Dilution Factorc 10 8.3
0.40
Concn. Limit,@ P.P.M. 8.5 2.4 0.39 0.25
0.48 1.12 1.67
0.21 0.09 0.06
20 10 5 or 10
10
4
4Nitrophenylhydrazine 702 97 Calculated for final volume. Where absorbance = 0.10 in a 3-ml. cell with a light path of 1 cm. Essentially the ratio of the final volume to the test solution volume. Value for a l O - 3 M solution; Amax. 550 m p with E 2400 has been obtained in original procedure ( I S ) . e HBT = 2-hydrazinobenzothiazole. NOF = p-nitrobenzenediazonium fluoborate. f Data from procedure B in reference ( 2 ) .
VOL. 34, NO. 4, APRIL 1962
507
the p-nitrophenylhydrazine method. Although pyruvaldehyde reacted with the diaminophenazine reagent to give a band a t 600 mp, this band has less than one tenth the intensity obtained with glyoxal. Pyruvaldehyde, unlike glyoxal, also gave a weak band near 715 mp in this procedure. The diaminophenazine procedure was approximately 20 times as sensitive as the 2-aminobenzenethiol method. The molar absorptivity obtained in the determination of glyoxal with 3methyl-2-benzothiazolone hydrazone (8) should be capable of being doubled with proper modification of a procedure which originally was set up to obtain optimum results with formaldehyde. However, the procedure is not very selective for glyoxal as m-ater soluble aliphatic aldehydes also give positive results with the reagent. The 2-
hydrazinobenzothiazole plus p-nitrobenzenediazonium fluoborate procedure for the determination of aldehydes can be used to analyze for glyoxal (IO). The method, however, has never been adequately investigated for the quantitative determination of glyoxal. ,4s the procedure has been used in the analysis for aliphatic, aromatic, and heterocyclic aldehydes, i t would have a poor selectivity in the determination of glyoxal. LITERATURE CITED
( 1 ) Banks, T., Vaughn, C., Marshall, L. M., ANAL. C H E M . 27, 1348 (1955). (2) Dechary, J. M., Kun, E., Pitot, H. C., Ibid., 26,449 (1954). (3) Fischer, O., Hepp, E., Ber. 22, 356 (~ 1889). _ _
_ .
(4) Neuberg, C., Straws, E., Arch. Biochem. 7,211 (1945).
( 5 ) Pesez, M., Bartos, J., Talanta 5 , 216 (1960). Ibid., 5 , 63 (6) Sawicki, E., Elbert, W., (1960). (71 Sawicki, E., Hauser, T. R., Stanley, T. W., ANAL.CHEM.31,2063 (1959). (8) Sawicki, E., Hauser, T. R., Stanley, T. W., Elbert, W., Ibid., 33,93 (1961). (9) Sawicki, E., Stanley, T. W., Chemist Analyst 49, 107 (1960). Xikro(10) Sawicki, E., Stanley, T. R., chim. Acta 1960,510. (11) Steigman, A., Brit. J . Phot. 93, 256 (1946). (12) Wanzlick, H. W., Lochel, W., Ber. 86, 1463 (1953). (13) Wise, C. S., Rlehltretter, C. L., Van Cleve. J. W.. ANAL. CHEY. 31. 1241 (1959j. RECEIVEDfor review October 6, 1961. Accepted January 16, 1962. Work per-
formed at the Laboratory of Engineering and Physical Sciencies, Division of Air Pollution, Robert A. Taft Sanitary Engineering Center, Public Health Service, U. S. Department 3f Health, Education, and Welfare, Cincinnati 26, Ohio.
Polarography of Diethyldithiocarbamate, a Titrant with the Rotated Dropping Mercury Indicator Electrode WALTER STRICKS and S. K. CHAKRAVARTI Department o f Chemistry, Marqoette University, Milwaukee
b Polarographic studies of diethyldithiocarbamate (RSH) at the dropping mercury electrode reveal that the product of the anodic reaction i s strongly adsorbed a t the mercury drop as indicated b y a prewave. The adsorbed film greatly affects the characteristics of the anodic wave of RSH in water. At RSH concentrations larger than 10-4M, the limiting current i s not proporiional to the concentration but becomes markedly larger. An explanation i s given for this. Also, in the presence of surface active agents and in ethyl alcohol solutions, a prewave i s observed, but the current of the main wave is proportional to the concentration over the entire concentration range investigated (6 X 10-5 to 2 X 10-'M). At the rotated dropping mercury electrode, the limiting current i s proportional to the RSH concentration up to a concentration of 5 X 10-4M in aqueous medium. This electtode i s used as an indicator electrode for amperometric titrations of metal ions with RSH. Proper buffer composition and pH are developed for the accurate determination of metal ions. The combined use of the cathodic current of metal ions and the anodic current of RSH under controlled pH conditions makes possible the amperometric titration of two metals in a mixfure.
508
ANALYTICAL CHEMISTRY
D
3, Wis.
(RSH) has been widely used as a n extracting agent for the determination of various metal ions (2, 5 ) . Also, visual titrations with RSH have been investigated ( 1 2 ) . S o attempt has been made to use this reagent for amperometric titrations of traces of metal ions. This paper deals 11ith the study of the polarography of RSH. Results $how that the rotated dropping nirrcury electrode (r.d.m.e.) can be used as an indicator electrode for amperometric titrations of metal ions with RSH as reagent. Two methods have been uscd for the amperometric titrations. The first utilizes the decrease in height of the reduction Ivave of the uncombined metal ion during the course of the titration. The second is based on t h r measurement of the anodic diffusion current of the excess titrating agent (RSH) after the end point. A combination of these t1i-o methods makes possible the determination of mixtures of metal ions. IETHYLDITHIOCARBA3IATE
EXPERIMENTAL
Reagents. R S H was a n Eastman white label product. -4 10-*M stock solution was prepared, and aliquot parts were diluted to obtain titrants which were and 2 x 10-3N in RSH. The solutions were standardized by amperometric titration with ethylmercury chloride (IO). Stock solutions, stored in the dark, were stable for a
week. Air oxidation of RSH is very slow. When purified air was passed through a 10-*X RSH solution for 3 days, the solution turned slightly turbid, and the titer decreased about 2%. The p H of the 0.1Jf RSH solution was 9.85. Stock solutions of copper, zinc, and tin (0.02X in metal ion) were prepared by dissolving a n accurately weighed amount of C.P. metal in nitric or hydrochloric acid. Part of the excess acid was evaporated, and the residue was diluted to the mark in a volumetric flask. All other metal solutions were prepared from the reagent grade chlorides of nitrates. Methods. Current-voltage curves nere obtained with a Sargent nianual Polarograph, Model 111, and n i t h a self-recording Sargent Polarograph, Model XXI. -111 potentials nere measured against the saturated calomel cell (S.C.E.).The titrations \!-ere performed n i t h the manual polarograph. Oxygen was removed ITith a stream of Linde nitrogen (99.996% pure). The pH of the solutions was measured with a Beckman Zeromatic p H meter. Five- and 10-ml. semimicroburets m-ith 0.01- and 0.02-ml. divisions u-ere used, Solution movements around the dropping electrode mere indicated by the presence of carbon particles in solution. Their motion nas observed through a low poll-er microscope focused on the drop. The conventional dropping mercury electrode (d.m.e.) had the following characteristics in 0.l.U potassium chlo-
