GROUP NUMBER 5
Presentation
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QUESTION
In a group of at least 8 students, prepare a detailed class presentation
on Precipitation titration:
Volhard titrations.
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Precipitation titrations
• Titration is a technique used in analytical chemistry to determine the
concentration of an unknown solution by using a solution of known
concentration
• Solution of known concentration is known as titrant while the solution of an
unknown concentration is known as analyte in titration technique.
• Precipitation titration is a type of titration which involves the formation of
precipitate during the titration technique
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Argentometric titration
• It is a type of precipitation titration which involves the use of silver ions. The
symbol of silver is Ag which is taken from its Latin name Argentum. So the
world argentometric is also taken from the Latin word Argentum.
Methods of Argentometric titration
• Volhard’s method
• Fajan’s method
• Mohr’s method
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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.
• Indirect titration means the process of reacting an analyte with moles of a
particular excess reagent and thereby titrating this reagent with a second
reagent. It is also known as back titration.
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Volhard’s method of titration conti…
• Volhard’s titration can be defined as a method for determination of chlorine,
bromine, and iodine in the form of halides by precipitating them with excess
silver nitrate and titrating the excess with a thiocyanate solution.
• It can also be defined as a method involves determination of halides(F⁻, Cl⁻,
Br⁻, I⁻) ions, anions like phosphate, chromate, in acidic medium by using
silver ions.
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Volhard’s method of titration conti…
• This titration must be performed in acidic medium otherwise iron ion get
precipitated as hydrated oxide.
• Iron Ions are used as indicators in Volhard’s method.
• In this method, first analyte (halide ion solution or any other anionic solution)
is titrated with a measured excess of AgNO₃.
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Procedures in Volhard’s titration method
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Procedure for determination of chloride ion concentration by titration
The burette is filled with 0.1M potassium thiocyanate solution and then is adjusted to
zero.
100mls of the sample is measured and poured into a conical flask.
1ml of ferric ammonium sulphate solution is then added as an indicator.
Using the potassium thiocyanate solution (KSCN), unreacted silver ions are titrated.
Because of the ferric thiocyanate complex, the endpoint is the appearance of a dark
red colour.
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Procedures in Volhard’s titration method con..
• The titration is repeated three times to get an accurate results.
• Then the moles of potassium thiocyanate used are calculated.
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Steps of Volhard’s method
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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 color of FeSCN²⁺:
Fe₃⁺ + SCN⁻ → FeSCN²⁺
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Conditions of Volhard’s method of titration
• 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.
• The temperature of the solution must be kept below 25°C as the red color of
the ferric thiocyanate complex fades quickly at higher temperatures.
• The concentration of nitric acid in the resultant solution should be between
0.5 and 1.00 M during the titration.
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Applications of Volhard’s method of titration
• Volhard’s method can be used for the determination of;
• Halides (Cl⁻, I⁻, Br⁻).
• Anions like Phosphate, Arsenate, Chromate, Sulphide, Carbonate and
Oxalate.
• Potassium as Potassium tetraphenyl borate.
• Fluoride as Leadchlorofluoride.
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Limitations of Volhard’s method
• It is time consuming.
• Sometimes adsorption of Ag˖ (Silver) may give false end point.
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Advantages of Volhard’s method
• It can analyze direct silver and indirect halide and has a very distinct color
change.
• This method is useful where titration is to be carried out at low pH.
• It can provide accurate results due to back titration.
• The strong acidic conditions give an advantage for analysis halide since ions
such as carbonate, oxalate, and arsenate do not interfere.
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References
• A.l. “A textbook of Quantitative Chemical Analysis”.
• D.A. Skoog, D.M. West, F.J. Holler, "Fundamentals of Analytical Chemistry”
Saunders Philadelphia.
• Gary D. Christian "Analytical Chemistry”.
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