ANALYTICAL CHEMISTRY, VOL. 50, (4) H. B. Herman, R. L. McNeeiy, P. Surana, C. M. Elliott, and R. W. Murray, Anal. Chem., 46, 1258 (1974). (5) C. M. Elliott and R. W. Murray, Anal. Chem., 48, 259 (1976). (6) F. C. Anson, Anal. Chem., 38, 54 (1966). (7) C. M. Elliott and R. W. Murray, Anal. Chem., 47, 908 (1975). (8) E. Blomgren, J. O'M Bockris, and C. Jesch, J . Phys. Chem., 65, 2000 (1961). (9) L. Pospisii and J. Kuta, Collect. Czech. Chem. Commun.,34,3047 (1969) (10) B. Damaskin, A. Frumkin, and A . Chizhov. J . Nectroanal. Chem., 28. 93 (1970). (1 1) B. A. Palkinson,Ph.D. Thesis, California Instiuie of Technology, Pasadena, Calif., 1978. (12) R. D. Armstrong, W. P. Race, and H. R. Thirsk, J . Electroanal. Chem., 23, 351 (1969). (13) M. Sluflers-Rehbach,J. Breukei, K. A. Gijsbersen, C. A . Wiznhorst, and J. H. Siuyters, J . Electroanal. Chem., 38, 17 (1972). (14) A. Napoii and P. L. Cignini, J . Inorg. Nucl. Chem., 38, 2013 (1976).
(15) (16) (17) (18) (19) (20) (21) (22)
NO. 13, NOVEMBER 1978
1891
R. D. Armstrong and E. Barr, J . Electroanal. Chem., 20, 173 (1969). S. Paul. Acta Crystallogr., 23, 491 (1967). S. H. Whitlow, Acta Crystallogr., Sect B , 31, 2531 (1975). W. Harrison and J. Trotter, J . Chem. Soc., Dalton Trans., 956 (1972); 1923 (1974). M. L. Post and J. Trotter, Acta Crystallogr. Sect. B , 30, 1880 (1974). P. J. Flook, H. C. Freeman, C. J. Moore, and M. L. Scudder, J . Chem. Soc., Chem. Commun., 753 (1973). P. DeMeester and D. J. Hodgson, J . Am Chem. Soc., 99, 6884 (1977). L. G. Sillen and A. E. Martell, "Stability Constants",The Chemical Society, London, 1964.
RECEIVEDfor review May 8, 1978. .4ccepted July 20, 1958. This work, Contribution No. 5783, was supported by a grant from the National Science Foundation.
Determination of Thiols by Conductometric Titration with Mercury(I1) Chloride in Water and in N,N-Dimethylformamide L. M. Doane and J. T. Stock" Department of Chemistry, University of Connecticut, Storrs, Connecticut 06268
A conductometric device that allows simultaneous ac bridge balance, without regard to capacitance effects, and generation of a signal proportional to solution conductance has been constructed and used to determine solution conductance in titrations of thiols with mercury(11) chloride. Quantitative data for determination of submillimolar solutions of 1-propanethiol, 2-propanethiol, and 2-methyl-2-propanethiol in water and in DMF were obtained with an accuracy and precision of fl%. Determinations of aqueous cysteine solutions gave similar results. Chloride, bromide, iodide, or thiocyanate up to concentrations of 1 mM did not generally interfere in the aqueous titrations. Similar titrations in DMF were unsuccessful. The accurate titration of thioglycolic acid in aqueous solution required the presence of bromide, iodide, or thiocyanate ion. With 3-mercaptopropanoic acid, accurate titration was successful with added thiocyanate, but not in the presence of bromide or iodide.
Apart from recent work with high-frequency signals (1,2), conductometric thiol titrimetry with precipitating or complexing reagents has received little attention. Although high-frequency methods permit measurements without solution electrolysis or electrode deactivation effects, the titration curves are rarely linear (3). A great advantage of conventional low-frequency conductometry is the ease and precision of end-point location. Mercury(I1) chloride and silver nitrate have found wide use in thiol titrimetry, since the mercaptides formed have very large formation constants, e.g., for cysteine, log KfiHg(RS)21 43.5 ( 4 ) ; log KspfAgRSI = ca. -20 ( 5 ) . The results obtained in the conductometric titration of thiols with silver nitrate were not very satisfactory (6)and similar titrations with mercury(I1) salts have not been attempted. The present work shows that mercury(I1) chloride is a n effective titrant in thiol conductometry both in aqueous and in DMF solution. The use of DMF minimizes electrode deactivation effects because mercaptide precipitation is prevented. 0003-2700/78/0350-1891$01.00/0
One of the problems in the conventional Wheatstone bridge conductance methods is obtaining a sharp "null". One approach is to use a switching circuit that is insensitive to phase shifts (7, 8). The present authors have modified this circuit to eliminate losses due to transistor lbias and have added an analog circuit to give fast conductance readouts. Because the two outputs are independent, they can be used simultaneously.
