The disodium tartrate dihydrate has probably not dissolved completely. The extraction time should be increased in order to prevent this.
In addition, care should be taken to ensure that the sample does not remain on the side of the vessel or on the electrode.
An increasing titer may be traced back to the limited capacity of the methanol to dissolve disodium tartrate dihydrate. We recommend that you replace the working medium at shorter intervals.
Another reason may be that the sample size selected was not optimal.