Volhard`s Method

1. Volhard’s Method
A Type Of Argentometric Titration

2. ARGENTOMETRIC TITRATION:
Argentometry is derived from a Latin word
Argentum, which means Silver. Argentometry is a type of
titration involving the Silver ion.
METHODS OF ARGENTOMETRIC TITRATIONS:
Argentometric methods involving precipitation titrimetry.
Mohr’s Method
Fajan’s Method
Volhard’s Method

3. VOLHARD’S METHOD
The Volhard’s method was first
described by the Jacob Volhard, a German
Chemist, in 1874.
This is an indirect titration procedure,
used for determining the anions that precipitate
with silver.

4. STEPS OF VOLHARD’S METHOD:

 Analyte is treated with the measured excess of Silver
nitrate:
X
-
+ Ag
+
AgX + Ag
+
(excess)
The unreacted Silver ions are titrated with a standard
solution of Thiocyanate ion, using Fe(III) as indicator:

Ag
+
+ SCN
-
AgSCN
The first slight excess of Thiocyanate ion gives Red
colour of FeSCN
2+
:
Fe
3+
+ SCN
-
FeSCN
2+

5. Indicators:
IndicatorEnd Point Colour
Ferric alum

Red colour
TartrazineBright lemon-
yellow colour

6. CONDITIONS FOR VOLHARD’S METHOD:
The titration must perform in acidic medium to prevent
the precipitation of Iron(III) as hydrated oxide.
The indicator concentration should not be more than
0.2M.
In case of I
-
, indicator should not add until all the I
-
is
precipitated with Ag
+
, since it would be oxidized by the
Fe(III).

2Fe
3+
+ 2I
-
2Fe
2+
+ I
2

7. If the AgX precipitate is less soluble than
AgSCN, they do not need to remove, before
titrating with SCN
-
, such is the case of I
-
, Br
-
.
If the AgX precipitate are more soluble, it will
react with the titrant, which result in the over
consumption of SCN
-
and cause the diffuse end
point. Such is the case with Cl
-
:

AgCl + SCN
-
AgSCN + Cl
-

8. To prevent this error the precipitate must be
filtered off, before titrating with SCN
-
OR
Add some volume of organic liquid (like
Nitrobenzene, Chloroform), which form a film
around the precipitates.
OR
Use tartrazine as indicator.

9. DETERMINATION OF ANIONS:
Anions whose Silver salts are
soluble in nitric acid like PO4
3-
, C2O4
2-
, AsO4
3-
, CrO4
2-
, S
2-

are precipitated in neutral solution with excess of Ag
+
.
The precipitate is filtered off , dissolve in Nitric acid and
the Ag
+
titrated with SCN
-
.
OR
The residual AgNO3 in the filtrate is titrated with SCN
-

after acidification with dilute Nitric acid.


10. PreciptateColour
AgCl White
AgBr White(creamy)
AgI Yellow
AgSCN white
Ag2CrO4 Red
Ag2C2O4 White
Ag2S Black
Ag2CO3 Yellow
Ag3AsO4 Brown
Ag3PO4 Yellow

11. APPLICATIONS OF VOLHARD’S METHOD
Volhard’s method can be used for the
determination of :
 Halides (Cl
-
, Br
-
,I
-
).
Anions like Phosphate, Arsenate, Chromate,
Sulphide, Carbonate and Oxalate.
Potassium as Potassium tetraphenyl borate.
Flouride as Leadchloroflouride.

12. ADVANTAGES OF VOLHARD’S METHOD
This method is useful where the titrations have to be
performed at low pH.
The strong acidic environment give advantage for halide
analysis because ions like carbonate, oxalate and
arsenate do not interfere.
Give accurate results due to back titration.

13. LIMITATIONS OF VOLHARD’S METHOD
Can not be used where the solution has
to be neutral.
Time consuming.
Sometimes adsorption of Ag
+
may give false end
point.

14. REFERENCES
D.A. Skoog, D.M. West, F.J. Holler, “Fundamentals
of Analytical Chemistry” Saunders Philadelphia.
A.I. Vogel “A Textbook of Quantitative Chemical Analysis”.
Gary D. Christian “Analytical Chemistry”.