EXPERIMENTAL Reagents. N,N-Dimethylformamide (DMF) was used as received from Eastman Kodak Co. I.-(+)-Cysteine (J. T. Baker Chemical Co.) was dried over P205. All 1hiols were obtained from the Aldrich Chemical Co. D,L-Penicillamine wm used as received. 1-Propanethiol, 2-propanethiol, and 2-methyl-2-propanethiolwere distilled under nitrogen. Thioglycolic acid and 3-mercaptopropanoic acid were distilled under reduced pressure. The thiols were stored under nitrogen. S t a n d a r d Solutions. Fresh standard 0.1 M thiol solutions were prepared daily. To avoid volatilization and oxidation of the liquid thiols, the following procedure was used. A volumetric flask having a Teflon stopper with a small center hole was flushed with nitrogen. The hole was then closed with a short plug of stainless steel rod and the flask was weighed. The appropriate amount of liquid thiol was then introduced by a :,yringe. The stopper was then plugged and unplugged to establish vapor pressure equilibrium, and the flask plus thiol weighed. To minimize loss of thiol, dilutions of the stock solutions were made by use of a syringe. With the exception of cysteine and D,L-peniCilhmine,which were diluted with outgassed water, 95% ethanol was used as diluent. Aqueous mercury(1I) chloride solution was standardized with potassium iodide (9). The standard solution in DMF was prepared by the direct weighing of mercury(I1) chloride that had been assayed with potassium iodide (9). Two-milliliter microburets (Roger Gilmont Instruments, Inc.) were used to deliver the thiol solutions, and the titrant. Instrumental. A circuit diagram of the conductometric device is shown in Figure 1. The device was assembled largely from dual operational amplifiers (National Semiconductor LM 1458 N) and standard electronic component:, (10). In the simplified block diagram in Figure 2, the signal from the bridge at point B is inverted by A3 and then fed into a precision limiter, which simulates ideal diode behavior (11). A similar limiter accepts the signal from point C. Positive half-cycles are thus removed from
0 1978 American Chemical Society
1892
*
ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
12K
I 1 1
‘IN
c
CIN
f Figure 1. Circuit diagrams of (A) rectifier and filter, (6)differencelanaiog divider, and (C) bridge balance circuit. Operational amplifiers A l - A 2 1 : National LM 1458 N. Match transistor pair T l - T 2 : National LM 394. Diodes I N 9 1 4
each signal. The two resultants are added by A4 and this combination signal is applied to the “vertical” input of an oscilloscope with a horizontal scan rate of 100 kHz. Two overlapping rasters appear. The bridge is balanced by changing R3 (Figure 2) until the bright-lit tops of the rasters coincide. Any capacitance changes merely shift the half-cycle across the screen, leaving the amplitudes essentially unaffected. The analog signal is obtained from E2,the voltage at point H, and E , the voltage applied to the other electrode at A. Provided
that essentially no current is drawn from B, the output at A14 (Figure 1B) is directly proportional to solution conductance. Phasing problems were removed by rectifying and smoothing both E 2 and E (10). The analog output is well suited to following fast conductance changes, e.g., driving a strip chart recorder in kinetic experiments. However, the bridge balance output was used exclusively in the titrations. This outpct gave an additional significant figure beyond the read-out of the available digital voltmeter attached to the analog output.
ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
1893
Figure 2. Block diagram of conductometric device. Amplifiers: Al, buffer; A2, difference amplifier; A3, inverter; and A4, signal amplifier
Table I. Results of Thiol Titrations in Aqueous Solution end point molar ratio, concn Hg:RSH range, mM
compound 1-propanethiol 2-propanethiol 2-methyl-2-propanethiol cysteine D , L-penicillamine
1 :2
0.1-1.0
1:2 1:2
0.1-1.0 0.3-1.0
1:2 1:l
0.3-1.0
1:2
thioglycolic acid
1 :1 1:2
3-mercaptopropanoic acid
1:2 1:2 1:2
0.3-0.7 1.0 0.3-0.7 0.3-1.0 0.3-1.0 0.3-1.0 0.3-1.0
accuracy and re1 std dev, % -0.36 I t0.16 i t0.79 i t0.32 * t0.03 t +0.15 i -8.28 i t1.05 i
0.92 0.61 0.97 0.53 0.20 0.25 0.77 0.98
t0.48
i
0.45
t0.45 t0.75
i
t
0.33 0.73
Procedure. Fifity milliliters of diluent were put in the titration cell, which was then mounted in a small waterbath maintained at fO.l degree of ambient temperature (23-25 "C). The cell contents were stirred magnetically and sparged with nitrogen. The gas was cut off when the cell contents attained constant temperature. Standard thiol solution was then added to the cell and the titration was started immediately. The wire electrodes (8) were continually checked during the course of the titration to determine if a precipitate was adhering to the electrodes, since an accumulation reduced the solution conductance below its correct value.
