Determining Salt in Food

Sodium ISEs are very specific to sodium measurement, and are prone to little interference. The startup cost of measurement with an ISE is moderate. However, the care involved with ISE tends to require a trained technical staff and a longer startup time before measurements may be taken.

Titration. This is the most common method of analysis in in-house laboratories for determining salt in foods. Titrimetric methods have been adopted as the reference method by organizations such as the Association of the Official Analytical Chemists (AOAC) for a variety of food matrices, which include cheeses, meats, and vegetables. A titration is a procedure where a solution of a known concentration (titrant) is used to determine the concentration of an unknown solution (analyte). Results are calculated based on the amount of titrant used to reach the endpoint. Endpoint can correspond to a color change of an indicator, or detected with a potentiometric sensor.

Titration: Mohr method. One way to determine salt content using titration is with the Mohr method. The Mohr method is a manual titration method using silver nitrate. In this titration, a burette is used to manually add silver nitrate to a sample, allowing for a reaction to occur between silver ions in the titrant and chloride in the sample between each dose. The pH of the sample must be buffered to around 7.0 in order for the reaction to occur. This reaction between silver and chloride produces an insoluble silver chloride (AgCl) precipitate.

An example of an ISE meter.
Image Credit: Hanna

Silver nitrate is added until chloride is no longer present in the sample solution. When silver nitrate is added to the sample in excess, it binds with a chromate ion indicator to produce a red color in solution, signifying the endpoint. Chloride concentration is calculated, which can then be used to infer sodium or sodium chloride content. This method has the benefit of high accuracy when performed by skilled operators, although determining when the color indicator has sufficiently changed makes this method prone to overestimation of salt content. The investment for manual titration is very low for silver nitrate titrant, color indicator, a manual burette, and other necessary volumetric glassware.

Titration: potentiometric method. Titration with silver nitrate may be automated with a potentiometric titration system. The titration system can be equipped with an ISE sensitive to the concentration of chloride or silver ions. However, this electrode would not be used to directly determine concentration during a titration. Instead, the electrode would monitor the solution for a change in the mV potential as a result of silver ions being in excess, or depletion of chloride ions in solution. As a result, calibration of ISEs is not necessary for titration, making the startup time for analysis immediate.

The amount of water used to extract the solid samples must be accounted for so that a dilution factor may be applied.

These titration systems automatically control titrant dosing and endpoint detection. Automatic endpoint detection increases titration precision by eliminating human subjectivity associated with manual titration. Instead of a visual color change indicator, the titrator will determine the endpoint by measuring changes in mV potential. Also, the automated dosing system dispenses smaller, more precise doses than a technician using a manual burette. Dynamic dosing is available on many titration units, which permits the unit to control how much titrant is dosed based on the progress of the titration. Dynamic dosing allows for larger doses to be dispensed in the beginning of the titration, with progressively smaller doses being dispensed as the endpoint is approached. This saves time and reduces the likelihood of overshooting the endpoint. Automatic titrators require a moderate to large investment.

Conclusions

Method selection is among the most important steps in establishing a protocol for monitoring salt in foods.