GB2074323A - Apparatus for Detecting Hydrogen Cyanide Gas - Google Patents

Abstract

Apparatus for detecting hydrogen cyanide includes a a cyanide ion selective electrode of the air gap type 13, a standard reference electrode 30, at least one salt bridge 27, 29, electrically connecting the ion selective electrode to the standard reference electrode, the ion selective electrode being suspended in a gas tight container which is provided with a gas inlet opening and a gas outlet opening, means 19 for providing a regular flow of an electrolyte onto the ion selective electrode and means for detecting a potential difference between the ion selective electrode and the standard reference electrode. Preferably the electrolyte is sodium hydroxide or potassium hydroxide and the cyanide electrode is a silver/silver salt electrode, Ag/Agl and Ag/Ag2S being preferred. The potential difference may be fed to a comparator, the output of which is connected to drive a visible or audible alarm. <IMAGE>

Description

SPECIFICATION Apparatus for the Detection of Hydrogen Cyanide Gas and Methods of Using the Apparatus The present invention relates to apparatus for the detection of hydrogen cyanide gas and to methods of using the apparatus.

Certain gases may be detected by means of a suitable gas sensing probe responsive to ions formed from the gas on interaction with an electrolyte solution wetting the probe. Such a probe forms one half of an electrochemical cell. A standard reference electrode forms the other half of the cell and is commonly connected to the first half by a salt bridge in contact with the wet surface of the probe. Typically the reference electrode may be a saturated calomel electrode or a silver/silver chloride electrode. The reference electrode supplies a constant and reproducible potential. The salt bridge is filled with a solution of a strong electrolyte which contains no ions which would influence the potential of either electrode. It is sometimes necessary to employ a double salt bridge in order to satisfy this requirement.The probe is commonly an ionselective electrode, the potential of which depends on the activities of the ions of the gas concerned.

Theoretically the potential of the cell measured by a very high input impedance meter is, for theanion of the gas given by

Equation 1 where Em is the measured potential.

E0, is the conditional potential for a given concentration of anions.

[X"-] is the concentration of the anion xn- in mol 1-1.

T is the Kelvin temperature.

R is the gas constant.

Nx is the number of electrons transferred in the electrode process.

F is the Faraday constant.

Er is the reference potential.

Ej is the potential of the liquid junctions.

The value of E01 changes if the conditions are changed and 2.3 RT/NxF is the slope factor which will also change if the conditions are changed. Under constant conditions the liquid junction potential of the salt bridge is constant, so that Equation 1 may be written as: Em=A+S log,,IX"-] Equation 2 where A is a constant.

S is the slope factor.

The constant A and the slope factor S may be determined by calibration.

The ion selective electrodes are generally of two types: those with 3 thin membrane and those without a membrane, the latter being known as "air gap" electrodes.

lon selective electrodes of both types are known for the detection of CO2, NH3, SO2 and the nitrogen oxides. These depend on the interaction of the gas with an ionisation of the gas in water or a buffer solution which produces hydrogen ions and the change in the latter is non-selectively detected with a pH glass electrode. Ion selective electrodes of both types are also known for the detection of HSS which is ionised in a strong alkaline solution and the sulphide ions are detected with a silver/silver sulphide ion selective electrode. Electrodes of the membrane type are known for the detection of cyanide ions using silver/silver iodide and silver/silver sulphide electrodes.

In accordance with a first aspect of the present invention apparatus for the detection of hydrogen cyanide includes (a) a ON- ion selective electrode of the air gap type, (b) a standard reference electrode, (c) at least one salt bridge electrically connecting the ion selective electrode to the standard reference electrode, (d) a gas tight container provided with a gas inlet opening, and a gas outlet opening, the ion selective electrode being suspended in the container, (e) means for providing a regular flow of electrolyte onto the ion selective electrode, and (f) means for detecting a potential difference between the ion selective electrode and the standard reference electrode when a gas in in contact with the ion selective electrode.

