Pure water pH measurement in low conductivity samples

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Courtesy of IC Controls Ltd.

Normally, pH measurements are made in water with 1,000 to 10,000 uS/cm conductivity. The conductivity conies from dissolved solids, typically, salts plus net acid or base which explains the exhibited pH. These dissolved substances are in the solution in ion form, which makes such solutions conducting. With 1,000 uS/cm conductivity of mobile ions pH electrodes have a good circuit and give fast, accurate pH readings.

In pure water, standard pH electrodes exhibit less reliable results. They respond more slowly, drift, and do not reproduce calibration results between buffers and grab sample. In highly pure water pH electrodes become jumpy, flow sensitive and inaccurate.

Pure Water is a relative term for low conductivity water of less than 500 uS/cm. IC CONTROLS lias observed these effects on waters between 50 and 500 uS/cm, most likely due to pH Buffer carry-over on the pH electrode. IC CONTROLS considers water below 50 uS/cm to be High Purity and recommends High Purity modification on all samples less than 15 uS/cm. We consider water below 5 uS/cm to be Ultra Pure and recommend Ultra Pure modification on all applications below 1 uS/cm
Normal pH buffers have strong chemical compositions in order to resist change and make a good standard. These chemicals can be measured in a general way relative to the sample by checking their conductivity. Normal buffer conductivities are in the 400,000 mSm/cni range, far higher than pure water.

Low Conductivity Symptoms

  • Low conductivity effects can be traced to
  • concentrated pH buffer carry-over into low conductivity sample;
  • differences between reference junction potential in low sample conductivity versus high buffer conductivity;
  • differences in apparent reference junction potential as rate of flow through the pH flow cell washes ions away from the reference;
  • absorption of carbon dioxide by low conductivity sample when exposed to air for calibration;
  • high resistance of the pure water producing a jumpy ungrounded 'antenna' effect in the pH electrode.

The effects will become more noticeable as the conductivity falls from simply Pure to High Purity and on to Ultra Pure, where they become predominant, below 1 uS/cnx Also, the effects become more noticeable as greater accuracy is required.

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