Factors Affecting the Determination of PH Value of Pure Water


1. Since the ion concentration in pure water is very low, and the reference electrode salt bridge solution selects Kcl with a high concentration of 3 mol/L, the concentration difference between them is large, which is very different from its situation in ordinary solutions. In pure water, the penetration rate of the salt bridge solution will increase, which will promote the loss of the salt bridge, thereby accelerating the reduction of the concentrations of K+ and CL-. Causes the change and instability of the liquid junction potential, while the potential of the Ag/AgCl reference electrode itself depends on the concentration of CL-. As the CL-concentration changes, the potential of the reference electrode itself also changes. As a result, the indication value drifts. This is especially true for composite electrodes that cannot be supplemented with an internal reference solution.

2. The pH value reflects the activity of H+, (H+) instead of the concentration of H+ [H+], and the relationship is (H+)=f×[H+]. F is the activity coefficient of H+. It is determined by the total concentration of all ions in the solution and not only by the concentration of the measured ions. In theoretical pure water, the activity coefficient f is equal to 1, but as long as other ions exist, the activity coefficient will change, and the pH value will also change. That is to say, the pH value is affected by the total ion concentration in the solution. If the total ion concentration changes, the pH value will change. Since the liquid junction of the composite electrode is very close to the pH-sensitive glass bulb, the salt bridge solution leaking from the liquid junction first gathers around the sensitive bulb, changing the total ion concentration in the vicinity. It’s just that the pH value near the sensitive bulb has been changed, which cannot reflect its true pH value. Although the method of stirring or shaking the beaker can change this situation, it has been proved that the value of the test will be different depending on the stirring speed. At the same time, stirring or shaking will accelerate the dissolution of CO2, so it is not advisable.

3. Pure water is easily polluted. When it is measured open in a beaker, it is easily affected by CO2 absorption, and the PH value will continue to drop. The relevant international standards stipulate that the measurement must be carried out in a special device in a closed state. But it is difficult to implement in general laboratory.

4. In order to ensure the pH zero potential of the composite electrode, the salt bridge must use a high concentration of Kcl. At the same time, in order to prevent the Ag/AgCl coating from being dissolved by a high concentration of Kcl, powdered AgCl must be added to the salt bridge to make the salt bridge solution. Saturated with AgCl. However, according to the above-mentioned item 1, due to the decrease of Kcl concentration in the salt bridge solution, AgCl originally dissolved in it is supersaturated and precipitated, thereby blocking the liquid junction.

5. Due to the high internal resistance of the glass resistance, the higher the internal resistance, the thicker the glass film, the asymmetric potential will increase, the inertia of the electrode will also increase, and the generation of electromotive force will be slower. At the same time, since pure water is a non-buffering liquid, its properties are completely different from that of standard buffer solution. The electrode that performs well in standard buffer solution is moved to pure water without buffering property at once, and the establishment time of electrode potential is undoubtedly will become sluggish.

How to overcome and improve:

Because it is difficult to test the PH value of pure water, it is mainly due to the structural performance of the electrode and the instability of the electrode liquid junction potential this time, as well as the unbuffered performance of pure water and CO2 in the air and other factors.

To this end, the following methods can be used to overcome and improve.

  1. Using a PH electrode suitable for pure water should be the first consideration. At present, some domestic manufacturers have developed some electrodes suitable for or specially used for measuring the pH value of pure water.
  2. When using a separate electrode, that is, a separate pH glass electrode and reference electrode for measurement, keep the reference electrode as far away as possible from the glass electrode to improve the phenomenon analyzed in item 4 above.
  3. Increase the sampling volume of the pure water to be tested, and possibly reduce the contact surface with the air. At the same time, do not stir and shake to reduce the absorption of CO2.
  4. Add neutral salt (such as Kcl) to the water sample to be measured as an ionic strength regulator to change the total ionic strength in the solution, increase the conductivity, and make the measurement fast and stable.

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