H Measurement with a Glass Electrode

A difference of 59.16 mV (at 25°C) builds up across a glass pH electrode for every factor-of-10 change in activity of H+ in the analyte solution. Because a factor-of-10 difference in activity of H+ is 1 pH unit, a difference of, say, 4.00 pH units would lead to a potential difference of 4.00 X 59.16 = 237 mV. The charge of a calcium ion is n = 2, so a potential difference of 59.16/2 = 29.58 mV is expected for every factor-of-10 change in activity of Ca2+ in the analyte measured with a calcium ion-selective electrode. 

Cremer at the Institute of Physiology at Munich discovered in 1906 that a potential difference of 0.2 V developed across a glass membrane with acid on one side and neutral saline solution on the other. The student Klemensiewicz, working with F. Haber in Karlsruhe in 1908, improved the glass electrode and carried out the first acid-base titration to be monitored with a glass electrode.13 

Outer reference HJ outside HJ inside Inner reference 

So little current flows across a glass electrode that it was not practical when discovered in 1906. One of the first people to use a vacuum tube amplifier to measure pH with a glass electrode was an undergraduate, W. H. Wright at the University of Illinois in 1928, who knew about electronics from amateur radio. Arnold Beckman at Caltech invented a portable, rugged, vacuum tube pH meter In 1935, which revolutionized chemical instrumentation.16 

To perform an electrical measurement, at least some tiny current must flow through the entire circuit—even across the glass pH electrode membrane. Studies with tritium (radioactive 3H) show that H+ does not cross the glass membrane. However, NaH sluggishly crosses the membrane. The H+-sensitive membrane may be thought of as two surfaces electrically connected by Na+ transport. The membrane s resistance is typically 108 SI, so little current actually flows across it. 

The most widely employed ion-selective electrode is the glass electrode for measuring pH. A pH electrode responds selectively to H , with a potential difference of 0.059 16 V for every factor-of-10 change in [H ]. A factor-of-10 difference in [H ] is one pH unit, so a change of, say, 4.00 pH units leads to a change in electrode potential of 4.00 X 0.059 16 V = 0.237 V. 

The pH-sensitive part of the electrode is the thin glass membrane in the shape of a bulb at the bottom of the electrode.  

A pH electrode must be calibrated before use. It should be calibrated every 2 h in sustained use. Ideally, calibration standards should bracket the pH of the unknown. 

Don t leave a glass electrode out of water (or in a nonaqueous solvent) longer than necessary. 

Store the glass electrode in aqueous solution to prevent dehydration of the glass. Ideally, the solution should be similar to that inside the reference compartment of the electrode. Distilled water is not a good storage medium. If the electrode is dry, recondition it in aqueous solution for several hours. If the electrode is to be used above pH 9, soak it in a high-pH buffer. 

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