Continuous monitoring using thiocyanate ion-selective electrodes

Introduction Ion-selective electrodes have been widely used in automating analyses for the potentiometric determination of traces as well as high concentrations of anions and cations. However, the use of electrodes for the routine and continuous monitoring of thiocyanate in ’environmental’ water samples has not been fully considered. Recently, the author proposed an automated potentiometric determination of thiocyanate at the ppm level in water using an ion-selective electrode with liquid membrane ]. In that paper, thiocyanate ion-selective membranes were basically examined for electrodes utilising long-chain quaternary ammonium cations such as tetradecyldimethylbenzylammonium and methyltrioctylammonium ions as the exchange sites in order to automate the procedure for the routine analysis of thiocyanate at the ppm level in water. However, an important additional requirement for such a procedure is that it should be easy to use, so that it would give precise results from a simply constructed apparatus. Hence, the problem was to devise a system which met all these requirements. Furthermore, it would have to be cheap to manufacture, since there are practical advantages in building on a modular basis with one module dedicated to each analysis. In this paper therefore, the practical problems of automating the system using thiocyanate ion-selective electrodes and also the continuous monitoring of thiocyanate in industrial waste water are described. The feasibility of using thiocyanate ion-selective electrodes with solid and liquid membranes for this purpose are evaluated. Also discussed are methods for overcoming difficulties of electrode maintenance during continuous monitoring. The electrodes compared are a commercial bromide ion-selective electrode with silver bromide solid membrane (Toa Model BR-125, obtained from Toa Electronics Ltd, Japan) and a thiocyanate ion-selective with tetradecyldimethylbenzylammonium thiocyanate liquid membrane using 1,2-dichloroethane described in the previous work ].


Introduction
Ion-selective electrodes have been widely used in automating analyses for the potentiometric determination of traces as well as high concentrations of anions and cations. However, the use of electrodes for the routine and continuous monitoring of thiocyanate in 'environmental' water samples has not been fully considered. Recently, the author proposed an automated potentiometric determination of thiocyanate at the ppm level in water using an ion-selective electrode with liquid membrane ]. In that paper, thiocyanate ion-selective membranes were basically examined for electrodes utilising long-chain quaternary ammonium cations such as tetradecyldimethylbenzylammonium and methyltrioctylammonium ions as the exchange sites in order to automate the procedure for the routine analysis of thiocyanate at the ppm level in water.
However, an important additional requirement for such a procedure is that it should be easy to use, so that it would give precise results from a simply constructed apparatus.
Hence, the problem was to devise a system which met all these requirements. Furthermore, it would have to be cheap to manufacture, since there are practical advantages in building on a modular basis with one module dedicated to each analysis. In this paper therefore, the practical problems of automating the system using thiocyanate ion-selective electrodes and also the continuous monitoring of thiocyanate in industrial waste water are described. The feasibility of using thiocyanate ion-selective electrodes with solid and liquid membranes for this purpose are evaluated. Also discussed are methods for overcoming difficulties of electrode maintenance during continuous monitoring. The electrodes compared are a commercial bromide ion-selective electrode with silver bromide solid membrane (Toa Model BR-125, obtained from Toa Electronics Ltd, Japan) and a thiocyanate ion-selective with tetradecyldimethylbenzylammonium thiocyanate liquid membrane using 1,2-dichloroethane described in the previous work ].

