FR2485200A1 - Coulometric substance concentration measurement appts. - has electrode exposed to fixed flow of substance identical to that analysed to provide constant background - Google Patents

Abstract

The under analysis substance undergoes an electrode reaction in the bath, and the resultant quantity of electricity is measured. In order to reduce the response time and to eliminate pauses or temporary concentration variations, the electrode is permanently exposed to a fixed flow of the same substance as the analysed substance. If the analysed substance is a gas or a liquid and the bath contains ions of that substance, the fixed flow is produced by electrolysis. The device contains a supplementary pair of electrodes (7,8) connected to a voltage higher than that of the electrolysis, producing a constant background signal used to give a zero reference. The supplementary electrodes are formed of platinum wires.

Description

Method and device for coulometric dosing
of a substance in an electronic cigarette - imlZlue
The present invention relates to a method and apparatus for the coulometric assay of a substance in an electrochemical bath.

 Coulometry is a well-known electrochemical technique for the determination of substances defined in electrochemical baths.

In general, in this assay technique, the substance to be assayed is subjected to an electrode reaction in the bath, for example to an oxidation or a reduction, and the quantity of electricity used by the reaction (Electrochemical methods of analysis - G. Charlot and D. Bézier - Masson & Cie,
Paris - 1954 - p.74 and ss.).

Coulometry generally has the advantage of being a fast and reliable method of analysis. It finds an interesting application in the analysis of gaseous mixtures where, to assay a component of the mixture (for example chlorine in air), it is reacted with an electrochemical bath and the substance liberated by coulometry is measured by bathing by reaction (Belgian patent 864 274 of 23 February 1978, in the name of Imperial Chemical Industries
Ltd.).

 In practice, however, it has been observed that commercially available coulometers generally have a response time which is not always uniform and which can sometimes vary very strongly between two successive dosages. In particular, it has been found that the response time of the known coulometers usually becomes very long, up to several minutes, when they have not been used for several days. The same phenomenon has been observed with coulometers used in continuous service to detect the accidental or intermittent occurrence of a particular predetermined substance in an electrochemical bath.

 This characteristic of the known coulometers is particularly disadvantageous when they are used for the control of gaseous atmospheres according to the technique described in the aforementioned Belgian patent 864 274, in particular for detecting the accidental presence of a toxic gas, such as chlorine, in the atmosphere. 'air.

 It is an object of the invention to overcome this disadvantage of known coulora meters by providing a method and a device for the coulometric assay of a defined substance in an electrochemical bath, whose response time is short and is not influenced by a momentary cessation of the reaction. dosage or by a momentary variation of the amount of substance to be assayed in the bath.

 The invention therefore relates to a method for the coulometric assay of a substance in an electrochemical bath, wherein the substance is subjected to an electrode reaction in the bath and the amount of electricity corresponding to the reaction is measured; according to the invention, the electrode is continuously scanned with a defined rate of substance identical to that which is dose.

 In the method according to the invention, the measurement of the quantity of electricity can be carried out by any suitable electrical measurement method, for example by means of an ammeter or a voltmeter, according to whether a coulometric assay is carried out. at constant potential or at constant current.

 The choice of the constant potential or the constant current intensity is imposed by the nature of the bath and that of the substance to be assayed, as well as by the constitution of the electrode and the electrode reaction implemented. In general, a value is chosen for which the accuracy of the measurement is optimum. For example, in the case of a constant potential measurement, the potential which corresponds to the diffusion plateau for the envisaged electrode reaction is preferably chosen.

 According to the invention, in addition to the substance contained in the bath and which is to be assayed, the electrode is subjected to a scan with a defined rate of substance identical to that which one seeks to dose.

 In the process according to the invention, the substance which is used for the scanning of the electrode must wander in the gaseous state or in the liquid state, for example in the state of a solution or 'a suspension.

 The magnitude of the substance flow rate used for the scanning of the electrode must be sufficient to ensure that the electrode reaction corresponding only to this flow rate of substance gives rise, on its own, to a detectable and constant quantity of electricity. However, excessive fluid flow should be avoided, so as not to affect the accuracy of the measurement.