Advantageously the apparatus is provided with means for supplying a continuous flow of gas across the ion selective electrode via the gas inlet opening and the gas outlet opening. Preferably the ion selective electrode is suspended in the container in such a way that it is directly in the gas flow between the gas inlet and the gas outlet openings of the container and it is therefore advantageous to provide the gas inlet and outlet openings in opposite walls of the container. Such apparatus is capable of continuously monitoring a gas flow for hydrogen cyanide.

It is preferred that the ion selective air gap electrode is a silver/silver salt electrode which advantageously comprises silver/silver iodide or silver/silver sulphide.

The layer of inorganic salt is usually porous and allows the sample gas to reach the base metal. In general when the electrodes were surrounded by an electrolyte containing the sample, interfacial potentials are developed which are associated with ion-exchange processes at the inorganic layer surfaces,Also diffusion potentials arise due to the diffusion of various ionic species within the layer. The iodide or sulphide layer itself takes no important part in the potential determining processes and only serves to ensure the prevalence of optimum conditions at the electrode surface for the solubility equilibrium between 1 or S2- ions and silver ions to be established.

Preferably the electrolyte is sodium hydroxide solution or potassium hydroxide solution.

The preferred means of providing a regular flow of electrolyte includes an open ended narrow bore tube arranged so that the open end is adjacent to and directed at the tip of the ion selective electrode. Preferably the open ended tube is supplied with electrolyte via a flexible tube from a reservoir containing the electrolyte. It is advantageous for the rate of flow of the electrolyte to be approximately 1 drip per 50 seconds, although the rate of flow of electrolyte may be adjusted to suit the rate of flow of the gas through the apparatus. It is preferred that means for restricting or increasing the flow of electrolyte is therefore provided between the reservoir and the open ended tube.

Maintaining a regular flow of electrolyte over the ion selective electrode ensures that the electrolyte/gas solution is regularly washed off the electrode by fresh electrolyte. This obviates an undesirable accumulation of cyanide ions in a single drop of electrolyte surrounding the electrode which would give rise to a steady increase in the potential difference while the flow of gas containing hydrogen cyanide is maintained.

Therefore by maintaining a regular flow of electrolyte to the ion selective electrode a measure of the concentration of hydrogen cyanide in the air flow may be determined.

An alternative to using a free flowing liquid electrolyte is to provide the electrolyte in the form of a viscous liquid or gel, the viscosity of which may be tailored to provide a continuous supply at whatever rate is desired.

Since the electrolyte will drip off the ion selective electrode onto the floor of the container it is advantageous'to provide a means for emptying the container, for example a weir.

The reference electrode may be connected to the ion selective electrode via a salt bridge comprising a glass capillary tube containing an electrolyte, for example potassium chloride solution, one end of the bridge being in continuous contact with the film of electrolyte surrounding the ion selective electrode.

It is preferred that the reference electrode is connected to the ion selective electrode via a double salt bridge comprising a potassium nitrate solution bridge and a potassium chloride solution bridge, the former in contact with the ion selective electrode and the latter in contact with the reference electrode. Contact between the two bridges may be effected by means of an electrolyte transfer vessel containing potassium nitrate solution.

When a double salt bridge comprising glass capillary tubes filled with electrolytes is used it is preferable that the ends of the capillaries in contact with the potassium nitrate contained in the electrolyte transfer vessel are plugged with porous plugs such as micro ceramic plugs of low leakage rate to minimise the flow of the solutions contained in the bridges comprising the double salt bridge. Alternatively an inert gel may be used.

When potassium nitrate solution is used in a salt bridge and in the electrolyte transfer vessel it is preferred that the potassium nitrate solution has a concentration of 0.01 M.

The potentials of the reference electrode and the liquid junction between the reference electrode and the ion selective electrode may be assumed to be constant although potassium chloride solution gives rise to small liquid junction potentials at the point of contact with the potassium nitrate solution in the electrolyte transfer vessel. Similarly a liquid junction potential exists at the tip of the ion selective electrode where the potassium nitrate comes into contact with the sodium hydroxide solution. Any change in liquid junction potentials with varying cyanide ion concentration constitutes an error.

