Getting on the Same Page: Calibration
When we look for answers in GlobalLab, we base our conclusions on data we all submit. But when we use probes and sensors, and since each of us uses our own instruments to collect data, how can we know whether our findings can be compared? How do we know our instruments even measure their target, in this case pH, or measure with comparable accuracy? What can be done so we can trust each other’s findings? For pH measurements scientists use a procedure called the calibration of pH probes. We check how accurately our probes and sensors measure pH by first measuring so-called ‘standard solutions,’ those with pH values we already know, since we either prepared them ourselves or purchased them from chemical catalogs.
The essence of the calibration of pH instruments is to make sure the measuring device we use is accurate enough to measure pH. Wee can then confidently combine other student’s readings with our own data and draw more accurate conclusions.
To calibrate our instrument, we use pH–samples, or so-called “standard solutions”, prepared beforehand with known pH-values. Then we measure the pH of these solutions with our device to determine if there is any difference between the ‘true’ pH values of the solutions with those measured by our device. Usually, for calibration purposes, several solutions with different pH values are used.
If we use an electronic pH-meter, and discover that there is a small discrepancy in the readings, we can adjust the meter using a special correction procedure described by the electronic device’s manufacturer. Often, it is a special ‘correction screw’ that needs to be turned until the screen reads the same pH as the ‘true’ pH’ values of the prepared (standard) solution). If the device we use to measure pH is connected to a computer, we can program a “correction value”, and an amendment can be made to the computer’s calculations.
If we use a paper test strip to measure pH, we need to keep in mind adding or subtracting this correction value from the actual readings. For example, if we measure a standard solution of pH = 4 with our probe, sensor, and discover that the instrument shows the pH = 4.3, it means our device is “off” by 0.3 pH units, and our future readings should be “corrected” by 0.3.
Keep in mind, that pH-measuring devices can behave differently within different pH- ranges. For example, in the region around pH = 8, the error cannot be not 0.3, but 0.5, and in the region around pH = 11, the error may be 0.7 units. That is why the calibration procedure usually is done with three or four standard solutions with pH in the acidic (below pH 5), neutral (around pH 7) and alkaline (above pH 8) regions.