Measuring pH of Yogurt

Step 1. Measure mV of pH 7.01 buffer and record value
Step 2. Measure mV value of pH 4.01 buffer and record value
Step 3. Calculate the absolute mV difference (pH 4.01 value – pH 7.01 value)
Step 4. Calculate the slope (mV difference/3)/59.16) = Slope)

Examples:
Electrode 1
pH 7.01 = -15 mV
pH 4.01 = +160 mV
Absolute mV difference is +160 mV – (+15 mV) = +175 mV
Slope = (175/3)/59.16=98%

Electrode 2
pH 7.01 = +15 mV
pH 4.01 = +160 mV
Absolute mV difference is +160 mV – (-15 mV) = +145 mV
Slope = (145/3)/59.16 = 82%

Conclusion: Electrode 1 is working properly while electrode 2 has an unacceptable slope. If changing the fill solution, cleaning the electrode, and calibrating does not help, replace the electrode.

Improve Your Measurement Technique

Consistency is key when performing any pH measurement. This is especially true when many different users are expected to perform the same measurement with the same electrode and meter. Even with a reliable, properly prepared sensor, careless measurement practices can have detrimental effects on a critical pH result. This is especially true for yogurt, where the final measurement window is only four tenths of a pH unit.

Maintain the electrolyte solution. Yet another advantage of owning an electrode filled with gel reference electrolyte is that it does not have to be refilled. The gel electrolyte should last for the lifetime of the sensor. On the other hand, if you own a refillable pH electrode without gel electrolyte, the level of electrolyte fill solution should be inspected before performing any calibration. Over time, the solution flows out of the reference junction, which can happen faster particularly if the electrode is not properly stored. Low electrolyte levels may lead to drifty or erratic readings, so it is good practice to ensure that your electrode fill solution level is no less than one half inch from the fill hole.

Also for refillable electrodes, the fill cap should be removed or loosened prior to calibration and measurement. Removing the cap creates positive head pressure in the reference chamber of the electrode, allowing for a greater flow rate of electrolyte through the junction. This is important for a faster and more stable reading, especially given the thicker consistency of yogurt.

Properly submerge and stir. It is crucial that both the pH sensing glass tip and the reference junction be completely immersed in your sample to function correctly. Make sure there is enough yogurt sample present to perform a suitable measurement.

For pH measurements and calibrations, it is important to stir the sample or buffer. While an automatic or magnetic stirrer is preferred, simply stirring the electrode within the sample can help. It is understandable that this can be tricky for thicker samples such as yogurt, but movement of the sample ensures it is well mixed and helps increase the response time of the analysis.

Slope is the change in mV per pH unit. The ideal slope of a pH electrode is 59.16, so the difference between pH 4 and 7 should be 177.48mV.

Credit: Hanna

Keep Up With Your Maintenance

Despite choosing an appropriate electrode and calibrating it correctly, poor maintenance and lack of proper care can reverse any diligence previously exercised.

Regularly clean your electrode. When pH is measured in dairy products such as yogurt, electrode fouling is a common challenge. Electrode fouling occurs when fats and proteins obstruct the reference junction or attach themselves to the sensing glass of the electrode. Electrode bodies and tips may also accumulate mineral deposits such as milkstone, a complex composed of organic matter, calcium, and magnesium.