• A special type of titremetric procedure
involves the formation of precipitates during
the course of titration. The titrant react with the
analyte forming an insoluble material and the
titration continues till the very last amount of
analyte is consumed. The first drop of titrant in
excess will react with an indicator resulting in
a color change and announcing the termination
of the titration.
• Quantitative precipitation can be used for
• The titration involves precipitation are called
• Two type
Reaction must satisfy this condition
• Precipitate must be practically insoluble.
• Precipitation must be rapid.
• Possible to detect equivalent point.
• Method based on precipitation of insoluble
silver is known as Argentiometry.
• Halogens can be determined by precipitation
as sparingly soluble mercurous salts HgCl2 and
HgI2 is called as mercurometry.
• Solubility product is the product of the
concentration of ions in the saturated solution
of a sparingly soluble salt as AgCl is constant
at a given temperature.
• When the ionic product exceeds the solubility
products the solution is super saturated and
precipitation will occur.
• When the ionic product is less than the
solubility product the solution is unsaturated.
• In quantitative analysis excess precipitating
agent is always employed to ensure complete
• If little excess of H2SO4 is employed, the ionic
product far exceeds the solubility product and
there is complete precipitation.
• Oxalic acid cause complete precipitation of
calciumoxalate from solution of calcium
acetate but not from calcium chloride and
• Acetic acid is a weak acid than oxalic acid thus it does
not suppress the dissociated oxalic acid. The
concentration of oxalate ion is sufficient to keep ionic
product greater than solubility product of calcium
• In case of calcium chloride HCl is formed which is
strong acid and highly dissociated. It suppresses the
dissociation of oxalic acid by common ion effect.
• The oxalate ion concentration falls below the value
required to exceed the solubility product of calcium
• The precipitation is therefore incomplete.
• This explains why calcium oxalate dissolves in HCl but
not in oxalic acid.
Effect of acid upon the solubility of a
• Sparingly soluble salt of a strong
acid, the effect of the addition of
acid will be similar to that of any
other indifferent electrolyte but if
the sparingly soluble salt of weak
acid will have solvent effect upon it.
Effect of temperature upon solubility
of a precipitate
• The solubility of the precipitate encountered
in the quantitative analysis increases with rise
of temperature is small but with other it is
• The solubility of silver chloride at 10 C and 100
C is 1.72 mg and 21.1 mg respectively.
• In case of barium sulphate at these two
temperature is 2.2 and 3.9 mg respectively.
• Where ever possible it is advantageous to
filter while the solution is hot; the rate of
filtration is increased.
• The solubility of foreign substances, thus
rendering their removal from the precipitate
Effect of solvent upon the solubility of
• The solubility of most inorganic compound is
reduced by the addition of organic solvent
such as methane, ethanol, propanol and
• Addition of 20 % ethanol renders solubility of
lead sulphate negligible thus permitting
• Similarly calcium sulphate a separates
quantitatively from 50 % ethanol.
• In 1856 Mohr introduced it.
• Determination of halide – chloride with silver
nitrate using potassium chromate solution as
• It is especially useful for the determination of
• Precipitated silver chromate, through
sparingly soluble in water is more soluble than
silver chloride and the red color due to silver
chromate does not appear until all the
chloride has been precipitate as silver chloride
• The sensitivity of indicator depends on
concentration, temperature H+ ion
concentration, concentration of electrolyte
and manner of observing the red coloration
• This method utilizes chromate as an indicator.
Chromate forms a precipilate with Ag+ but this
precipitate has a greater solubility than that of
AgCl, for example. Therefore, AgCl is formed
first and after all Cl- is consumed, the first drop
of Ag+ in excess will react with the chromate
indicator giving a reddish precipitate.
2 Ag+ + CrO4
• In this method, neutral medium should be used
since, in alkaline solutions, silver will react with
the hydroxide ions forming AgOH. In acidic
solutions, chromate will be converted to
dichromate. Therefore, the pH of solution should
be kept at about 7. There is always some error in
this method because a dilute chromate solution is
used due to the intense color of the indicator.
This will require additional amount of Ag+ for the
Ag2CrO4 to form.
• Allowable PH range is 6.5 to 10 .
• Below PH 6.5 there is increased in solubility of
silver chromate .
• above PH 10 the end point comes too late and
Silver hydroxide is also precipitated.
