University of Babylon
College of
Engineering / Al-Musayab
Chemistry Manual / First stage
PREPARED BY
Dr. Ali Jassim Al-Zuhairi
M.sc. Mohammed Thamer Al-Zubaidi
3- Precipitation titration
Experiment No. (4)
Preparation and standardization of AgNO3 solution by Mohr's
method
Theory:The precipitation method is based on titration with the use of
reactions accompanied by formation of sparingly soluble compounds.
The most important methods are those based on precipitation of insoluble
silver salts in accordance with the equation
Ag+ + X
AgX
The method is based on titration of a halide, such as NaCl with
AgNO3 solutions in the presence of K2CrO4 as indicator.
The end point of the titration is the point at which the color of suspension
changes from pure yellow (due to the presence of CrO42- ions in solution)
to reddish brown of AgCrO4.
While AgNO3 can be used (p.s.s), but is quite expensive and used
after standardization against pure dry NaCl, not KCl because of the traces
of water locked in crystals.
Notes:1- Solution of AgNO3 should in a dark colored bottle to prevent photo
chemical reduction to silver metal
Ag+ + e-
hʋ
Ag
2- In Mohar method Cl- or Br- ion is titrated with Ag+ ion using K2CrO4
as an indicator.
Ag+ + Cl-
AgCl
White p.p.t
Ksp= 1.2 ×1010-
University of Babylon
College of
Engineering / Al-Musayab
Chemistry Manual / First stage
2Ag+ + CrO-
Then
At end point
While
PREPARED BY
Dr. Ali Jassim Al-Zuhairi
M.sc. Mohammed Thamer Al-Zubaidi
Ag2CrO4
Ksp =1.7 ×1010-
Ksp (AgCl) = [Ag+][Cl-] = 1.82×1010[Ag+] = [Cl-] = 1.82×1010- = 1.35×105-M
Ksp (Ag2CrO4) = [Ag+] 2[CrO4-2]= 1.2×1012[CrO4-2]=(1.1×1012- )/(1.35×105-)= 6×103-M
If CrO42- concentration is kept at (6×103-M) the red (p.s.s) of Ag2CrO4
will appear at the end point.
Conditions for Mohr's method.
1- The Mohr's method is used for determination of silver, chloride and
bromide (it cannot use for iodides and thiocyanates because the results
are greatly distorted by adsorption effect.
2- The Mohr's method is suitable in neutral or weakly alkaline solutions
(PH = 6.5-10) because Ag2CrO4 is soluble in acids.
3- Absence of cations which from precipitates with CrO42- (such as Pb2+
Ba2+) and anions forming slightly soluble silver salts such as PO43,AsO4,SO32-,S2-,CO32-,C2O42- in Mohr's method.
4- Avoid the formation of Ag2CrO4 by local excess of the titrant before
the end point, by shaking the flask.
5- Pure NH4Cl can be titrated by Mohr's method providing that PH lives
between (6.5-7.2), at higher PH, too much NH3 is evolved increasing the
solubility of AgCl and Ag2CrO4.
6- In concentrated solutions of Ca2+ and Mg2+, the end point appears too
soon, ant not sharp, addition of (5-10ml) of (1%) agar solution prevents
the agglomeration of the precipitate.
7- Mohr's method can not be used for chlorides of cations which are
hydrolyzed such as Al3+, Fe3+,Bi3+, Sn2-, In2+,.......... .
University of Babylon
College of
Engineering / Al-Musayab
Chemistry Manual / First stage
PREPARED BY
Dr. Ali Jassim Al-Zuhairi
M.sc. Mohammed Thamer Al-Zubaidi
Procedure:standardization of AgNO3:1.
2.
3.
4.
Put (5ml) from standard (0.1N) NaCl solution in to a conical flask.
Add (1ml) of the chromate indicator solution.
Titrate with silver nitrate solution until the color change to reddish.
Calculate the molars of AgNO3.
Questions:1- What is the principle of titration of bromide and chloride with AgNO3
solution without indicators ?
2- How does K2CrO4 act an indicator in titration of chlorides with
AgNO3 solution ?
3- State the conditions for application of the Mohr's method ?
4- State the blank indicator ?
5- Why the solution of AgNO3 is stored in a dark colored bottle ?
6- Why Mohr's method is take place in neutral or weakly alkaline
solution ?
University of Babylon
College of
Engineering / Al-Musayab
Chemistry Manual / First stage
PREPARED BY
Dr. Ali Jassim Al-Zuhairi
M.sc. Mohammed Thamer Al-Zubaidi
Experiment No.(5)
Determination of chlorides in presence of Adsorption indicator
(Fajan's method).
Theory:An adsorption indicator is an organic compound that is adsorbed
on or desorbed from the surface of the solid formed during a precipitation
titration . Titration involving adsorption indicators are rapid, a curate and
reliable.
Condition of adsorption indicator:1- The precipitate must be colloidal to maximize the quantity of indicator
adsorbed.
2- The (p.p.t) must adsorb it's own ions strongly.
3- The indicator dye must be primary adsorbed ion.
4- The PH of the solution must be such as to assume that the ionic form
of the indicator predominates.
Examples on adsorption indicator:(a) fluorescein.
(b) dichloro fluorescein
In this exp. we use fluorescein as adsorption indicator, it is very weak
acid, it is used in PH (7-10).
University of Babylon
College of
Engineering / Al-Musayab
Chemistry Manual / First stage
PREPARED BY
Dr. Ali Jassim Al-Zuhairi
M.sc. Mohammed Thamer Al-Zubaidi
Questions:1. What's mean of adsorption indicator?
2. Write the structure of fluorescein?
3. In which PH the adsorption indicator fluorscein work?
4. What is the condition of adsorption indicator?