Chemistry: electrical and wave energy – Processes and products – Processes of treating materials by wave energy
Patent
1994-08-26
1995-12-12
Tung, T.
Chemistry: electrical and wave energy
Processes and products
Processes of treating materials by wave energy
2041531, 204400, 204415, 257414, G01N 2726
Patent
active
054746604
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method and apparatus for determining the concentration of ammonium ions in solution and has particular, although not exclusive, applicability to analysis of aqueous solutions such as contaminated water supplies and bodily fluids where ammonium ions may be produced by enzymatic action as for urea in blood.
Techniques for sensing the concentration of ammonia in a gas phase, and in a gas phase above and in equilibrium with a solution containing dissolved ammonia are known. Such techniques may then be used to determine the concentration of ammonium ions in the solution by utilising the pH-dependent relationship which exists between the concentration of ammonium ions and ammonia in solution and the relationship which exists between ammonia gas and dissolved ammonia in equilibrium at the gas/liquid interface. It is known that by increasing the alkalinity of a solution containing ammonium ions, then ammonia may be generated within the solution. Increasing the alkalinity of such a solution may most conveniently be achieved by adding to the solution either alkali or buffer reagents such as sodium hydroxide or potassium hydroxide as disclosed in EP-A-359158. However, addition of such reagents to the solution may be undesirable because the use of consumables is inconvenient and may also cause unwanted hydrolysis of biochemical materials leading to an increase in ammonium ion and ammonia levels.
Alternatively, the concentration of ammonium ions in solution may themselves be directly sensed by, for example, using an ammonium ion sensitive device within the solution.
Both the above known techniques suffer from various shortcomings. The former, for example, because of the addition of alkali or buffer reagents. The latter, for example, because an ion sensitive device in the solution will have at least a degree of cross-sensitivity to Na.sup.+ or K.sup.+ ions as well as being predominantly sensitive to NH.sub.4.sup.+ ions. Similarly Na.sup.+ and K.sup.+ ion sensitive electrodes show a degree of cross-sensitivity to NH.sub.4.sup.+ ions. As a result of the mutual cross-sensitivity of these ion sensitive devices, an analysis based upon a combination of such electrodes may be unsatisfactory, especially for an ion only present at low concentrations.
It is thus an object of the present invention to at least alleviate the aforementioned shortcomings by avoiding the need to add alkali or buffer reagents to the solution under test and also to avoid the need to sense the ion concentration in the solution itself.
Hence a first aspect of the present invention provides a method of determining the concentration of ammonium ions in solution, comprising: least a portion of the solution alkaline thereby to convert ammonium ions to ammonia; region remote from the solution; and is representative of the concentration of ammonium ions in the solution. Thus there need be no contaminants added to the solution and also a sensor can be used which is not generally prone to cross-sensitivity to cations other than ammonium ions in the solution.
Preferably the electrochemical generation of hydroxyl ions occurs adjacent to the gas-permeable member. This is because the ammonia in the gas phase arises through a partition between the gas phase and that part of the solution which is adjacent the gas phase, which boundary is formed at the gas-permeable member. Adjustment of the pH of only a restricted region close to the gas-permeable member limits the quantity of electrochemically generated hydroxyl ions required and hence reduces the charge which needs to be passed through electrodes employed in the generation.
Advantageously the gas-permeable member is a membrane structure.
Preferably, sensing of the ammonia is achieved by employing an ammonia gas sensor device such as a gas-sensitive field effect transistor device.
A second aspect of the present invention provides apparatus for the determination of the concentration of ammonium ions in solution, comprising: to render the solution alkaline thereby to convert ammonium io
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Robins Ian
Shaw John E. A.
Central Research Laboratories Limited
Tung T.
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