ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
2337
Electrocatalytic Detection of Cobalt Separated by Ion Exchange Chromatography Yoshinori Takata, Fumio Mizuniwa, and Chiaki Maekoya Hitachi Research Laboratoiy, Hitachi, Ltd., 4026 Kuji-machi, Hitachi-shi, Ibaraki-ken 3 19- 12, Japan
A liquid chromatographic method Is described for the detectlon of cobalt ion. The reactlon tor the detectlon was the electrocatalytic oxidation of tartrate eluent and a carbon working electrode with cobalt ion acting as a catalyst. The detectlon limit was 5 ng and the llnear dynamic range was more than 10’. Reproduciblltty at 1 ppm (sample dze 0.5 mL) was 2.2% by coefficient ot variance. Cerous, chromic, ferrous, ferric, manganese, lead, and stannous ions were also detected by their catalytic actlon. The method was applied to the analyds of a boiling water reactor coolant.
Recently, it has become apparent that trace amounts of h e a w metal ions in the coolant water of reactors cause operational, safety, and maintenance problems. For example, the operator’s radiation exposure is increased by accumulation and deposition of activated ionic products of corrosion on the inside surfaces of reactor pipes. In order to reduce the amount of corrosion products carried into the reactor, adequate anticorrosion steps are necessary which are based on a survey of plant water quality. The most important element t o be monitored from the viewpoint of radioactivity is cobalt which produces 6oCo. Since the concentration of cobalt ion in the primary coolant normally ranges from parts per trillion to parts per billion, quantitative analysis usually requires preconcentration. If the analytical sensitivity is high enough, of course, no preconcentration is necessary. Accordingly, a more sensitive analytical method is desirable for shortening the analysis time and for the reduction of operator radiation exposure. Flameless atomic absorption spectrophotometry and inductively coupled plasma emission spectroscopy are very sensitive. However, it is necessary to be very careful of laboratory contamination with radioactive isotopes by dispersion of the atomic vapor a t high temperatures. A novel and sensitive electrocatalytic method for the detection of cobalt ion was found unexpectedly in the process of detecting ferrous ion by oxidation using a coulometric detector for liquid chromatography (1-3). A large peak appeared before the ferrous ion peak when the sample solution contained cobalt ion. The area of this peak indicated that the quantity of electricity for the electrochemical oxidation was 50 times higher than the value calculated for Co(I1) by Faraday’s law. The reaction was the electrocatalytic oxidation of tartrate in the eluent a t a carbon working electrode with Co2+ ion acting as a catalyst. This paper presents the optimum conditions for detection of the cobalt ion and describes its application to the primary coolant of a hoiling water reactor.
EXPERIMENTAL Apparatus. The experiments were carried out with a Hitachi Model 635 liquid chromatograph,a Hitachi Model 630 coulometric detector, and a concentrator. A schematic diagram of the apparatus is shown in Figure 1. The concentrator consists of a peristaltic pump, two mini-columns,coupled 4-way valves, and a timer which controls the time required for preconcentration.
Mini-column. PTFE columns of 10-mm length and 9-mT i.d. were filled with Dowex 50W-X8, 1OC-200 mesh cation-exchange resin. Separation Column. A water-jacketed glass column (9-mm i.d. X 10 cm) was filled with Hitachi Custom Ion-ExchangeResin No. 2611, a strongly acidic cation-exchange resin (sulfonated polystyrene resin of nominal 10% divinylbenzenecross-linking). Column Operation and Detection of Metal Ions. The eluent (sodium tartrate-sodium chloride) is pumped through the mini-column to the sampling valve and then the separation column. The sample is pumped through the other mini-column for preconcentration of the sample constituents. The effluent from the column is passed directly into the electrolytic cell of the coulometric detector, and the electrolytic current based on the electrochemical oxidation reaction is recorded. The component quantities are obtained by comparison with a calibration graph. The electrolyte for the counter electrode of the electrolytic cell was 0.1 M potassium ferricyanide-0.1 M potassium ferrocyanide-0.1 M potassium n i t r a t e 4 2 M ammonium hydroxide. The working and counter electrodes were carbon cloth. The detection potential was chosen between +0.8 and +1.05 V vs. ferriferrocyanide depending on the eluent pH. The electrolytic cell effluent was mixed with buffered cupric diethylenetriaminepentaacetate(DTPA) solution before it passed into a second coulometric detector when the method presented was compared with the normal coulometric detection method as described in Refs. 4 and 5 .