 According to a preferred embodiment of the process according to the invention, applied in the case where the substance to be assayed is a gas or a liquid, and where the bath contains ions of the substance to be assayed, the substance which is The process for scanning the electrode is generated by electrolysis of the bath.

 This embodiment of the invention finds an interesting application for the determination of a halogen in an aqueous bath containing ions of this halogen, for example for the determination of bromine or iodine released in an aqueous solution of bromide or of alkali metal or alkaline earth metal iodide by reaction of this solution with an oxidant. It finds a particularly interesting application when it is used for the detection of an oxidizing gas of a gaseous mixture, for example for the detection of chlorine in air, by the detection method described above, with reference to the Belgian patent 864 274.

 The method according to the invention can be implemented by means of a device for the coulometric measurement of a substance in an electrochemical bath, comprising, in a manner known per se, an enclosure for the electrochemical bath, a pair of electrodes in contact with the bath and connected to a current source whose voltage is lower than the electrolysis voltage of the bath and an amperemeter mounted on the circuit of the electrodes, said device further comprising, in accordance with the invention, a means for admission, in contact with the electrodes, a defined flow of substance identical to that which one seeks to dose.

 In the device according to the invention, the choice of the means of admission of the substance in contact with the electrodes is not critical and depends essentially on the nature of the substance.

 In a preferred embodiment of the device according to the invention, applied especially in the case where the substance to be assayed is a gas or a liquid and where the electrochemical bath contains ions of the substance to be assayed, the admission means comprises a second pair of electrodes in contact with the bath and connected to a second current source whose voltage is at least equal to the electrolysis voltage of the bath.

 According to a variant of this embodiment of the invention, the two pairs of electrodes may have a common electrode.

 Features and details of the invention will become apparent from the following description of the single figure of the accompanying drawing, which shows schematically, in elevation, partially broken away, a particular embodiment of the device according to the invention.

 The device shown in the figure is intended for the determination of bromine in a calcium bromide bath.

 It is specially designed for the monitoring of the atmosphere of a room or an industrial site, and more specifically for the detection of an accidental presence of chlorine in the atmosphere, by coulometric determination of the bromine released by the reaction of the chlorine with the calcium bromide bath.

 For this purpose, the device comprises a tubular sleeve 1 made of an absorbent fibrous material, for example asbestos, saturated with a concentrated aqueous solution of calcium bromide.

The tubular sleeve 1 is engaged in a sleeve 2, where its lower end is immersed in a calcium bromide bath, so that the entire sleeve is constantly saturated with this solution, by capillary effect.

 A first pair of electrodes consists of two platinum wires 3 and 4 wound separately spirally on the sleeve 1 and respectively connected to two connection terminals 10 and 11 to a DC source 5 whose voltage is less than the electrolysis voltage of the calcium bromide bath.

 The electrical circuit of the electrodes 3 and 4 and the current source 5 comprises an amperemetre 6.

 According to the invention, the device comprises a second pair of electrodes 7 and 8, which consist of two platinum wires, one end of which is applied to the sleeve 1, without touching the electrodes 3 and 4. The electrodes 7 and 8 are connected to a second source of direct current 9, whose voltage is sufficient to cause the electrolysis of the calcium bromide bath.

 During the operation of the device, the electrodes 3 and 4 are coupled to their current source 5 and the electrodes 7 and 8 are coupled to the current source 9. The electrolysis of the bath, in contact with the second pair of electrodes 7 and 8 causes the formation of a constant flow rate of bromine, which is reduced in contact with the cathode 4 of the first pair of electrodes, so that the ammeter 6 records a substantially constant background signal.

 As soon as chlorine is present in the gaseous atmosphere surrounding the sleeve 1, a corresponding amount of bromine is released by reaction of the chlorine with the bath and immediately reduced in contact with the cathode 4, causing a corresponding increase, almost instantaneous, of the intensity of the current measured at the ammeter 6.

 According to a modified embodiment, not shown, of the device, the two pairs of electrodes have a common electrode, for example 11 electrode 4; in this case, the electrode 4 is coupled to the two negative terminals of the two current sources 5 and 9, the electrode 7 being suppressed.

 The interest of the invention will appear on reading the following application examples.

