Practical investigations

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I am doing an investigation on the salt content in crisps. My first experiment is "Volhards method for determinig chloride ions". Part of this is a titration using potassium thiocyanate and silver nitrate using ammonium iron(III) sulphate as an indicator. However the technician accidently made up ammonium iron(II) sulphate. Why did this not work?

Jenkin replies
In the ‘Volhard’ titration, adding an excess of silver nitrate solution to a solution containing chloride ions, results in the precipitation of silver chloride.
 
The concentration of chloride can then be determined by back-titrating of the excess (unreacted) silver ions with thiocyanate solution to form a silver thiocyanate precipitate.
 
Ag⁺ + CNS^- (aq) -> AgCNS (s)
 
When this reaction is complete, the slightest excess of thiocyanateproduces a reddish-brown coloration, due to the formation of a complex ion: this marks the endpoint of the titration.
 
Fe³⁺(aq) + 6CNS^-(aq) -> [Fe(CNS)₆]^3-
 
Since this a specific reaction of Fe³⁺ ions, it is not surprising that it did not work with the Fe²⁺ ions in a solution of ammonium iron(II) sulphate.
 
Always carry out a risk assessment and check with your teacher before starting any practical work.

Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website. For further guidance see our tutorial on Risk Assessment.

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updated: 03 March 2006