pH via Meter with Combination Electrode


Background:  A pH (potential of Hydrogen) measurement reveals if a solution is acidic or alkaline, or neutral.  The pH scale is logarithmic and runs from 0.0 (acidic) to 14.0 (alkaline or basic) with 7.0 being neutral.


Method pH Water

Method pH Food

Method pH Dairy

Apparatus

Turn Around Time

Sample Required

Type of Test

Reportable Units

Measurement Range

Related Resource

APHA SM 4500-H+B  ISO Accredited

AOAC 981.12, USDA/FSIS MLG Ch 2  ISO Accredited

APHA SMEDP 15.022  ISO Accredited

pH meter with a resolution of 0.01 pH units with combination glass electrode

Same day if submitted before 1 pm

100 mL liquid samples of 10 to 100 g solid samples of about 4 oz semi solid samples

Potentiometric

pH

0.0 to 14.0

What is Water Activity and pH?


Analysis Description:  Samples are prepped according to various techniques which are appropriate to the sample submitted.

 

pH electrode

  1. Sensing bulb
  2. Glass (or measuring, sensor) calomel electrode
  3. Internal solution (saturated, neutral KCL)
  4. NA
  5. Reference calomel electrode
  6. Reference internal solution (saturated, neutral KCL)
  7. Ceramic junction
  8. Body of Electrode

The glass electrode is the pH-sensing electrode (#2).  During measurements, an electrical potential is produced across the thin “glass membrane” tip of the glass electrode, which is a bulb of special glass that is electrically conductive (#1).  The rest of the glass body of the electrode is non-conductive.  The potential is proportional to the H+ concentration, and the pH meter converts the potential differences directly into pH units.

The reference electrode (#5) serves as a stable potential standard for comparison with the glass electrode.  It also completes the electrical measuring circuit by using saturated KCl (#6) as a liquid junction to do so.  To complete the circuit, a very small amount of this electrolyte solution must pass through a very small hole (ceramic junction #7) at the bottom of the electrode continuously while pH measurements are being made.  Proper flow of KCl requires that an air hole near the top of the electrode be opened up prior to making measurements.