pH meter, experts in acidity meters, come on in! !
Thread Content
In the principle of a pH meter, is there a linear relationship between potential difference and pH value? The pH meter I use shows a pH value of 5.8 when measuring a solution with a pH of 7, and it displays 4.1 when the solution’s pH is 2.5. What could be the reason for this? Also, was this question posted in the wrong forum?:)
Calibrate it again with a standard solution to see if it’s a problem with the pH meter!
It has just been calibrated, and the standard solution is definitely fine. The situation before calibration was the opposite: when the actual pH was 7, the reading was around 8.6, while when the pH was 2.5, the reading was around 1.5 – it was completely the opposite. That’s why I’m puzzled! !
:) It’s likely that your standard buffer wasn’t prepared properly; generally, pH is related to temperature, so you need to check whether there is automatic compensation for pH and temperature. First, calibrate the zero point at pH 7 – at 25°C, this value is 6.85 – then calibrate at pH 4, and that’s it. pH isn’t linear; typically, the zero point is set at 7, with 4 and 9 forming the range of measurement.
Since our operating temperature is maintained between 30 and 35 degrees, can it be assumed that there is a linear relationship between potential and pH? Putting it another way, even if there is a slight change in the pH of the buffer during calibration, should the trend of change remain consistent in both cases? Isn’t it abnormal to obtain two opposite trends after calibrating with the same buffer twice?
Is it an online meter? What is the slope after marking? Try using the 6.86PH and 9.18PH standard solutions; if it doesn’t work, there is likely a problem with the electrode.
Based on the results you obtained, it seems there is a problem with the pH meter’s electrode!
Can such electrode calibration be considered satisfactory?
The pH value and the potential difference seem to have a logarithmic relationship, being approximately linear within a certain range. Generally, during the measurement process, the pH value being tested should fall within the standard range of two specific pH values. For example, when measuring the pH of pure water, the calibrated pH values should be 6.86 and 9.18; only in this way can the obtained Z value be accurate. It is best not to perform simultaneous calibration of the pH value at 3 points, as it is too troublesome and inaccurate. Based on what you’ve described, there might be a problem with your pH electrode; it’s also possible that the instrument itself is faulty. It’s worth checking both possibilities.
Glass electrodes have a lifespan. Check the electrode parameters, such as glass impedance and slope; if the values deviate significantly from the normal range after verification, it indicates that the electrode needs to be replaced.
The problem remains unsolved. Now I manually adjust the electrode zero point (currently around 8.5) and the slope (1.235) to ensure accuracy at pH levels above 7. Here, materials with low pH values are not common, so we make do with what’s available. Also, what would happen if acid entered the glass electrode (the port where the KCl solution is added doesn’t seem to be very airtight, and rapid stirring might allow the acid to get in)? Weren’t the electrodes of Leici considered pretty good?
I’ve used domestic desktop models, and their stability is much worse than that of online versions. The length of time the electrodes remain in the solution, as well as whether shaking occurs, can affect the measurement results; differences of up to 2 units can occur.