How to avoid fluctuating concentration values of KF titrants if di-sodium tartrate is used as the standard?

The concentration determination of the KF titrant is fluctuating; the concentration values are constantly out of specifications even if a new KF titrant is used?

Possible reasons:

The possible reason might be that di-sodium tartrate is not completely dissolved in the solvent. The stirring time is too low to ensure this resulting in incomplete dissolution revealing a cloudy solution.

Recommendations:

The concentration determination is correct only if di-sodium tartrate (DST) has completely dissolved. The maximum solubility of DST in 40 mL Methanol is 0.12 g, This maximum value shall not be exceeded and means that a maximum of three determinations, with 40 mg of sample can be performed in sequence on the same solvent. It is highly recommendable to increase the stirring time up to 180 s in order to ensure complete dissolution. Please note that DST from different manufactures can have different physical features and can require very different stirring time for the complete dissolution. Grinding of DST prior to weighing fosters dissolution. The standard amount shall be added into the KF titration cell with the aid of the weighing boat. When adding the standard with the weighing boat, care must be taken that no standard is adhering to the beaker wall or the sensor. Use the back weighing technique, i.e. weigh the sample boat after addition of the standard to the KF cell in order to determine the accurate sample amount.

If using pure water as a standard, the accuracy of the balance or micro-syringe may not be adequate and is causing the error.

As an alternative, KF titrant concentration determination can also be performed with liquid standards. Liquid standards do not show solubility issue as the solid standards.