In accordance with a second aspect of the present invention a method of detecting hydrogen cyanide using the apparatus of the first aspect includes (a) presenting a gas in contact with the ion selective electrode, (b) supplying the ion selective electrode with a regular flow of an electrolyte, (c) dissolving and ionising any hydrogen cyanide present in the electrolyte surrounding the electrode and (d) detecting the potential difference between the ion selective electrode and the standard reference electrode.

It is preferable to pass a flow of gas over the ion selective electrode via the gas inlet and outlet openings provided in the apparatus by any suitable means eg. a pump.

It is preferable that the rate of flow of gas over the electrode is constant whilst the apparatus is running, otherwise undesirable fluctuations in potential difference between the electrodes will result. Similarly it is advantageous for the electrolyte to be supplied at a constant rate whilst the apparatus is running because of the accumulative effect described above. Different rates of flow of gas and different rates of supply of electrolyte may be employed, depending on the amount of hydrogen cyanide present in the air flow, to avoid the possibility of an increase in the potential difference due to the accumulative effect described above. One or both of the rates may be altered to achieve the most accurate and consistent results and it is advantageous to allow the apparatus to achieve equilibrium each time one or both rates of flow are altered.

A potential difference between the ion selective electrode and the standard reference electrode may be detected by any suitable means, for example, a pH meter connected across the two electrodes or a low impedance millivoltmeter, but it is preferred that an electrical comparator circuit containing an alarm indicator is employed.

In accordance with a third aspect of the present invention apparatus for the detection of hydrogen cyanide according to the first aspect and wherein the means for detecting a potential difference between the ion selective electrode and the standard reference electrode includes (a) means for generating a reference voltage, (b) comparator means for comparing the reference voltage with a potential difference between the two electrodes when a gas in in contact with the ion selective electrode, (c) a switching device operable by an output signal from the comparator means and (d) an alarm operable by an output from the switching device.

The means for producing a reference voltage preferably includes an amplifier using 100% negative feedback connected in series with a variable resistance which may be set to the precise voltage corresponding to a concentration of cyanide ions at which it is desired to produce an output from the comparator means to the switching device which triggers the alarm.

Advantageously the comparator means for comparing the reference voltage with the potential difference between the ion selective electrode and the standard reference electrode includes an amplifier operating without feedback.

The switching device may be any suitable electrical switch preferably it is a thyristor. It is advantageous to incorporate a reset switch when using a thyristor as a triggering means in order to break the circuit to stop the alarm once it has been triggered.

The alarm may be any suitable device designed to give a visual and/or audio signal when triggered electrically, for example an electric lamp, an electric bell, or a piezoceramic vibrator.

A visual display device such as a chart recorder or a cathode ray oscilloscope may also be connected across the electrodes.

In accordance with a fourth aspect of the present invention a method of detecting hydrogen cyanide using the apparatus of the third aspect includes (a) presenting a gas in contact with the ion selective electrode, (b) supplying the ion selective electrode with a regular flow of an electrolyte, (c) dissolving and ionising any hydrogen cyanide present in the electrolyte surrounding the electrode, (d) detecting the potential difference between the ion selective electrode and the standard reference electrode when a gas is in contact with the ion selective electrode, (e) comparing potential difference with the reference voltage and (f) triggering the alarm when the potential difference between the two electrodes exceeds the reference voltage.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a partly schematic cross-sectional view of apparatus for the detection of hydrogen cyanide gas in an airflow, Figure 2 is a cross-sectional view of an electrode assembly shown in Figure 1, Figure 3 is a graph showing a calibration plot of potential change versus logXO [No of ,u moles HCN per cm3 of gas] for the purpose of calibrating a silver/silver sulphide electrode.

Figure 4 is a schematic representation of an electrical alarm circuit.

