SOP: Salt Content by Mohr Titration

Salt shapes flavor, controls water activity, and appears on every nutrition label. The Mohr titration, with its brick-red silver chromate endpoint, gives a direct chloride measurement that converts neatly to salt. It is fast, cheap, and reliable for routine production control.

Chemical analysis converts invisible composition into numbers that specifications, labels, and safety decisions depend on. Consistent technique matters as much as good instruments: small variations in timing, temperature, or reagent preparation shift results enough to fail a batch that should pass, or pass one that should fail.

Purpose

To determine chloride and calculate sodium chloride content by silver nitrate titration for labeling and formulation control.

Scope

Applies to brines, sauces, dressings, processed meats, snacks, and dairy products. Samples with high protein or fat need clarification before titration.

Equipment and Materials

• Burette, 25 mL, Class A (amber glass preferred for silver nitrate)
• Standardized silver nitrate solution, 0.1 N, stored in the dark
• Potassium chromate indicator, 5 percent aqueous
• Conical flasks, 250 mL
• Volumetric pipettes and flasks
• Deionized water
• Carrez reagents or equivalent clarifier for protein-rich samples
• pH paper or meter

Procedure

1. Prepare a clear sample solution. For most products, blend 5 to 10 g of sample with warm deionized water, transfer to a 250 mL volumetric flask, and make to volume.
2. For protein-rich samples, add Carrez I and II to precipitate proteins, then filter. The filtrate must be clear and near neutral pH.
3. Check that the solution pH is between 6.5 and 9. Adjust with dilute sodium hydroxide or nitric acid if needed; the chromate endpoint fails outside this range.
4. Pipette 25 mL of the prepared solution into a conical flask and add 1 mL of potassium chromate indicator.
5. Titrate with 0.1 N silver nitrate, swirling, until the first permanent reddish-brown tinge appears throughout the solution.
6. Record the volume. Run a blank on deionized water and subtract it.
7. Standardize the silver nitrate against dried sodium chloride monthly.
8. Run duplicates; agreement within 0.3 mL is required.

Recording Results

Percent sodium chloride equals (sample titration minus blank) times normality times 0.05844 times dilution factor times 100, divided by sample weight. Report to two decimal places.

Quality Control

Run a reagent blank with every batch and subtract it from sample results. Analyze every sample in duplicate; the relative percent difference between duplicates must not exceed 10 percent for concentrations well above the detection limit. Include a certified reference material or a validated in-house control sample with each batch and plot the result on a Shewhart control chart. Investigate any point outside two standard deviations and reject the batch if a point falls outside three standard deviations. Verify the calibration of volumetric glassware annually against Class A tolerances, and check the analytical balance daily with certified weights. Standardize titrants monthly and label with the expiry date. Document all quality control results in the batch record; sample results must not be reported when associated QC fails.

Troubleshooting

• No sharp endpoint: pH is out of range or phosphate interferes. Verify pH and clarify the sample properly.
• Endpoint fades: light is degrading the silver nitrate. Use amber glassware and work away from direct sunlight.
• High blanks: chloride in the water or reagents. Use fresh deionized water and analytical grade reagents.
• Cloudy filtrate: repeat clarification with a finer filter paper.

Safety Notes

• Silver nitrate stains skin and clothing black and is corrosive to eyes. Wear gloves and safety glasses.
• Collect silver-containing waste separately; do not pour down the drain.
• Store silver nitrate in the dark; it is light sensitive.