Conventional Ag/AgX electrodes, responsive to halide X, cannot be used to monitor the addition of a second halide Y since such additions result in a slow chemical conversion of the macroscopic halide coating AgX to AgY. This is a serious problem in the manufacture of photographic emulsions that frequently contain more than one silver halide. The thesis describes a new electrochemical measurement technique with the ability to make appropriate determinations in solutions of mixed halides. In the new technique (termed "clean/coat/measure"), silver electrodes were prepared "in situ" by applying square wave pulses to the electrode. First the previous halide layer was removed, then the electrode was coated in situ with a new layer of silver halide and this was used to measure the open circuit potential before the cycle was repeated. In this way the halide coating reflected the composition of the measurement solution. Existing commercial instrumentation was inappropriate for the proposed measurement sequence. Thus, a range of instrument hardware and software was designed and built by the author and used to study the influences of a multitude of parameters on the measurement performance. 1. A stable and accurate measurement system was designed and fabricated allowing the potentials of eight electrodes to be measured simultaneously in grounded solutions. Data was collected and stored on a PC using custom written software. Calibration curves for conventional silver/silver chloride, bromide and iodide electrodes were obtained over a range of concentrations and temperatures. Silver/silver halides electrodes with small surface areas (< 9 mm2) and thin halide coatings (< 1 nm thick) were studied to ensure that such electrodes performed as conventional large, thickly coated electrodes. Calibration curves showed no deterioration of response due to small surface areas and, over short time scales (< 2 min), no deterioration due to thin layers. 2. A laboratory instrument was designed and built to apply potential pulses, control a rotating disc electrode (RDE) and collect data. The system allowed both controlled potential pulses to be applied to the electrodes and open circuit potentiometric measurements to be made. Measurements of potential and current were collected at a rate of 10,000 measurements per second. The system used custom software running on a PC to control the instrumentation and to store data on the PC. Using this instrumentation a RDE was used to study the new "clean/coat/measure" pulsed technique. Results from the RDE study indicated that an electrode capable of sensing halide could be produced by this technique if an applied potential pulse with sufficient charge was applied. This minimum charge (11 x w-s C cm-2) produced a coating thickness approximately equivalent to a monolayer. The study also indicated that the technique was independent of the speed of rotation of the silver electrode and was successful over a wide range of conditions of pulse time, applied potential and cycle times for solution of potassium bromide in the range 0.001 to 0.05 M. The technique also successfully measured the addition of potassium iodide to a solution of potassium bromide while conventional thickly coated electrodes did not. 3. Two further instrumentation systems were designed and built to be used in a grounded stainless steel emulsion making vessel , one to apply controlled potential pulses and one to apply constant current pulses. Using these instruments and the conditions found for the RDE, static cylindrical electrodes in stirred solutions were investigated using both controlled potential and constant current square wave pulses of between 50 and 500 ms. Both potential step and current step techniques successfully measured the halide concentration of solutions of potassium chloride and bromide (0.001 to 0.5 M) and potassium iodide (0.0001 to 0.5 M). Both methods were also shown to be able to successfully monitor the addition of iodide to bromide and chloride solutions. With respect to future work, modifications to the instrumentation are proposed, including the replacement of the PC by an on-board microprocessor, the design of a multi-channel system and use of intelligent software to determine the optimum potential or current to apply. Areas of work required to be carried out before the system could be used in a production environment are given.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:720672 |
| Date | January 2003 |
| Creators | Edwards, Stephen John |
| Publisher | University of Bedfordshire |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/10547/622158 |
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