In Figure 1 a chamber 5 is provided with a gas inlet opening 7, a gas outlet opening 8 and an excess electrolyte drain 9. The lower surface 25 of a cover 2 has a recess 4 along its perimeter and an O-ring 6 is placed in the recess 4 such that when the cover 2 is placed on the chamber 5 the wall 12 of the chamber 5 fits into the recess 4 of the cover 2 and the O-ring 6 is squeezed between the inner surface 11 of the wall 12 and the recess 4 and by so doing effects a gas tight seal.

The cover 2 is provided with an opening 3 through which an electrode assembly 1 is passed until a shoulder 10 provided on the electrode assembly 1 rests on the cover 2. By virtue of this arrangement an electrode 13 projecting from the lower part of the assembly 1 is suspended in the chamber 5 between the gas inlet opening 7 and the gas outlet opening 8.

In Figure 2 the electrode assembly 1 is shown in more detail to consist of an upper electrode holder 1 5 which is fitted on a lower electrode holder 1 7 by means of a screw thread 16. An electrode 1 3 is provided with a terminal twist 14 forming an arc of a circle and is push fitted through a silicone rubber plug 21 which is in turn fitted inside the upper electrode holder 1 5. A narrow bore tube 18 has an open end 22 bent towards the electrode 13 maintaining a gap 23 between the open end 22 of the tube 18 and the electrode 13 and is also push fitted through the plug 21.

A salt bridge 27 comprising a glass capillary tube 29 filled with potassium nitrate solution is push fitted through the plug 21. The capillary tube 29 is positioned in the plug 21 adjacent to the ion selective electrode 13 so that the open end of the capillary tube 29 maintains contact with the surface of the electrode 13. One end of the capillary tube 29 is plugged by a porous ceramic plug 33 and is immersed in an electrolyte transfer vessel 28 (shown schematically) filled with potassium nitrate solution. One end of a second salt bridge 29 filled with potassium chloride solution is connected to a standard reference electrode 30 (shown schematically). One end of the salt bridge 29 is plugged with a porous ceramic plug 34 and immersed in the potassium nitrate contained in the electrolyte transfer vessel 28.Electrical contact between the ion selective electrode 1 3 and the reference electrode 30 is thus effected via the salt bridges 27 and 29 and the electrolyte transfer vessel 28.

A flexible tube 1 9 connects the narrow bore tube 1 8 to an electrolyte reservoir 25 (shown schematically) via a flow regulator 26 (shown schematically).

Means for detecting a potential difference, described more fully below with reference to Figure 4, is shown schematically as a pH meter in Figure 2.

A pH meter 32 is electrically connected across the electrode 13 and the electrode 30 via insulated screened wires 20 and 31.

A gas flow containing a sample of hydrogen cyanide gas is maintained through the chamber 5 via the openings 7 and 8, by means of a pump (not shown). A regular flow of electrolyte is maintained over the tip of the ion selective electrode 13 via the narrow bore tube 18. The electrolyte surrounding the electrode 13 firstly dissolves the hydrogen cyanide and secondly ionizes the gas into negatively charged cyanide ions which are neutralised by the electrolyte and positively charged hydrogen ions. The cyanide ions migrate towards the metal component of the electrode 13 creating a potential difference between the ion selective electrode 13 and the standard reference electrode 23 which provides an input to the circuit shown in Figure 4.

The salt bridge 27 containing potassium nitrate solution and the salt bridge 29 containing potassium chloride solution maintain electrical contact between the ion selective electrode 13 and the reference electrode 30 via the electrolyte transfer vessel 28.

In Figure 4 a screened input lead 39 from the gas detection apparatus 41 (shown schematically) as shown in Figures 1 and 2 is connected to a comparator 35. A reference voltage generator 40 supplies a reference voltage to the comparator 35. A switching device 38 eg a thyristor is connected to the output from the comparator 35. An alarm 37 is connected to the output from the switching device 38. The alarm 37 may be one of the specific alarms mentioned above. A manual reset switch 36 is also connected to the alarm 37 to enable the alarm 37 to be turned off once it has been triggered.

