Variation in results and repeatability of result is poor

What is the reason for variation in results and poor result repeatability.

Possible reasons:

The sensor was not calibrated in case of an EP titration and it may also be due to sensor contamination or improper response. Incomplete dissolution of a sample can also be responsible for variation of results. The amount of solvent added is too low or the dissolution capacity of the solvent is not sufficient to dissolve the total amount of sample added. Stirring time is too short in order to dissolve the amount of sample added. Air bubblse may be present in the tubes and burette, which lead to inaccurate dosing thus affecting the dosing accuracy. Cross contamination (contaminated solvent) or sample carry over in which previously analyzed samples may stick to the equipment (sensor, dosing tube, magnetic stirrer bar) can therefore lead to wrong titrant consumption and wrong results. The most common problem lies in inaccurate sampling which leads to variation in results.

Recommendations:

Prepare fresh titrant and determine the titer value before sample analysis. Low concentration titrants e.g.0.01M NaOH require more frequent standardization.To keep the titer of sodium hydroxide constant, it is important to avoid intake of CO₂ from air which in turn leads to formation of carbonate. This can be avoided by using a suitable adsorbing medium i.e ascarite. Make sure to calibrate the pH sensor with fresh, non-expired pH buffers whose pH values encompass the required pH range. If a sensor is not performing well, check if the membrane is dehydrated, contaminated or damaged, electrolyte contaminated or missing, an air bubble behind the ceramic junction, crystallized KCl blocking the outlet of electrolyte; all these may affect the sensor’s performance. Verify that your burette and all tubes are bubble free and perform a rinsing procedure of the titrant in the burette before starting the titration. Make sure that the equipment is thoroughly rinsed with deionized water and is free from any adhering sample. Increase the solvent volume in steps of 10 mL and observe the dissolution: a solid free solution must be obtained at the end. Increase the stirring time and verify that a solid freesample solution is obtained. Sample size, burette size and titrant concentration shall be selected to use 30 to 90% of the burette volume to reach the endpoint. Burette refilling during a titration should be avoided since it can lead to signal instability or irregularities in the curve profile.