First series of tests (comparative)
In the following tests, a commercial coulometric dosing device, sold by Sella Control Ltd and specially adapted for the detection of the accidental presence of chlorine in the ambient air, was used.

 The device used was of the type described in Belgian Patent 864 274 cited above; it was analogous to the device shown in the figure and described above, except that it was not provided with the second pair of electrodes 7 and 8 nor the second current source 9. The spiral electrodes 3 and 4 were in platinum, the bath consisted of a saturated solution of calcium bromide at room temperature and the potential of the current source 5 was set at 600 mV. Amperemeter 6 was calibrated directly in chlorine content in the air.

Example 1
The device was placed in ambient air, free of chlorine, for 18 hours.

 At the end of 18 hours, a flow of air containing 3 ppm of chlorine was circulated in contact with the device, and the chlorine content indicated by the amperemet was periodically recorded over time. . The results of the measurements are given in Table I below.

TABLE I

<September>
<tb><SEP> Time <SEP> Content <SEP> in <SEP> Chlorine <SEP> measured
<tb><SEP> (s) <SEP> (PP) <SEP>
<tb><SEP> O <SEP> O <SEP>
<tb>(<SEP> introductory <SEP> moment)
<tb> chlorine <SEP> in <SEP> air)
<tb><SEP> 15 <SEP> 193 <SEP>
<tb><SEP> 60 <SEP> 1.9
<tb><SEP> 120 <SEP> 2,2
<tb><SEP> 240 <SEP> 2.5
<tb><SEP> 840 <SEP> 2.9
<tb> Example 2
The test of Example 1 was repeated, with the only difference that it was waited four days before subjecting the device to a stream of air charged with 3 ppm chlorine.

 The results of the measurements are recorded in Table II.

TABLE II

<tb> Time <SEP> Content <SEP> in <SEP> Chlorine <SEP> measured
<tb><SEP> (s) <SEP> (ppm)
<tb><SEP> O <SEP> O <SEP>
<tb><SEP> 15 <SEP> 0.7
<tb><SEP> 60 <SEP> 1,1
<tb> 110 <SEP> 1.4
<tb> 240 <SEP> 1.8
<tb> 500 <SEP> 2,3
<Tb>
Example 3
The test of Example 2 was continued for about ten minutes, then the chlorine inlet was stopped in the vicinity of the device, but the electrodes 3 and 4 connected to the current source 5 were held. After 18 hours The device was again subjected to a stream of air containing 3 ppm chlorine and the chlorine content indicated by the ammeter 6 was periodically read from that moment. The results are shown in Table III.

TABLE III

<tb> Time <SEP> Content <SEP> in <SEP> Chlorine <SEP> measured
<tb><SEP> (s) <SEP> (ppm)
<tb><SEP> o <SEP> 0
<tb><SEP> 15 <SEP> 1,3
<tb><SEP> 60 <SEP> 2.1
<tb> 120 <SEP> 2,4
<Tb>
An examination of Tests 1 to 3 shows that the response time of the commercial detection device varies greatly from one test to another and depends largely on the use that has been made of the device before the test.

Second series of tests (according to the invention)
For carrying out the following tests, the same commercial device as used for the first series of tests was used, and it was converted to conform to the invention. To this end, between the turns of the electrodes 3 and 4, two auxiliary electrodes 7 and 8 formed of two platinum wires have been inserted, one edge end being applied against the absorbent sleeve 1, as shown in FIG. The voltage of the auxiliary power source 9 has been adjusted to obtain an electrical current of approximately 10 A in the auxiliary electrodes 7 and 8.

Example 4
The electrodes 3 and 4 on the one hand, 7 and 8 on the other hand, being respectively coupled to their current sources 5 and 9, the device was placed in the ambient air free of chlorine for four days.

 Under the effect of the bromine released by the partial electrolysis of the bath in contact with the auxiliary electrodes 7 and 8, the heat sink has recorded a background signal which has stabilized at a constant value. The scale of the ammeter has been moved so that the "zero" value of the scale corresponds to the constant value of the background signal.

 At the end of the four days, the device was subjected to a stream of air, charged with 3 ppm of chlorine by volume, and the chlorine content indicated by the ammeter was read periodically from this moment onwards. 6. The results of the measurements are given in Table IV.

