This is a general presentation about Argentometric Titration or well known as Precipitation Titration. Contain Mohr Methods, Volhard Methods, and Fajans Methods.
3. The Goal Determine the
number of titrant
needed to reach the
equivalence point of
the titration reaction
Argentometric is a process of
determining the content of a substance
based on precipitation reaction from
component of the test substance, using
precipitation formation of Ag+.
4. Influence Factors Temperature
Solvent Characteristic
Ion effect of a kind
The effect of other ion
The effect of pH
Complex Influence
Hydrolysis Influence
5. Mohr Methods
Definition
The Use
Limited to a solution with a
pH value of between
6 until 10
determine the content of
Chloride and Bromide
The Principle Using Chromate indicator
6. Mohr Methods
The standard solution is silver nitrate solution
using Potassium Chromate indicator
NaCl + AgNO3 AgCl (precipitate) + NaNO3
2AgNO3 + K2CrO4 (precipitate) + 2KNO3
7. Mohr Methods
Titration should be done in neutral or semi-
alkalis atmosphere because:
1. In the acid precipitation atmosphere of AgCrO4 will
dissolve because the silver dichromate are formed
(Ag2Cr2O7)
2. In the silver nitrate base atmosphere will reacted
with hydroxide ion to form silver hydroxide
precipitation
AgNO3 + NaOH AgOH (precipitation) + NaNO3
8. Mohr Methods
The disturbance of this titration:
1. Ion that will precipitate sooner than AgCl, example :
F, Br, CNS-
2. Ion that form complex with Ag+, example : CN-, NH3,
above pH 7
3. Ion that form complex with Cl-, example : Hg2+
4. Cation that precipitate the Chromate, example : Ba2+
9. Volhard Methods
• Indicator : Fe3+ (Ferry
Ammonium Nitrate) that
formed the Red color
from the iron (Fe) when it
react with SCN-.
• The materials that used:
– Standard solution Volume
of AgNO3
– Ion Halida (X-) example: Cl-
– Standard Solution Tiosianat
(KSCN or NH4SCN) as a
titrant.
– Indicator iron (III) (Fe3+).
• The certainty:
– This titration are occur in
Acid condition with pH
under 3.
– If not, the end point can’t
be showed.
10. • Process:
Here are the reaction :
Volhard Methods
leftover standard
solution of AgNO3
solution which
contains halogen
ion ,ex : (X-)
The excess of ion
Ag+ from AgNO3
which not react
with Halogen Ion
Will Experience
The titration
With Standard
solution of
Triosanat
(KSCN or NH4SCN)
By using the
indicator of Iron
(III) (Fe3+).
Ag+ + Cl AgCl(s) Ksp = 1.82 x 10-10
titrant #1 white precipitate
(excess)
Ag+ + SCN AgSCN(s) Ksp = 1.1 x 10-12
titrant #2 white precipitate
Fe3+ + SCN FeSCN2+ Kf = 1.4 x 10+2
indicator red precipitate
11. • The used of this methods :
– Determining the amount of Chloride,
bromine, and Iodide or the other
Halogen Ions on Acid condition.
Volhard Methods
13. The Differences of Methods in
Argentometri Titration
Methods Indicator Conditio
n
Reaction
Methods
Substanc
e in
Buret
The
endpoint
of
titration
Mohr K2CrO4 Netral Direct AgNO3 Dark Red
Precipita
te
Volhard Fe3+ / Fe
Allum
Acid Not
direct
KCNS Dark Red
Solution
Fajans Adsorpsi
(fluorescein
)
Netral Not
Direct
AgNO3 Pink
Solution
• The most stable method
are Volhard, but this
method are used
Cyanide Acid (HCN) that
toxic characterize.
• The existence of the
precipitation while the
practical are influenced
by sun light as
catalisator.