An input from the gas detection apparatus 41 is compared with a reference voltage by the comparator 35. When the input to the comparator 35 from the gas sensing probe 41 exceeds the reference voltage an output from the comparator 35 operates the switch 38 allowing the alarm 37 to be operated by the comparator output. The reset switch 36 may be used to manually break the circuit in order to turn off the alarm.

Examples of the use of the apparatus of the forms shown in Figures 1,2 and 4 are as follows: Example 1 An ion selective electrode was prepared by taking a length of 22 SWG mint grade 5 silver wire supplied by Johnson Matthey and Co Ltd, sealing it in a soft glass sheath and precision cutting the exposed part of the wire to a length of 2 cm. Half of the length of the exposed wire was coiled into 1 - turns to ensure that the replenishing electrolyte would cover the entire exposed surface of the electrode. The silver wire was cleaned in 50% AnalaR (Trade Mark) nitric acid for 10 seconds. The cleaned wire was then coated with a layer of silver sulphide by placing it in an electrochemical cell in which it was made to function as an anode against a silver cathode.The electrolyte was a solution of 0.05M potassium sulphide to which had been added one drop of 0.01 m sulphuric acid as a supporting electrolyte.

A current of 1 5.9 yA was applied for 15 minutes after which time the silver sulphide coated silver wire was removed and excess electrolyte washed off.

A silicone rubber cone was squeeze fitted onto the newly made electrode and an 8 cm 20 gauge hypodermic needle, the termination of which had previously been bent at right angles, was pushed through the cone adjacent to the electrode, such that the open end of the needle faced the electrode but did not make a metal to metal contact.

A glass capillary salt bridge was pushed through the cone adjacent to the ion selective electrode such that the end was sufficiently close to the electrode that it contacted the electrolyte film surrounding the electrode.

An electrode holder was constructed from two hollow cylindrical pieces of PTFE. The upper section of the holder was provided with an external screw thread. The lower section of the holder was provided with a turned shoulder and an internal screw thread. The rubber plug containing the needle, the capillary salt bridge and the electrode was pushed into the upper section adjacent to the screw thread compressing the plug and by so doing tightly gripping the components fitted through it and the lower section was screwed onto the upper section.

A container with an approximate volume of 250 cm3 was constructed with a gas inlet port and a gas outlet port from Pyrex (Trade Mark) glass and a cover to fit it was made from Perspex (Trade Mark).

The silver/silver sulphide electrode and a saturated calomel electrode were connected to a Corning EEL Model 12 Research pH meter via screened insulated wires ensuring that grease and dust were excluded from the solder contacts.

A length of 1 mm diameter polyethylene tubing was connected to the needle and a reservoir of electrolyte which was filled with 0.1 mol 1-1 NaOH solution.

The glass capillary salt bridge adjacent to the ion selective electrode was filled with saturated potassium nitrate solution and led to an electrolyte transfer vessel containing the same solution. A second salt bridge filled with saturated potassium chloride solution was connected to the reference electrode and led to the electrolyte transfer vessel.

To calibrate the electrode it was first allowed to stand in its operative condition (ie with 0.1 m NaOH dripping over its surface) for 12 hours or until a reproducible and constant potential to better than 5 mV was achieved. The cell was connected to a gas flow of 970 cm3/min provided by an electric pump.

Hydrogen cyanide was generated by dropping 1.84 g/cm3 sulphuric acid from a pressure adjusted dropping funnel into AnalaR KCN situated in a 250 m3 flask. This was connected via.

a tap to a second flask the neck of which was sealed by a subaseal through which the HCN at atmospheric pressure could be extracted. After sealing the reagents were allowed to react and the HCN was trapped by an acetone/C02 trap surrounding the second flask by which it was liquefied.

