21143
Anal. Chem. 1981, 53, 2143-2144
Baumann, E. W. Anal. Chim. Acta 1971, 54, 189-197. Durst, R. A. Ed. “Ion Selective Electrodes”; Department of Commerce, National Bureau of Standards: Washington, DC, 1969; NBS Spec. Publ. (US.)No. 314, pp 154-155. MkJgley, D.; Torrance, K. “Potentiometrlc Water Analysts”; Wlley: London, 1978, pp 318-319. Waldbott, G. L.; Burgstahier, A. W.; McKinney, H. L. ”Fluoridation: The Great Dilemma”, Coronado Press: Lawrence, KS, 1978; pp 35-36. Liberti, A.; Masclnl, M. Anal. Chem. 1969, 41, 676-679. Reference 11, p 361.
Tusl, J. Anal. Chem. 1972, 44, 1693-1694. Iriweck, K.;Sorantin, ti. Mikrochim. Acta 1977, 2 , 25-31. &em. Abstr. 1977 87, 149230d. Wassenaar, J. E.; Binnefts, W. 1. Voedins 1976, 39, 18-20. Chem. Absfv. 1978. 89. 194650. Reference 19, pp 51-62.
(20) Holaday, D. A.; Rudofsky, S.; Treuhaft, P. S. Anesthesiology 1970, 33, 579-593. (21) Mazze, R. I.; Trudell, J. R.; Cousins, M. J. Anesthesiology 1971, 35, 247-252. (22) Cousins, M. J.; Mazze, R. I. Anaesfhesia Intensive Care 1973, 1 , 355-373. (23) Yoshimura, N.; Holaday, D. A,; Fiserova-Bergerova, V. Anesthesiology 1976, 44, 372-379.
RECEIVED for review November 26, 1980. Resubmitted June 15,1981. Accepted July 13,1981. We are grateful to the Ro:yal Melbourne InstituteofTechnology for the award ofa grant.
Determination of Mercury in the Presence of Iron(1II) by Iodide Ion Selective Electrode Guler Somer Chemistry Department, Hacettepe University, Beytepe, Ankara, Turkey
It has been tried for a long time to develop an electrode for determining mercury. Since it was known that mercury was an interfering ion for the silver/sulfide electrode, we tried to use this electrode for the determination of mercury. However, the Ag/S electrode was not reversible, and even after brief exposure to mercury it became completely unresponsive. It was found later that an iodide electrode could be used for the determination of mercury and that this electrode responds to mercuric ion down to about loW8M (1). The mechanism appears to involve the reaction of mercuric ion with silver iodide on the electrode membrane surface to release silver, which is sensed by the electrode. Because the electrode actually senses silver, a monovalent ion, it exhibits a monovalent (60 mV) slope, in spite of”the fact that mercuric ion is divalent. Since mercuric ion readily forms chloride complexes, this ion should be removed before measurement by passing the sample through an anion exchange column (I). Overman, on the other hand, used potentiometric titration of mercury with sodium iodide using the same electrode as the sensor (2). As he also discusses, ferric ion, peroxide, and chelating agents are interfering ions and have to be eliminated. Therefore for the determination of mercury, especially in natural matrices in which considerable amounts of iron are present, interference of Fe3+should be eliminated. Recently some new electrodes which have a response to mercury have been suggested, one in liquid state (3) and some in solid state ( 4 , 5 ) but they are not commercially available. In this work the determination of mercury with an iodide ion selective electrode and the interference of iron(II1) is investigated. For this purpose a potentiometric titration method is applied with sodium iodide solution as the titrant. EXPERIMENTAL SECTION Apparatus. A Corning Model 12 Research pH meter with Orion (Model 94-53) solid-state iodide ion selective electrode and a Corning fiber junction saturated calomel electrode was used. No difference in results was observed when an Orion double junction reference electrode was used instead of the fiber junction electrode. Reagen1,s. The mercury@) solution was prepared by dissolving Hg(NO& in 0.1 N “0% This stock solution was diluted with triple distilled water and used in concentrations of W--10-* M. Merck pro analysis grade NaI was dissolved in triple distilled water in appropriate concentration with mercury solution and used as the titrant. 0003-2700/81/0353-2143$01.25/0
Table I. Change of Electrode Potential with Hg2+Concentration
10-4 10-3
210 21 2 24 5 29 5 338
140 179 247 302 358
160 175 235 290 349
Procedure. Solutions of mercuric ion (10-2-10-8M) are titrated with NaI using a microburet of 5-10 mL volume. Before ealch new experiment the bottom of the ion-selective electrode is cleaned with a specific polishing paper which is suggested by the Orion Co. Potential readings are taken while stirring with a magnetic stirrer.
