GB2233768A - Ion selective sensor with failure indication - Google Patents

Abstract

Failure of an ion selective sensor is indicated when the sensor output falls outside a predetermined range. A pH sensor is described. It comprises a pH selective glass bulb 18 with silver/silver chloride electrode 24 in potassium acid phthalate reference electrolyte 20, and a reference electrode of silver/silver chloride 36 in standard reference electrolyte 32 connected to the sensing volume 50 by liquid junction 30. Amplifier 44, connected to the sensor output is programmed to give a normal output for pH range 6.5-8.2. If a zero voltage output occurs, corresponding to a pH of 5.5 and probable rupture of tube 16, signal circuit 46 produces an output. <IMAGE>

Description

SAFE FAILURE ION SELECTIVE COMBINATION ELECTRODES This invention relates

to ion selective combination electrodes, and more particularly to such devices which can detect and signal their own breakdown.

General purpose pH combination electrodes that operate over the full pH range and provide an output of about minus 60 millivolts (mV) per unit of pH, output at pH 7.0 being zero mV are known. It is known also to shift the zero mV point, as to pH 4. 0 or 10. 0, to avoid automatic temperature compensating metering circuits, by varying internal solution formulation.

We have now discovered that an ion selective combination electrode in which a predetermined output (preferably zel.70 voltage) corresponds to ion concentrations, s,,.i(,,h as pH, outside a desired range makes it possible to detect an electrode's internal breakdown.

The present invention provides an ion selective combination elec.trode comprising: an ion selective electrode, said electrode including a container, a liquid in said container, and an ion sensitive element in said liquid; a referenc.e electrode; and a control unit, said unit being constructed to provide a signal if the output from the electrOCIE.'3 is outside a predetermined range.

In preferred embodiments, sensitivity is to pH. Also in preferred embodiments, the output from the electrodes giving rise to the signal from the control unit is zero voltage.

The in-,,cjltion is hereinafter described by way of example only, wit:h ref6rence to the single figure of drawings, which is a diagrammatic representation of an embodiment of elec-crode according to the invention.

Ion selective combination electrode 10 has as two functional parts an ion selective electrode indicated generally at 12 and a reference electrode indicated generally at 14.

Ion selc-ictive electrode 12 has a glass stem 16 in - 2 communication with ion selective glass bulb 18. Within the vessel defined by stem 16 and bulb 18 is solution 20, in which hang silver wire 22 with dependent therefrom. silver chloride billet 24. Inner solution 20 is a pH 5.0 solution prepared by adding Potassium Acide Phthalate (Merck, AR Primary StandARd), 0.05 mols phthalate, to 1.8 mols of potassium chloride, the whole made up to a one liter quantity with deionized water, and adjusted to its final p11 with NaOH. Seal 26 provides a seal around tubular portion 16 and wire 22.

Bushing 28, which has liquid junction 30 extending therethrough, is an annulus which positions electrode 12 within electrode 14. The latter contains, above bushing 28, a standard prior art reference electrolyte solution 32. Silver wire 34 extends into solution 32, and has dependent from it silver chloride billet 36. Seal 38, which is annular, seals wire 34 and both electrodes therearound.

Container 40 is shown with, within it, a solution 42 the p11 of which is being monitored.

Wires 22 and 34 are connected with amplifier 44, which provides a voltage output reflecting the input from the two wires. The unit indicated diagrammatically and generally at 46 has control and alarm circuits and provides a warning signal if voltage goes to zero; the unit 46 is programmed so that predetermined desired voltages are in the pH range of 6.5 to 8.2, while zero voltage is at pH 5.5. Outer wall 48 provides both the wall for reference electrode 14 and a zone 50. whereat solution being tested can be in contact with bulb 18 and junction 30, for sealed electrical contact therethrough (through holes not shown). 30 The f01101,7ing chart compares the preferred embodiment and prior art above referred to:

pH of a solution 4.0 5.0 being tested Preferred Example safety range Preferred Example << low pH alarm range Preferred Example high pH alarm range Prior Art electrode MV output

Prior Art electrode failure point

Preferred Example electrode mV output Preferred Example electrode failure point Operation 120 30 0 -30 X 1 (p11 5.5) 1 6.0 7.0 (6.5) 0 1 X (pH 7. 0) 8.0 9.0 -60: -120 In normal use, with the combination electrode 10 in the test liquid 42, 1)11 is within the safe range set into unit 46, and is therein monitored. If tube 16. -- ruptures, voltage goes to zero, which in the invention is outside the safe or predetermined operating range, and a signal circuit in unit 46 gives its signal. The signal is also given at other voltages outside the predeterminedly desired range.

Claims (4)

Claims Other embodiments within the following claims will appear to those skilled in the art. What is clalmed.is: -210 CLAIMS:

1. An ion selective combination electrode comprising: an ion selective electrode, said electrode including a container, a liquid in said container, and an ion sensitive element in said liquid; a reference electrode; and a control unit, said unit being constructed to provide a signal if the output from the electrodes is outside a predetermined range.

2. The combination electrode of claim 1 in which sensitivity is to pH.

3. The combination of Claim 1 or Claim 2 in which said output is zero voltage, and zero voltage is outside said range.

4. An ion selective combination electrode substantially as hereinbefore described, with reference to and as shown in the drawings.

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