Basic concepts of pH Meter

A pH meter  is an analytical instrument used to determine acidity or alkalinity of a solution - also known as pH. pH is the unit of measure that describes the degree of acidity or alkalinity. It is measured on a scale of 0 to 14.

What is Centrifugation?

Centrifugation is atechnique to separate particles from a solution by the application of centrifugal forces and the separation is achieved based upon theparticle size, shape, density, viscosity of the solution, and centrifugal rotor speed.

Figure -1 A typical pH meter

pH is represented in the form of an equation as the negative logarithm of the Hydrogen ion concentration.

For example in pure water or neutral solution,

The quantitative data provided by the pH calculation expresses the degree of the activity of an acid or base in terms of hydrogen ion activity. The pH value of a substance is directly associatedwith the ratio of the hydrogen ion [H+] and the hydroxyl ion [OH-] concentrations in the solution. If the H+ions concentration is more than OH-, the material is acidic (pH < 7). If the OH- concentration is greater than H+ions, the material is basic (pH > 7). If equal amounts of H+ and OH- ions are present in the solution, itis neutral (pH = 7).

pH Measurement

The pH value of a solution can be measured by pH papers or indicators, which change colour as the pH level varies, but it won't give an accurate reading. Theprecise pH measurements are calculated with an electronic pH meter. A pH meter consists of three parts: a pH measuring electrode, a reference electrode, and a high input impedance meter. The pH electrode can be assumed of as a battery, with a voltage that varies with the pH of the measured solution. The pH measuring electrode is a hydrogen ion sensitive glass bulb, with a millivolt output that contrasts with the changes in the relative hydrogen ion concentration inside and outside of the bulb. The reference electrode output does not vary with the activity of the hydrogen ion. The pH electrode has a very high internal resistance, making the voltage change with pH difficult to estimate. The input impedance of the pH meter and leakage resistances are, therefore,vital factors. The pH meter is a high impedance amplifier that accurately measures the minute electrode voltages and displays the results directly in pH units on either an analog or digital display. In some cases, voltages can also be read for special applications or for use with ion-selective or Oxidation-Reduction Potential (ORP) electrodes.

pH Electrodes

pH electrode technology has not reformed much in the past 50 to 60 years. Even with the modern technological developments of the last 30 to 40 years, pH electrode manufacturing remains an art. The unique glass body of the electrode is blown to its configuration by glassblowers; it's not an advanced or high-techtechnique but a very much critical and essential step in the electrode manufacturing. The thickness of the glass determines its resistance and affects its output.

A typical modern pH probe is a combination electrode, which combines both the glass and reference electrodes into one body. The combination electrode consists of the following parts:

Figure-2: Scheme of the typical pH glass electrode. ( image source Wikipedia)

1. a sensing part of the electrode, a bulb made from a specific glass
2. internal electrode, usually a silver chloride electrode or a calomel electrode
3. The internal solution, usually a pH=7 buffered solution of 0.1 mol/L KCl for pH electrodes or 0.1 mol/L MCl for pM electrodes
4. When using the silver chloride electrode, a small amount of AgCl can precipitate inside the glass electrode
5. The reference electrode, usually the same type as 2
6. Reference internal solution, usually 0.1 mol/L KCl
7. Junction with studied solution, usually made from ceramics or capillary with asbestos or quartz fiber.
8. Body of electrode, made from non-conductive glass or plastics.

The bottom of a pH electrode balloons out into a round thin glass bulb. The pH electrode is best assumed to bea tube within a tube. The inner tube contains an unchanging 1×10−7 mol/L HCl solution. Also inside the inner tube is the cathode terminus of the reference probe. The anodic terminus wraps itself around the outside of the inner tube and ends with the same sort of reference probe as was on the inside of the inner tube. It is filled with a reference solution of KCl and has contact with the solution on the outside of the pH probe by way of a porous plug that serves as a salt bridge.

Calibration

The pH meter must be calibrated before each measurement using at least two standard buffer solutions of known pH values (typically around 4 and 7).

Some typical applications requiring pH Measurements

• pH Measurement is very crucial in the agriculture industry for soil evaluation. Major crops require an alkaline environment, and hence pH Measurement becomes necessary.
• It is also used in the food industry, especially for dairy products like cheese, curds, yogurts, etc.
• It becomes mandatory for the chemical and pharmaceutical industries.
• It becomes a significant factor in the production of detergents.
• pH level monitoring is essential in water treatment plants and RO water purifiers.