FAQs on pH Measurement Technology

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FAQs on pH measurement technology: How much does temperature affect the accuracy of pH measurement? For pH electrodes, the effect of temperature is 0.003 pH/°C per pH unit. For example, a pH meter with an accuracy of 0.2 grades is calibrated in a pH 7.00 buffer at 30°C, and then tested in a solution at 60°C (assuming that the pH of the solution ranges from pH 6 to pH 8, which is one pH unit away from pH 7.0). The maximum error caused by temperature is then 30 × 0.003 = 0.09 pH. For a change of 3 pH units (within the range of pH 4 to 10), the maximum error is 0.27 pH, which shows that temperature has a significant impact on pH. Of course, we can also conclude that, in order to reduce the errors caused by temperature on pH measurements, we should pay attention to the following three points: (a) Choose a buffer solution whose pH is as close as possible to that of the solution being measured, in order to calibrate the pH meter; (b) Ensure that the temperature of the calibration solution is the same as or as close as possible to that of the solution being measured; (c) Use a pH meter with temperature compensation. pH meters with an accuracy of over 0.1 pH come equipped with temperature compensation functions, while pH meters with an accuracy of 0.2 pH do not have such compensation. Some pH meters of the 0.2 grade claim to have a precision of 0.1 grade, but this is impossible; some people confuse the concepts of resolution of 0.1 pH with precision of 0.1 pH. Even considering just one pH unit, the pH error for a difference of 60°C is 0.03×60=0.18 pH; therefore, a pH meter without temperature compensation can achieve a maximum accuracy of only 0.2 pH. Can temperature compensation eliminate all errors caused by temperature? It must be specifically noted that the temperature compensation set on the pH meter only compensates for the slope term of the electrode (2.303RT/F). Affected by temperature are also the standard potential of glass electrodes, the potential of reference electrodes, the junction potential, etc.; these do not have a strictly linear relationship with temperature. At the same time, the pH electrode also needs some time to reach equilibrium at the new temperature. Therefore, neither manual temperature compensation nor automatic temperature compensation is very sufficient. According to the operational definition of pH measurement, in order to obtain accurate results, the sample solution and the standard solution must be measured at the same constant temperature; this is the principle of isothermal measurement. For pH measurements with general accuracy requirements, when the temperatures of the sample solution and the standard solution differ, temperature compensation can be used. How to use a pH electrode stand? Why use a pH electrode stand with a stirring function? Using a pH electrode stand for testing is a habit inherited from previous measurement methods. Since the electrodes used in the past were separate measurement electrodes and reference electrodes, testing with two electrodes necessarily required the use of an electrode holder. These days, pH composite electrodes are generally used. In fact, an electrode holder is not necessarily required, but its use is beneficial in that it serves as a habit that makes measurements easier, and it also helps to prevent glass pH composite electrodes from breaking. However, there is a problem here: after the pH composite electrode is immersed in the solution, it needs to be stirred to accelerate the response. This is especially true for plastic-shell pH electrodes, as it helps to remove bubbles from the cavity surrounding the electrode bulb; otherwise, the response will be slow or the test results may be inaccurate. Therefore, when using an electrode holder for testing, a relative motion is required to gently shake the beaker containing the solution; however, this approach is rather inconvenient. To address this issue, Shanghai Sains Instrument Factory designed an electrode holder with stirring functionality, essentially combining a magnetic stirrer and an electrode holder into one unit. After immersing the electrode in the solution, turn on the stirrer and stir for about ten seconds, then stop stirring and take a reading; this method should be the easiest and most reasonable. Moreover, the purpose of stirring goes beyond that; for solutions containing other reagents, stirring helps to achieve uniformity quickly or ensure complete reaction. For some solutions with very low ionic strength, such as pure water or organic solutions, as well as for the testing of some viscous or suspended solutions, stirring yields better results. How to test the pH of a trace solution? Among some foreign pH electrode samples, we can find electrodes designed specifically for testing the pH of trace solutions; these typically have a very small outer diameter, but they are quite expensive. Here we recommend a simple method: use the 2015P-C flat pH composite electrode produced by Shanghai Sixin Instrument Factory. A small flat-bottomed beaker slightly larger than the outer diameter of the electrode (Φ15) can be used, allowing the testing of at least 0.3 ml of solution. The testing results are excellent, and this electrode can be used in conjunction with various models of portable or laboratory pH meters. How to test the pH value of skin, paper, leather, meat, fruits, etc.? Using the same 2015P-C planar pH composite electrode, materials such as skin, paper, and leather can be tested as long as they contain water and their surfaces are cleaned. For testing meat and fruits, there is also a method used abroad, which involves the use of needle electrodes equipped with stainless steel conical blades or glass electrodes with a conical shape. However, such electrodes are equally expensive, and their performance is not necessarily ideal, as friction occurs on the surface of the glass membrane during penetration, resulting in induced effects. The simplest method is also to use the 2015P-C planar pH composite electrode; isn’t it enough to just cut the meat and fruit with a knife? Next, let’s introduce another interesting test: Engineers at the Shanghai Food Research Institute found through testing that by cutting a piece of meat horizontally with a clean knife, and then measuring the pH value of the resulting cross-section. The pH level is between 7.0 and 7.4 in fresh meat; it then gradually decreases to 5.4–5.5, marking the onset of the rigor period. After that, the pH rises again to 5.6–6.2, indicating maturity – this is the optimal time for consuming the meat. If proper preservation measures are not taken during this period, the pH level will gradually increase, and the \"fresh meat\" will turn into \"spoiled meat\". This method is suitable for pork, beef, and aquatic fish; interested readers might want to give it a try. How to test the pH value of high-purity water? pH measurement of high-purity waters such as boiler feed water, steam condensate, and deionized water is a rather complicated testing task. The reason is that the conductivity of the sample is low, and the resistance of the solution becomes significant when compared to the other resistances in the measurement circuit. At the same time, due to the instability of the liquid junction potential and changes in pH caused by the dissolution of carbon dioxide in the air, it is difficult to obtain reproducible results. To solve this problem, glass pH composite electrodes with low internal resistance, a large liquid junction area, and a high permeation rate of the reference solution are typically used. The glass sensitive film of the composite electrode is close to and fixed at a short distance from the liquid junction, which helps to reduce the influence of solution resistance and achieve stable potential. A large liquid junction area and a fast infiltration rate of the reference solution help to reduce resistance and achieve a stable and reproducible liquid junction potential. At the same time, do not use a potassium chloride solution saturated with silver chloride as the reference solution, because silver chloride will precipitate out in pure water and block the liquid junction. Generally, the ionic strength of rainwater is very low (not exceeding 0.0001 mol/L), and its pH measurement method (for monitoring acid rain) is similar to that used for measuring the pH of high-purity water.