Sodium chloride testing methods
Sodium chloride (NaCl), often referred to as table salt is naturally present in many foods and is also added to foods to enhance flavor. Nutritional aspects of salt are of interest to the general population. While sodium and salt are often thought of as being synonymous, this is not accurate. The Code of Federal Regulations (CFR) makes this distinction. Overconsumption/underconsumption of salt as per the recommended daily intake (RDI) of 2400mg sodium / day is of interest to people, and salt and sodium content should be checked by food producers.
Many test methods are available on the market to determine salt content. The AOAC method 971.27 "sodium chloride in canned vegetables" has withstood the test of time, and is very accurate. High performance liquid chromatography (HPLC) is a newer method that provides even greater accuracy. Sodium ion concentration measurements are also a valuable tool, as a sodium ion concentration can be easily converted to a NaCl concentration. Although, it should be pointed out that a sodium ion concentration measurement measures all forms of sodium in a sample, not just sodium chloride. This should be noted especially when testing will be done on vegetable stocks, and broths as monosodium glutamate, and its derivatives will give you false reports of NaCl content.
I'll review the silver nitrate method in this article.
Principle of sodium chloride reaction with silver nitrate. Balanced chemical reaction. Potentiometric review.
Argentometric titration: Titrations involving silver nitrate. Silver nitrate has a very well established, and understood reaction mechanisms with halide salts. This particular chemical reaction can be described as a precipitation reaction. An insoluble ionic solid will form as the titrant is added, making the appearance of the solution cloudy. If the titration is going to be done manually, then a potassium chromate indicator or equivalent should be selected. A potentiometric titration review will require the use of an electrode to monitor the progress of the reaction. A silver sulfide electrode with a 1M potassium nitrate electrolyte will be selected to monitor the conductivity changes of the chemical reaction.
The reaction: The Ag+ cation reacts quickly with Cl- anion to produce a white, insoluble silver chloride salt.
AgNO3(aq) + NaCl(aq) ---> AgCl(s) + NaNO3(aq)
The electrode selected to monitor the electric potential of the solution is a silver sulfide combination ion selective electrode (ISE) with a 1M potassium nitrate electrolyte.
The point in the reaction where the added silver nitrate is stoichiometrically equal to the moles of the sodium chloride in solution is referred to as the equivalence point, which is used to determine the amount of sodium chloride in the test sample.
After each dose of silver nitrate is dispensed into the sample via the peristaltic pump:
The reaction progress is monitored – A stable electric potential is measured (mV) by the electrode.
After a stable reading is determined – the auto-titrator will compare this measurement to the prior measurement(s) and will dynamically adjust (up/down) the next dose of silver nitrate dependent on the magnitude of conductivity change.
A plot of electric potential (mV) vs. standardized silver nitrate (mL) is approximated by a curve that is processed in real-time. The inflection point on the curve is the titration equivalence point.
The unit calculates the 1st derivative of the approximated curve in an effort to determine the inflection point. The maximum ΔmV of the 1st derivative curve is the titration equivalence point.
The titrant volume dispensed to achieve the equivalence point is then used to automatically calculate the sodium chloride content of the sample using the following formula:
%NaCl (g/100g) = [(VAgNO3*NAgNO3*EQratio*MWNaCl*(1LAgNO3/1000mLAgNO3)]/(wgt)
VAgNO3 = Volume of silver nitrate titrant dispensed (mL)
NAgNO3 = The normality of the standardized silver nitrate solution (mol/L)
EQratio = Equivalence ratio from the reaction (1molNaCl/1molAgNO3)
MWNaCl = Molecular (formula) weight of sodium chloride (58.44g/mol)
wgt = Sample weight (g)
Normality check of silver nitrate with a salt standard: Utilizing the same principle from the argentometric reaction combined with a dilution factor, a precise normality of the silver nitrate can be determined from a silver nitrate titration with a standardized salt solution as follows:
NAgNO3 = (WgtNaCl*EQratio*Av/Vf)/(MWNaCl*VAgNO3)
NAgNO3 = Calculated normality of silver nitrate (mol/L)
WgtNaCl = Weight of sodium chloride standard (g)
EQratio = Equivalence ratio (1mol AgNO3)/(1mol NaCl)
Av = Aliquot volume. Sample amount drawn from final dilution volume Vf
Vf = Final dilution volume. Standardized NaCl dissolved into the final volume Vf
Equipment needed for the titration. Auto-titrator and manual titration.
