Back Titatrations
A back titration is used when its not possible to use a standard titration to
get an accurate determination of concentration. It is used when the substance
being analysed is volatile, ie will evaporate easily ir is unstable in some
other way. The classic example used is measuring the amount of ammonium salts
in lawn fertiliser
See the example on p 38 / 39
The steps we use in analysing the ammonium salts in Lawn fertilizer is as follows
· A known, excess amount of a strong base (like NaOH) is added to the
sample the moment the container is unsealed. This ensures that the acid completely
reacts before it can evaporate. The mixture will now contain some left over
base plus the products of the reaction
· The mixture is then titrated against an acid of known concentration
to determine the amount of base remaining after the first reaction
· From this we can calculate the amount of base that must have reacted
with the weak acid
· So we can work backwards to deduce the concentration of the original
weak acid
Do Questions 45, 46 and 47 on page 42