Total Acidity (TA) determination in honey

Before you read this article you must understand the following concepts:
Titration, pH, Normality

It’s know that’s extremely difficult to measure total acidity (TA) in honey using standard titration methods. The pH endpoint was unstable and was impossible to obtain accurate TA measurements in honey. In an article published in 1958, D. John W. White identified the source of the unstable endpoint as due to a reaction between gluconolactone and gluconic acid (see: Acid–base titration endpoint considerations in honey). Dr. John W. White developed a method for measuring TA in honey that accounted for the lactone reaction and is accepted today by the AOAC International as a Official Methods of Analysis (method 962.19).

Equipment

  • pH meter (precision 0.01 unit) and electrodes,
  • Analytical balance,
  • 25 ml volumetric pipette,
  • Two 10 mL buret (± 0.05 mL),
  • 250 ml beaker,
  • Magnetic stir and stir bars,

Reagents

  • 0.05N NaOH
  • 0.05N HCl

Procedure

  1. Accurately weigh 10 g of honey and dissolve it in 75 ml of deionized water in a 250 ml beaker
  2. Place calibrated pH/reference electrodes and a stir bar into the solution, mix, and titrate to pH 8.5 with 0.05N NaOH (rate of 5 ml/minute) and record.
  3. Add to the solution 10 ml of 0.05N NaOH.
  4. Back titrate to pH 8.3 using 0.05N HCl from the second buret and record.
  5. Run a blank determination procedure [note 1] of 75 ml of deionized water to pH 8.5 and record.

Calculation

The free acidity (F.A.) Is the acidity titratable with sodium hydroxide up to the equivalence point:

The lactone acidity (L.A.) corresponds to the combined acidity which is not directly titratable:

The total acidity (T.A.) is the sum of the free acidity (F.A.) and the lactone acidity (L.A):

Notes
[1] A blank titration is done without the analyte present to check for possible sources of error in the “blank” solution. For example, deionized water is slightly acidic and may affect the results of an acid base titration (if you want a highly accurate concentration). So you would do a blank titration to find the concentration of H3O+ in the water and use it to correct the concentration of your analyte.