Pure Water pH Made Easy

Corrosion by acids in process waters can be a costly affair in many industries. In the power generation business it can cost a small fortune if slack pH control lets even a little corrosion occur over a long period of time. pH control is relatively easy but pH measurement in the type of ultra-pure waters found in power stations is no easy matter.

pH electrodes measure ions and when there are few ions of any kind, as in ultra-pure water, ordinary electrodes tend to stabilise very slowly and drift frequently. This makes precise measurement difficult and any small change in pH impossible to distinguish. Thus dosing control cannot follow pH changes immediately but lags behind.

At Peterhead, the Scottish Hydro-Electric power station is a traditional gas powered one. The drum water, economiser inlet and extraction pumps/condenser are the principal points where pH is measured. Mike Riddle is the Technical Assistant to the Engineering Department responsible for water and environmental matters. ‘Ordinary electrodes weren't reliable enough, solid state electrodes weren't responsive enough and sleeve junction electrodes leaked too much,' he explained.

So when Thermo Electron announced the launch of the K-series, a new range of high performance pH electrodes, Scottish Hydro-Electric were keen to trial it. ‘We had recognised the industry's need for a pH electrode that worked well in pure waters,' said Richard Newberry, then Regional Sales Manager for Thermo Electron, ‘so we designed an electrode specifically for that application'.

The K-series electrodes all employ a unique cross-linked polymer as reference material. Patented chemistry results in a sufficiently firm gel that no physical junction is required (e.g. a ceramic frit); a drilled hole suffices. Consequently the surface area of the reference in contact with the sample is much larger than on conventional electrodes. This helps but it is the remarkable stability of the polymer that means drift, even in pure waters, is minimal.

‘Since we started using the K-series electrode we've noticed a much better real-time response to small changes in pH,' observed David Barbour, Assistant Chemist, ‘it's certainly helped to make the investment in a new ammonia dosing system worthwhile.'

‘On calibration, the slope of these electrodes is reproducible and consistently good. As for maintenance, usually we only need to clean the electrodes after a shutdown. These electrodes have saved us time yet increased our confidence in the accuracy of our pH measurement', commented Mike Riddle.

For more information contact Thermo Electron, Station Road, Auchtermuchty, Fife, KY14 7DP
Tel. 01337 828871 Fax. 01337 828972, e-mail sales.water.uk@thermo.com.