3) Titration of excess K2Cr2O7 in the sample with FAS
Initial Volume of FAS (ml) = 0.00
Final Volume of FAS (ml) = 17.20
Volume of FAS used (ml) = 17.20
Volume of FAS used to titrate with the blank solution = 17.20 ml
- Data treatment and questions:
1. The reaction between K2Cr2O7 and FAS can be described by the following equation:
6Fe2+ + Cr2O72- + 14H+ → 6Fe3+ + 2Cr3+ + 7H2O
Given that the molarity of K2Cr2O7 is 0.0417M
The volume of K2Cr2O7 used = 10ml
No.of moles of K2Cr2O7 used = molarity x volume in ml/ 1000
= 0.0417M x 10/1000 = 0.000417 mol
From the above equation, mole ratio of Fe2+ : Cr2O72- = 6 : 1
No. of moles of FAS solution in titrant = 4.17 x 10-4 mol x 6 = 0.002502 mol
The molarity of FAS solution= no. of moles x 1000/ volume in ml
= 0.002502 x 1000/ 10= 0.2502M
By standardization, the molarity of the FAS solution is 0.2502M
-
K2Cr2O7 reacts with organic matter during reflux as the following equation
Cr2O72- + 8 H+ → 2Cr3+ + 4H2O +3O
Organic matter is being oxidised.
mole ratio of Cr2O72- : O = 1:3
molecular weight of oxygen= 16gmol-1
Weight of oxygen generated = 3 x 16g = 48g
As volume of FAS used to titrate with excess K2Cr2O7 in the sample is 17.20ml.
no. of mole of FAS titrant used to titrate with excess K2Cr2O7 = 0.2502 x 17.20 /1000
= 0.0043 mole
mole ratio of Fe2+: Cr2O72- = 6 : 1
hence, no. of mole of excess K2Cr2O7 in the sample= 0.0043/ 6
= 0.00072mole
As volume of FAS titrant used to titrate with the blank solution = 25.20cm3 (given)
Hence, no. of mole of FAS titrant used to titrate with blank solution
= 0.2502 x 25.20/ 1000
= 0.0063 mole
no. of mole of K2Cr2O7 = 0.0063 / 6
= 0.0011mole
Volume of water sample used = 25 ml = 0.025dm3
Weight of oxygen generated = 48,000 mg
COD of my sample =〔(0.0011 –0.00072 ) x 48000〕/ 0..025
= 729.60 mg O2 L-1
The chemical oxygen demand of the sample is 729.60 mg O2 L-1.
3. Ag2SO4 can act as a catalyst to speed up the oxidizing reaction. However, problem arises as samples containing halide ions will form precipitates with Ag+, as the following equation:
Ag+ (aq) + X- (aq) → AgX (s) where X is halide ions.
These precipitates are insoluble. As from the principle of catalysis, catalysts should not take part in the reaction, this principle would be violated if the silver precipitates are formed, which is not desirable. And, the silver sulphate may be used up in the reaction that cannot act as a catalyst. As the amount of silver sulphate decreases, it affects the rate of the reaction.
Mercuric sulphate was added, as it can remove halide ions in the sample solution before refluxing instead of adding Ag2SO4. It is because HgSO4 can form complexes with the halide ions. So HgSO4 will not from precipitate with halide ion that can also suppress the oxidation of chloride ions in the COD test mixture.
How did the reaction take place?
For the observation of colour change during titration, the colour change should be as the following:
Orange yellow→ blue green → reddish brown
End- point
Fe3+ Fe2+
The colour change arose from the oxidation state of the iron complex.
When FAS was continually added, dichromate started to oxidize the Fe2+ ions into Fe 3+ ions, so it appeared as blue green colour from the colour of Fe3+ ions.
When it reached end point, all dichromate ions were used up, no more oxidising agent was present, hence Fe2+ in the FAS remained in such form, and appeared as reddish brown colour due to the presence of Fe2+ ions.
In the process of oxidizing the organic substances found in the water sample, potassium dichromate is reduced, forming Cr3+. The amount of Cr3+ is determined after oxidization is complete, and is used as an indirect measure of the organic contents of the water sample.
What is the use of the blank solution?
As COD measures the oxygen demand of organic compounds in a sample of water, it is important that no foreign organic material entered the flask of sample. Hence a blank sample is required in the determination of COD. A blank sample is created by adding all reagents to a volume of . COD is measured for both the water and blank samples, and the two are compared. The oxygen demand in the blank sample is subtracted from the COD for the original sample to ensure a true measurement of organic matter.
What method can we use to determine COD apart from titrimetric method?
To investigate the COD value, apart from titration method, we can use a quite
new and convenient invention, which is a set of COD analyzer, it is a powerful
multi-parameter ion specific meter ideal for wastewater treatment facilities.
On the other hand, we can also use micro sample digestion method to measure COD. The sample is firstly being oxidized, then it is placed into reflux system. Afterwards, the sample is being undertaken the colorimetric measurement using spectrophotometer.
Can we use other reagent to oxidise the sample apart from potassium dichromate?
The main point to choose the reagent is to consider if it is a strong oxidising agent or not. Thus, potassium permanganate solution can also be considered as a choice of oxidising agent.
Precaution
- Whenever adding concentrated sulphuric acid, add it little at a time slowly along the side of the flask with swirling, so that to release the heat gently and lower the temperature inside the flask. Continuous addition and swirling throughout.
- Handle with care of the concentrated sulphuric acid. Wear gloves and safety goggles all the time.
- Point the mouth of the flask away from any person including ourselves.
- Ensure swirling is in a proper manner, not a very vigorous nor very gently ones.
- Cool down the flask by water bath until cool if required.
- Do not put the flask directly into the ice otherwise it may cause breakage of the glassware.
- Watch the heating mantle tightly. Time is counted from the first drop condensed. Turn the power a bit lower with continuous dropping of the condensed liquid.
- A small beaker was placed on the reflux tube to prevent foreign substances falling into it.
The volume of FAS used to titrate with standard K2Cr2O7 and excess K2Cr2O7 in the sample solution are 10.00cm3 and 17.20cm3 respectively.
The molarity of FAS titrant is 0.2502 M while the chemical oxygen demand (COD) of the sample is 729.60 mg O2 L-1.
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