TABLE IV

<tb><SEP> Time <SEP> Content <SEP> in <SEP> Chlorine <SEP> measured
<tb><SEP> (s) <SEP> (ppm)
<tb><SEP> 0 <SEP> 0
<tb>(<SEP> introductory <SEP> moment)
<tb> chlorine <SEP> in <SEP> air)
<tb><SEP><1<SEP> 1,2
<tb><SEP> 15 <SEP> 2.1
<tb><SEP> 60 <SEP> 2.5
<Tb>
Example 5
The test of Example 4 was continued for about 15 minutes, then chlorine was discontinued in the air surrounding the device for 24 hours.

 At the end of the 24 hours, chlorine was again introduced into the air in the vicinity of the device, so as to maintain permanently 3 ppm of chlorine therein.

The results recorded at ammeter 6 are recorded in
Table V.

TABLE V

<tb> Time <SEP> Content <SEP> in <SEP> Chlorine <SEP> measured
<tb><SEP> (s) <SEP> (ppm)
<tb><SEP> 0 <SEP> O <SEP>
<tb><1<SEP> 1,2
<tb><SEP> 60 <SEP> 2,4
<Tb>
Example 6
The test of Example 4 was repeated, but waiting seven days before introducing chlorine into the air surrounding the device.

 The results of the measurements are shown in Table VI.

TABLE VI

<tb> Time <SEP> Content <SEP> in <SEP> Chlorine <SEP> measured
<tb><SEP> (s) <SEP> (ppm)
<tb><SEP> O <SEP>
<tb><1<SEP> 1,2
<tb><SEP> 60 <SEP> 2,6
<Tb>
An examination of the tests 4 to 6 shows that the response time of the device according to the invention is virtually constant, regardless of the use that was made before the test. This response time is also excessively short, the device being indeed able to detect more than 40 x of the total content of chlorine in the analyzed air, in less than 1 s.

 A comparative examination of the results of tests 1 to 3, carried out with the commercial detector, with those of tests 4 to 6, carried out with the detector according to the invention, immediately shows the progress made by the invention with regard to the reliability, speed and accuracy of measurements.

Claims (10)

 1 - Method for the coulometric determination of a substance in an electrochemical bath, according to which the substance is subjected to an electrode reaction in the bath and the quantity of electricity corresponding to the reaction is measured, characterized in that continuously subject the electrode to a scan with a defined flow rate of substance identical to that which is dose.  2 - Process according to claim 1, characterized in that, in the case where the substance to be assayed is a gas or a liquid and where the bath contains ions of the substance to be assayed, the substance that is used to the scanning of the electrode is generated by electrolysis of the bath.  3 - Process according to claim 2, characterized in that it is used for the determination of a halogen released by reaction of the bath with an oxidized component of a gaseous mixture  4 - Process according to claim 3, characterized in that it is used for the determination of bromine or iodine released by reaction of a bath of bromide or iodide of alkali metal or alkaline earth with chlorine contained in the air.  5 - Process according to any one of claims 1 to 4, characterized in that performs the constant potential electrode reaction.  6 - Device for the coulometric assay of a substance in an electrochemical bath, comprising a chamber for the bath, a pair of electrodes in contact with the bath and connected to a source of direct current whose voltage is lower than the voltage d electrolysis of the bath and an ammeter mounted on the circuit of the electrodes, characterized in that it comprises an inlet means in contact with the electrodes, a defined flow of substance identical to that which is to be determined.  7 - Device according to claim 6, characterized in that the intake means comprises a second pair of electrodes in contact with the bath and connected to a second source of direct current, whose voltage is at least equal to the voltage d electrolysis of the bath.  8 - Device according to claim 7, characterized in that the two pairs of electrodes have a common electrode.  9 - Device according to claim 7 or 8, characterized in that the two pairs of electrodes are applied on a sleeve of an absorbent material whose end is engaged in the chamber, where it is immersed in the bath.  10 - Device according to claim 9, characterized in that the first pair of electrodes is spirally wound on the sleeve and the second pair of electrodes comprises two metal son, one end is applied edge on the sleeve between two successive turns of the first pair of electrodes.