Materials and methods
Thiocyanate ion-selective electrode with liquid membrane The liquid membrane containing an ion-associate formed between tetradecyldimethylbenzylammonium (zephiramine; obtained from Dojin Chemical Laboratories Co Ltd, Japan) and thiocyanate was prepared by extraction with 1,2dichloroethane (DCE). The thiocyanate ion-selective electrode with liquid membrane was then prepared as follows [1]. 100 ml of a x 10 -a M sodium thiocyanate aqueous solution was transferred into a 500 ml separation funnel. 100 ml of x 10 -a M zephiramine (chloride) in DCE solution (obtained by directly dissolving 36.9 mg of dried zephiramine chloride [2 into 100 ml of DCE) was added and the resulting solution was shaken for 60 mins in an Iwaki Model KM shaker. The aqueous phase was discarded and the organic phase shaken *Present address: School of Engineering, Okayama University, Tsushima-naka, Okayama-shi, 700, Japan again with another 100 ml aliquot of the x 10 -a M sodium thiocyanate aqueous solution in order to purify it. After phase separation, the organic phase was filtered through a dry filter paper to remove droplets of water. The organic solution was then diluted with DCE to give a x 10 -4 M solution which was used to make the thiocyanate ionselective liquid membrane. The electrode was constructed using the barrel of an Orion Model 92-07 nitrate ion-selective electrode, and the x 10 -4 M zephiramine thiocyanate in DCE solution plus a x 10 -2 M aqueous sodium thiocyanate solution as organic liquid membrane and internal reference solutions respectively. The liquid is supported by a cellulose membrane filter used as a barrier to keep the liquid membrane solution and aqueous sample solution separate. The liquid membrane potentials with a Yokogawa Model MR-Y511 reference electrode are measured using a Hitachi-Horiba Model F-5 pH meter equipped with a Yokogawa Model 3046 laboratory recorder.
Procedure for continuous monitoring A schematic diagram of the continuous thiocyanate monitoring equipment used in this work is given in Figure 1. The waste water sample was pumped up to a filtration unit (C) with an Iwaki Model LP-15 laboratory pump (Figure A) at a flow rate of 5 1/min, which was adjusted by a flow meter (B). The sample was pre-treated by the filtration unit which has three different meshed plastic filters ( Figure 1, a: 10 mesh, b: 40 mesh, c: 200 mesh) to avoid contamination of the electrode by suspended solids. The resulting sample solution was then thermostated at 25 -+ 0.5C with a Mitamura circulator-type thermostated unit (Dvolume capacity: 5 1) and well stirred by a circulator (circulation capacity: 5 1/min). The sample obtained by these pretreatments was used for the measurement of thiocyanate under a Denkikagakukeiki ultrasonic wave cleaner (E).
The membrane potential, based on the thiocyanate concentration, was detected by a Hitachi-Horiba Model F-5 pH meter (H) equipped with a thiocyanate ion-selective electrode (F) and a Yokogawa Model MR-Y511 reference electrode (G). The data obtained was continuously recorded by a fitted Yokogawa Model 3046 laboratory recorder (I).

