Measuring pH of Yogurt

Maintaining electrolyte fill levels helps obtain more stable readings.

Credit: Hanna

Conic electrode tip. Conventional pH electrodes have a spherical sensing bulb that provides an increased surface area for the sample to interact with the sensing glass. This bulb shape is ideal for measurement in aqueous solutions. However, other tip designs exist on the market, and each shape offers an advantage in certain applications. For example, conical tipped pH electrodes are pointed so that they may easily penetrate semisolid or emulsified objects, including yogurts.

If measuring the pH of yogurt with an electrode constructed of a spherical bulb and ceramic reference junction, a homogenized slurry of yogurt and deionized water should be prepared. A slurry is necessary because the flow rate of electrolyte into a semisolid yogurt alone is too slow to enable a direct measurement. An electrode utilizing a conical tip shape in combination with an open reference junction allows for direct measurements of thick yogurt samples, thus saving on preparation time and eliminating a potential source of error. For thinner yogurts or other dairy products such as milk or cream, the spherical tip may be suitable due to its wider area of contact that permits a faster stabilization time. Ultimately, the selection of the tip should be based on the nature of the sample matrix.

Make Sure Your Sensor Works

Calibrate your electrode often. Prior to measurement, pH meters must be calibrated. Calibration adjusts how pH values are assigned to incoming mV (millivolt) readings from the electrode. The pH electrodes generate a mV potential based on hydrogen ion activity. This activity is determined by pH glass, which is specially formulated to measure the hydrogen ion. Hydrogen ions (H+) contribute to how acidic a sample is, while hydroxide ions (OH-) contribute to how basic a sample is. The pH scale ranges from 0 to 14, with pH values less than 7 being acidic, pH values greater than 7 being basic, and pH 7 being neutral.

As pH glass breaks down and changes over time due to normal wear and tear, calibration of the meter corrects for changes in the glass. The quality and frequency of calibration procedures will ultimately determine the accuracy of your data. For best results, it is important to calibrate the pH meter at least once per day with standards that bracket the expected pH range of the samples. Because milk and yogurt typically have a pH range from pH 6.7 to 4.0, ideal calibration standards are pH 4.01 and 7.01; a third buffer such as pH 1.68 or 10.01 may be incorporated for higher precision.

The mV reading at pH 7 is the offset. The ideal offset of an electrode is 0.0mV and should never be outside of ±30mV.

Credit: Hanna

Perform periodic slope checks. The theoretical relationship between pH and mV is defined by the Nernst equation. Based on this equation, a theoretical electrode will read 0 mV in pH 7.0 buffer (the value of which is known as the offset), and will have a slope of -59.16 mV per change in pH unit. Calibration corrects for deviations of electrode behavior from this theoretical relationship, but the extent of this correction is finite before the accuracy of the measurement is affected. Many meters will have indications of electrode condition or slope condition, but it is recommended to use the mV mode on a pH meter to periodically check electrode offset and slope.

To perform an electrode offset and slope check, first measure and record the mV value in pH 7.0 buffer; this is the electrode offset. Next, measure the mV value in a second buffer, such as pH 4.0. To determine the electrode slope, calculate the difference in mV between the two buffers and then divide this by the difference of pH units between buffers. To convert this result to electrode slope percentage, divide the electrode slope by the theoretical slope of 59.16, and multiply by 100. An acceptable offset range is ±30 mV and slope percentage is 85 to 105 percent; anything outside of these ranges may result in inaccurate measurements.