General Purpose Glass Sensor

HANDHELD pH INSTRUMENTS

Technical Reference

What is pH?

Definition: pH is the negative logarithm of hydrogen ion activity in a solution.

The Concentration ratio of hydrogen ions (H+) and hydroxyl ions (OH-) determine the pH value of a solution. Any hydrogen activity will produce a 59.16 mV/ pH unit across the glass membrane. The measurement is expressed on a scale of 0.0 to 14.0. Water with a pH of 7 is considered neutral (H+ ions = 10-7 and OHions =10-7). A solution is considered acidic when the hydrogen ions (H+) exceed the hydroxyl ions (OH-), and a solution is considered an alkaline (base) when the hydroxyl ions (OH-) exceed Hydrogen ions (H+).

How is pH measured?

A pH instrument consists of three main components, refer to Figure 1.

  1. The pH measuring cell: Hydrogen sensitive glass is blown onto the end of an inert glass stem. A silver wire, treated with silver chloride (Ag/AgCl) is sealed inside the glass (cell) with a solution of potassium chloride saturated with Silver chloride. The measuring solution has a neutral pH level of 7 or 0 mV. A properly hydrated glass sensor will produce a “Gel Layer” on the inside and outside of the glass membrane. The “Gel Layer” enables hydrogen ions to develop an electrical potential across the pH glass sensor; a millivolt signal varies with hydrogen ion activity on the glass membrane while submerged in the solution being tested.
  2. The Reference cell: A silver wire treated with silver chloride (Ag/AgCl) is sealed inside an inert glass housing (cell) with a solution of potassium chloride saturated with silver chloride. The inert glass prevents hydrogen ion activity from test solutions to influence the reference cells constant millivolt signal. The combination of the reference electrode silversilver chloride wire, and the saturated potassium chloride solution develops a constant 199-millivolt reference signal. The millivolt signal produced inside the reference electrode does not vary as long as the chloride concentration remains constant. The reference voltage is used as a baseline to compare variations or changes in the solution being tested. The reference cell is in contact with the test solution through a reference junction that is commonly made of porous Teflon®*‚ ceramic, or a wick type material called a Pelon strip. This junction completes the measuring circuit of the pH sensor.
  3. Display meter: When the pH sensor is placed in a solution, the pH-measuring cell develops a millivolt signal that reflects the hydrogen ion activity of the test solution. A high impedance meter accurately measures the small millivolt changes and displays the results in pH units on either an analog meter or digital display.

Temperature considerations:

The pH glass membrane is sensitive to the temperatures of solutions being tested. Prolonged use and/or exposure to temperatures (above 35°C) will accelerate the aging, and increase chemical attack to the glass membrane which will shorten the overall service life of the sensor.

ELEVATED TEMPERATURES WILL SHORTEN THE SERVICE LIFE OF A pH SENSOR.

Increase temperatures also decreases the impedance of the glass membrane. The decrease of the impedance affects the millivolt output of glass membrane. Temperature changes close to neutral (pH 7) usually do not affect pH levels; however, when levels are < pH 3 and > pH 11 a dramatic error may occur. This problem is resolved using a built in ATC (Automatic Temperature compensation) which uses a mathematical formula (Nernst equation) to correct pH errors due to temperature factors.