chem - gravimetric analysis

what does gravimetric analysis do?

• gravimetric analysis uses the mass of solids to determine the concentration of a species in a commercial product or water sample

• precipitation reactions are used to remove the desired ions from sample

• concentration is presented as a percentage by weight or in mgL^-1 (ppm)

choice of precipitate + requirements

• a suitable precipitate must be formed to analyse a particular ion in the sample

• the precipitate must:

  • have a known formula - desired ion

  • have low solubility - should not dissolve

  • be stable when heated (so it can be dried)

  • not form precipitates with other ions which could be present

precipitates formed for gravimetric analysis table

SNAPE

all nitrates, ammonium compounds, ethanoates and compounds of group 1 metals are soluble

solubilities of common ionic compounds

steps for typical gravimetric analysis (solid samples or solution samples)

for solid samples:

• start with solid sample to be analysed - e.g. fertiliser, there identify lots of different ions within it

• grind sample to a powder - increase SA, easy to dissolve

• accurately weight mass of sample (this is used later to calculate the concentration of analyte), analyte = substance you are analysing for

• add a solvent to dissolve the ion under analysis

for solution samples or continue through for solid samples:

• filter solution to remove insoluble substances - don’t want to add to the final mass by leaving insoluble substances in there

  • discard insoluble residue

  • keep filtrate - solution which has been filtered

• add a substance to filtrate which will form a precipitate with the analyte

• filter under vacuum to remove as much solvent as possible (can also rinse with distilled water)

  • precipitate can be washed at this time to remove all soluble ions

• dry precipitate in oven (100-110 degrees celsius) - remove all water

  • ensure the precipitate does not melt below this temperature

  • drying should be completed to a constant mass - e.g. you dry it for a few days and measure it at each day, and when the mass of ppt. becomes constant over a few days you know all the water has evaporated

• weigh precipitate and record mass

• use stoichiometry to determine mass of ion in precipitate

calculating the concentration of an analyte

the conc. of an analyte is generally stated in percentage by mass

illustrated steps of gravimetric analysis

*for a water sample would start at ‘add a substance to precipitate A’

errors in gravimetric analysis

• assumptions made:

  • analyte is only substance which dissolves in solvent - FROM SOLID

  • analyte = only substance which is precipitated *ask about this

• drying precipitate:

  • any solvent leftover will add to final mass

  • dry, weigh, repeat until constant mass

• ensure all of analyte is precipitated from solution:

  • add excess reactant to ensure full precipitation - to ensure reaction is completed fully

• rinsing equipment properly

• spillage or splashing of solution may lose analyte

• not weighing all of precipitate:

  • some may be left in funnel or on glass rod etc.