pH measurement in high purity water has met with variable success. The high resistivity of pure water, the vulnerability to contamination, the variability of reference electrodes and additional temperature effects converge to make tins a particularly challenging measurement. Overcoming these obstacles requires a thorough understanding of the potential problems and their systematic solution. Provided here is a practical tutorial on instrument and electrode design, installation, calibration and operation which have proven successful.
Why is pH measured in the fust place? A good case can be made that in pure water it is unnecessary to monitor pH since a conductivity measurement is simpler and assures high purity. For example, if the conductivity is less than 0.06 uS/cm then the pH must be between 6.9 and 7.2—the extremes possible with strong acid and base contaminants, respectively. This pH vs. conductivity relationship has been documented in graphic form for strongly ionized acids and bases and for weakly ionized carbon dioxide and ammonia typical of power plant samples, l*-*^
If water treatment systems always produced pure water there would be no need for pH measurement of the product. Conductivity measurement would suffice. However, conductivity' is non-specific: it cannot distinguish among acids, bases and salts. Thus when conductivity does increase, the more specific nature of pH provides additional information useful in identifying the source of contamination. pH can be useful in diagnosing deionization system problems.
Despite the term 'specific conductivity.' it is conductivity's non-specific nature that makes pH measurement necessary, hi cycle chemistry control the level of pH-adjusting ammonia or amine requires pH in addition to specific conductivity to give confirmation of the treatment level.
One of the first difficulties in making any pure water measurement is preserving the integrity of the sample. Water is property called the universal solvent. It will dissolve traces of contaminants from sample lines, flow chambers and containers as well as the atmosphere. It is necessary to rinse new or unused sample lines a surprisingly long period of time before representative samples can be obtained.
A constant threat of contamination comes from carbon dioxide in air. Although air contains only 0.03% carbon dioxide, pure water in equilibrium (saturated) with ah* absorbs enough CO2 to yield a pH of approximately 5.6 and a conductivity of approximately 1 uS/cm. Carbon dioxide ionizes in water to form a weak solution of carbonic acid.