Samples of shredded, grated, or sliced cheese are dispersed with a high-speed disintegrator in a solution of trichloroacetic acid, which denatures protein and assists in the liberation of all sodium from the matrix. Toluene is added in a second step of the dispersion process to assist in the solubilization of fat. Ammonium fluoride solution is then added, and the sodium titrated with a titrant comprising 0.5 mol/L Al(NO3)3 and 1.1 mol/L KNO3 to an exothermic endpoint.
Na+ + 2K+ + Al3+ + 6F- ↔ NaK2AlF6 ↓
In this determination, ammonium fluoride (NH4F) has been found to give sharper endpoints than ammonium bifluoride (NH4F ∙HF). In addition to this application note, you can find more information on thermometric sodium determination in foods in our application video available on YouTube: https://youtu.be/lnCp9jBxoEs