This invention relates to the simultaneous measurement of the concentration of a selected ion species in a solution and the pH of the solution. The invention particularly, though not exclusively, relates to photographic solutions, and particularly, though not exclusively, where the selected ion species is silver. In general, however, the invention relates to the simultaneous potentiometric measurement of the concentration of any ion species in a solution and measurement of the pH of the solution using an ISFET (Ion Selective Field Effect Transistor).
For the present purpose the tern xe2x80x9csolutionxe2x80x9d is to be understood as also including an emulsion, for example a mixture of a silver compound suspended in gelatin, or a dispersion. The invention will be particularly described, by way of example only, with reference to photographic solutions.
It is known simultaneously to measure silver ion concentration in, and the pH of, an aqueous solution. In one arrangement, a single reference electrode is connected into a first potentiometer circuit with a conventional glass pH electrode, and is connected into a second potentiometer circuit with a conventional silver electrode, all three electrodes being immersed in the solution. In another arrangement, an ISFET is used instead of the glass pH electrode. This necessitates the use of a separate reference electrode for each measuring circuit in order to provide electrical isolation between the circuits since the ISFET is a current carrying device whose presence would otherwise interfere with the voltage measurement of the silver electrode.
A glass pH electrode has the disadvantage that it can be damaged under conditions of high temperature and high pH, so that its readings become unreliable or inconsistent. An ISFET overcomes this disadvantage. However, the conventional arrangement including an ISFET described above is complicated by the requirement of the additional reference electrode, especially when applied in a large scale production vessel, as used in the preparation of photographic emulsions for example, where the electrodes are configured in a unitary probe. This can lead to difficulties for maintenance and for calibration. Furthermore, existing probe structures would require extensive modification to accommodate the additional reference electrode, which would be expensive.
It will be appreciated that if, on the other hand, measurement of ion concentration and pH were not required simultaneously, then the measurements would not interfere with each other and a single reference electrode could be used successively in combination with an ion concentration electrode and an ISFET.
In accordance with one aspect of the present invention, there is provided apparatus for simultaneously measuring the concentration of a selected ion species in a solution and the pH of the solution, comprising: a first electrical circuit that is arranged to receive signals from both a reference electrode and an ion selective electrode immersed in the solution and to derive therefrom an output signal representative of the concentration of the selected ion in the solution; a second electrical circuit that is arranged to receive signals from both said reference electrode and an ISFET immersed in the solution and to derive therefrom an output signal representative of the pH of the solution; wherein any d.c. input signal to said first electrical circuit from the reference electrode is substantially electrically isolated from the input of the second circuit; wherein a signal representative of the voltage, usually earth potential, of the solution is supplied (a) directly to the first circuit so as to establish a reference, usually earth, potential for the first circuit, and (b) to the second circuit through a.c. coupling means so as to establish a corresponding virtual reference, usually earth, potential for the second circuit; and wherein the first and second electrical circuits are arranged to be provided with electrical power from supplies that are electrically isolated from each other.
The apparatus may comprise means for displaying a representation of said ion concentration and pH output signals, wherein said second electrical circuit includes an isolation amplifier, and wherein said display means is arranged to receive said pH output signal of the second circuit through the isolation amplifier. Preferably, the apparatus includes a further isolation amplifier through which the ion concentration output signal of the first circuit is supplied to the display means. Advantageously, the apparatus comprises a low pass filter, wherein said pH output signal from the second electrical circuit is arranged to be passed to the display means through the low pass filter.
Preferably, the apparatus comprises a high value resistor, for example of about 1 Mxcexa9 or greater, that is arranged to effect said electrical isolation of d.c. input signals to said first and second electrical circuits. Also said a.c. coupling means may comprise a high value capacitor, for example of about 1 xcexcF or greater.
In accordance with another aspect of the present invention, there is provided a method of simultaneously measuring the concentration of a selected ion species in a solution and the pH of the solution, comprising the steps of: measuring in a first electrical circuit the potential difference between an ion selective electrode and a reference electrode both immersed in the solution, and deriving therefrom the concentration of the ions in the solution; measuring in a second electrical circuit the current flowing between an ISFET and the reference electrode both immersed in the solution, and deriving therefrom the pH of the solution; connecting the reference electrode to the first and second electrical circuits such that any d.c. signal from the reference electrode is electrically isolated from the second circuit; making an electrical connection between the solution and the first circuit so as to provide the solution potential as a reference, preferably earth, potential therefore, and making an electrical connection between the solution and the second circuit through a.c. coupling means so as to provide a corresponding virtual reference, preferably earth, potential therefore; and supplying the first and second circuits with electrical power from sources that are electrically isolated from each other.
The method of the invention is advantageously carried out using the apparatus of the invention.
Details of electrodes suitable for use in the present invention as ion selective and reference electrodes, and of ISFETs, can be found in the book xe2x80x9cpH Measurementxe2x80x9d by Helmuth Galster (VCH,1991).
The electrical isolation of the two circuits provided in the present invention allows an ISFET to be used in the pH measuring circuit, whilst needing only a single, common, reference electrode. The disadvantages of the known arrangements for simultaneous ion concentration and pH measurement are thus overcome in a particularly convenient manner.
The isolation is provided at several stages. Initially this is done by arranging that the signal from the reference electrode is used in the ion concentration circuit as a potentiometric measurement, and is supplied to the pH measuring circuit only as an a.c. input, i.e. after having any d.c. component isolated therefrom. An actual reference potential, the potential, usually earth, of the solution, is applied to the first circuit, and a virtual reference potential derived therefrom is applied to the second circuit. The two circuits have separate isolated power supplies. Furthermore, when the resulting ion concentration and pH signals are supplied to a display means, such as a multi-channel voltmeter, this is done through respective isolation amplifiers, which are preferably supplied from a third, isolated power supply.
The ability to use a single reference electrode means that a single, unitary measurement probe can be constructed, in which the ISFET can be installed relatively easily along with the ion selective and reference electrodes. Furthermore, the measuring apparatus can be calibrated more easily than is the case with the known arrangement using two reference electrodes.
Although described with reference to a single ion selective electrode and a single ISFET, it is envisaged that the present invention may comprise two or more ion concentration electrodes and/or two or more ISFETs, each type of electrode being connected into the respective first or second electrical circuit.
Apparatus for, and a method of, simultaneously measuring the concentration of a selected ion species in a solution and the pH of the solution, will now be described, by way of example, with reference to the accompanying schematic circuit diagram.