RESULTS AND DISCUSSION The amount of mercury can theoretically be determined directly, by measuring the potential of a solution into which an iodide-selective electrode and a reference electrode are immersed. However it is observed that, especially at low concentrations of mercury, the proportionality between potential and the logarithm of the mercury concentration does not exist and therefore the results are not reliable. Tablo I summarizes the data obtained for the change of potential with Hg2+concentration. As it is observed, the change of potential with concentration if3 not proportional since theoretically for a 10-fold change of concentration a potential change of 59 mV is expected. In addition, potential measurements of identical solutions a t different times were not reproducible. They changed depending on the surface of the electrode. The results of three sets of experiments (EI,EII, EIII)which show the nonreproducibility are also given in Table I. According to Overman (2) mercuric ion down to 5 X 104 M can be determined by using a potentiometric titration. Because of the nonreproducible results as shown in Table I, it is preferable to use the NaI titration of mercury(I1) ions. In this method for each mercury(I1) ion, two iodide ions aire consumed. During the titration, mercury ions form Hg12,and after the equivalence point they form HgId2-complexes (I). Figure 1 shows the titration curve of 50 mL of M mercury(I1) solution with IO-$ M NaI. Because of a 200 mV change of potential about the end point, this titration can be 0 1981 American Chemical ,Society
2144
Anal. Chem. 1081, 53, 2144-2146
260
t
I
I80
- 240
7-.u
140
-
E.
W
::I
100.
- 360
- 4000
- 20
-601
02
4
6
8mL NoIOI
12
14
16
I8
Figure 2. Tltration of mercury(II), in an Iron(1II) containing medium (lron(II1) Is complexed each time with NaF): I, titration of 50 mL of lo-' M Hg2+with lo3 M NaI (E, = 278 mV); 11, titration of 50 mL of lom5M Hg2+with lo-' M NaI (E, = 232 mV); 111, titration of 50 mL of M Hg2+ with lo-' M NaI ( E , = 182 mV). E , Is the Initial potential of solution (vs. SCE).
0
0
2
04
06
08
IO
12
14
16
mL N o 1
Figure 1. Tltration of 50 mL of
M mercury(I1) with
M NaI,
Eo = 238 mV.
done very safely. With 5 X lo4 M, lo4 M, and 5 X M mercury(I1) solutions, the change of potential about the end point was 100,75, and 40 mV, respectively. The error of the 5 X M solutions was about 16%; lo-' M mercury(I1) solutions however could not be titrated with confidence. Therefore it can be concluded that lo4 M mercury(I1) solutions can easily be titrated with NaI using an iodide-selective electrode. Solutions containing 5 X M Hg2+,on the other hand, can be determined between some limits of confidence. When the procedure was applied to the determination of mercury in coal, some difficulties appeared. It was discovered that iron(III), which is present in coal, reacts with the titrant NaI and produces a positive bias. Although according Overman (2) 8 X M Fe3+ does not interfere with the determination of M mercury(II), higher amounts of iron present in natural matrices do interfere. Therefore for the determination of mercury(I1) in solutions containing Fe3+the effect of iron(II1) should be eliminated. This can be done by using complexing agents, which complex only the iron(II1)and not mercury(I1). Oxalate, fluoride, and TETA (triethylenetetramine) are the most convenient complexing agents.
Among them, TETA could not be used because of its basic character, since it forms mercury oxides. Oxalate could be used only if no alkaline earth element ions were present since they form precipitates with Cz042-.In coal samples therefore F- is found to be the best complexing agent. In one experiment, 3 mL of 0.1 M Fe2(S04)3was added to 30 mL of lo4 M mercury(I1)solution. Under these conditions it was impossible to find an end point using the normal procedure. Then NaF was added until the yellow color disappeared. After the Fe3+was complexed, the Hg2+was titrated, and the change of potential about the end point was about 200 mV. The same procedure was used with 10" and 5 X lo4 M mercury(I1) solutions with same amount of Fe3+, and reliable results were obtained. With lo4 M mercury(I1) solutions the sensitivity was low. Some of the titration curves are given in Figure 2. Since in many natural substances iron(II1) is the major interfering ion, complexing it with NaF will solve the problem. This procedure is applied to coal solutions and 5 X lo4 M mercury(I1) is determined, which confirms that this method may be applied to natural substances.
LITERATURE CITED (1) (2) (3) (4) (5)
Orion Newsletter 1970, 2(98), 41. Overman, R. F. Anel. Chem. 1971, 43, 618-617. Ballescu, G. E.; Cosofret, V. V. Tehnta 1978, 23(9), 877-681. Van de Leest, R. E. Analyst (London) 1977, 102, 509-514. Kopytin, A. V.: Zhukov, A. F.; Urusov, Yu. I; Kopytlna, L. A.; Gordievskii, A. V. Zh. Anal. Khim. 1979, 34 (3), 465-468.
RECEIVED for review May 8, 1981. Accepted July 6, 1981.
Determination of Arsenic(II1) by Computerized Potentiometric Stripping Analysis Daniel Jagner, * Mats Josefson, and Stig Westerlund Department of Analytical and Marine Chemistty, Chalmers University of Technology and University of Goteborg, S-4 72 96 Goteborg, Sweden
Davis et al. describe a preconcentration method for As(V) and k(II1) to be used in connection with the electroanalytical determination of total arsenic ( I ) . The method is based on sample digestion with mineral acids and subsequent reduction of arsenic(V) to arsenic(II1) chloride by copper(1) chloride. 0003-2700/81/0353-2144$01.25/0
Volatile arsenic(II1) chloride is then transferred by a stream of nitrogen to a trap solution for subsequent analysis. Davis et al. used high speed anodic stripping voltammetry for the determination of arsenic(II1) in the trap solution using a polar planimeter to evaluate the anodic stripping peak area. 0 1981 American Chemical Society
