General matters regarding measurement methods using ion selective electrodes

Measurement range

In general, the maximum ion concentration that ion selective electrodes can measure is around 10^-1 mol/L, and the minimum is 10^-4 to 10^-7 mol/L.
Since the minimum measurable concentration differs depending on the type and structure of the ion electrode, it is necessary to check the reproducibility of measurements near the minimum limit using reference solution. In addition, at the minimum and maximum limits, the gradient of potential difference versus concentration is likely to be small, so it is necessary to compensate for errors using reference solutions that have concentrations close to that of the sample to be measured.

Ionic strength

Because the activity coefficient changes under the influence of ionic strength, thereby causing errors in measurement, the ionic strength of the sample solution must be kept constant. In order to achieve this, an indifferent salt (supporting electrolyte)—which does not react with the target ions or have any impact on electrode potential—needs to be added to the sample. It is necessary to select different types and add different amounts of indifferent salts depending on the type and concentration of the ions to be measured.

Response time

The time it takes for the electrode potential of an ion selective electrode to become stable within a range of variation of 1 mV depends on electrode type and structure, ion type, concentration, and ionic strength.
The response time when measuring a high ion concentration after measuring low ion concentrations is relatively short, while the opposite case tends to produce a longer response time. In addition, at around the minimum measurement limit, the response time is generally relatively long, being of the order of several minutes. With regard to a 1mV variations in potential difference, a one digit change in output gives a measurement error equivalent to an error in concentration of around 4% for monovalent ions and 8% for divalent ions. Hence, when using the ion electrode method, it is necessary to wait until the potentiometer indicates a stable value before taking a reading.

Impact of pH

The measurable pH range depends on the type and structure of the ion selective electrode. In general, this range is smaller when the concentration of the ions to be measured is lower.
For some types and structures of ion electrode, the components of the sensitive membranes may dissolve or electrode potentials may change under the influence of pH. Furthermore, some pH levels may cause a decrease in the sensitivity of the ion electrode, or parallel shifting of the calibration curves. To prevent these effects, keep the pH level of the sample solution as constant as possible while carrying out measurements.

Impact of temperature

The temperature of the sample solution has an impact on the potential gradient measured by the ion electrode method. Every 10°C increase in the temperature of the solution results in a change in potential of around 2 mV for monovalent ions and 1 mV for divalent ions. The temperature must be the same for both the reference solution used to determine the calibration curves and the sample solution being measured.
Note that stirrers can produce heat, thereby changing the temperature of the solution.

Potential gradients calculated by Nernst's equation (mV/pX)

Impact of stirring

Stirring the sample solution may have an impact on potential measurements performed using an ion selective electrode, since it can result in changes in electrode potential, response time, and the minimum measurement limit. To avoid this, stirring of the solution should be performed as quickly and steadily as possible, and should not hinder the process of measurement.

Impact of light

Light can cause a change in potential in some types of ion selective electrodes. If using one of these types, use a brown beaker to exclude light. Solid-state membrane ion electrodes containing mainly silver halides are particularly prone to being affected by light.

Impact of coexisting ions

Although ion selective electrodes have good selectivity, no ion electrode can avoid the effects of all ions. When using the ion electrode method, it is important to know the effects of coexisting ions and take measures to avoid them.
The effect of coexisting ions on electrode potential can be predicted from the extent to which they react with the components of the response membrane. For example, solid-state membrane electrodes can be seriously affected by coexisting ions that form insoluble compounds or complex salts with the response membrane material; and liquid membrane electrodes can be affected by coexisting ions that form ionic associates with components in the response membrane.

Accuracy of ion selective electrodes

The rate of change of concentration against potential is obtained from Nernst's equation as follows:
△Ｃ／Ｃo＝１０△E /(2.303 RT/ZF)－1
A change in potential of 1 mV produces an error in concentration of 3.97% for monovalent ions and 8.22% for divalent ions at 25°C. When carrying out measurements with an ion selective electrode, use a potentiometer that can be read accurately to a precision of 1 mV or better.

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