The Required Purity of the Water Used in the Experiment

An experiment, in essence, is the act of verifying hypotheses derived from a phenomenon. Whether a hypothesis can be proven true depends crucially on its reproducibility. The reproducibility of an experiment requires not only skillful technique but also the purity of the chemical reagents used and the precision of the analytical instruments. The purity of the chemical reagents used to prepare solutions and the water used in the experiment is also very important. If contaminants in the water are assumed to affect the experimental results, these substances must be removed. Furthermore, to obtain good reproducible results, using pure water with stable quality is necessary. With the increasing sensitivity of analytical systems used in experiments, higher requirements have been placed on the purity of the water.

In water, an electric current is applied to two electrodes with a surface area of 1 cm², spaced 1 cm apart, to monitor the conductivity between the electrodes. The resistance between the electrodes can be determined by the applied voltage and the measured current. This value is commonly referred to as resistivity or specific resistance in water quality analysis, and its unit is MΩ·cm. The reciprocal of resistivity is called conductivity, expressed in μS/cm. These two parameters are the most commonly used parameters for expressing the purity of water.

Removing ions from tap water increases resistivity (decreases conductivity), but this increase is not unlimited. This is because some water molecules ionize into hydrogen and hydroxide ions, with a resistivity limit of 18.248 MΩ·cm (25°C). Furthermore, resistivity changes with the ionization constant of water and is therefore affected by water temperature. For example, ultrapure water at 25°C has a resistivity of 18.2 MΩ·cm, but it is 84.2 MΩ·cm at 0°C and 1.3 MΩ·cm at 100°C. Around 25°C, a 1°C increase in temperature will decrease the resistivity by 0.84 MΩ·cm. Therefore, resistivity compensated to 25°C is often used as a benchmark.

In addition, parameters such as Total Organic Carbon (TOC), pyrogen and endotoxin content, bacterial content, particulate matter content, microbial content, and total dissolved solids (TDS) are also frequently used as important supplementary parameters for describing water quality. Therefore, the purity standard of water is usually described and graded by one or more of these parameters.