The generated HCN was introduced to a gas flow via a venturi meter connected between the electric pump and the detection apparatus. The rate of flow of HCN was controlled by altering the temperature of the liquid HCN and was calculated from the Poiseuille equation.

Calibration of the electrode to the amount of hydrogen cyanide entering the system at a fixed point in linear flow distance from the electrode was achieved by a best straight line fit to measured data by least squares analysis. The electrode response gave a mean slope of 124.4 with a standard deviation of 2.6 for the plot of potential change versus log,, [number of y moles HCN per cm3 of gas]. (Figure 3.) The durability of the ion selective electrode was tested by measuring the consistency of the electrode over a long period of operation and by measuring the reproducibility of the potential measurement after allowing the electrode to remain inoperative for certain periods of time.

Analytical evaluation of the silver/silver sulphide electrode as a continuous monitor for cyanide ions has shown that it is well suited for the measurement of cyanide in a glass flow of about 100 cm3/min for amounts of HCN introduced into the flow system of up to 241 y moles.

Continuous monitoring over a period of two days produced a base line reproducible to better than 10 mV. Allowing the electrode to stand in a redundant state for up to twenty days produced no noticeable drop in potential for a standard addition of HCN concentration.

The potential was observed to fluctuate with the stage of droplet formation of NaOH at the end of the coiled electrode and could be as much as 6 mV depending on the sodium hydroxide flow rate and cyanide concentration. The size of the sawtooth fluctuations increased with cyanide concentration but were always regular and reproducible and did not seriously hamper the recording of results.

The droplet formation was affected by varying the gas flow rate between 0 and 970 cm3/min and this in turn affected the shape of the sawtooth trace. Sudden changes in the flow rate caused irregular and unpredictable changes in the electrode response but the trace turned to a steady base line after two or three minutes.

Example 2 A silver/silver iodide electrode was made in the same way as the silver/silver sulphide electrode and the same procedure as described in Example 1 was carried out.

The silver iodide layer was slowly dissolved by the sodium hydroxide solution and although the electrode performed adequately it was inferior to the silver/silver sulphide electrode.

Example 3 A silver/silver sulphide electrode made in the way described in Example 1 was evaluated in the same way as described in Example 1 using an electrical comparator circuit with an alarm indicator designed to respond at a predetermined voltage.

The electrical comparator and alarm circuit was made up of the following components.

Means for providing a reference voltage comprised a 741 amplifier, a 5.1K ohm resistance and a 500 ohm variable resistance. A similar amplifier was used as a voltage comparator, the output lead from which was connected via an IN4148 diode and a 24K ohm resistance to a C103W thyristor. The alarm means was provided by an electric lamp.

To balance variations of current gain and baseemitter voltages of the input bipolar transistors within the amplifiers, 5.6K ohm resistances were included in the circuitry.

All the components were chosen to accommodate a cell output voltage range of between 0 and 1000 mV.

The input lead to the comparator from the gas sensing probe was made as short as possible and screened to reduce noise. For the same reason the upper half of the ion selective electrode holder was accommodated in an aluminium box which housed the circuitry.

Example 4 A silver/silver sulphide electrode was prepared by taking a length of 22 SWG mint grade 5 silver wire supplied by Johnson Matthey and Co Ltd, sealing it in a soft glass sheath and precision cutting the exposed part of the wire to a length of 2 cm. Half of the length of the exposed wire was coiled into 14 turns to ensure that the replenishing electrolyte would cover the entire exposed surface of the electrode. The silver wire was cleaned in 50% AnalaR (Trade Mark) nitric acid for 10 seconds. The cleaned wire was soaked in 0.1 M sodium hydroxide solution for 5 minutes and then placed in a sealed flask containing hydrogen sulphide gas for 10 minutes. The electrode was evaluated in the same way as described in Example 1.