• If the solution is alkaline make it acidic with
nitric acid then neutralise it by adding sodium
bi carbonate or borax
• If appreciable amount of Ammonium salts are
present the PH should not exceed 7.2.
• In reverse titration iodides and bromides
cannot be titrated.
Preparation of 0.1 M silver nitrate –
weigh 17 g of silver nitrate dissolved it
in 1000 ml of distilled water
• Weigh accurately 0.1 g of sodium chloride
dissolve in 5 ml of water, 5 ml of acetic acid ,
50 ml of methanol, 0.15 ml of eosin stirr
preferably with magnetic stirrer and titrate
with silver nitrate. End point appearance of
pink colour [ Rose milk colour ]
• In 1874 volhard designed the method of
estimation of silver ions [ AgNO3 ] in dilute
acid solutions by titrating against a standard
thiocyanate solution in the presence of ferric
salt [ Ferric ammonium sulphate ] as indicator.
• It has been extended to estimate chloride,
bromide and other several analysis.
• Ammonium or potassium thiocyanate solution is used
in conjunction with 0.1 M AgNO3 in the assay of
substances which react with nitrate but which cannot
be determined by direct titrations with silver nitrate
• In this method to the halide solution, a known excess
of silver nitrate is titrated with 0.1M ammonia or
potassium thiocyanate solution is called Volhard’s
• In this method the precipitate of Silver chloride is
filtered off and the filtrate is titrated with standard
thiocyanate solution using ferric ammonium sulphate
solution as indicator. At the endpoint a permanent red
colour is produced due to the formation of ferric thio
• This is an indirect method for chloride
determination where an excess amount of
standard Ag+ is added to the chloride solution
containing Fe3+ as an indicator. The excess
Ag+ is then titrated with standard SCN-
solution untill a red color is obtained which
results from the reaction:
Fe3+ + SCN- Fe(SCN)2+
The indicator system is very sensitive and usually
good results are obtained. The medium should be
acidic to avoid the formation of Fe(OH)3
• However, the use of acidic medium together
with added SCN- titrant increase the solubility of
the precipitate leading to significant errors. This
problem had been overcome by two main
Removal of precipitate of silver
• The reason for removing the
precipitate of silver chloride react
with thiocyanate SCN to form the
change in Titre value .
• In determination of iodide and
bromide is not needed because
the reaction is negligible.
Modified volhard’s method
cold well’s method
• Especially NaCl or KCl are determined.
• In case of chloride it is usual to filter of the
silver chloride or coagulate the precipitate by
means of either dibutyl phthalate preferred or
• The excess of silver nitrate is back titrated
with potassium or ammonium thiocyanate
using ferric alum as indicator.
• The nitro benzene or dibutyl phthalate is
added to coagulate the silver chloride
precipitate so that it will not interfere with the
titration of excess of silver nitrate by forming a
layer over silver chloride and this avoids the
need for filtration.
• Preparation of 0.1 M Ammonium thio
Dissolve 7.612g of Ammonium thio cyanate in
1000ml of distilled water.
• Pipette 30 ml of silver nitrate into a
flask dilute with 50 ml of water, add
2 ml of nitric acid, 2 ml of ferric
ammonium sulphate solution and
titrate with ammonium thio cyanate
solution to the first appearance of
reddish brown colour.
• In 1923-24 Fajan introduced the method
• Adsorption indicator is used
• The action of these indicators are based on
the simple fact that the endpoint the
indicators get adsorbed by the precipitate [
AgCl] and during the process of adsorption, a
change in colour of the indicator will takes
place which may result in a substance of
• Fluorescein and its derivatives are adsorbed to
the surface of colloidal AgCl. After all chloride
is used, the first drop of Ag+ will react with
fluorescein (FI-) forming a reddish color.
• Ag+ + FI- AgF
• Among these methods, the Volhard
Method is widely used because we can
detect the end point of precepitation
titration very well.
Limitations of Precipitation Titration
A few number of ions such as halide ions (Cl-
, Br-, l-) can be titrated by precipitation method.
Co-precipitation may be occurred.
It is very difficult to detect the end point.
Determination of endpoint
• Formation of coloured precipitate
• In mohr’s method a small quantity of
potassium chromate is added as indicator. At
the end point the chromate ion combines with
silver ion to form the sparingly soluble red
• Silver chromate sparingly soluble in water but
more soluble in silver chloride.