- Auto-titrator with calibrated board, and burette.
- Silver sulfide combination electrode with potassium nitrate electrolyte.
- Nitric acid. Preferably diluted down to 1.5-3M.
- Silver nitrate solution. Pre-made is convenient, although you still have to verify the normality of the solution as there is potential for light degradation.
- Stir plate, stir bar.
- Sodium chloride standard, with all traces of water, removed.
- Dark container to store the silver nitrate.
- Class A volumetric flask.
- Class A 5mL pipette.
- 4 point scale.
- Manual only - Potassium chromate indicator.
Procedure - Standardization of silver nitrate, and titration procedure.
Silver nitrate normality check for each received lot of AgNO3
- Using a desiccated sodium chloride standard – weigh 0.2000±0.0050g in a 100mL Class A volumetric flask.
- Dilute to volume of 100mL with distilled water. Mix well until salt is dissolved.
- Measure 5mL of the salt standard solution with a class A pipette and dispense into a 150mL glass beaker.
- Dilute to volume of 50mL with distilled water.
- Add 50mL of 1.5M HNO3, add a stir bar, and place the beaker on the stir plate.
- Start titration on auto-titration unit, or manual titration w/indicator.
- If you programmed the channel correctly using the calculation provided previously, you should get a result that matches the normality of you silver nitrate standard.
Titration procedure of test sample
- Weigh 5.0000±0.0500g of the homogenized, well-mixed sample on a 4 point scale into a 150mL glass beaker.
- Dilute to 50mL with distilled water.
- Add 50mL of 1.5M nitric acid.
- Place stir bar in beaker, and set the beaker on the stir plate.
- Lower the silver sulfide combination electrode and temperature probe into the beaker taking care to avoid touching the stir bar.
- Turn on the stir plate and verify sample agitation is vigorous.
- Initiate titration.
- Titration result should have a distinct endpoint. The 1st derivative of the plot of conductivity vs. titrant volume should peak. This peak is usually around 260mV.
- A titration of canned beef broth is carried out in the following manner: A 5.031g sample of an undiluted sample is titrated with 6.037mL of 0.0855N AgNO3. (A) Calculated the final salt concentration. (B) Based on this result, what percentage of the recommended daily intake (RDI) of sodium is this if the serving size is 240g? With the understanding that the declared contents of sodium on nutritional labels is regulated by the CFR - Is this report accurate?
Part (A) - Plug into equation:
%NaCl (g/100g) = [(VAgNO3*NAgNO3*EQratio*MWNaCl*(1LAgNO3/1000mLAgNO3)]/(wgt)
%NaCl (g/100g) = [(6.037mL)AgNO3*(0.0855molAgNO3/1LAgNO3)*(1mol)NaCl/(1mol)AgNO3*(58.44gNaCl/1molNaCl)*(1LAgNO3/1000mLAgNO3)/(5.031g sample)]*100
%NaCl (g/100g) = 0.5996%
Part (B) - RDI for sodium is 2400mg/day.
To calculate this next part, 1st we must take our answer from (A) and multiply it by the serving size to get the total salt.
(0.5996gNaCl)/(100g serving)*(240g serving) = 1.439g NaCl or 1439mg NaCl
2nd we must convert our weight of salt to the weight of sodium, as the distinction of salt vs. sodium is different as detailed in the CFR.
MW of NaCl is 58.44g/mol; MW of Na = 22.99g/mol; MW of Cl = 35.45g/mol
Therefore, to easily make this conversion --> 22.99/58.44*1439 = 566.1mg sodium
Is this accurate? The answer is maybe. While this test method is highly accurate for testing salt content in foods - there are additional compounds of sodium that can be present in the sample that this test method cannot detect. Flavor enhancing compounds such as monosodium glutamate, disodium inosinate, disodium guanylate and popular flavor enhancing additives and can be present in the sample. The sodium content of the food would actually higher than calculated with the silver nitrate method. Unless you know, or are able to prove that the compounds are not present in your sample then the %NaCl is as far as you can go.