Chemistry: electrical and wave energy – Apparatus – Electrolytic
Patent
1993-02-11
1994-04-19
Niebling, John
Chemistry: electrical and wave energy
Apparatus
Electrolytic
204416, 204435, 20415321, 2041829, G01N 2726
Patent
active
053042964
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to sensors which include an external reference electrode and a glass pH electrode, in particular a glass pH electrode with improved temperature characteristics.
Currently available sensors include glass electrodes which have characteristics that do not vary linearly with temperature, thus limiting the range over which the currently available pH meters can apply a temperature correction with a specified accuracy.
A pH meter is a voltmeter that measures the D.C. potential between a glass pH electrode and a reference electrode and converts the measurement so obtained into a pH value. The sensing and reference electrodes may be combined into a single unit or pH probe. The reference electrode supplies a known and stable reference potential against which the potential of the glass electrode may be measured. The difference between the glass electrode potential and the reference electrode potential varies in a manner which is known from the Nernst equation. The potential difference is a function of hydrogen ion activity and the "slope factor" which varies according to the Nernst equation.
The temperature compensation of pH measurements with glass electrodes is becoming of increasing importance as more measurements are made outside the laboratory, e.g. in environmental work, where temperatures in the range of from 0.degree. to 10.degree. C. are common, and in flue gas desulphurization liquors, where the temperature may be .gtoreq.50.degree. C.
The ideal characteristics of a potentiometric pH cell (given that a non-zero temperature coefficient is inevitable) are listed below:
1. Slope factor (k) varies linearly with temperature, in accordance with the theoretical Nernst slope factor ##EQU1## where R is the gas constant, T is the absolute temperature and F the Faraday constant.
2. The standard potential (E.degree.) varies linearly with temperature.
3. The e.m.f. is independent of temperature at pH 7, i.e. the isopotential pH (pH.sub.iso) has the value 7.0.
4. The pH at which the cell e.m.f. is zero should be 7.0.
5. The cell should have a low thermal capacity, enabling temperature equilibrium to be reached quickly.
6. The response to changes in temperature should be monotonic, i.e. if individual components of the cell have temperature coefficients of opposite sign, the design of the cell should not permit these components to change in temperature at such different rates that the overall cell e.m.f. changes direction en route to its new equilibrium value.
7. The system should not exhibit thermal hysteresis.
Apart from these characteristics, a pH cell must also satisfy the specification for measurement at constant temperature, e.g. precision, accuracy, range and freedom from interference.
Whilst characteristic (1) can be achieved and characteristic (4) presents relatively few problems in isolation, realizing characteristics (2) to (4) simultaneously is more difficult.
Characteristic (2) is rarely achieved in practice, because of the chemistry of the glass electrode's internal filling solution. In most cases this characteristic is assumed to be approximately achieved over a range of about 20.degree. C.
Characteristics (5), (6) and (7) generally receive a low priority in the design of electrodes.
We have now developed a sensor including a glass pH electrode in which the linearity of the change in standard potential (characteristic (2)) is optimised, whilst generally satisfying the requirements of characteristics (3) and (4).
Accordingly, the present invention provides a sensor which includes an internal reference electrode and a glass pH electrode, the glass pH electrode comprising a silver-silver chloride or a mercury-mercurous chloride inner electrode and an electrode filling solution for the inner electrode, which electrode filling solution is a zwitterionic buffer solution containing chloride ions which is chosen so as to satisfy as nearly as possible one of the following external reference electrode is a non-isothermal calomel reference electrode the solution fulfills the requirements t
REFERENCES:
patent: 4175020 (1979-11-01), Janata et al.
patent: 4279724 (1981-07-01), Hearn et al.
patent: 4913793 (1990-03-01), Leonard
Bell Bruce F.
National Power PLC
Niebling John
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