14. Equipment and Reagents :
• The solid reagents used in this experiment were
NaCl, CaCO3 3, NaHCO , K2CrO4 and AgNO3. The
equipment used is listed below:
– A burette Transfer pipette 25 mL and pipette pump
– Erlenmeyer flasks 250 mL Desiccator
– Volumetric flask 500 mL Amber bottle
– Graduated cylinder 100 mL Wash bottle
The Procedure of Mohr Method Titration
15. Preparation of 5% K2CrO4(indicator):
1.0 g of K2 4CrO was dissolved in 20
mL of distilled water.
The Procedure of Mohr Method Titration
1
16. Preparation of standard AgNO3 solution:
• Dilute sea water by pipetting 20 ml sample into a 100 mL
volumetric flask and making it up to the mark with
distilled water
• Pipette a 10 mL aliquot of diluted sea water into a conical
flask and add about 50 mL distilled water and 1 mL of
Chromate indicator
The Procedure of Mohr Method Titration
2
17. Preparation of standard AgNO3 solution:
• Titrate the sample with 0.1 mole Liter -1 silver nitrate solution. Although the
silver chloride that forms is a white precipitate, the chromate indicator initially
gift the cloudy solution faint lemon-yellow color.
• The endpoint of the titration is identified as the first appearance of a red-brown
color of silver chromate
• Repeat the titration with further aliquots of diluted sea water until concordant
result (tetras agreeing within 0.1 mL) are obtained.
The Procedure of Mohr Method Titration
2
18. • Mohr Methods
– In the precipitation of chloride by silver ion, chromate
ion (CrO4
2) is used as an indicator in the formation of
Ag2CrO4, a reddish-brown precipitate formed when
excess Ag+ is present.
– The concentration of titrant rises sharply near the
equivalence point, and the solubility of Ag2CrO4 is
exceeded
– Limitation : The concentration of titrant rises sharply
near the equivalence point, and the solubility of
Ag2CrO4 is exceeded, and it has to be avoid from direct
sunlight or neon light, because silver nitrate solution is
sensitive to the light (Photochemistry reduction).
Discussion
19. • Volhard Methods
–Determination of Cl . For titration of silver ion
with thiocyanate (SCN) and iron(III) as an
indicator.
–The endpoint is routinely used for halide
determinations where a known excess of silver
ion is added to precipitate the halide ion.
–The titration is usually done in acidic pH
medium to prevent precipitation of iron
hydroxides, Fe(OH)3.
Discussion
20. • Fajans Methods
– Adsorption indicators are organic compounds that tend to
be adsorbed onto the surface of the solid precipitate in a
precipitation titration.
– Adsorption indicators work best when:
• They do not precipitate out silver ion when the indicators are at
low concentration.
• They bind to the precipitate only when excess silver ion is present
to produce color.
– Example of Adsorption Indicators: Fluorescein
– The limitations of Fajans Method :
• It has to be performed at near neutral pH values
• The endpoint determination is operator-dependent
• It cannot be easily automated
Discussion
21. Conclusion
• Titrations with Ag+ are called argentometric titrations.
• For argentometric titrations, three classical methods
based on color indicators can be used for end point
detection:
• Mohr titration – formation of colored precipitate at the
end point.
22. Conclusion
• Volhard titration – formation of a soluble,
colored complex at the end point.
• Fajans titration – adsorption of a colored
indicator on the precipitate at the end point.
23. References
• Marshall Cavendish Education. (2014). Qualitative Analysis of Anion
and Cation. Retrieved from https://www.mceducation.sg/resource-
for-parents/more-tips/secondary-science/understanding-
qualitative-analysis-experiments
• Anne Marie Helmenstine Ph.D. (2015). Quantitative Analysis
Definition. About Education. Retrieved from
http://chemistry.about.com/od/chemistryglossary/a/quantitativede
f.htm
• What is a Titration. Retrieved from
http://chemed.chem.purdue.edu/genchem/lab/techniques/titratio
n/what.html
• University of Canterbury. Determination of Chloride Ion
Concentration by Titration (Mohr’s Method). Retrieved from
http://www.outreach.canterbury.ac.nz/chemistry/documents/chlori
de_mohr.pdf
