Test kits for water analysis
Test kits for water analysis

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Test kits for water analysis

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.

Download this video clip.Video player: Video 6
Skip transcript: Video 6 Measuring pH.

Transcript: Video 6 Measuring pH.

Narrator
A frequent operation carried out in an analytical laboratory is measuring pH. This is a pH meter - it is essentially a voltmeter, whose output is in pH units rather than volts. It's linked to a combination glass and reference electrode - essentially a tube within a tube - an inner glass electrode sensitive to protons, and an outer reference electrode which provides a constant potential. We measure the potential difference between the analyte solution and the internal reference. At the bottom is a fragile glass membrane - this is the sensing part of the probe and must be treated with great care. You'll notice it's surrounded by a protective shield.
First, we need to calibrate the system. To do this we need solutions with fixed and known pHs - these are called buffers. Generally (but not exclusively) buffers having pHs of 7, 4 and 10 are used. Ideally we recommend checking all three pHs, although two is generally deemed to be sufficient. In this example we are going to measure the pH of a soft drink we know to be acidic, so we'll calibrate with the pH 4 and 7 buffers
So let's carry out the calibration. First we thoroughly rinse the electrode with distilled/deionised water, and dry off the excess moisture. Although it's not absolutely necessary, we're using a magnetic stirrer here. Now we dip the electrode into the pH buffer, being careful to prevent the stirrer flea from touching the delicate probe. After waiting for the reading to stabilise we adjust the reading to pH 7.00 using the buffer control. Notice this meter has a built-in temperature control which automatically compensates for any variation of the pH with temperature. Now we remove the electrode and once again thoroughly rinse it with distilled/deionised water and make sure that it's dry. Then we dip it into the pH 4 buffer solution, leaving the reading to stabilise, and then we adjust the display on the meter to give a reading of 4.00.
Make sure you throw away any used buffer and store the stock in a refrigerator.
So the pH meter is now calibrated. After another rinse and dry with distilled/deionised water we can use it to measure the pH of our soft drink. A pH of 2.27, in other words highly acidic. Also note this is the value at 18.5 °C.
Finally a few words about storage and maintenance of pH electrodes. Ideally the storage solution should be similar to that inside the reference compartment. Here we are using potassium chloride acidified with hydrochloric acid to a pH of 1 to 2. One thing you mustn't do is keep the electrode in distilled/deionised water; this will cause excessive leaching of chloride ions out of the reference probe eventually rendering it unstable. Some laboratories use a pH 7 phosphate buffer in which to store the electrode, this is fine except be aware that microbial growth can occur, particularly in hot conditions. The potassium chloride reference solution in the outer tube is open to the environment; as such some of this solution will be lost - due to evaporation, and during contact with the test solution - so it needs to be regularly replenished. If you notice your readings fluctuating significantly - this is a tell-tale sign that your reference solution needs topping up.
End transcript: Video 6 Measuring pH.
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.

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