Question

Discuss proper operation and maintenance of glass pH electrodes.

Answer 1

Proper Operation:

The glass electrode can be used with strong acids; however, it is attacked by strong alkaline solutions. Therefore, glass electrodes should never be left in alkaline solutions for longer than is necessary to measure the pH.
The glass electrode responds rapidly to large pH changes in buffered solutions, but the response is slower in poorly buffered, or unbuffered, solutions. Equilibrium is reached slowly and may require several seconds. Poorly buffered solutions should be stirred vigorously during measurement to prevent stagnation at the electrode. Measurements can be made in partly aqueous solutions but the degree of hydration of the outer surface of the membrane will alter the potential across the membrane. Hence, values obtained in non-aqueous, or highly ionic, solutions will be incorrect.

Maintenance of Glass pH electrodes:

Many types of pH electrode are available but the standard glass or epoxy-bodied combination electrode is ideal for the majority of tests carried out on aqueous solutions with a reasonable ionic strength at ambient temperatures and with limited use in strongly acidic or alkaline solutions.
The following general guidelines indicate the care and maintenance required for pH electrodes:

To dry the electrode, use clean soft tissues and blot the liquid from the electrode. Immerse in pH 4 buffer for short-term storage. For longer-term storage use the same solution as the reference electrolyte of the electrode. In most cases this is a 3 mol/L KCl solution. Most manufacturers supply a plastic protection cap which is filled with this solution. Close off the filler opening if there is one. If the electrode will be not used for a long period of time, you may store it dry to prevent ageing (ageing takes place only when the electrode is wet). This does not apply to combination or gel electrodes, as these must be stored in a concentrated solution of KCl only. Never store your electrode in water (see below). Always rinse thoroughly with deionised water after use. If the response of a glass electrode has become sluggish, the recommended treatment (which should only be performed when other measures have failed) consists of 1 minute in 20% ammonium bifluoride solution, followed by 15 seconds in 6 mol/L hydrochloric acid. Care should be exercised when carrying out this treatment as the risk of the formation of hydrofluoric acid is present. The electrode should then be rinsed thoroughly and soaked for 24 hours in water or in an acidic buffer solution. Electrodes that have been allowed to dry out (often indicated by a hard, dry deposit of KCl crystals) should be soaked overnight in warm deionised water. Liquid junctions with fibres or ceramic pins occasionally can become blocked due to crystallisation (eg KCl). If soaking in KCl solution does not solve the problem, raising the temperature to the maximum allowable for the reference system will often help. Other types of blockage can also occur (eg in the form of a precipitate [black] of silver chloride or mercury sulphide in the porous pin). Gentle use of abrasive paper can sometimes remove the precipitate. In other cases, chemical procedures such as soaking the electrode for a few hours in an acidic solution of thiourea (1 mol/L thiourea in 0.1 mol/L HCl) can be used. Ensure that the electrode is used and stored within its specified temperature range. Extreme changes in temperature between samples will affect response time, and electrodes used above their temperature range will age rapidly. Ensure that air bubbles are not trapped at the bottom of the electrode. If present, bubbles should be removed by holding the electrode vertically and gently tapping the electrode body. If the air bubbles are trapped by KCl crystals, heating the electrode gently to 60°C (maximum) in a water bath may also prove beneficial.

Handled carefully – the normal lifetime of glass electrodes is approximately two years.
Occasionally, functional failure occurs before mechanical failure. This is recognised by a gradually increasing electrode response time, with increasingly erratic readings. This is a different effect from electrode shock, which also produces increased response time. Electrode shock is produced by dipping the electrode into a high-concentration solution and then immediately afterwards into a lowconcentration solution, or vice versa. Thus, if one tries to measure pH 2 and then pH 11, an increased response time should be expected.