1. Gravimetric Analysis

2. What is gravimetric analysis?
Precipitation reactions can be used to determine the amount of a salt in a sample of a solution. This is gravimetric analysis.
It is a cheap and simple method to perform but has largely been replaced by faster instrumental methods in industry.
In gravimetric analysis
precipitation solution (a solution that will form a precipitate with the ion being analysed) is added in excess to make sure all of the ion being analysed has precipitated
the precipitate is filtered and dried to a constant mass to ensure all water is removed.
the mass of the precipitate is used to calculate the moles of the original salt in solution and then the composition of the sample can he determined using
The molar ratio in the equation
n = 𝑚 𝑀𝑟 where n = moles, m = mass and Mr = molar mass (mass of one mole)

3. How the technique works….

4. An example of gravimetric analysis
The procedure to calculate the % NaCl in a sample is fairly straight forward.
In this process:
- a known mass of foodstuff is diluted with water to dissolve all the NaCl
- the dissolved NaCl is then separated from the rest of the foodstuff by filtering
- the Cl- ions (in the filtrate) are precipitated by reacting them with AgNO3 (forming insoluble AgCl)
- filtering this precipitate and drying then leaves solid
- dry AgCl which is weighed.
- from the mass the number of mole of AgCl can be determined and hence the number of mole of NaCl in the foodstuff and then the mass of NaCl.

5. The % by mass of many ions and ionic compounds can be determined using this precipitation method as long as the:
- ppt has a known formula
- ppt has very low solubility
- ppt is stable when heated

6. Solubility
It is important that the specific ion doesn't form precipitates with other ions which may be present. This means you will need to be confident in identifying spectator ions and producing ionic equations.
element ppt name
chlorine AgCl silver chloride
bromine AgBr silver bromide
iodine AgI silver iodide
iron Fe2O3 iron (III) oxide
phosphorus Mg2P2O7 magnesium pyrophosphate
magnesium Mg2P2O7
sulfur BaSO4 barium sulfate
barium BaSO4
Sources of Error - What are the potential sources of error in a typical analysis?