runs
ions tolerated
22 16 15 13
c1Cl-, Br-, SCNCl-, Br-, I-, SCNCl-, Br-, I-, SCN-
IBrSCNSC!N-
/I
A
15
RESULTS AND DISCUSSION Titrations in Aqueous Solution. A typical titration curve has t h e form shown a t A in Figure 3. Since thiols are very weak electrolytes [pK, > 81 ( I Z ) , the initial conductance is low. The highly mobile hydrogen ion produced according to:
HgC12
+ ZRSH
Hg(RS), + ZH+ + 2C1-
(1)
should cause an immediate rise in conductance. Inexplicably, however, rapid rise in conductance did not occur until approximately 0.5 equivalents of titrant had been added, as shown at B in Figure 3. In most cases, little or no curvature was detectable in t h e region of the end point. As expected, the excess-titrant line is essentially horizontal. When the titration of thiols at concentrations less than 0.1 m M was attempted, curves such as C in Figure 2 were obtained, so that precise end-point location was impossible. At such low concentrations, the conductance was not stable at a n d beyond t h e end point. T h e results in Table I show t h a t 1-propanethiol, 2propanethiol, 2-methyl-2-propanethio1, and cysteine can be titrated with precision and accuracy to better than fl%.
0
05
10
16
EQUIVALENTS OF TIT RAN^ ADDED
Figure 3. Titration curves of 1-propanethjol with HgCI, in water. Concentration of 1-propanethiolis (A) 0.3 mM, (B) 0 1 mM. and (C) 0.5 rnM
Cysteine and 2-methyl-2-propanethiolcan be titrated with no loss of accuracy in the presence of up to 1 m M chloride, bromide, iodide, or thiocyanate ion, despite the fact that these anions form very stable compounds with mercury(I1). However, iodide must not be present when z-propanethiol is to be titrated, and only chloride did not interfere with ti-
ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
1894
.--___I
7
f
'
/,'
D ~
I
!.L05
_ ~ -_ ~ T~~
~.. -~ ...__
10
15
0
J
~ _ ~ . __.__.__ _ ~ _ _ 20 2 5
EQUIVALENTS OF TITRANT ADDED
Figure 4. Titration curves of 0.3 mM D,L-penicillaminewith HgCI, in water and in the presence of 1.0 mM (A) KCI, (B) KSCN, (C) KBr, and
2
1 EQUIVALENTS
OF TITRANT ADDED
Figure 5. Titration curves of 1-propanethiol with HgCI, in DMF. Concentration of 1-propanethiol is (A) 1.0 mM, (B) 0.7 rnM, and (C) 0.3
mM
(D) K I
trations of 1-propanethiol. Foreign electrolyte concentrations greater than approximately 1.0 mM gave large background conductances, with corresponding loss of precision and accuracy. Negative errors of a t least 1% occurred when the thiol concentrations were above approximately 1 mM. Even the titration of cysteine, in which no precipitation occurs, showed this effect. D,t-Penicillamine and the two mercapto carboxylic acids gave titration curves of the form shown at A in Figure 4. The first end point corresponds to a mercury/thiol molar ratio of 1:2 and the second to a ratio of 1:l. However, the first end point is generally unusable. Only in the titration of D,Lpenicillamine a t a concentration of 1.0 mM (Table I) could the first end point be effectively used. The addition of chloride ion had little effect, but the addition of bromide, iodide, or thiocyanate suppressed the second end point (Figure 4, curves B, C, and D). Titrations of D,L-penicillaminein the presence of these anions were low by greater than 2 % . Attempts to titrate thioglycolic acid or 3-mercaptopropanoic acid without added electrolytes were unsuccessful because of erratic conductance data. The addition of bromide. iodide, or thiocyanate ion permitted the accurate titration of thioglycolic acid. Only in t,he presence of thiocyanate were the results of 3-mercaptopropanoic acid accurate. The added electrolytes suppressed the second end point, so that titrations performed in the presence of these electrolytes were based on the 1:2 combining ratio (Figure 4, curves B, C, and D). Equation 1 indicates a 1:2 combining ratio. Presumably, the continued addition of titrant can bring about the reaction indicated by Equation 2
HgClz + Hg(RS)Z 2HgRS' + 2C1(2) For this reaction to occur, the 1:2 mercaptide from which the thiol anion is abstracted must be less stable than similar mercaptides formed by cysteine and the aliphatic thiols. The products of Equation 2 are written as ions to explain the steady increase in conductivity past the first end point. T h e suppression of the second end point by thiocyanate ion and some of the halide ions implies that the mercaptide interacts with the excess of these ions, as indicated by Equation 3, where X- is bromide, iodide, or thiocyanate ion.
