Determining Salt in Food

In mechanical refractometers, the sample is placed on a prism, and the user looks through an eyepiece to determine the “shadow line” to determine this critical angle. Since temperature greatly affects refractive index, temperature compensation is achieved using bimetal strips that move the lens or scale as they expand or contract due to changing temperature. Manual refractometers are a low cost investment, but have limited accuracy due to subjectivity of determining the “shadow line,” variations in ambient light wavelengths, and limited temperature compensation.

Digital refractometers utilize an internal light source at a fixed wavelength. This internal light passes through a prism and into the sample and an internal light detector identifies the critical angle and therefore, the refractive index. Digital refractometers eliminate the subjectivity of determining the shadow line manually and have improved temperature compensation due to the use of programmed algorithms. As a result, digital refractometers can perform measurements in wider temperature ranges at a low-moderate price investment.

Refractometers are beneficial due to their low startup cost and lack of chemical reagents required to perform tests. However, this method is not specific to salt, and therefore prone to interferences from substances present in the sample that alter refractive index. These substances include fats, sugars, and salts other than sodium chloride. If salt is the only variable present in a complex sample, refractometers can be useful for qualitative measurements.

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Ion-selective electrode. Another method used for determining salt content in food is through the use of an ion-selective electrode, more commonly referred to as an ISE. An ISE is a chemical sensor with a sensing tip used to determine the concentration of a specific ion in a solution. In sodium ISEs, the sensing tip is a specially formulated sodium-specific glass bulb. ISEs obey the Nernst Equation, which allows us to correlate a millivolt (mV) reading to a proportional concentration value. However, much like refractometry, changes in temperature can also affect measurement accuracy. This is mitigated one of two ways: by monitoring temperature and applying a temperature correction using the electrode’s isopotential point or by maintaining a constant temperature between standards and samples during calibration and measurement.

In both manual and potentiometric titrations, sodium content is inferred from chloride concentration.

Like a pH meter, ISEs require care to ensure accurate measurements. The glass bulb of the sodium ISE must be hydrated at all times in an electrolyte solution. In addition, the electrode bulb needs periodic etching to ensure that a fresh layer of sensing glass is exposed prior to measurement. Proper function of the electrode can be validated by performing a slope check using sodium standards. The slope check ensures that the electrode conforms to Nernstian behavior and is operating correctly.

The ISE must be calibrated daily in order to ensure accurate measurements. Calibration standards should bracket the expected concentration of the sodium content of the food measured. For example, one calibration standard should have a higher concentration than the expected concentration, and another standard should have a lower concentration than your expected value. The standards should also be a decade apart from one another (i.e. 100 parts per million, or ppm, and 1,000 ppm).

Ionic strength adjuster (ISA) must also be added in a fixed ratio to both calibration standards and samples for accurate readings. Electrode response is affected both by ion concentration, as well as ion activity. The ISA standardizes ion activity between calibration standards and samples, therefore ensuring changes in the electrode response are based on changes in ion concentration, rather than ion activity. Once calibration is complete, measurements on liquid or solid samples can be performed. Solid samples can be extracted with water. The amount of water used to extract the solid samples must be accounted for so that a dilution factor may be applied.