• The red colour does not appear until all the
chlorides are precipitated.
• In determination of neutral halide 0.5 ml of 5
% w/v of potassium dichromate is used.
• In acid solution potassium di chromate cannot
be used as indicator because of solubility of
potassium dichromate in acid.
• Acid solution neutralised with chloride free
calcium carbonate and sodium bi carbonate.
• Alkali is acidified with acetic acid and slight
excess of calcium carbonate.
• The solubility of silver chromate is increased
with temperature then the titration must be
performed at room temperature.
• Formation of soluble coloured compound
• It is exemplified by Volhard for titration of
silver in the presence of free nitric acid with
standard potassium or ammonium
• The indicator is ferric nitrate or ferric
• Addition of thiocyanate produces first a
precipitate of silver thiocyanate
• When this reaction is complete, the slight
excess of thiocyanate produces reddish brown
colour due to formation of complex.
• It is applied to the determination of chlorides,
bromides and iodides in acid solution.
When excess of silver has reacted,
thiocyanate may react with silver
chloride, since silver thiocyanate is less
soluble. It will takes place before the
reaction occurs with Iron III ions which
cause titration error
• It is necessary to prevent reaction between
the thiocyanate and silver chloride.
• Silver chloride is filtered off before back
titrating since at this stage the precipitate will
be contaminated with adsorbed silver ions.
• The suspension should be boiled for a few
minutes to coagulate the silver and then
remove the most adsorbed silver ions from its
surface before filtration. The cold filtrate is
• After the addition of silver nitrate , potassium
nitrate is added as coagulant the suspension is
boiled for about 3 minutes, cooled and then
• Desorption of silver ions occurs and on cooling re
adsorption is largely prevented by presence of
• An immiscible liquid is added to coat the silver
chloride particles and thereby protect them from
interaction with the thiocyanate.
• The most sucessful liquid is
• Nitro benzene – 1 ml for each 50 mg of
• Dibutyl phthalate
• The suspension is well shaken to coagulate the
precipitate before back titration.
• At the equivalence point the indicator is
adsorbed by the precipitate and during the
process of adsorption a change occurs in the
indicator which leads to a substance of
different colour. Thus they termed as
• Fluorescein , Eosin, Tatrazine , Rhodamine
AgCl precipitated in the presence
of excess of Chloride ions
AgCl precipitated in the presence
of excess of silver ions
• If fluorescein will strongly adsorbed than the
nitrate ion will reveal its presence on
precipitate not by its own colour, which is that
of the solution.
• Another view is adsorption of fluorescein ion a
rearrangement of the structure of the ion
occurs with formation of coloured substance.
• Important dye with phthalein group
• Condensing phthalic anhydride with resorcinol
• 0.2% solution of sodium salt of fluorescein in
• Fajan and wolff recommended 1-2 drops of
indicator for 10 ml of neutral 0.1 N halide
• It is rapidly decomposed by light.
• It can be used for chlorides bromides iodides and
• It is weakly acidic solution.
• Condensing phthalic anhydride
and mono chloro resorcinol.
• 0.1 % solution in 70% alcohol
or 0.1% solution as indicator.
• It is orange colour powder dye sparingly
soluble in alcohol. Bromide, iodide and
thiocyanate even in dilute solutions may be
• It is good adsorption indicator for titration of
chloride and bromide solution.
• The dye is green crystalline product which
dissolves in water to give a bright red solution.
• During the titration of chloride or bromide
with silver nitrate most of the indicator is
adsorbed on the precipitate in its red form, at
the equivalence point the colour suddenly
changes to blue.
• It is not useful in the pressence of sulphuric
acid because of bleaching effect on the colour.
• Good results obtained only when halogen is
present in the ionic form.
• Undissociated compounds such as chromic
chloride and mercuric chloride cannot be
analysed by this procedure. The same is of
course true of any adsorption indicator
• 0.2% alcoholic solution of diphenyl carbazone is
used as adsorption indicator.
• In titration of chloride a colour change from
bright red to violet is obtained.
• With bromide and iodide the change is from
yellow to green and with thiocyanate from pink
• Since no color change is obtained in the presence
• An acid solution must be first neutralised.
What types of drug are analysed by this method