GB1584549A - Electrolytic indicator cell - Google Patents

Description

(52) ELECTROLYTIC INDICATOR CELL (71) We, STANDARD TELEPHONES AND CABLES LIMITED a British Company of 190 Stand, London WC2R 1DU, England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to indicator cells, and in particular to indicator cells providing a visual indication that electrical charge passing therethrough has exceeded a predetermined threshold value.

According to the present invention there is provided an indicator cell providing a visual indication that an electrical charge passing therethrough has reached à predetermined threshold value which cell has a sealed enclosure through the wall of which are sealed two inert electrodes, which enclosure is transparent or is provided with a transparent window, wherein the cell contains an electrolyte providing an electrical connection between the electrodes which electrolyte consists of an iodide solution, solubilized starch, an excess of anti-oxidant such that said predetermined charge is required to flow through the cell before the onset of any liberation of iodine at the anode, and an acid present in an amount at least equal to that calculated for the neutralisation of hydroxyl ions produced at the cathode electrode when said predetermined threshold charge has been caused to flow through the cell.

The cell makes use of the well known reaction of iodine in a starch solution to produce an intense blue coloured reaction product. Such iodine may be liberated at the inert anode of an electrolytic cell containing an iodine salt when current is passed through the cell.

Therefore it is possible to provide an indicator cell having a pair of electrodes sealed through the wall of an enclosure containing an electrolyte consisting solely of a solution of an iodine salt such as potassium iodide and solubilized starch. However, as a visual indicator of the flow of charge through the device, such a cell suffers from two significant disadvantages. One disadvantage is that in the presence of oxygen which may leak into the cell or be contained in solution in the electrolyte the iodine salt is liable to decompose in the absence of any current flow to liberate iodine and so after a period a false indication may be obtained. Another disadvantage is that after a flow of current the liberated iodine or starch-iodine complex diffuses from the anode region of the cell to the cathode region while the alkali liberated diffuses from the cathode region to the anode, and where they meet the starch-iodine complex is decomposed and decolourized by the alkali. These reactions may be set out in the following form:

1. 21- - 2 (h) anode) 12, I2 + starch o 12.starch, deep blue 2. 2K+ + 2 (ffi) cathode 2K, 2K + H20 < 2KOH + H2 3. 2KOH + I2.starche KI + KIO3 + starch deep blue colourless The first of these disadvantages is overcome according to the teachings of the present invention by the addition of a small amount of anti-oxidant such as sodium sulphite or sodium thiosulphate which will prevent the liberation of iodine in the absence of a current.

The quantity necessary can be determined empirically so that it will be just adequate to prevent the liberating of a visibly detectable quantity of iodine produced by contained oxygen and oxygen leaking into the cell during a specific period. Another feature of the addition of the anti-oxidant is that by the addition of more than is necessary to "getter' the oxygen the excess will be oxidised at the anode before iodine begins to be liberated. Thus it is possible to produce a predetermined threshold value of charge flow through the cell before any iodine is liberated. Thus

no indication will be visible

full indication will develop.

The second mentioned disadvantage of the simple 2-component electrolyte is overcome according to the teachings of the present invention by the addition of a quantity of acid to the electrolyte. The minimum quantity of acid that needs to be added is determined as the amount that is necessary to neutralise the alkali produced during the passage of the total quantity of charge necessary to produce the maximum visual indication. By this means the colour indication is made permanent because the decolourization reaction 3 above is no longer possible.

The biological decomposition of the starch can be prevented by means well known to those skilled in the art. For instance suitable amounts of bacteriocides and fungicides or of broad spectrum antibiotics may be added to the electrolyte.

There follows a description of an indicator cell embodying the invention in a preferred form. The description refers to the accompanying drawings in which: Figure 1 is a plan view of the cell, and Figure 2 is a sectional view of the cell.

Referring to the drawings a 20 mm in diameter and 6 mm in depth container 1 for the electrolyte of the cell is provided with a lid 2, 10 mm in diameter, which also functions as a window through which the interior of the indicator cell may be observed. The container 1 and lid 2 can be made of glass and both parts may be transparent. In the present instance both are made of an acrylic resin, and the body 1 incorporates a white pigment to make it opaque and provide a good contrast for the starch-iodine dye. The window is sealed to the container with chloroform. An anode electrode 3 and a cathode electrode 4 made of an inert metal such as platinum are fitted through the container 1 and sealed in position for instance with epoxy resin. Between the anode 4 and the window 2 is located a layer 5 of absorbent material which serves to hold and spread the dye, and an electrically insulating spacer 6 is placed between the anode and cathode electrodes 3 and 4 to prevent their accidental direct connection. Conveniently the layer 5 and spacer 6 may be made of the same material, for instance filter paper. Once the cell has been assembled it is filled with electrolyte through a filler hole (not shown) in the container 1. This is then sealed off with epoxy resin to form a hermetically sealed enclosure.

The electrolyte typically consists of three parts of 5 gm per litre potassium iodide solution to one part of 5 gm per litre starch solution. To each 100 ml of this mixture is added 5 ml of 1 Normal sulphuric acid, and for every 5 ml of potassium iodide solution in the mixture is added 0.4 mg of sodium sulphite.

