Lab.3
Precipitation titration
Titrations with precipitating agents are useful for determining certain analytes containing halides e.g. Cl can be determined when titrated with AgNO
3
.
Detection of end point:
–
Formation of Precipitation - Mohr’s method
–
Formation of colored adsorped on the ppt. – Fajan’s method
– Formation of colored solution –Volhard method
1-
Mohr’s method for determining chloride:
Mohr’s method is used for determination of chloride and bromide ions (but it is not used for iodide and cyanide ions why?). principle:
Mohr’s method is a direct titration using standard solution of silver nitrate in a neutral medium (why?).
Chloride is titrated with AgNO
3
solution. A soluble chromate salt (K
2
CrO
4
) is added as the indicator. This produces a yellow color solution. When the precipitation of the chloride is complete.
NaCl + AgNo3 ⇆ AgCl + NaNo3
At the end point: The first excess of Ag
+ reacts with the indicator to precipitate red silver chromate as a second precipitate after precipitation of all chlorides as silver chloride.
+
(aq)
4
2–
(aq)
2
4 (s)
Yellow red ppt
The Mohr’s method must be performed at a pH about 8. This method is useful for determining Cl
-
in neutral or unbuffered solutions such as drinking water.
1. Pipette 10 ml of sodium chloride solution (NaCL) into stoppered conical flask.
3. Add 1 ml of 2% neutral potassium chromate indicator.
4. Titrate with 0.05 N AgNO
3 solution, swirling the liquid constantly, until the red colour-formed by addition of each drop of AgNO
3 solution begins to disappear more slowly; this is an indication that most of chloride has been precipitated and that the end point is near.
5. Continue the titration, dropwise, until a faint, but distinct, brick red color is formed and does not disappear on vigorous shaking.
End point: brick red color
Calculation:
Conc. Of NaCl (g/L)= (ml of titrant) (F) (f) (1000) / ml of sample

2Volhard method for determining of bromide:
Principle:
This is an indirect titration procedure for the determination of anions that precipitate with silver like CL
-
, Br
-
, I
-
, SCN
-
, and it is preferred in acid (HNO
3
) solution. A measured excess of AgNO
3
is added to ppt the anion, and the excess of Ag+ is determined by back titration with standard NH
4
SCN solution:
+
(aq)
–
(aq)
(s)
+
+
(aq)
–
(aq)
(s)
The end point is detected by adding iron III (Fe3+) as ferric ammonium sulfate which forms a soluble red complex with the first excess of titrant.
3+
(aq)
–
(aq)
2+
(aq)
These indicators must not form a compound with the titrant that is more stable than the precipitate or the color reaction would occur on addition of the first drop of titrant.
Procedure
1. Pipette 10 ml of potassium bromide solution (KBr) into a glass stoppered conical flask.
2. Add 2 ml conc. HNO
3
.
3. Add accurately measured 20 ml of 0.05N AgNO
3 and shake vigorously for one minute.
4. Add 1 ml of ferric alum indicator.
5. Titrate with o.1 N NH
4
SCN.
End point : orange solution is formed
Calculation:
Conc. Of KBr (g/L)= (20 - ml of titrant) (F) (f) (1000) / ml of sample

The indicator reaction takes place on the surface of the precipitate. The indicator, which is a dye, exists in solution as the ionized form, usually an anion.
Iodide can be determined by direct titration with standard silver nitrate using an adsorption indicator such as eosin, fluorescein or diiodofluorescein. kI + AgNO
3
→ AgI + kNO
3
The titration of I with Ag + :
Before the equivalent point, I is in excess and the primary layer is I (go back to precipitation process in gravimetry). This repulses the indicator anions; and the more loosely held the secondary (counter) layer of adsorbed ions is cations
Beyond the equivalent point (end point as well), Ag
+
is in excess and the surface of the precipitate becomes positively charged, with the 1

layer being Ag + . This will now attract the indicator anion and adsorb it in the 2

(counter) layer:
AgI : Ag
+
: : indicator
-
The color of the adsorbed indicator is different from that of the un-adsorbed indicator, and this difference signals the completion of the titration.
1. Pipette 10 ml of KI solution in a conical flask, dilute with 100 ml distilled water.
2. Add 10 drops of eosin indicator.
3. Titrate with 0.05N AgNO
3 solution with constant rotation until the first separation of precipitate from colloid state and assuming a pronounced pink colour.
Calculation:
Conc. of KI (g/L)= (ml of titrant) (F) (f) (1000) / ml of sample