Gravimetric Analysis

Gravimetric Analysis  to nd the percentage of one particular component in a m lves forming a suitable precipitate with the ion in the sampl

steps in gravimetric analysis:

 This procedur procedure e is especially especially suitable for determining determining of metal ions and anions such as sulphates.

The precipitate should: 1. Have a known formula 2. Have a low solubility 3. Be stable when heated so it does not decompose and can be easily dried !. "ot form precipitates with other ions that are likely to be present

When designing a gravimetric procedure, a knowledge of the !"U#I"ITI$ of the precipitates likely to be produced is important

% simple form of gravimetric analysis

% more complicated form of gravimetric analysis Iron, in the form of dried iron&II' sulfate, is sometimes prescribed by doctors to patients su(ering from anaemia.  To determine the iron content of some such iron tablets, )* tablets were crushed and then weight. Steps Why?  Their mass was +.+- g. ). Tablets were dissolved in water and the insoluble residues ltered o( 

/. 0otassium dichromate was added to oxidise all the e/1 present to e+1 . 2ext, sodium hydroxide was added until no more iron&III'

 The iron is often present as e&II' sulfate and can be easily dissolved in water.  There can be other components of the sample that are insoluble. It is important to remove these before precipitate forms. 0otassium dichromate is a strong oxidant and can therefore oxidise e&II' to e&III' 3uite easily. %lthough e/1 forms a precipitate, it is more soluble than more iron&III' hydroxide. 4ence losses due to this

Steps +. 5ixture &e&III' and 2a!4' was boiled

Why?  The e&!4'+ precipitate is gelatinous. #oiling helps to granulate it making ltration easier.

. 0recipitate was collected by ltration

-. ilter paper and precipitate were heated in crucible to form e&III' oxide and destroy the lter paper

 The e&!4'+ is dehydrated on heating and forms e/!+ .  The lter paper is a type that produces carbon dioxide and no ash, hence its mass will not interfere with the weighing of the ppt.

6. The repeated drying and weighing until a constant mass of precipitate is obtained.  mass of iron&III' oxide was found

to be )./ g.

 can now calculate The mass of iron per tablet. The percentage of iron in these tablets

 Two more examples7

xamining the source of error It is important to evaluate the limits of your results If certain things happen during the analysis you need to be able to predict how these may a(ect the nal results &see Table below'

4einemann Textbook, pg )8, )9, /-