Understanding pH Meters & pH Measurement

• A pH meter is a scientific instrument used to measure hydrogen-ion activity
• The pH meter indicates acidity or alkalinity in water-based solutions
• Acidity or alkalinity is expressed as pH
• The pH meter measures the difference in electrical potential between a pH electrode and a reference electrode
• A pH meter is referred to as a "potentiometric pH meter"
• pHis the unit of measure describing the degree of acidity or alkalinity
• pH is measured on a scale from 0 to 14
• The pH meter scale ranges from 0 to 14
• Material is acidic if the concentration of H⁺ is greater than the concentration of OH⁻
• An acidic pH value is less than 7
• Material is neutral if the concentration of H⁺ is equal to OH⁻
• A neutral pH value is 7
• Material is basic if the concentration of H⁺ is less than OH⁻
• A basic pH value is greater than 7
• pH equals the negative logarithm of hydrogen ion concentration expressed in molarity

Formula for pH

• pH = -log[H⁺]

Main components

• A pH meter consists of a glass electrode, a reference electrode, and an electrometer

Glass Electrode

• A glass electrode consists of a very thin bulb, about 0.1 mm thick
• The thin bulb is blown onto a hard glass tube of high resistance
• The bulb contains 0.1 mol/litre of HCl
• The HCL connects to a platinum wire via a silver-silver chloride combination

Reference Electrode (Calomel Electrode)

• A reference electrode has a stable and well-known electrode potential
• It is a glass tube containing saturated KCl connected to platinum wires
• The platinum wires run through mercury-mercurous chloride paste

Electrometer

• An electrometer measures very small differences in electrical potentials
• It does this in a circuit of extremely high resistance

pH calibration

• A pH meter should be calibrated before each measurement for precise work.
• Calibration should be performed with at least two standard buffer solutions
• The standard buffer solutions span the pH values to be measured
• General purpose buffers should be standard at pH 4.01 and pH 10.00
• For more precise measurements, a three buffer solution calibration is best
• For more precise measurements, use pH 4.01, pH 7, and pH 10.00 buffers
• Buffers are solutions that have constant pH values
• Buffer solutions have the capacity resist changes in pH

Acid-Base Titration with pH Meter

• Titration is an analytical chemistry technique
• Titration determines the concentration of an unknown acid or base
• An acid/base titration can be monitored with an indicator or with a pH meter
• Titrant is the substance or reagent with known concentration and volume
• Titrant is usually a strong acid or base
• Analyte is the substance, usually with an unknown concentration, can be either acid or base, strong or weak
• Equivalence Point is the point in a titration where the amount of solute reacts fully with the titrant
• An acid/base titration can be monitored with an indicator or a pH meter
• When using a pH meter, the pH of the solution is recorded as titrant is added
• pH is plotted against the volume of titrant added to get a titration curve
• Equivalence point occurs where very small additions of titrant cause a rapid rise in pH

Procedure

• Fill a burette with 25ml of 0.1M NaOH solution
• Take 20 ml of HCl solution (unknown concentration) in a suitable beaker
• Construct the system as shown in the diagrams
• Merge the operated pH electrode in the HCl beaker and record pH Value.
• Drop 2ml of NaOH from the burette and record the pH Value.
• Repeat step until reaching the Equivalence Point (2ml each time).
• Calculate the HCl concentration

Formula to calculate HCL Levels after titration.

• (MV)Acid = (MV)Base