Different solution pH value test and selection of pH electrode

This article will focus on revealing the problems encountered in pH measurement and introduce the characteristics of electrodes that can solve these problems after development. In most cases, the pH value of the sample that cannot be measured by ordinary electrodes can now be completed by some special pH electrodes. Determination of standard pH value Laboratory determination is generally done by a conventional composite pH electrode. Usually the following conditions are required: the measurement range is between PH2 and PH12, the temperature is between 10°C and 50°C, and the ion concentration is between 0.5 and 4mol/L. Moreover, the analyte is a buffered homogeneous aqueous solution. Low ion concentration samples A sample with an ion concentration of only a few mmol or less belongs to the low ion type. Such samples will have poor electrical conductivity. In low ion samples the liquid junction will produce increased resistance. This transfer resistance causes contact problems between the reference electrolyte and the solution under test, which in turn induces a diffusion potential. Also, the signal can be affected by stirring. The appeal problem can be solved by using an electrode with a ring-shaped grounded junction (see Figure 1) that enables contact between the reference electrolyte and the solution being measured. Figure 1: Combination electrode with annular grounded junction. Semi-aqueous or anhydrous solutions Relative pH measurements are only possible in anhydrous solutions (less than 5% water). Semi-aqueous solutions are basically also low-ionic. If the sample contains more than 5% moisture, the traditional definition of pH measurement applies, ie: the values ​​obtained are values ​​and not relative values. The contact area (junction) between the electrolyte and the solution to be measured is usually separated, causing signal instability. Furthermore, there is a possibility of sedimentation at the liquid junction. This can easily occur when a saturated KCL solution is used as a reference solution. To prevent this phenomenon to the greatest extent, ensure the fluidity and compatibility between the electrolyte and the sample to be tested. Using LiCl-containing ethanol or LiCl-containing acetic acid as reference electrolyte for anhydrous samples can solve the appeal problem. Protein-rich solutions When high concentrations of protein come into contact with the KCL reference electrolyte, it may precipitate at the junction. Using a special electrolyte can solve this contradiction. A protein-contaminated junction can generally be cleaned by soaking the electrode in a mixture of pepsin and HCl for several hours. Sulfide-containing solutions When using Ag/AgCL reference systems, the reference electrolyte always contains dissolved silver. When the solution silver in the liquid junction contacts with the sulfide in the measurement solution, insoluble silver sulfide will be generated. Silver sulfide clogs the liquid junction and turns it black, which results in a slow and unstable measurement signal. In the case of reference systems with silver ion traps, AgCl-free electrolytes can be used. Connections contaminated by sulfide can sometimes be soaked in thiourea/HCL solution through the electrode to form silver sulfide. Solutions containing hydrofluoric acid Hydrofluoric acid can damage the glass membrane and at low concentrations can also hinder the formation of a gel layer. This can lead to unstable measured values ​​and shorten the service life of the electrodes. Hydrofluoric acid has a damaging effect only when the pH value is lower than 5. When the total fluoride ion concentration is between 0.2g/L (PH3; 20°C) and 1g/L (PH1; 20°C). There are special electrodes for this type of application. When the concentration of fluoride ion is high, antimony (Sb) electrode and special reference electrode are used.