TEST METHOD: CHLORIDE IN WATERS

Date updated: 31 May 2007 (first issue)

RISK ASSESSMENT

Assess Safety, Health, Quality and Environmental aspects of each specific step.

TASK SAFETY REQUIREMENTS

Safety boots are mandatory

Safety glasses to be worn whenever handling reagents

INTRODUCTION

Chloride presence in waters may be determined by several methods. For freshwaters and acid waters the Vega spectrophotometer provides a range of analytical methods.For estuarine, marine and hypersaline waters the spectrophotometric methods require dilutions in excess of those appropriate for good laboratory practice. For these waters titration is recommended.

This method (Mohr's) determines the 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.

 SPECIFIC JOB STEPS

RISK ASSESSMENT

RISK CONTROL

Equipment and reagents required:

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

 

Contaminated reagents or deionised water

 

Chloride indicator is TOXIC

 

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 shall be immediately rinsed with water.

Determine appropriate sample size and burette size

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.

I do not have any idea what range of chloride may be in my sample

 

 

Dilutions much above 1:10 introduce errors

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 to determine the likely concentration range of chloride.

 

 

Consider using a larger burette rather than a larger dilution.

Titrate the sample

  • 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.

I am unsure of recognising the end-point

 

 

 

I overtitrated.

The following illustration shows (from left)

  • the colour of the indicator in the untitrated solution
  • the milkiness occuring early in the titration
  • the end point where a tiny red aspect is observed
  • the colour resulting from overtitration

Either start with a new sample, or 'back-titrate' using a standard solution of chloride.

Calculations:

   
     

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