Claims (48)

Claims

1. Apparatus for detecting hydrogen cyanide including (a) a CN- ion selective electrode of the air gap type, (b) a standard reference electrode, (c) at least one salt bridge electrically connecting the ion selective electrode to the standard reference electrode, (d) a gas tight container provided with a gas inlet opening and a gas outlet opening, the ion selective electrode being suspended in the container, (e) means for providing a regular flow of an electrolyte onto the ion selective electrode and (f) means for detecting a potential difference between the ion selective electrode and the standard reference electrode when a gas is in contact with the ion selective electrode.

2. Apparatus as claimed in claim 1 and which also includes means for supplying a continuous flow of gas across the ion selective electrode.

3. Apparatus as claimed in claim 1 or claim 2 and wherein the ion selective electrode is suspended directly in the gas flow between the gas inlet and gas outlet openings of the container.

4. Apparatus as claimed in any one preceding claim and wherein the gas inlet opening is provided in an opposite wall of the container to the gas outlet opening.

5. Apparatus as claimed in any one preceding claim and wherein the ion selective electrode comprises a silver/silver salt electrode.

6. Apparatus as claimed in claim 5 and wherein the ion selective electrode comprises a silver/silver iodide electrode.

7. Apparatus as claimed in claim 5 and wherein the ion selective electrode comprises a silver/silver sulphide electrode.

8. Apparatus as claimed in any one preceding claim and wherein the means for providing a regular flow of electrolyte onto the ion selective electrode comprises an open ended narrow bore tube arranged with the open end adjacent to and directed at the tip of the ion selective electrode, the tube being connected via a flexible tube to an electrolyte reservoir.

9. Apparatus as claimed in claim 8 and wherein means for regulating the flow of electrolyte is provided between the electrolyte reservoir and the open ended narrow bore tube.

10. Apparatus as claimed in any one preceding claim and which aiso includes means for emptying the container of liquid.

11. Apparatus as claimed in claim 10 and wherein the means for emptying the container of liquid comprises a weir.

12. Apparatus as claimed in any one preceding claim and wherein the ion selective electrode is electrically connected to the standard reference electrode by a single salt bridge.

13. Apparatus as claimed in claim 12 and wherein the salt bridge comprises a potassium chloride solution salt bridge.

14. Apparatus as claimed in any one of claims 1 to 11 and wherein the ion selective electrode is electrically connected to the standard reference electrode by a double salt bridge.

1 5. Apparatus as claimed in claim 14 and wherein the double salt bridge comprises a potassium nitrate solution bridge and a potassium chloride solution bridge, the potassium nitrate solution bridge being in electrical contact with the ion selective electrode and the potassium chloride solution bridge being in electrical contact with the standard reference electrode, the two salt bridges being electrically connected via an electrolyte transfer vessel.

1 6. Apparatus as claimed in claim 1 5 and wherein the electrolyte transfer vessel contains potassium nitrate solution.

17. Apparatus as claimed in claims 1 5 or 16 and wherein the potassium nitrate solution has a concentration of about 0.01 M.

18. Apparatus as claimed in any one preceding claim and wherein the means for detecting a potential difference is either a pH meter or a low impedance millivoltmeter.

1 9. Apparatus as claimed in any one of claims 1-17 and wherein the means for detecting a potential difference is an electrical comparator circuit.

20. Apparatus as claimed in claim 1 9 and wherein the electrical comparator circuit includes (a) means for generating a reference voltage, (b) comparator means for comparing the reference voltage with a potential difference between the ion selective electrode and the standard reference electrode, when a gas is in contact with the ion selective electrode, (c) a switching device operable by an output signal from the comparator means and (d) an alarm operable by an output from the switching device.

21. Apparatus as claimed in claim 20 and wherein the means for producing a reference voltage includes an amplifier using negative feedback and which is connected in series with a variable resistance.

22. Apparatus as claimed in claim 20 or claim 21 and wherein the comparator means for comparing the reference voltage with the potential difference between the ion selective electrode and the standard reference electrode includes an amplifier operating without feedback.