Results and discussion
Preliminary tests by batch work Before use in the continuous monitoring system the basic characteristics and performance of the thiocyanate ionselective electrode was evaluated with both liquid and solid membranes. The electromotive forces of a solid membrane electrode (commercially available bromide ion-selective electrode: Toa Model BR-125) and a liquid membrane electrode obtained according to the previous paper ], were measured using a pH meter after the constant membrane potentials were obtained. The temperature of sample solutions was controlled at 25-+0.5C, and the solutions were stirred during the measurements.
The response ranges of both electrodes were first examined and it was found that the membrane potentials of both Volume 3 No. 4 October 1981 electrodes against the logarithmic activity of the thiocyanate ion held a Nernstian relationship down to 10 -s M sodium thiocyanate solution (0.58 ppm as SCN-). However, the response range of the liquid membrane electrode was slightly wider than that of the solid one because linearity was maintained at higher concentrations.
The response rate was also examined to determine the time necessary for the membrane potentials to equilibrate. Constant responses were found at 40 and 80 sec respectively when the liquid and solid membrane electrodes were used with 10-4 M sodium thiocyanate sample solution. In this sense, the thiocyanate ion-selective electrode with liquid membrane was superior to the solid membrane. The effect of pH on the liquid and solid membrane potentials was also examined. The pH was adjusted by using sulphuric acid and sodium hydroxide. The variations of pH had no effect on the membrane potentials over the pH ranges from to 13 and from 2 to 12 against the liquid or the solid membranes. However, both electrodes were found to be affected by a high ionic strength in the sample solutions.
The effect of temperature was examined and both electrodes were found to be affected by variations of sample temperature. The results obtained by varying the sample temperature in the determination of thiocyanate agreed with those obtained by calculation.
The obtained was less than 5%, expressed as a relative standard deviation using a solid membrane electrode). It is concluded that the liquid membrane is superior to the solid membrane in this respect.
During continuous monitoring variations in pH had no effect on results. Accordingly, the pH of sample solutions need not be adjusted. Temperature should be accurately maintained at 25+0.5C to obtain satisfactory results.
Precision using the continuous monitoring process The precision of within-day and day-to-day results obtained with both electrode methods and a spectrophotometric method [3] (see Appendix), is summarised in Table 2. Using 10 -4 M sodium thiocyanate aqueous solution (5.8 ppm as SCN"), day-to-day precision with a liquid membrane electrode demonstrated worse precision than within-day one; day-to-day standard deviation ranged from 1.8 times the corresponding within-day value. Day-to-day precision with a solid membrane electrode demonstrated considerably better precision than that with a liquid membrane electrode as shown in Table 2. Furthermore, day-to-day and withinday precision obtained by the spectrophotometric method demonstrated the highest precision.
However, the author found no difference in precision using both electrodes for continuous monitoring because of poorer overall precision, whereas the spectrophotometric method in this study was considerably more precise on the batch work.
Continuous monitoring of the waste water samples As shown in Figure 1, only the thiocyanate ion-selective electrode with a zephiramine thiocyanate in DCE solution as the liquid membrane solution was applied to the continuous monitoring of thiocyanate in waste water from the Saidaiji Plant of Japan Exlan Co, Ltd. This water sample could not be monitored using a thiocyanate ion-selective electrode with a silver bromide solid membrane since it contained about 2000 ppm chloride ion from sea water.
A typical example of the results for the continuous monitoring of the waste water sample was given in the previous paper [1] where the results obtained by using both the proposed liquid membrane electrode method and the spectrophotometric method [3] were compared in detail to evaluate the reliability of the electrode method as shown in Table 3. The correlation coefficient between the electrode and spectrophotometric methods was found to be significant though the thiocyanate concentrations with the electrode method were a little higher than those with the spectrophotometric method because of a small positive In the liquid membrane electrode method described here, thiocyanate present as thiocyanate ion in such water samples could automatically be analysed without any interference from co-existing ions (at 5.8 ppm level of thiocyanate, less than 2000 ppm chloride and less than 6 ppm nitrate did not interfere with this determination as the error incurred was less than 6% which was the relative standard deviation of this method). But for water samples containing less than 8 ppm chloride and more than 6 ppm nitrate, a thiocyanate ion- selective electrode with a silver bromide or thiocyanate as the liquid membrane solution might preferably be used for continuous monitoring of thiocyanate ion.
Automatic cleaning of the electrode As shown in Figure 1, an ultrasonic wave cleaner was used to clean the electrode and its operating conditions were studied for the liquid and solid membrane thiocyanate ion-selective electrodes.
It was found that both electrodes were best cleaned by an ultrasonic wave cleaner in order to prevent contamination from suspended solids in waste water samples. When this method was not used, the electrodes had to be washed by a soft brush every day, whereas the electrodes could be used continuously for about three weeks when the ultrasonic wave cleaner was used.

Life of electrode
To examine day-to-day error, results from the control samples given by a newly constructed electrode on one day were arbitrarily chosen as being correct. Results obtained on other days were related to these expected values. When 10-4 M sodium thiocyanate aqueous solution (5.8 ppm as SCN) was used as a control sample, the membrane potential of liquid membrane electrode on day one was 108 mV. The life of the electrode was therefore established by the following criteria. The membrane potential should be within the range 103 to 113 mV and the error compared with the previous day's data should not be more than 6% (relative standard deviation of the liquid membrane electrode method). It was found that when both electrodes were operating automatically for 24 hr a day in 5.8 ppm thiocyanate control sample solution, the life of a liquid membrane electrode was 15 (variation -3 to +11) days and that of solid membrane electrode was found to be 20 (variation -6 to +10)days without any electrode washings. However, when the actual waste water sample was used, the former rose to 22 (variation -4 to +6) days and the latter to 23 (variation -5 to +4)days even when using an ultrasonic wave cleaner. Even for environmental water samples, contamination caused from suspended solids under the ultrasonic cleaner need not necessarily shorten the lifetime of both types of electrode.

