FEATURES OF CONDUCTOMETER CALIBRATION FOR DISTILLED WATER QUALITY CONTROL

Abstract

The aim of the work. Determination of the influence of container dimensions, in which the conductometer is calibrated, on the correctness of measuring the specific electrical conductivity of test solutions and distilled water for pharmacopoeial needs.

Materials and Methods. The specific electrical conductivity measurement of standard solutions and distilled water was carried out on a Hanna HI 2300 conductometer at a temperature of (25.0±0.5) °C. The conductivity meter was calibrated using a "Conductivity Standard 84 µSm/sm" solution (Mettler Toledo). An additional check of the correct operation of the conductometer was done using a solution of  29.4 µS/cm. Distilled water was obtained on a GFL-2008 aquadistiller. The temperature of standard solutions and distilled water was controlled by a TFA 30.1018 thermometer and maintained by a LOIP LT-112 circulation thermostat. Standard solutions and distilled water were stored and tested in high-pressure polyethylene containers.

Results and Discussion. The experiment proved that the value of the specific electrical conductivity of  the test solutions and distilled water depends on the container volume in which the conductometer was calibrated and in which container the measurement was made. In the case of small volumes containers (20 and 50 ml), the correct result is obtained only under conditions of calibration and measuring in the containers of the same volume. In case of measuring the specific electrical conductivity of distilled water in a smaller container than the one in which the calibration was performed, the device shows underestimated values. In larger containers, the values of specific electrical conductivity are overestimated. In case of calibration in a 100 ml container, the correct result is achieved for measuring the specific electrical conductivity of distilled water in 100 and 1000 ml containers. The obtained results can be explained by the so-called "field effect" – the electromagnetic field is partially outside the geometric space of the measuring cell, and it can be affected by inhomogeneities and interference, such as vessel walls.

Conclusions. The correct conductivity measurements for small volumes of solutions (20 and 50 ml) can be obtained by calibrating the conductometer and measuring the conductivity in containers of the same size and volume. The best approach is to calibrate and measure the electrical conductivity of solutions in containers with a volume of at least 100 ml, in order to minimize the influence of the walls of the container on the correct conductometric measurements.