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 As(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
JALYTICAL CHEMISTRY, VOL. 53, NO. 13, NOVEMBER 1981
Lee and MBranger used a charge-transfer analyzer to obtain the anodic stripping signals (2). In the present paper the determination of arsenic(II1) by means of computerized potentiometric stripping analysis is described and the technique is compared with the anodic stripping techniques. Since the reduction technique has been described in detail by Davis et al. and by Lee and MBranger, this aspect will not be discussed further here. THEORY In potentiometric stripping analysis arsenic(II1) is preconcentrat,ed as elemental arsenic on a working electrode by means of potentioatatic deposition (preelectrolysis)in the same way as in anodic stripping voltammetry. In potentiometric stripping analysis the elemental arsenic is, however, reoxidized (“stripped’) chemically at a controlled rate and the potential vs. time curve monitored (3-6). The oxidation rate is controlled by diffusion of oxidants to the electrode surface. In the electroanalytical determination of arsenic by stripping techniques, the use of gold electrodes, either a solid gold electrode or a thin film on a carbon substrate, is recommended (I). Since, however, the conductivity of elemental arsenic is very low, the arsenic deposited on the working electrode will finally block the current through this electrode. For this reason elemental gold has been codeposited on the working electrode during preelectrolysis. This was achieved by adding Au(II1) to the sample prior to analysis. The Au(II1) added to the sample is, moreover, a suitable reagent for reoxidation of the potentiostatically deposited arsenic ( 7 , B ) . EXPERIMENTAL SECTION Instrumentation. A potentiometric stripping analyzer (9) (Radiometer ISS 820) was used in combination with a microcomputer system based on an Intel 8085 processor (IO). The working electrode was manufactured by pressure-fitting a 10-mm glassy-carbon rod with a diameter of 2.1 mm into a 120-mmTeflon rod with a diameter of 12 mm. The center of the glassy-carbon rod was placed 4 mm from the center of the Teflon rod. The working electrode was rotated at a constant rate of 1200 rpm during both preelectrolysis and stripping. A saturated calomel electrode (Radiometer K 4040)was used as reference and a gold rod of diameter 2 mm as counterelectrode. Reagents. The hydrochloric acid was of Suprapur (Merck) grade. A stock solution of Au(II1) containing 5 g L-’ was prepared by dissolving the metal in aqua regia. Recommended Procedure. Add hydrochloric acid to the sample to a total concentration of approximately 7 M and Au(1II) to a total concentration of approximately 0.4 mM. If As(II1) concentrations above 10” M are to be determined, the Au(II1) concentration is increased to 1.2 mM. If the glassy-carbon electrode has not been precoated with a gold film in a previous sample, this is achieved by preelectrolysis at -0.05 V vs. SCE for 4-8 min. Once gold precoated, the working electrode can be used for a large number of samples. After a total of 2-3 h of preelectrolysis the old gold film is wiped off and a new precoating procedure performed. After gold precoating, the sample is preelectrolyzed for 2 min at -0.05 V vs. SCE prior to the registration of the stripping curve. A standard aliquot of arsenic(II1) is added, and the preelectrolysis/stripping cycle repeated. The arsenic(II1) concentration is evaluated by means of the normal equations for standard addition. RESUETS AND DISCUSSION Medium. In order to investigate the effect of the hydrochloric acid concentration, we preelectrolyzed four different samples containing 0.8 mM of Au(III), 1.3 pM of As(II1) (100 pg L-l), and 1,3,5, or 7 M of hydrochloric acid for 2 min at -0.05 V vs. SCE. The results are shown in Figure 1. As can be seen from the figure, the stripping plateaus are more well defined the higher the hydrochloric acid concentration. Similar observations were made for anodic stripping voltammetry (I). Hydrochloric acid concentrations above 7 M were
2145
E vs SCE
-0.051
I
I
I
I
111 \
t0.35
Figure 1. Potentiometric stripping curves registered after 2 rnin of preelectroiysls at -0.05) V vs. SCE in solutions containing 1.3 pIM As(”, 0.8 mM Au(II1) and (a) 1 M, (b) 3 M, (c) 5 M, and (d) 7 M hydrochloric acid. In curve d the half-width of the Stripping piateeiu has been estimated. E vs SCE V I
c035-
-
(a) 0
(b)
50
(c)
100 rns
2148
Anal. Chem. 1981, 53,2146-2147
Table I. Elements Interfering with the Potentiometric Stripping Determination of As(II1) in 7 M Hydrochloric Acid
a
stripping concn ratio for element potential, 10%interference M(n) V vs. SCE [As(III)]:[M(n)] As( 111) 0.13 Sb(II1) 0.05 1 :2000 Bi(II1) 0.06 1 :2000 Cu(I1) a 1:20 Hg(II) 0.29 1 :500 Ag(1) 0.25-0.35 1:50 Not well defined due to high chloride concentration.