HgC12
+ Hg(RS)2
""
2HgRSX
+ 2C1-
(3)
This reaction has little effect on the conductance, because Xand chloride ion have similar ionic conductances. Titrations in DMF. Although mercaptide precipitation did not occur in DMF, inaccurate results were found at thiol
Table 11. Results of Thiol Titrations in DMF Solution
compound 1-propanethiol 8-propanethiol 2-methyl-2propane thiol cysteine
end point molar ratio, Hg: RSH
concn range, mM
accuracy and relstd dev, % runs
3:2 3:2 3:2
0.3-1.0
-0.10
0.3-1.0
-0.36 t0.92
1:l
0.3-1.0 -12.40 t 3.45 0.3-1.0 -8.55 L 1.64 0.3-0.7 -2.73 i 2.04 0.3-0.7 -0.32 I0.55 0.3-1.0 t 0 . 5 3 t 2 . 2 4 0.3-1.0 +8.63 c 0.92
2:l
D,L-penicillamine thioglycolic 3-mercaptopropanoic acid
1:1 2:l 3:2 3:2
0.3-1.0
i i i
0.47 0.45
1.06
9 9 10
9 9
6 6 9
9
concentrations greater than 1.0 mM. Some titrations that are possible in water failed when the medium was changed to DMF. In DMF, the mercury/thiol stoichiometry of the aliphatic thiols was found to be 3:2. Figure 5 shows typical aliphatic thiol titration curves. The linearity of the pre-equivalence line from the beginning of the titration is noticeable, but so is the curvature in the region of the end point. In fact, if enough readings are not obtained prior to the half-titrated point, accurate extrapolation to the end point is impossible. Results obtained in DMF media, summarized in Table 11, show that aliphatic thiols can be titrated with precision and accuracy of f l % . It is probable that DMF influences the form of these titration curves, since the titration curves of cysteine and D,L-penicillamine differ radically when the medium is changed from water to DMF. Titrations of cysteine and D,L-penicillamine exhibited three end points as shown in Figure 6. This is in sharp contrast to the behavior in aqueous solution. The first end point is concentration dependent and is useless. In the case of cysteine, neither of the other end points gave accurate results. However, the third end point, in which the mercury/thiol stoichiometry is 2:1, could be used to determine D,L-peniCillamine with errors less than 1%. Titration curves of thioglycolic acid and 3-mercaptopropanoic acid were similar to those of the aliphatic thiols and involved a 3:2 mercury/thiol stoichiometry. Since the end point of thioglycolic acid was concentration dependent (indicated by its large standard deviation) and that of 3-mercaptopropanoic acid had a large (although precise) error in accuracy, standardizing the titrant against the thiol a t a thiol con-
ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
1895
suggested t h a t Reaction 1 is followed by Reaction 4 A
2HgC12
1 0
1
2
EQUIVALENTS
3
4
OF T I T R A N T ADDED
Figure 6. Titration curves of cysteine with HgCl, in DMF. Concentration of cysteine is (A) 1.0 rnM, (B) 0.7 rnM, and (C) 0.3 rnM
centration close to t h a t in the sample is recommended to obtain accuracies within 1% . T h e titrations of cysteine and D,L-penicillamine in DMF undoubtedly were affected by the amino group, since titration curves of 3-mercaptopropanoic acid (I) [a compound similar to cysteine (II), but with the amino group absent] CH,-CH, -COOH SH
CH,-CH-COOH I SH NH, (11)
(1)
Cysteine
3-Mercaptopropanoic acid
had the form and stoichiometry of the aliphatic thiol titration curves. Attempts were made to titrate various thiols in the presence of bromide, iodide, or thiocyanate ion a t a concentration of 1.0 mM. All attempts failed, because interpretation of the titration curves was impossible. Stricks and Kolthoff ( 4 ) examined the interaction of cysteine and mercury(I1) chloride in aqueous solution and
+ Hg(RS)Z
+
Hg(RS)2*2HgC12
(4)
This implies 3:2 stoichiometry. These workers stated that an excess of mercury(I1) chloride was required to obtain the addition compound. If the 3:2 stoichiometry in our work implies the formation of the same addition compound, this compound must be considerably msore stable in DMF than in aqueous solution, since it had to be formed during the titration, Le., without excess of mercury(I1) chloride. Only Reaction 1 contributes to the solution conductance. If Reaction 4 occurs after the thiol has been consumed by Reaction 1, the solution conductance would not change and the mercury/thiol molar ratio would appear to be 1:2. Only when Reaction 4 occurs simultaneously with Reaction 1 can a 3:2 stoichiometry be indicated by the end point. Thus, the addition compound actually appeared to form in an excess of thiol in DMF. However, it also seemed apparent that there was some unknown role that DMF was playing in forming this apparent addition compound.
