Wouldn’t it be nice if things always go exactly how we expect them to? In our minds, we always create the ideal conditions for major events in our lives before they happen. We picture the perfect job, the perfect spouse, and the perfect home. However, life routinely throws us curve balls, and reality is much different from what we had hoped. It would be nice if there was a way to calibrate ourselves to prevent this let down. Luckily with pH measurement, we can adjust the sensor slope through calibration. This ensures that performance reality meets our expectations even though conditions are not ideal.
What is Slope?
A pH sensor’s slope is the linear correlation between the raw voltage reading and a pH value. The slope is what determines how much the raw voltage reading must change in order to see a change of one pH. The theoretical slope value, or ideal condition, is 59.16 mV at 25 °C. This means that for every change of 59.16 mV the pH value will change by one pH unit. At a pH value of 7 the theoretical mV value should be 0 mV. In perfect conditions, the mV value will step change by +59.16 mV for each decreasing pH value from 7 and -59.16 mV for each increasing pH value from 7. This means that in perfect conditions the raw voltage at 0 pH would be +414.12 mV and the raw voltage at 14 pH would be -414.12 mV. As we know though, the ideal conditions rarely take place. Every pH sensor is unique and comes from the factory with a slight offset. As the sensor ages from exposure to the process, this offset will increase.
The Importance of Calibration
Every time you calibrate, you determine a new slope for each pH sensor. This adjusts the sensor for its new reality. We recommend that you perform at least a two-point calibration to ensure you are calibrating your slope correctly. In a previous post, we discussed the benefits of doing a two-point calibration in comparison to doing a one-point or process calibration.
When performing a two-point calibration, you show your pH sensor two different known pH values to verify the raw voltage difference. It is best practice that one of these pH values is 7 pH, so you adjust for your zero point offset as well. Remember, at a pH of 7 the theoretical raw voltage value is 0 mV. Placing the sensor in the second pH value will give you the raw voltage compared to what it should be under perfect conditions. This allows you to adjust for that unique sensor’s change in raw voltage per pH compared to the ideal value of 59.16 mV.
pH slope is important because it is the numerical indication of how the change in voltage correlates to a change in pH. In ideal conditions, the raw voltage will step change by 59.16 mV for every unit of change in pH value. Although we always expect the ideal conditions to happen, this is rarely the case. With the slope of a pH sensor, we can calibrate for this offset by doing a two-point calibration before use. This ensures that we get the most accurate performance from each pH sensor rather than just hoping for the best.
