Activity and Concentration
Ion selective electrodes (ISE) measure the activity of ions in solution. Activity depends on the ionic strength (IS) of the solution and is virtually equivalent (close) to the concentration when in very dilute solutions. In more concentrated solutions with high IS, the activity decreases and will differ from the concentration. Differences in IS between standard and samples can cause errors in the ISE measurement.
Use of ISA or Special ISA Reagent
To minimize this difference, an Ionic Strength Adjuster (ISA) solution is added to samples and standard solutions before measurement, so that the IS is similar in all. In addition to IS adjustment, ISA reagents can contain pH adjustor's, decomplexing agents, or species that remove interferences.
Strong Sample Backgrounds
Most ISAs increase the IS to about 0.1M; however, this strength may not be sufficient to match the background of samples with very high IS, such as seawater, brines, strong acids, strong bases, foods, beverages, etc. and won’t give a uniform IS in samples and standards. Depending on the composition and properties of the high ionic strength samples, certain measurement techniques are recommended, such as DKA, sample dilution, etc.
Notes on Necessary Conditions for DKA Analysis
Complexing agents should be absent or in excess (at least 1000 times the sample species concentration).
- The ratio of free species to complexed species must remain constant throughout the additions of standard
- Indication that a complexing agent is present, but not in a great enough quantity to maintain a constant free/complexed ratio, is an abnormally high or low slope. In this case, an excess of complexing or decomplexing agent may be added to the sample prior to analysis
- If you do not know whether complexing agents are present, assume that they are not present
Electrode interference should be minimized, because the effect of interference may change as the concentration of species of interest changes.
- This may result in loss of precision, high, or low slope. Use of special ionic strength adjustor's that remove interference is recommended
- See above section on special ISA reagents and consult the appropriate electrode user guide for specific information
Measurement Hints for Double Known Addition Method
1/ Any sample size may be used depending on availability and expected sample concentration. If sample is a liquid, recommended sample volume should be 100 mL or 50 mL. If the sample is a solid, dilute a weight of 1-3 grams to a known volume with DI water.
2/ The first standard addition should approximately double the initial sample concentration. For a 100 mL sample size, this can be done by adding 1 mL of a standard that is about 100 times higher than the expected sample concentration (or 0.5 mL for a 50 mL sample volume)
3/ Alternatively, calculate the volume of standard required to double the initial sample concentration using this formula:
Vfi = (Csample * Vsample)/Cstandard,
- Vfi – volume of first increment, mL
- Csample – expected sample concentration
- Vsample – sample volume in the analysis beaker
- Cstandard – standard concentration
For example, if your samples are estimated to be near 10 ppm, use a spiking solution about 100 times higher (e.g. 1000 ppm) and calculate the first standard addition as follows:
Vfi = (10 ppm sample * 100 mL sample) / 1000 ppm standard = 1 mL first addition
4/ The volume of the second addition should be ten times the first spike, e.g. 10 mL, if first spike is 1 mL.
5/ The total volume of the added standard should not be more than 11% of the initial sample volume to avoid significant dilution effects.
Troubleshooting DKA Testing
1/ Use logged slope and mV values to troubleshoot DKA testing, if results are not as expected.
2/ If the first spike volume does double the concentration as expected, then the observed change in potential from the initial reading will be about 16-18 mV for monovalent ions and about 8 – 9 mV for divalent ions. If not, try adjusting the spike amount higher or lower to be closer to this range.
3/ The observed change in potential between the first and second spike should be about 46 mV for monovalent ions and about 23 mV for divalent ions (if the first addition did about double the sample concentration).
4/ If the calculated slope value is much higher or lower than expected, there may be interference.
- Spike a sample and run DKA on the spiked sample. Is the recovery of the spike as expected?
- If not, there may be an interference or a complexing agent present. See the electrode user guide to determine appropriate methods for removing interference and for known complexing and decomplexing agents