23. Apparatus as claimed in any one of claims 20-22 and wherein the switching device is a thyristor.

24. Apparatus as claimed in any one of claims 20 to 23 and wherein the comparator circuit also includes a reset switch in order to break the circuit to stop the alarm once it has been triggered.

25. Apparatus as claimed in any one of claims 20 to 24 and wherein the alarm comprises a device designed to give a visual signal when triggered by the switching device.

26. Apparatus as claimed in any one of claims 20 to 24 and wherein the alarm comprises a device designed to give an audio signal when triggered by the switching device.

27. Apparatus as claimed in any one of claims 20 to 24 and wherein the alarm comprises a device design to give a visual and an audio signal when triggered by the switching device.

28. Apparatus as claimed in any one of claims 20 to 25 and wherein the alarm includes one or more electric lamps.

29. Apparatus as claimed in any one of claims 20 to 24 or claim 26 and wherein the alarm includes an electric bell or a piezoceramic vibrator.

30. Apparatus as claimed in any one of claims 20 to 24 or claim 27 and wherein the alarm includes an electric lamp and an electric bell or an electric lamp and a piezoceramic vibrator.

31. Apparatus as claimed in any one preceding claim and which includes a visual display device connected to the means for detecting a potential difference.

32. Apparatus as claimed in claim 31 and wherein the visual display device is a chart recorder, a cathode ray oscilloscope or a combination of the two.

33. A method of detecting hydrogen cyanide using the apparatus claimed in claim 1 including the steps of (a) presenting a gas in contact with the ion selective electrode of the air gap type, (b) supplying the ion selective electrode with a regular flow of electrolyte, (c) dissolving and ionising any hydrogen cyanide present in the electrolyte surrounding the ion selective electrode and (d) detecting any change in potential difference between the ion selective electrode and a standard reference electrode.

34. A method of detecting hydrogen cyanide using the apparatus claimed in claim 20 including the steps of (a) presenting a gas in contact with the ion selective electrode of the air gap type, (b) supplying the ion selective electrode with a regular flow of electrolyte, (c) dissolving and ionising any hydrogen cyanide present in the electrolyte surrounding the ion selective electrode, (d) detecting the potential difference between the ion selective electrode and the standard reference electrode, when a gas is in contact with the ion selective electrode, (e) generating a reference voltage, (f) comparing the reference voltage with the potential difference between the two electrodes and (g) triggering the alarm when the potential difference between the two electrodes exceeds the reference voltage.

35. A method as claimed in claim 34 and wherein the reference voltage generated corresponds to a concentration of cyanide ions at which it is desired to produce an output from the comparator means to the switching device which triggers the alarm.

36. A method as claimed in claim 35 and wherein the setting of the variable resistance determines the size of the reference voltage.

37. A method as claimed in any one of claims 33 to 36 and wherein a flow of gas is passed over the ion selective electrode.

38. A method as claimed in claim 37 and wherein the flow of gas is maintained at a constant rate.

39. A method as claimed in any one of claims 33 to 38 and wherein the electrolyte flow is maintained at a constant rate.

40. A method as claimed in claim 39 insofaras dependent on claim 38 and wherein the two rates of flow are in equilibrium.

41. A method as claimed in any one of claims 33 to 40 and wherein the electrolyte is hydroxide solution.

42. A method as claimed in claim 41 and wherein the electrolyte is sodium hydroxide solution.

43. A method as claimed in claim 41 and wherein the electrolyte is potassium hydroxide solution.

44. A method as claimed in any one of claims 33 to 43 and wherein the electrolyte is in the form of a free flowing liquid.

45. A method as claimed in any one of claims 33 to 43 and wherein the electrolyte is in the form of a viscous liquid or gel.

46. A method as claimed in claim 39 and wherein the rate of flow of electrolyte is about 1 drip per 50 seconds.

47. Apparatus substantially as hereinbefore described with reference to the accompanying drawings.

48. A method of detecting hydrogen cyanide substantially as hereinbefore described with reference to the accompanying drawings.