LITERATURE CITED (1) L. Serrano Berges and T. Fernandez Perez, An. Real SOC.€span. Fis. Ouim., Ser. 6.. 62. 807-820 (1966). (2) F. Oehme, Erdol Kohle, 13, 394-396 (1960). (3) H. Willard, L. Merritt, and J. Dean, "Instrumental Methods of Analysis", 5th ed., D. Van Nostrand, New York, N.Y., 1974,Chapter 26. (4) W. Sbicks and I. M. Kokhoff, J. Am. Chem. Scc., 75,5673-5681 (1953). (5) R. Cecil and J. R. MePhee, Adv. Protein Chem., 14, 262 (1959). (6) W. Scheele and C. Gensch, Kaufsch. Gummi, 6, 147-157 (1953). (7) M. Michaelis, fract. Electronics (London), 5, 587-593 (1969). (8) J. T. Stock, J . Chem. Educ., 52, A165-Al68 (1975). (9) W. Scott, "Standard Methods of Chemical Analysis", 5th ed., D. Van Nostrand, New York, N.Y., 1944,p 580. (IO) L. M. Doane, PhD Thesis, University of Connecticut, Storrs, Conn., 1977. (11) H. Malmstadt, C. Enke, and S. Crouch, "Electronic Measurements for Scientists", W. A. Benjamin, Reading, Mass., 1974,p 436. (12) S. Patai, "The Chemistry of the Thiol Group", Interscience Publishers, New York, N.Y., 1974,pp 398-400.
RECEIVED for review April 26, 1978. Accepted August 4, 1978.
Voltammetric Behavior of the (Pt)H20,C02/H2,C032-System in Molten Alkali Nitrates Elio Desimoni, Francesco Palmisano, Luigia Sabbatini, and Pier Giorgio Zambonin * Cattedra di Chimica Analitica della Universiti di Bari, Via Amendola 773, 70726 Bari, Italy
An electrochemical investigation was performed on the H20,C0,/H2,C032- system in the ( Na-K)N03 equimolar melt at 513 K by RDE and peak voltammetry and DME polarogH,, CO, H20, raphy. Solutions containing C03,-, C0:and C032- H2 CO, H20 were investigated. The reported experimental results represent a quite complete qualltatlve description of the mentioned system for which a reversible or quasi-reversible voltammetric behavior can be hypothesized. C032- = H,O The possibility to employ the reaction H, CO, 2e for the analytical detectlon of hydrogen and carbon dioxide in the given solvent is briefly discussed.
+ +
+
+
+ +
+
+
A systematic investigation on hydrogen systems in the (Na-K)NO, equimolar mixture was recently initiated in our laboratory in the course of a wide study on gaseous systems 0003-2700/78/0350-1895$01 .OO/O
in fused salts. Solubility ( I ) , potentiometric ( 2 ) ,and voltammetric ( 3 , 4 )data were already published together with the results of a kinetic investigation ( 5 ) on the slow reaction of hydrogen with the solvent anions
H2
+ NOS- = HzO .t NOz-
(1)
A brief review of preliminary results on all these topics is contained in reference 6. Interesting results, showing an unusual behavior in the field of hydrogen electrooxidation were obtained in the course of rotating disk electrode (RDE) voltammetric investigations (3, 4 ) performed on the (Pt)H,O/H,,OH- and (Au)H20/H2,0Hsystems in the (Na-K)N03equimolar melt. The mechanistic model proposed (3, 4 ) to explain the experimental results involves the chemical consumption of the electroactive species (H,) catalyzed by electrochemically produced radicals (OH-) in the diffusion layer. The direct involvement of the electrode C 1978 American Chemical Society
