pH meter

pH meter Principle Operation:

pH meters operate by measuring the Hydrogen ion activity in a solution. This translates to pH measurements and our understanding of acids (pH less than 7) and bases (pH greater than 7) by the equation pH = -log[H+]. Refer to wiki for more information.

In order to use a benchtop pH meter one would need:
1) pH meter
2) pH electrode/probe
3) Calibration buffers (usually of pH 4 , 7, and 10)
4) Electrode filling solution (if your electrode is refillable)

The pH meter:
The pH meter interfaces signal detected by the pH electrode translates it to digital signal and outputs information to the user in a readable form (a pH reading). Today’s modern pH meter’s have added bell’s and whistles that I will refer to as “bonus features.” These include:

   portability (able to run on battery)

   usb connection to a computer

   storage of thousands of data

   Customized methods

   security passwords

    and so on

The more important improvements are their ability to include automatic temperature compensation (ATC), along with being more accurate and precise.

The Glass pH electrode:
The most common electrode found in pH meters is the glass electrode. This is a type of ion-selective electrode that is specific to proton concentration or [H+], and beside it is a reference electrode, which together are referred to as a combination electrode.

A line diagram of the combination electrode:

Ag(s) | AgCl(s) | Cl || H+(aq, out) ⸽ H+(aq,in), Cl(aq) | AgCl(s) | Ag(s)

Where the outer reference electrode consists of Ag(s), AgCL(s) and Cl(aq)

The analyte has H+(aq,out)

The H+ and Cl outside

Inner reference electrode AgCl(s) and Ag(s)

Note:
The glass membrane is the pH-sensitive part of the electrode which consists of SiO4. The oxygen anions on the membrane surface can bind to H+ from the outside (analyte) and from the inside (inner solution)

H+ protons do not move across the membrane, but rather the change between the concentration of protons outside and inside the membrane determines the pH measurement. This response of the glass electrode is the potential (E):

E = constant + B(0.05916)ΔpH

The ΔpH is the difference between analyte and inner solution

B is ideally one, but in reality, can range from 0.98 to 1.

The constant is the asymmetry potential that needs to be corrected by calibrating the pH with known standard solutions.

The slow-moving sodium ions (Cl) allows electric current to flow across the membrane, and completes the circuit.

Cons:
The cons of glass combination electrodes is a constant need to correct junction potential drift by calibration from every two hours to every use depending on the analyte being measured.

Standards also determine the accuracy of the pH measurements, and have an uncertainty of ±0.01 pH units. In addition to, junction potential error adding another ±0.01 pH unit of uncertainty. This translates to a total of ±0.02 pH units of uncertainty or a 5% uncertainty in concentration of protons H

Lastly, the temperature at which the standard is measured should be the same temperature as when the analyte will be measured.

Pros:

It is becoming more common for pH meters and electrodes to have ATC (automatic temperature compensation) functionality built in. Therefore, the changes of the temperature between measurements can be accounted for.

More accurate standards are available to improve the pH accuracy, but at a higher cost.  


Chemstack’s 8157BNUMD electrode probe


Diagram of pH electrode¹

Chemstack's Thermo Orion pH Meter A211 with pH/ATC Electrode

A lab benchtop pH meter with 8157BNUMD pH/ATC electrode has the following specs:

  • Automatic temperature compensation (ATC)
  • Temperature range from 22°F to 221°F
  • Accuracy of ±0.002pH with temp. compensation
  • pH range of -2.000 to +20.000
  • Resolution of 0.1, 0.01, or 0.001
  • Max of 5 point calibration

References:

  1. Harris, Daniel C. Exploring Chemical Analysis. 5th ed., W.H. Freeman and Company, 2013.
  2. Thermo Scientific Orion Star A211 PH Benchtop Meter. Thermo Fisher Scientific, www.thermofisher.com/order/catalog/product/STARA2110.