What are side reactions?
These are reactions with substances in the sample that:
- interfere with the stoichiometry of the KF reaction
- change the pH value of the KF reagent
- either produce or use up water themselves
- oxidize on the anode of the generator electrode
- reduce on the cathode of the generator electrode
- react with the ingredients of the KF reagent
Recognizing side reactions
One of the worst things that can happen with KFT is not knowing that a side reaction is falsifying your results. Below are some characteristic signs of side reactions.
Titration time and titration curve
Some indications of side reactions include longer titration times compared to the titration of a water standard, slow endpoint detection, and a higher drift value after the titration finishes than at the titration start. Comparing the titration curves of the sample and a water standard with a similar water quantity makes it easier to evaluate the situation. Just plot a graph of the volume against time (or µg water against time, in the case of coulometry). If the graph exhibits a curve that increases steadily as illustrated in Figure 1 (in orange), this can indicate a side reaction.
Figure 1. Side reactions can often be identified from checking the titration time and the titration curve, as shown in this graph.
If you notice that the water content depends on the sample weight or the titrant consumption (µg water for coulometry), then you can check the slope of a regression line after plotting water content against titrant consumption (µg water).
Ideally, the slope (b) should be 0. Significantly positive or negative values can indicate a side reaction, as shown in Figure 2.
Figure 2. If the slope of the regression line for the water content / titrant consumption value pairs deviates significantly from 0, this indicates a side reaction.
If the water recovery value found after spiking the samples is not within 100 ± 3%, this can indicate a side reaction. Depending on the type and speed of the side reaction, the recovery may be too high or too low. For example, samples which contain DMSO (dimethyl sulfoxide) change the stoichiometry of the Karl Fischer reaction and therefore result in false low readings.
Please note that a recovery rate of almost 100% does not guarantee the absence of a side reaction. Side reactions that take place very rapidly will not be detected, since the side reaction is already complete when the spiking process begins. A spiking procedure is described in detail in chapter 2.5.12 of the European Pharmacopoeia.
The oxidation of iodide or reduction of iodine leads to incorrect results.
How can you check whether your sample is undergoing a side reaction with iodine or iodide? A simple preliminary test can clarify the situation. Dissolve the sample in a weakly acidic (alcoholic) solution and then add some drops of iodine or potassium iodide solution. Based on the coloring (a discoloration of iodine or the formation of brown iodine), a side reaction can be detected.
Evaluating redox potentials
Comparing the redox potentials of the redox pairs of sample substances with the redox potential of iodine/iodide can be helpful to assess whether an undesired redox reaction may occur.
If the standard potential is higher than that of iodine/iodide, as in the case of e.g., chlorine, the oxidation of the iodide may result in false low readings.
If it is lower (e.g., lead), the reduction of the iodine may result in values that are too high.
|Element changing oxidation state||oxidized form + x e– → reduced form||Standard electrode potential E°|
|Cl||Cl2 + 2e– ⇌ 2 Cl–||+1.36 V|
|I||I2 + 2e– ⇌ 2 I–||+0.54 V|
|Pb||Pb2+ + 2e– ⇌ Pb||-0.13 V|
Avoiding side reactions
Most side reactions can be suppressed by taking suitable measures, such as those listed here.
- For ketones and substances that react with the methanol present in the KF reagent: Use methanol-free reagents.
- For samples that lower the pH range of the KF reagent: Add buffer solution for acids or a stoichiometric excess of imidazole.
- For samples that increase the pH value (e.g., aminic bases): Add buffer solution for bases or a stoichiometric excess of salicylic acid / benzoic acid.
- High drift after titration: Postdrift correction may help. This is done by stopping the titration at a defined time and recording the additional consumption over several minutes. This allows the calculation of the drift after the titration. This postdrift is then used to correct the water quantity found.
- Samples that reduce iodine: Subtract the iodine consumption of the reductant in the sample from the overall iodine consumption of the sample.
- Samples that oxidize iodide: Reduce the oxidant, e.g., Cl2, in advance with an excess of SO2, for example, by treating the sample with the solvent of a two-component reagent.
- General: Carry out the titration in a thermostatically controlled cell connected to a circulation thermostat at, e.g., -20 °C in order to slow down the side reaction. Note that the titration parameters should be adjusted to the low temperatures.
- General: Extract the water with the KF oven method if the interfering components are thermally stable at oven temperature.
- General: Mask or eliminate the interfering component, e.g., by adding N-ethylmaleimide in the case of thiols.
Side reactions can negatively influence and falsify your results. Recognizing and avoiding side reactions in KF titration is therefore crucial for the most accurate determinations.
For more information, check out our blog series about frequently asked questions in Karl Fischer titration.