RESULTS AND DISCUSSION Separation and Detection of a Standard Solution. An example of the separation and detection of a standard solution containing 5 ppm of ferrous and 0.5 ppm of cobalt ion is shown in Figure 2. The eluent was a 0.15 M sodium tartrate-0.05 M tartaric acid-0.05 M sodium chloride solution. In this medium, the cobalt ion is not expected to be detectable from its electrolytic oxidation, but current flows whenever cobalt ion is in the column effluent. The cobalt peak was about 50 times higher than the calculated value assuming a one-electron reaction. Mechanism of the Detection. It was thought that the tartrate used as an eluent was being electrocatalytically oxidized. To make sure, the tartrate concentration was varied from 0 to 100 mM while keeping the concentration of sodium chloride a t 100 mM. The electrolytic oxidation current was mol of cobalt ion was injected a t a measured when 5 X point just before the cell. Figure 3 shows background current and peak current plotted against working electrode potential. The cobalt peak was not observed when tartrate was not in the eluent and the peak current increased with tartrate concentration. For the data shown in Figure 3, the working electrode used was carbon cloth. The carbon cloth electrode deteriorated after using it for a few weeks. However, when the carbon cloth electrode was replaced with a platinum screen, the cobalt peak could not be observed. I t was therefore concluded that the oxidation reaction of tartrate took place only on a carbon cloth electrode and the carbon cloth was also oxidized. Effect of Eluent pH on the Detection. The effect of eluent pH on the peak current and on the background current was also studied. The results are shown in Figure 4. These
0003-2700/79/0351-2337$01.00/00 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
Flgure 1. Schematic diagram of apparatus: (a) sample, (b) filter, (c) peristaltic pump, (d) coupled 4-way valves, (e) eluent pump, (f) eluent, (9) mini-columns, (h) sample valve, (i) sampling coil, (j) standard solution, (k) separation column, (I) coulometric detector
0.7
Potential
0.8
0.9
1.0
1.1
( V v s . ferri-ferrocyanide )
Figure 4. Effect of pH on background and peak currents. Tartrate concentration: 0.1 M; other conditions same as in Figure 3
T
4: 7
0
i
,--A
10
15 Time
20
25
30
( min)
Flgure 2. Chromatogram of standard solution. Sample: 0.5 ppm Co2+ and 5.0 ppm Fe2+,0.5 mL; column: 100 mm X 9 mmi.d.; resin: Hitachi Custom Ion-Exchange Resin No. 2611; eluent: 0.15 M Na-tartrate-0.05 M tartaric acid-0.05 M NaCI; flow rate: 1 mL/min; detection potential: 0.95 V vs. ferriferrocyanide
4t
foo T
1 5
1
2
1
3
4
Flow r a t e (mumin
)
Figure 5. Effect of flow rate. Eluent: pH 3.8; detection potential: 1.05 V; other conditions same as in Figure 4
(V
vs. ferri-ferrocyanide
)
Figure 3. Effect of tartrate concentration on background and peak currents. Sample: 1 mM Co2+, 0.5 mL; eluent: 0.1 M NaCI-tartrate, pH 3.8; flow rate: 1 mL/min
21
currents were measured against the working electrode potential while the pH of the eluent was varied from 3 to 7 . The eluent was a 100 mM sodium tartrate-100 mM sodium chloride solution. These graphs show the reaction is greatly affected by pH so that the working electrode potential must be chosen for an optimum value depending on the eluent pH. Effect of Flow Rate. The flow rate of the eluent also has an effect on peak current and peak area as Figure 5 demonstrates. If the electrolysis takes place quantitatively, the peak area must be constant since it is independent of the flow rate. But in Figure 5, it is seen that the peak current increases and peak area decreases with increasing flow rate. Taking into consideration that the residence time of the ions in the cell is shortened by increasing the flow rate, the catalytic oxidation reaction is thought to continue whenever cobalt ion remains in the cell. The catalytic activity is thought to be gradually reduced, because the peak current would be constant vs. the
k
Background
1 It
L
O0
1
,
1
20
40
60
Temperature
(OC
)
Figure 6. Effect of flow electrolytic cell temperature. Detection potential: 0.9 V; other conditions same as in Figure 5
flow rate if there were no reduction of the catalytic activity. Effect of Cell Temperature. The plot in Figure 6 shows the effect of the cell temperature on the peak current and the background current. Both currents are increased with increases in temperature. This means that the reaction is accelerated by higher temperatures, so that the cell must be kept at a constant temperature.
ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
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~
Table I. Detection of Ions by Electrocatalytic Reactiona ion
reagent &NO, BaC1, . 2 H 2 0 CaCl, Ce(NO,);GH,O CoC1,.6H2O Cr (NO ,) ,YH,0 K,Cr,O. FeS0;7H20 FeCl,.GH,O MnC1;4H20
Ag'
Ba2 Ca2+ Ce3+ CO2+ Cr3+ Cr(V1) +
Fe2+ Fei+
Mn'+ Ni? +