Maintenance of electrode
In order to check the results, the liquid membrane electrode used in this work should be standardised against a standard sodium thiocyanate aqueous solution (10 -4 M; 5.8 ppm as SCN-) as a control sample solution every day. Since there was no provision for auto-standardisation, the standardisation was carried out manually each day to guarantee the certainty of the electrode method. If the membrane potential of a liquid membrane electrode used in the standardisation did not range from 103 to 113 mV, the electrode was checked in detail for contamination from suspended solid and so on, even though the samples were filtered with three different meshed plastic filters (first: 10 mesh, second: 40 mesh, third: 200 mesh) and the ultrasonic wave cleaner was used to prevent electrode contamination. When the electrode did not show a Nernstian relationship and did not recover, the assemblies of the liquid membrane electrode such as the porous membrane filter, the organic liquid membrane and internal reference solutions could be replaced. The solid membrane electrode however has to be exchanged for a new electrode. With the former, in general, the assemblies described above needed renewing at least once a month since the baseline in the recordings was gradually raised. With the latter, the body of the electrode should also be replaced each month as this becomes contaminated by suspended solids in environmental water samples.

Conclusion
Thiocyanate in environmental water samples could be continuously monitored with satisfactory results according to this procedure using a thiocyanate ion-selective electrode with a liquid membrane of zephiramine thiocyanate in DCE solution. A commercially available bromide ion-selective electrode with a solid membrane of silver bromide was compared with this electrode and both electrodes were then evaluated in detail for continous monitoring of thiocyanate in environmental water samples. The main conclusions reached using the proposed liquid membrane electrode are as follows: 1. The linearity range of electrode response for the liquid membrane electrode is to 112 -s M thiocyanate (slope -58 mV). For the solid membrane electrode it is 10 -x to 10 -s M thiocyanate (slope -56 mV).
2. The time needed to achieve constant membrane potentials for 10 -4 M thiocyanate (5.5 ppm as SCN-) is 40 and 80 sec for the liquid and solid membrane electrodes respectively. 3. pH variations have no effect on the membrane potentials over the pH ranges to 13 and 2 to 12 for the liquid and solid membrane electrodes respectively.  The life of electrode is 22 and 23. days for the liquid and solid membrane electrodes respectively, using an ultrasonic wave cleaner. During the continuous monitoringofthiocyanate at 5.8 ppm in actual industrial waste water samples, the maximum permissible concentrations of co-existing chloride and nitrate ions are 2000 (C1-) and 6 (NOa-) ppm and 8 (C 1-) and >3000 (NOa-) ppm for the liquid and solid membrane electrodes respectively. The effect of temperature, within-day and day-to-day precision, automatic cleaning of electrode, and maintenance of electrode were also evaluated, but the author can find no difference between the liquid and solid membrane electrodes in this respect.
The method of using a thiocyanate ion-selective electrode with liquid membrane recommended here seems to be superior to that of using a cyanide ion-selective electrode for the determination of thiocyanate since the latter has a three stop procedure for a quantitative transformation of thiocyanate into cyanide 4].
It is therefore concluded that for continuous monitoring of thiocyanate in environmental water samples such as natural and waste waters containing less than 2000 ppm chloride and a few ppm nitrate only a liquid membrane electrode gives satisfactory results.

APPENDIX
The chemistry of the spectrophotometric method for the determination of thiocyanate [3]  the text but all authors should be named in the list of references. When reference is made to a chapter in a book the reference should take the following form: [7] Malmstadt, H.V. in "Topics in Automatic Chemistry" Ed. Stockwell P.B. and Foreman J.K. 1978 Horwood, Chichester, pp. 68-70. Only work which has been published or has been accepted for publication should be cited. Avoid the citation of documents which are subject to restricted circulation, patent literature, unpublished work and personal communications. The latter can be mentioned in the text in parenthesis.
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