8.1 c: Precipitation Titrations and Gravimetric Analysis

Precipitation Titrations and Gravimetric Analysis

​What will we be learning...

c	conduct investigations and/or process data involving:  – gravimetric analysis – precipitation titrations

​Precipitation Titrations

• used to determine quantity of a particular ion present in a solution

• can use either direct or back titration method

• often use the silver cation to precipitate the silver salts of the anion being investigated

• issue: difficult to identify endpoint

  • we use 3 methods to determine the endpoint

  • Mohr's, Volhard's, Fajan's

​Mohr's Method

• for use with chlorides, bromides and cyanides

• uses direct titration

• uses silver nitrate

• chromate ions as the indicator

• for determination of endpoint:

  • once all halide ions have precipitated, additional silver ions react with the chromate indicator and form a red-brown precipitate (silver chromate) - indicates endpoint

• errors occur: because more titrant is added to reach the endpoint. Can be addressed by running a blank titration

• in the blank titration, silver nitrate solution is titrated against a solution that contains the indicator but no halide ion, and is done until the indicator changes colour

• the volume obtained in this blank titration is equal to the additional silver nitrate

• the volume is then subtracted from the initial titration volume

Open Ended

​Volhard's Method

• back titration

• a known amount of silver nitrate is added to the sample which precipitates all the ions being investigated

• excess silver nitrate is titrated against potassium thiocyanate - produces AgSCN

• to detect the endpoint: Fe3+ ions are added to the silver nitrate solution, and the endpoint gives a distinctive dark blood red colour

• Errors:

  • when the precipitate is more soluble than the AgSCN endpoint precipitate

  • need to consult the Ksp values to determine solubility, if it is more soluble, then it needs to be filtered from the solution before titration
    

Open Ended

​Fajan's Method

• direct titration

• endpoint determined by a colour change using an absorption indicator

• indicator absorbs onto the precipitate at the end point and changes colour

  • don't actually need to fully understand how this works - beyond the course

• different indicator depends on the usage

• Cannot be used:

  • if the ion being analysed is in low concentrations

  • lots of non-reacting ions - indicator will attach to these and not to the precipitate

​Gravimetric Analysis

• same as earlier in the course

• need to separate out the species you want to measure

• often see examples with composition of fertilisers

  • fertiliser is dissolved in water

  • add in chemicals to precipitate out certain parts

  • solid precipitate is dried, weighed, and compared to the initial mass of the fertiliser

  • can then determine concentration or percentage composition

​Up next:

conduct investigations and/or process data to determine the concentration of coloured species and/or metal ions in aqueous solution, including but not limited to, the use of:

– colourimetry

 – ultraviolet-visible spectrophotometry

– atomic absorption spectroscopy