Introduction to Standard Addition and Sample Addition Methods.
Also see: Alternative Procedures for Highest Possible Precision.

These methods involve measuring the voltage in a relatively large volume (e.g. 25 - 100 ml) of sample (for Standard Addition) or standard (for Sample Addition) then adding a much smaller volume (e.g. 1 - 10 ml) of standard (or sample) and taking a second reading after the voltage has stabilised in the mixture. The volume and concentration of the standard and the volume of sample must be chosen carefully according to the following criteria:

1) The volume of the first solution must be large enough to cover the tips of both electrodes and allow room for a magnetic stirring rod to continuously stir the solution during measurement and mixing of the additive - whilst ensuring the electrodes remain immersed.

2) The concentration and volume of the additive must be sufficient to cause a significant and measurable increase in the measured voltage of the first solution (ideally, 20 to 30 mV - achieved by increasing the original concentration by 5 to 10 times - depending on the electrode slope at the concentration of the first solution).

3) The volume of the additive must be small enough so that it does not cause a significant change in the Ionic Strength of the first solution.

4) The volume of the additive must be large enough so that volumetric errors are not significant - or use Gravimetric Addition - i.e add standard from a weighed "dropping bottle" then re-weigh.

Summary of Advantages over Direct Potentiometry

• The electrodes remain immersed throughout the process so that there is little change in the liquid junction potential of the reference electrode (which can often be changed by several millivolts when the electrodes are removed from one solution and placed in another) between calibration and sample measurement - and therefore this source of measurement error is virtually eliminated.
  
  
• Calibration and sample measurement are both made essentially at the same time and in the same solution so that ionic strength and temperature differences between standard and sample are reduced and ISAB is not generally required to compensate for this - although it may be required to help reduce the time needed to obtain a stable reading; particularly for low concentration samples, in the non-linear range of the electrode calibration, or for highest possible precision measurements (see later).
  
  
• Once the approximate concentration for the samples is known, the calibration (slope) can be "fine tuned" by analysing a standard with a concentration that lies within the range of the samples (and is at the same temperature) and then adjusting the slope and re-calculating the results until the standard gives the correct answer. This "fine tune" procedure is very quick and easy using the ELIT ISE/pH Ion Analyser Software.
  Alternatively, the slope can be calculated off-line using a modified version of the Standard Addition equation and then typed in to the Standard Addition software.
  See Alternative Procedures for more details.
  
  
• Measuring the slope at or very near to the sample concentration means that these methods can achieve more reliable results in the non-linear range and can be used with old or worn electrodes which may not be completely linear over their whole range, as long as the slope is stable and reproducible over the limited range of the samples.

The main disadvantage of these methods is that it is necessary to mix together accurately measured volumes of standard and sample, and they involve more complex calculations than simple direct potentiometry. This makes them a more skilled and time-consuming procedure and they have traditionally been less popular with most ISE users. It must be noted, however, that these analyses can now be made much more easily and quickly using the ELIT ISE/pH Ion Analyser (2-channel computer interface) software to calculate a suitable volume and concentration for the standard, measure the electrode potentials, and calculate the results - and by using automatic syringe pipettes to dispense the measured volumes.

A further slight disadvantage is that the approximate concentration of the sample must be known before commencing the analysis in order to choose an appropriate standard concentration and suitable volumes for the two solutions. Nevertheless, if the sample concentration is completely unknown, it can easily be determined by making a quick Direct Potentiometry measurement first. This can be done using an old calibration graph - but the most accurate assessment can only be made if the electrodes are calibrated using two standards which span the expected range of the samples immediately prior to analysis.

The Standard Addition (also known as "Known Addition") method is the most common and involves adding a small volume of a concentrated standard (e.g. 2, 5 or 10 ml) to a much larger volume of sample (e.g. 25, 50 or 100 ml). The voltage is first measured in the pristine sample. Then the standard is added, the solutions are mixed well, a second reading is taken and the software calculates the concentration of the sample.

The main limitation to the Standard Addition method is that it requires relatively large volumes of sample (at least 20ml, preferably 100). Moreover, the measurements are made at the ionic strength of the sample and it is possible that a slightly reduced slope would be more appropriate for samples with high Ionic Strength. Ideally, therefore, for the highest possible accuracy, the slope should be determined using standards made up in a similar matrix to the samples, but not including the detected ion or any that would interfere with it.

The Sample Addition method is used in exactly the same way as Standard Addition, except that a small volume of sample is added to a larger volume of standard. There are three main advantages of this method over Standard Addition:

1) The sample matrix is almost irrelevant (it is normally diluted at least 1:10) and therefore the measurements are made at the ionic strength of the standard and the normal electrode slope is appropriate.

2) The pure standard solution is always measured first so that the operator can choose a standard concentration to ensure that all measurements are made within the optimum linear range for the electrode.

3) For greatest possible accuracy, the standard can be measured by Standard Addition and the electrode slope adjusted to give the correct concentration, (then the new concentration, after the Standard Addition calculated) before adding the sample.

This method is most useful when only small amounts of sample are available, or for samples with relatively high concentration (generally needs to be above about 100ppm for method to work well), or those with a complex matrix. In the latter case it is important to achieve a sufficient dilution of the sample matrix so that it does not significantly alter the ionic strength of the standard and it may be beneficial to dilute the sample first before adding it to the standard; but care must be taken to ensure that the concentration of the measured ion is still sufficient to cause a detectable change in the mV reading in the pure standard. This requirement is the reason why, in most cases, this method is not suitable for samples with less than about 100ppm.

Basis of Calculations

These methods depend on the fact that the plot of electrode potential (mV) against concentration (ppm or mol/L) is a logarithmic curve. Thus any particular ratio of the amount of increase in mV in response to a particular increase in concentration (i.e. the slope of the curve) will only fit in one unique part of the curve.

The formulae for the Standard Addition calculation is:

And for Sample Addition:

Alternative procedures for highest possible precision.

Partly based on ideas published by Denis Rice, Consulting Analytical Chemist,
64 Cliff Drive, Katoomba, NSW 2780, Australia. tdrice@pnc.au

" Gravimetric Analyte Addition Ion-Selective Electrode Potentiometry with Initial Spiking"
ISBN 0-646-37757-4.