U.S. flag

An official website of the United States government, Department of Justice.

Laboratory Orientation and Testing of Body Fluids and Tissues for Forensic Analysts

pH Meters

Home  |  Glossary  |  Resources  |  Help  |  Contact Us  |  Course Map

Basic Acid – Base Chemistry

Acid Definitions:

  • Brønsted-Lowry – a compound capable of donating hydrogen ions
  • Lewis – a compound that can accept an electron pair from a base
  • Arrhenius – a compound that releases hydrogen ions in solution

Base Definitions:

  • Brønsted-Lowry – a molecule or ion that accepts hydrogen ions from solution
  • Lewis – a molecule or ion that donates an electron pair to an acid
  • Arrhenius – a compound that releases hydroxide ions in solution

Acid – Base Reactions

  • Neutralization
    • The neutralization reaction of acid and base results in water and salt.
    • H+ (aq) + OH- (aq) → H2O
  • Examples of acid – base reactions:
    • HBr + KOH → H2O + KBr
    • HCl + NaOH → H2O + NaCl

In keeping with the Brønsted-Lowry definitions of acids and bases, the criterion is the concentration of hydrogen ions present. Acids are proton donors and bases proton acceptors. The pH scale denotes the acidity or basicity of something through measurement of hydrogen ion concentration.

Given pH = -log [H+], a solution where [H+] = 1 x 10-7 moles/liter is of pH 7. Thus, solutions with lower pH designations are more acidic, and those with higher pH are more basic. Ranging from pH 0 (e.g., HCl) to pH 14 (e.g., NaOH), pH 7 is considered neutral. Acidic and/or basic solutions may require titration to a desired pH through the addition of acid and/or base. Thus, the pH meter is a tool in determining the pH of the solution in question.

Image of pH Meter
National Institute of Justice (NIJ) (see reuse policy).

pH Meters

pH meters utilize a probe bearing a thin glass bulb at the tip. Two electrodes inside the probe measure voltage. One electrode is contained within a liquid of fixed pH, while the other is free to respond to the [H+] of the sample. The voltage difference is measured by a voltmeter and translated into pH. Trainees must be able to properly care for, calibrate, and use the pH meters available in the laboratory before making reagents. Most pH meters are readily calibrated using standard pH solutions available from the manufacturer or a variety of suppliers; however these are not traceable to a national standard of measurement so the laboratory must provide satisfactory evidence of correlation of results.

Back Forward