86 FOOD SCIENCE
H
Extra electrode features
anna Instruments offers a number of additional electrode features. These include a built-in temperature sensor for the automatic compensation
of temperature variations and a microprocessor-based digital version that stores GLP data and serial number for traceability. High-temperature glass for use in cooking processes is another feature. Having the right pH electrode for your food product will ensure the best possible results and a long life.
Hanna offers pH electrodes with an open junction design. With this style of junction, there is a hard gel known as viscolene that separates the sample from the internal reference. Te open junction design is extremely resistant to clogging making it the best design for semi- solids, emulsions and direct measurement of food products.
Calibration
When calibrating a pH electrode it is important to choose buffers that ‘bracket’ the expected reading. Bracketing is the process of calibrating a pH meter to points above and below the expected pH value of the sample being tested.
When a pH probe is placed in any solution, a voltage is generated and converted to a pH value. A pH buffer is a solution of a known value. It is the voltage response in a pH buffer that is being standardised during the calibration process.
A pH meter will typically accept ±60 mV for a pH 7.01 buffer. Tis value is known as the offset voltage. Large changes in an offset voltage can indicate that either the buffer is no longer the pH value that is stated on the bottle or that there is a coating on the pH electrode Hanna Instruments recommends that the offset should be ±30 mV. Te calibration of a pH
electrode to pH 4.01 or 10.01
buffer is known as the slope adjustment. Tis value, relative to the offset, determines the slope of the line used by the meter to correlate between the mV of a sample to its pH reading. A probe with 100% slope will generate 59.16 mV/pH unit away from pH 7.01 at 25°C. Most pH meters will calibrate to a slope between 85 and 105% (50 mV – 62 mV/ pH unit at 25°C). Te slope of a pH electrode should be greater than 90%.
All pH electrodes will have their slope degrade over time. It is typically a gradual process. Any large change in slope from one calibration to the next is an indicator that the pH buffer is contaminated.
Many portable and benchtop pH meters now have storage for data logging and advanced diagnostic capabilities
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