Why calibrate your pH meter and how!

Why calibrate your pH meter and how!

When it comes to measuring pH you deserve good data. In order to obtain accurate pH data you need to calibrate well and often. In order to obtain precise pH measurement a calibration should be performed before each measurement. In saying this, sometimes this is not feasible and a good rule to abide by is to calibrate at the beginning of each day. This will offer you repeatable results with a high degree of accuracy.

What is calibration?
A pH calibration is the process of adjusting your pH meter by measuring solutions of a known pH value. This is because the characteristics of your electrode will change over time and this needs to be compensated for. A calibration does this by matching your pH meter to the current characteristics of your pH sensor.

What types of calibration are there?
There are single point, two point and multi-point calibration when it comes to pH meters. Essentially this is how many points a calibration is performed at. A single point calibration can be used when you are looking to measure a consistent pH value with little variation. This method involves only using a single buffer solution as a reference for calibration.

The most common pH meter calibration is the two point calibration and is best suited when you have a range of pH samples. The buffer solutions should bracket your expected pH sample. In this process the pH meter determines the slope and offset error for the actual pH electrode in use. This information then allows the meter to adjust the mV/pH equation of the pH meter to match the characteristics of the electrode in use. 

If you are bracketing a wider range of the pH scale a multi-point calibration covering this range will give you the most accurate and repeatable results. This is where 3 or more calibration points are done and gives the pH meter a more accurate mV/pH equation over the range you are looking to cover.

Performing a Two Point Calibration
  1. Turn the pH meter on and enter the calibration mode on the pH meter
  2. Select two pH buffers that bracket the expected sample pH. The first buffer should be pH 7.00 (your neutral/zero point) and the second buffer should be near the expected sample pH (pH 4.01 or pH 10.01). 
  3. Before starting calibration, be sure the sensor and the buffer solution are at the same temperature. If not, allow time for temperature equilibration. If you have a meter with ATC (Automatic temperature compensation), the meter will compensate for the temperature difference and as such you can skip this step.
  4. Pour the necessary amount of buffer solutions into individual glass beakers. Buffer solutions will remain stable in a glass beaker for a maximum of 2 hours. Please note: Never carry out calibrations in the bottle of buffer solution as this will taint your solution and significantly reduce accuracy and do not pour used solutions back into the bottle, discard them and use new solution each calibration.
  5. Place the electrode into the pH7 solution you have decanted. When the reading is stable, set the pH meter to the pH7 value. 
  6. Remove your electrode and rinse it with distilled water
  7. Repeat step 5 with your next buffer.
  8. When the pH meter calibration is done, rinse the electrode and place into the sample and take your pH measurement.
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