Interferences. Since the “reductillation” procedure described by Davis et al. ( I ) eliminates interference from other elements, this aspect is of importance only if the arsenic(II1) is to be determined without prior preconcentration. Possible
interferents are those which can be reduced at the gold electrode at -0.05 V vs. SCE. These elements are specified in Table I together with the concentrations at which serious interference from them commences. As seen from the table the most serious interferent is copper(I1). LITERATURE CITED (1) Davis, P. H.; Dulude, G. R.; Griffin, R. M.; Matson, W. R.; Zinc, E. W. Anal. Chem. 1978, 50, 137-143. (2) Lee, S. W.; MBranger, J. C. Anal. Chem. 1981, 53, 130-131. (3) Jagner, D.; A h , K. Anal. Chlm. Acta 1978, 100, 375-388. (4) Jagner, D. Anal. Chem. 1878, 5b, 1924-1929. (5) Jagner, D. Anal. Chem. 1979, 51, 342-345. (8) Jagner, D.; Arb, K. Anal. Chim. Acta 1979, 107, 29-38. (7) Skibsted, L. H.; Bjerrum, J. Indisn Chem. SOC.J. 1977, 54. 102-108. (8) Skibsted, L. H.;Bjerrum, J. Acta Chem. S a n d . 1977, A31, 155-156. (9) Graabaek, A. M.; Jensen, 0. J. J . Ind. Res. Dev. 1979, 2Y, 124-127. (10) GranBli, A.; Jagner, D.; Josefson, M. Anal. Chem. 1980, 52, 2220-2223.
RE~FJVED for review March 30,1981. Accepted August 6,1981.
Reactivation of an Amino Bonded Phase Liquid Chromatographic Column Dorl Karlesky, Dennis C. Shelly, and Isiah Warner” Department of Chemistty, Texas A&M Universit)’, College Station, Texas 77843
The use of amino bonded phase packings for preparative HPLC is well documented. For example, Wise and co-workers have reported the utility of n-propylamine stationary phase for preparative separations of crude oil and sediment extracts (1). A cyclic amine functionality, n-propylpyrrolidone, has been applied to preparative fractionation of shale oil (2). These separations, which are performed in the normal phase mode, provide aromatic ring fractionation based on the configuration and extent of ring condensation. Also, aromatic fractionation can be effected by using reversed-phase conditions as explored by Mourey and co-workers (2). In this study, the two separation modes were found to be complementary with respect to selectivity. A consistent mechanism of separation in the normal phase mode appears to be a charge transfer electronic interaction between the stationary phase nitrogen lone pair electrons and solute r-electron clokd. Abbott has reviewed polar bonded stationary phases as important choices for analytical separations (3). Commonly, alkylamine bonded phases are employed in the analysis of saccharides and oligosaccharides (4,5). Although this column packing shows reasonably good selectivity for these an&es, the analysis is complicated by the reaction of reducing sugars with the amine bonded phase functionality (6). Schiffs base formation by amine-carbonyl condensation is a well-documented reaction which occurs during the chromatography typically resulting in decreased recoveries of the solutes. This reaction may also limit the usefulness of this column for oil fractionation unless the sample is first extracted to remove polar constituents possibly containing aldehyde or ketone moieties (7). Although efforts are continuing to minimize the on-column reaction (a),there is currently no method for regenerating an alkylamine bonded phase column which has undergone significant reaction-deactivation by either sample or mobile phase carbonyls. The conversion of the Schiff base (an imine) back to the amine and carbonyl-bearingspecies occurs readily in aqueous acid conditions (9). The reaction is reported to follow a hydrolytic pathway in which water is added to the C=N group, and then the amine is eliminated from a tetrahedral intermediate (10). The reaction, where acetone is the carbonyl0003-2700/81/0353-2146$01.25/0
containing hydrolysis product is shown in eq 1 and 2. In this
H
0-
B case, acetone is the reactant for the condensation reaction in which I was formed. In acid solution the rate-limiting step is thought to be breakdown of I1 (the tetrahedral intermediate). One would expect similar reactions for reducing sugm and aromatic ketones and aldehydes, excepting, of course, different rates of hydrolysis of the parent imine (9). Alkylamine bonded phases can be operated in the normal and reversed phase as well as ion exchange modes. Converting from one mode to another necessitates the use of intermediate solvents. When changing from cyclohexane (normal phase) to methanol/water (reversed phase), we inadvertently used acetone as an intermediate polarity solvent, resulting in extensive on-column Schiff base reaction. This paper describes our experiences in reactivating an n-propylamine bonded phase preparative HPLC column. EXPERIMENTAL SECTION Chromatographic System. All chromatography was performed on a microprocessor controlled Altex Model 312 MF liquid chromatograph equipped with a UV detector monitoring at 254 nm. The column (30 cm X 10 mm id.) was packed with 10-pm Chromosorb LC-9 (SupelcoInc., Belefonte, PA), an n-propylamine bonded stationary phase. Reagents. Polycyclic aromatic hydrocarbon (PAH) standards were prepared by dissolving appropriate quantities of benzene (Fischer Scientific Corp, Fair Lawn, NJ), anthracene (Aldrich Chemical Corp., Milwaukee, WI), naphthacene, and pyrene (Matheson, Coleman and Bell Inc., Cincinnati, OH) to make solutions of approximately lo4 M. Reagent grade 2-propanol (Fischer Scientific Corp., Fair Lawn, NJ) was used without purification as an intermediate polarity solvent. Spectrograde 0 1981 American Chemical Society
