Methods Manual for Salt Lake Studies/Salinity/measuring chloride concentration

Chloride titration measures the chloride content of salt waters. This is useful if the brines are marine derived, as it is possible to compare the resulting chloride concentration against tables such as Gino Baseggio’s Composition of Seawater (Baseggio, 1974) to estimate the salinity based on the chloride reading. The limitations of the method relate to two aspects – the chloride titration itself, and the mix of ions in the brine.

Chloride titrations may be based on silver nitrate or a mercuric reagent. The latter is more accurate and has a clearer end-point. The former is safer to use and environmentally less damaging if some reagents are spilled. However the end-point is notoriously hard to determine. Practice at determining the end point, against a standard, is essential.

In saline lakes where the ionic mixture is not marine derived, the use of chloride as a surrogate salinity method could underestimate salinity significantly.

Mohr's method for chloride titration

Mohr's method, often called the argentometric method (Eaton et al, 1995), determines chloride ion concentration of a solution by titration with silver nitrate, and is suitable for highly saline brines. As the silver nitrate solution is slowly added, a white precipitate of silver chloride forms. The end point of the titration occurs when all the chloride ions in the sample are precipitated. Then additional silver nitrate ions from the titre react with the chromate ions of the indicator, potassium chromate, to form a red-brown precipitate of silver chromate. The same reaction occurs with bromide, so this analysis will provide:

• chloride concentrations for waters with negligible bromide,
• bromide concentrations for waters with negligible chloride and
• combined chloride/bromide concentrations for waters containing both elements.

Determine appropriate sample size and burette size before starting. Smaller burettes have a finer resolution than larger burettes. Choose a burette based on the resolution you will require. Given that 1mL 0.1M AgNO3 is equivalent to 3.545 mg Cl-, estimate how large your sample size needs to be, to use no more than a full burette of titre.

Should the sample size be very small, prepare a dilution instead and use a larger aliquot.

In situations where the approximate range of chloride in the sample is unknown, measure the specific gravity of the sample to gain an estimate of its salinity. If the water has any marine derivation, use a reference table such as Baseggio's (1974) to determine the likely concentration range of chloride.

Things to keep in mind

• Test against a standard (made from AR grade NaCl) monthly.
• Record standard results on the appropriate standards chart.
• Analyse a blank (demineralised water) each run, record it and use in calculation.
• Use disposable gloves. Take care not to splash chemicals. Any splashes should be immediately rinsed with water.
• Dilutions much above 1:10 introduce errors
• Consider using a larger burette rather than a larger dilution.

Equipment and reagents required

• Beaker and stirrer bar
• induction stirrer,
• self zeroing burette,
• sample and some distilled water,
• chloride indicator solution (10%w/v potassium chromate OR 4.2%w/v potassium chromate with 0.7%w/v potassium dichromate) in dropper bottle (caution - this reagentis toxic),
• 0.1M silver nitrate solution.

Titrate the sample

Indicator color - precipitate - endpoint - overtitration

• Add the sample aliquot to the beaker and place on stirrer. If the stirrer bar is not covered by the sample, add a little distilled water
• Add several drops of chloride indicator until a lemon yellow colour appears
• Ensure the burette of siver nitrate titre is zeroed.
• Add the titre slowly to the beaker. At first the precipitating chlorides will make the solution white. At the end-point a tiny amount of red will appear (the "baked biscuit" endpoint). Stop here, read the burette.
• Record the mLs of titre used.

Calculation

$Cl^{-}(mg/L)=3545\times {\frac {V_{1}}{V_{2}}}$

OR

$NaCl(mg/L)=5844\times {\frac {V_{1}}{V_{2}}}$

where

${\text{V}}_{1}={\text{volume of titre}}$ and

${\text{V}}_{2}={\text{volume of sample}}$