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POTENTIOMETRIC TITRATION OF IRON IN MOHR'S SALT
Transcript of POTENTIOMETRIC TITRATION OF IRON IN MOHR'S SALT
Apparatus and Materials
Become familiarized with the experimental requirements of a REDOX TITRATION and the calculations necessary to determine the equivalence point of a potentiometric redox reaction
Real-World Applications of the Potentiometric titration technique
Advantages and disadvantage of potentiometric technique
Equipment and reagents
The iron content on a sample of
ferrous ammonium sulfate (Mohr's salt)
can be detemined by means of a
The titration must be performed in the presence of sulfuric and phosphoric acids.
The H3PO4 forms a complex with the Fe3+.
This reaction lowers the Fe3+ concentration
and, in turn, the potential of the
Mohr's salt is unstable because
ion is easily oxidized
The sample should be protected
as much as possible from:
In this experiment, we will focus our attention on the use of a redox reaction.
Because most potentiometric titrations are asymmetrical
and time consuming, the titration will be performed using dyphenylAMINE sulfonate as a redox indicator for the analysis of an unknown Mohr's salt sample.
More expensive:: you need a stirrer and bar, electrodes, and a
Slower to set up: – making sure the electrodes are correctly in
place and functioning properly, getting the burette above the beaker
and out of the way of the electrodes, making sure the stirrer bar
isn’t going to smash the end off the electrode
• Background colour :– the colour
of the sample is too great to be able to see an
indicator change colour.
• slower to perform: – having to record all the data (in most cases)
slower to get the endpoint volume: – No need to read the volume off the burette at the end (in most cases), you have to process the data in some way
Solution is too dilute: –
indicator titrations are not
Magnetic stirrer/stirring bar
50(+/-).02 mL Buret
250 mL beakers
Potassium dichromate, K2Cr2O7 (MW = 294.185)
50% Sulfuric Acid (MW = 98.079; specific gravity =1.84 g/mL)
Mohr's Salt Unknown, FeSO4(NH4)2SO46H2O (MW = 392.143) Phosphoric Acid (85%)
For Concentration of Standard Potassium Dichromate (K2Cr2O7)
For Classic Titration
100mL sulfuric acid/phosphoric acid solution.
1. Add enough titrant to induce a 10 mV change on the pH meter (~1 mL).
2. When the titration is close to the endpoint (≤0.20 mL for a 10 mV change in potential), titrant must be added at 0.10 mL intervals until a hugh jump in voltage is observed (in some cases, between 200- 300 mV).
3.As soon as the volume of titrant needed to attain a 10 mV change reaches 1.00 mL again, add three 1.00 mL portions of titrant.
diphenylamine sulfonate as indicator (0.3%)
Preparation of Standard K2Cr2O7
1.Dry a sample of standard K2Cr2O7 for an hour at 150 C.
2. Allow the sample to cool to room temperature in a desiccator.
4. Calculate the exact molarity of this solution.
3. Accurately weigh about 1.25 g, dissolve in a beaker with 30 or 40 mL of distilled water. Transfer
quantitatively into a 250 mL volumetric flask and dilute to the mark.
Preparation of 0.5 M H2SO4 / H3PO4 solution
1.Add 15 mL of 50% sulfuric acid to 300 mL of distilled water, and mix well. (Do this slowly and carefully, since the solution will get hot). Store in a close container.
2.Add 45 mL of phosphoric acid (85%), to the abovementioned mixture and mix thoroughly.
The substance may be toxic to blood, kidneys, lungs, liver, upper respiratory tract, skin, eyes. Repeated or prolonged exposure to the substance can produce target organs damage. Repeated exposure to a highly toxic material may produce general deterioration of health by an accumulation in one or many human organs.
sTANDARD Potassium Dichromate (K2Cr2O7)
Incompatibility with various substances:
Reactive with oxidizing agents, reducing agents, combustible materials, organic materials, metals, acids, alkalis, moisture.
Determine the equivalence point of a redox titration using different methods: indicators, midpoint, first derivative and second derivative plots.
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Monosegemented flow potentiometric titration for the determination of chloride in milk and wine
Inst. Mike Vazquez
Potassium dichromate is an excellent oxidizing agent for Iron(11)
Adding phosphoric acid to the titration creates the change in potential around the final point region more drastic
pH is proportional to the oxidation efficiency in potassium dichromate
The potentiometric titration is more precise and accurate than the classical titration
The potentiometric titration error was .19 mL
With the theorical data from the potentiometric titration,it was obtained that the equivalence point for all three graphics was 8.19
The %Fe at the unknown Mohr's Salt for the poteniometric data,was9.91(+/-).0001% with and RSD of 6.2