Chemistry: electrical and wave energy – Apparatus – Electrolytic
Patent
1993-04-05
1994-09-27
Niebling, John
Chemistry: electrical and wave energy
Apparatus
Electrolytic
204412, 204415, 204130, 204132, 204252, 324 711, 324425, 324450, G01N 2726
Patent
active
053505000
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a measuring cell of the kind used in the measurement of an electrokinetic potential, for example of a fibrous or granular dispersion, and to a measuring system incorporating such a cell. It also relates to a measurement system for the measurement of an electrokinetic potential incorporating such a cell, and to a method of maintaining the condition of a measurement electrode or a pair of electrodes in such a cell.
More particularly, electrical characteristics due to electrokinetic phenomena are of industrial interest. Electrokinetic potentials are of importance in so far as they occur in fibrous and granular dispersions, such as stock for making paper, mineral ores or flocculated sewage sludge. Paper stock is a dispersion of cellulose fibres, together with fillers and other additives, in water. In paper making a particular manifestation of an electrokinetic potential is Zeta potential which is conventionally measured by way of another potential called streaming potential. European Patent EP B1 0 079 726 (Wiggins Teape) describes the construction of a measuring cell for the measurement of streaming potential, and the arrangement of the complete cell system including flow circuitry and manner of operation with a view to obtaining consistent and reliable measurement from the feed stock or furnish as supplied to the wire of a paper-making machine.
BACKGROUND OF THE INVENTION
FIG. 1 of EP B1 079 726 shows the main elements of the described measuring cell. The cell cavity is divided into two compartments by a mesh. As illustrated the mesh is horizontal, providing upper and lower compartments. A respective measuring electrode projects into each compartment. The operation of the cell in brief is that the paper stock is fed to the lower compartment to build a pad of fibrous material on the underside of the mesh, the cell being generally full of the water component of the stock. The pad is then subject to controlled pressure conditions across the pad and a potential difference measurement is made between the electrodes at the established pressures to obtain a measurement of streaming potential. This is discussed in detail in the prior patent.
The described measurement cell and its manner of operation are considered to provide a substantial advance in the art of measuring streaming potential, particularly in providing an effective means of on-line measurement on a paper-making machine. The cell particularly described in the patent utilises stainless steel electrodes. It was found that in long-term use, irregularities and drifts could arise in measurements made with the cell. Investigation has revealed that these irregularities were related to an electrochemical potential effects arising from oxidation of the stainless steel. The nature of the surface oxide film that forms on stainless steel is very variable.
In seeking for alternative materials it has also been found that avoiding the build up of an oxide film is not necessarily the answer. The present investigation has shown that there may also be a capacitance effect at the electrode/water interface which causes drift in potential measurement. This capacitance is very high. Measurements on gold electrodes, which do not readily oxidize, have indicated a capacitance value of 20 .mu.F/cm.sup.2.
This capacitance is a direct consequence of polarization of the electrode in the stock being measured. There is an inevitable but quantifiable polarization in tap water. However, polarization effects also result from chemicals being absorbed onto the electrodes from the stock being measured. These are not quantifiable, particularly having regard to the wide variation that occurs in the constituents of paper-stock. It has been found that gold electrodes suffer from such absorption. The electrode then forms the equivalent of a polarized electrolytic capacitor with an ionised layer in the stock within a few molecules of the electrode surface. This gives rise to the high capacitance per unit area quoted above and the better the qu
REFERENCES:
patent: 4535285 (1985-08-01), Evans et al.
Bell Bruce F.
Niebling John
The Wiggins Teape Group Limited
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