RETURN TO
AGGIE
HORTICULTURE


  


This article appeared in the December 2001 issue of Texas Food Processor,
edited by Dr. Al B. Wagner, and produced by Extension Horticulture,
Texas Cooperative Extension, The Texas A&M University System, College Station, Texas.


Common Mistakes Made When Measuring pH

Electrochemical Techniques In Food
And Beverage Analysis

Excerpt from an article by Clifford L. Nilsen, entitled "Electrochemical Techniques in Food and Beverage Analysis," appearing in "Food Quality" magazine.

There are a number of electrochemical procedures applied to food and beverage analyses. The most widely used are pH, potentiometric titrations and Karl Fischer titrations. As with any other laboratory procedure, electrochemical techniques are only as good as the chemist performing the procedures. Following are common mistakes that can be easily avoided.

pH

  • Not rinsing the electrode and wiping it dry prior to use. This can result in possible contamination and an unreliable pH reading.

  • Failure to calibrate with buffers prior to each use. Uncertain pH readings will result since the state of calibration is uncertain.

  • Storing electrodes in distilled or deionized water. Doing so can deplete salts from the electrode filling solution, resulting in poor electrode performance. Buffers or tap water should be used for electrode storage.

  • Not maintaining a proper level of filling solution (such as saturated KC1) in the electrode. Also causing poor electrode performance, this can also make it difficult to achieve a suitable slope correction during buffer calibration.

  • Storing the electrode dry. Salt deposits will from around the ceramic junction, interfering with electrode response.

  • Incorrect calibration. Failure to calibrate correctly with at least two buffers will most likely give inaccurate pH readings.

  • Using expired buffers. pH readings resulting from using expired buffers cannot be considered reliable.

It is necessary that pH meters are calibrated with buffers in order to match the non-linearity of the electrode(s) to the perfect linearity of the pH meter amplifier. The slope is the adjustment for electrode non-linearity. A typical procedure for proper pH calibration of a pH meter is a follows;

  1. Fill a clean and dry 10 mL beaker with about 80 mL of pH 7.0 buffer.

  2. Add a small, coated magnetic stir bar, place the beaker on a stir plate and begin gentle stirring.

  3. Rinse the electrode thoroughly with deionized or distilled water and part it dry with a soft tissue.

  4. Immerse the electrode into the pH 7.0 buffer as far as possible without touching the bottom or sides of the beaker and without the stirring bar banging into it.

  5. Set the pH meter slope control to exactly 100 percent.

  6. Read the pH of the buffer. If it is not exactly 7.0, adjust the calibration knob on the meter to produce a 7.0 reading.

  7. Rinse and dry the electrode, then repeat the above three steps using either pH 4.0 buffer if readings are expected to be below pH 7.0m, or with pH 10.0 buffer if readings are expected to be above pH 7.0. Read the pH of the buffer.

  8. If the pH is not exactly 4.0 or 10.0, depending upon which buffer was used, adust the slope control of the meter to read exactly 4.0 or 10.0.
The meter is now ready for use. If the pH 4.0 or 10.0 buffer cannot be sloped to read 4.0 or 10.0, then recondition or replace the electrode.


 


 RETURN TO
TEXAS FOOD
PROCESSOR