Refractometers can be the easiest to use with low equipment cost and no chemicals required, but are not selective to sodium chloride and therefore, can only be used for quantitative measurements in binary solutions.
The sodium ISE is can be beneficial to a QC protocol due to its high accuracy and direct determination of sodium. Several other methods, including titration and refractometry, infer sodium from another measurement. The sodium ISE is the only method truly specific for sodium, making it ideal for foodstuffs with complex matrices. However, the required daily prep time for calibration and electrode care is high, and requires excellent laboratory technique in order to obtain accurate measurements.
In both manual and potentiometric titrations, sodium content is inferred from chloride concentration. This can be problematic for complex samples that contain other chloride salts or other sources of chloride that are not sodium chloride. For example, magnesium chloride and calcium chloride are commonly added to tofu as coagulants, therefore rendering a chloride titration infeasible at determining sodium content.
Manual titrations may be insufficient in accuracy and repeatability due to the subjectivity of determining the titration endpoint from a color change, and the coarse dosing resolution of manual burettes. Automatic titrators can be the easiest to use with the potential for the most accurate methods, with typical %RSD of <1% to 2%. Potentiometric titration is also recommended for many of the titrimetric methods contained within the AOAC standard methods of analysis. However, the capital investment can be the highest among the other mentioned methods.
The ideal method can change depending on the specific product, but in all cases, all available methods should be reviewed for their ease of use, accuracy, and costs.
Masulli is application engineer at HANNA instruments. Reach him at dmasulli@hannainst.com.
Greetings,
Is the Mohr’s Method a AOAC method of analysis
FYR
http://www.aoac.org/aoac_prod_imis/AOAC_Docs/ISPAM/7.Codex.pdf
How does salt analysis using refractive index compare to silver nitrate titration for accuracy of NaCL percentage?
Hi Jim,
Analysis with a refractometer will match up very closely in simple solutions like NaCl brines (on the order of within 1-2% of one another).
However in finished goods like sauces, jams, or dressings the disparity between the two is much greater. Refractometry is less reliable for quantitative measurements in these cases because other compounds besides NaCl influence the refractive index. As a result, some users of refractive index use it as a rapid qualitative consistency check.