It has been found that a charge (total integrated current) of a few millicoulombs will give a clearly defined dark indication in this particular cell. The potential across the cell electrodes was in the range 0-5 to 1-5 volts. It will be clear that many modifications may be made to the design and layout of the cell. For example the outer ring of the container surrounding the window may be provided with colour matching scale from white to deep blue for the purposes of comparison with the colour displayed at the centre. This may be arranged so that the position of the match on the scale gives an indication of the amount of charge that at any particular time has flowed through the cell.

WHAT WE CLAIM IS: 1. An indicator cell providing a visual indication that an electrical charge passing therethrough has reached a predetermined threshold value which cell has a sealed enclosure through the wall of which are sealed two inert electrodes, which enclosure is transparent or is provided with a transparent window, wherein the cell contains an electrolyte providing an electrical connection between the electrodes which electrolyte consists of an iodide solution, solubilized starch, an excess of anti-oxidant such that said predetermined charge is required to flow through the cell before the onset of any liberation of iodine at the anode, and an acid present in an amount at least equal to that calculated for the neutralisation of hydroxyl ions produced at the cathode electrode when said predetermined threshold charge has been caused to flow through the cell.

2. An indicator cell as claimed in claim 1 wherein the enclosure is provided with a window around which is disposed a colour matching scale.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. prevent the liberating of a visibly detectable quantity of iodine produced by contained oxygen and oxygen leaking into the cell during a specific period. Another feature of the addition of the anti-oxidant is that by the addition of more than is necessary to "getter' the oxygen the excess will be oxidised at the anode before iodine begins to be liberated. Thus it is possible to produce a predetermined threshold value of charge flow through the cell before any iodine is liberated. Thus no indication will be visible full indication will develop. The second mentioned disadvantage of the simple 2-component electrolyte is overcome according to the teachings of the present invention by the addition of a quantity of acid to the electrolyte. The minimum quantity of acid that needs to be added is determined as the amount that is necessary to neutralise the alkali produced during the passage of the total quantity of charge necessary to produce the maximum visual indication. By this means the colour indication is made permanent because the decolourization reaction 3 above is no longer possible. The biological decomposition of the starch can be prevented by means well known to those skilled in the art. For instance suitable amounts of bacteriocides and fungicides or of broad spectrum antibiotics may be added to the electrolyte. There follows a description of an indicator cell embodying the invention in a preferred form. The description refers to the accompanying drawings in which: Figure 1 is a plan view of the cell, and Figure 2 is a sectional view of the cell. Referring to the drawings a 20 mm in diameter and 6 mm in depth container 1 for the electrolyte of the cell is provided with a lid 2, 10 mm in diameter, which also functions as a window through which the interior of the indicator cell may be observed. The container 1 and lid 2 can be made of glass and both parts may be transparent. In the present instance both are made of an acrylic resin, and the body 1 incorporates a white pigment to make it opaque and provide a good contrast for the starch-iodine dye. The window is sealed to the container with chloroform. An anode electrode 3 and a cathode electrode 4 made of an inert metal such as platinum are fitted through the container 1 and sealed in position for instance with epoxy resin. Between the anode 4 and the window 2 is located a layer 5 of absorbent material which serves to hold and spread the dye, and an electrically insulating spacer 6 is placed between the anode and cathode electrodes 3 and 4 to prevent their accidental direct connection. Conveniently the layer 5 and spacer 6 may be made of the same material, for instance filter paper. Once the cell has been assembled it is filled with electrolyte through a filler hole (not shown) in the container 1. This is then sealed off with epoxy resin to form a hermetically sealed enclosure. The electrolyte typically consists of three parts of 5 gm per litre potassium iodide solution to one part of 5 gm per litre starch solution. To each 100 ml of this mixture is added 5 ml of 1 Normal sulphuric acid, and for every 5 ml of potassium iodide solution in the mixture is added 0.4 mg of sodium sulphite. It has been found that a charge (total integrated current) of a few millicoulombs will give a clearly defined dark indication in this particular cell. The potential across the cell electrodes was in the range 0-5 to 1-5 volts. It will be clear that many modifications may be made to the design and layout of the cell. For example the outer ring of the container surrounding the window may be provided with colour matching scale from white to deep blue for the purposes of comparison with the colour displayed at the centre. This may be arranged so that the position of the match on the scale gives an indication of the amount of charge that at any particular time has flowed through the cell. WHAT WE CLAIM IS:

1. An indicator cell providing a visual indication that an electrical charge passing therethrough has reached a predetermined threshold value which cell has a sealed enclosure through the wall of which are sealed two inert electrodes, which enclosure is transparent or is provided with a transparent window, wherein the cell contains an electrolyte providing an electrical connection between the electrodes which electrolyte consists of an iodide solution, solubilized starch, an excess of anti-oxidant such that said predetermined charge is required to flow through the cell before the onset of any liberation of iodine at the anode, and an acid present in an amount at least equal to that calculated for the neutralisation of hydroxyl ions produced at the cathode electrode when said predetermined threshold charge has been caused to flow through the cell.

2. An indicator cell as claimed in claim 1 wherein the enclosure is provided with a window around which is disposed a colour matching scale.

3. An indicator cell as claimed in claim 1 or 2, wherein the cell is planar and the

electrodes are separated by a porous spacer.

4. An indicator cell as claimed in any preceding claim wherein the soluble iodide is potassium iodide.

5. An indicator cell substantially as hereinbefore described with reference to the accompanying drawings.