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Test kits for water analysis
Test kits for water analysis

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6.2 pH probes

The pH of an aqueous solution is an expression of its acidic or alkaline nature, and is defined as

pH = - log [H+(aq)]
Equation label: (21)

where [H+(aq)] is the concentration of hydrogen ions in the aqueous solution, in mol l−1.

The pH scale is a direct measure of the hydrogen ion concentration, with a pH value of 1 representing a hydrogen ion concentration of 1 × 10−1 mol l−1, and a pH of 14 indicating a hydrogen ion concentration of 1 × 10−14 mol l−1.

Question 12

What is the hydrogen ion concentration in an aqueous solution of pH 5?

Answer

pH = − log10 [H+] = 5
so [H+] = 1 × 105 mol l1

To measure pH accurately we use a voltmeter (referred to as a pH meter) linked to two electrodes, one of which is sensitive to H+ ions and the other of which is a reference. For ease of use, the two electrodes are usually housed within one glass casing. These are often referred to as combination electrodes. A cross section is shown in Figure 10, which also includes a brief description of some of the components. The voltage difference between the electrodes is proportional to the pH outside the glass membrane.

Figure 10  Cross-section of a combination electrode for pH measurement.

Question 13

Suggest how you would calibrate a pH meter.

Answer

You could take readings from solutions whose pHs are known.

Fortunately solutions are available to us that have known pHs. These are buffers - solutions that resist changes in pH either on the addition of small quantities of acids and alkalis, or when diluted. A further consideration is the temperature of the solution you are measuring, as the sensitivity of the electrode varies with temperature. Compensation for such changes is either automatic, whereby a built-in temperature sensor (thermistor) signals the meter, or manual, whereby the operator has to key in the temperature measured by an external thermometer. Either way, the meter makes the appropriate adjustment to the output.

Let's now look in detail at this process and how pH measurements are actually carried out using a meter and electrode. Watch Video 6 Measuring pH.

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Video 6 Measuring pH.
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The concentration of acid or alkali affects many chemical or biological properties of water, for example it can influence the growth of fish populations, and is one of the most important indicators of quality. Industrial wastewaters may contain significant concentration of mineral acids which will lower the pH of water.

Portable handheld and weatherproof pH meters encased in plastic suitable for outdoor work are available for use in the field. These are particularly suitable for field work due to one feature of the combination pH electrode:

A ceramic plug is responsible for allowing electrical contact between the sample and the reference solution. This plug is actually a porous piece of ceramic material pushed through the glass shaft of the electrode, and which acts as a restriction, allowing reference material to slowly flow out of the electrode rather than flood out. It is prone to blockages, however, particularly with highly turbid or dirty samples - the type of water you're likely to sample in the field.

Field pH probes tend to have open junctions, where the reference electrolyte is completely exposed to the environment. It doesn't run straight out because the reference material is not a liquid. These probes use a gel-type electrolyte, which is permanent (no refilling is necessary) and ideal for measuring dirty samples.