pH Electrode Technical Education

H₂O -> H+ + OH- = 1 x 10^-14 (mol/L)2 = Kw

Hydrogen Ion Concentration in Moles/Liter at 25° C

pH of Common Materials

Why is pH an Important Measurement?

It determines product quality in:

• Sugar refining
• Pulp and paper mills
• Latex coagulation
• Photo developing

It enhances product efficiency of:

• Flue gas scrubbing
• Circuit board etching
• Fermentation processing
• Assuring product safety
• Chromate and cyanide destruct systems
• Potable and waste waters
• Food, low pH to prevent botulism

How Can pH be Measured?

Colorimetric Methods

• Reagent addition
• pH paper

Electrochemical Methods (pH electrodes)

A pH Measuring System Consists of:

1. A pH electrode: an electrode whose output voltage changes as the pH (hydrogen ion concentration) changes.
2. A reference electrode: an electrode whose voltage output stays constant.
3. A pH meter: a millivolt meter with a special high impedance input circuit and circuits to change the electrode's millivolts into pH unit readouts.
4. Optionally, an automatic temperature compensator: a device which senses temperature so that the meter can correct for the effects of temperature changes.

How Does A pH Electrode Work?

• Special composition glass senses H+ and a millivoltage is generated (59.2 mV per pH unit at 25° C).
• A filling solution picks up the signal from the special pH glass.
• A pure silver wire dipped in silver chloride passes the signal from the solution whose pH is being measured to the electrode's cable or connector.
• Sodium Ion error for solutions > pH 12.3.

How Does A Reference Electrode Work?

• A porous reference junction separates the filling solution in the electrode from the solution whose pH is to be measured.
• The filling solution's constant chloride ion concentration generates a millivoltage at a pure silver wire with silver chloride on it.
• The silver wire passes the signal from the solution being measured to the electrode's cable or connector.

Single vs. Double Junction Reference

Chemicals that cause silver to precipitate at the reference junction will contaminate and plug single junctions. These may be such compounds as sulfides, mercaptans, cyanides, Iodides, and proteins. Other compounds such as silver, lead, mercury, and other heavy metal compounds will react with the chloride in the gel, causing a reduction in the reference output. Selection of the proper chemistry in the lower (double) junction will prevent or at least minimize the negative effects of these reactive compounds.

How Does A Combination pH Electrode Work?

A combination pH electrode consists of a pH electrode and a reference electrode built into a single body or housing. A combination electrode therefore works like the pH and reference electrodes combined!

How Does a pH Meter Work?

A pH meter takes the input from the pH glass (high impedance mV) and the input from the reference sensor and compares these two values to get a resulting millivolt reading. The reading in mV is converted to pH by the following guidleine:

• Zero mV = pH 7
• 59.2mV per pH unit change
• mV are + for pH <7 and mV are - for pH >7

Example of pH Circuit

Note that the pH and reference input go into an operational amplifier (op amp) due to the very high resistance of the pH glass. The meter will also adjust zero and span offsets and can do automatic temperature compensation for pH error (discussed below).

Temperature Compensation

When measuring pH using a pH electrode the temperature error from the electrode varies based on the Nernst Equation as 0.03pH/10C/unit of pH away from pH7. As shown in the table below, the error due to temperature is a function of both temperature and the pH being measured. Note that there is no error at pH7 and 25° C. Temperature compensation can be achieved manually or automatically. Manual temperature compensation is usually achieved by entering the temperature of the fluid being measured into the instruments menu and then the instrument will display a "Temperature Compensated" pH reading. This means that the pH value is corrected to the value expected at 25° C. Automatic temperature compensation requires input from a temperature sensor and constantly sends a compensated pH signal to the display. Automatic temperature compensation is useful for measuring pH in systems with wide variations in temperature.

Note: Values in light blue are less than 0.1 error and may not require temperature compensation. Values in gray are temperature and pH in which there is no error in pH from temperature.

How are pH Systems Calibrated?

• The electrodes are placed in buffers. Buffers are solutions of know, stable pH value 
  The commonly used buffers have pH values of 4.01, 7.00 and 10.00 
  The pH values of buffers change with changes of temperature (and so does the pH of solutions being measured)
• First the the system's (electrode and meter together) Zero Point is adjusted 
  Zero point is usually determined with pH 7.00 buffer 
  Ideally, a buffer is used whose value is close to that of the material or solution being measured 
  A "calibrate "or "standardize" adjustment potentiometer knob or push button on the pH meter is used to set the system (electrode and meter together) to read the buffer's pH value
• Next, the system's span is checked and/or adjusted. The electrodes are rinsed and put into a second buffer 
  The system should read close to the pH value of the second buffer. 
  Most meters have controls labeled "SPAN" or "SLOPE" which compensates for electrodes with spans that are too short.
• The frequency of calibration is at the discretion of the user.
• NOTE: Two-point calibration is necessary to ensure electrode works properly since a broken electrode can give acceptable pH7 output in calibration mode.

pH Applications

• Wastewater neutralization
• Electroplating
• Chemical manufacturing
• Circuit board etching
• Flue gas scrubbers
• Boilers and cooling towers
• Pulp and paper mfg.
• Food and beverage
• Pharmaceuticals
• Inks, paints latex
• Water treatment (sewage)
• Aquariums, aquaculture
• Fermentation (wine, beer, alcohol)