The current experiment has been developed and tested with students in class XII as part of the Chemistry Olympiad programme conducted by HBCSE. This experiment aims to familiarize students with complexometric titration, introduce them to the various parameters that play an important role in complexometric titrations.
Learning Objectives of this experiment
1) Preparation and dilution of sample from given tablet.
2) Understand the principles of complex formation reaction, importance of conditions to be maintained in complexometric titrations, EDTA as a versatile ligand, metallochromic indicators, and their role, factors affecting the sharpness of the endpoint and different types of titration (back, direct etc)
3) Stoichiometric calculations
The full task takes about 2.5 hours to complete along with calculations if done by an individual student. The detailed notes about the task are given below.
Part A: Preparation of sample
A synthetic sample was prepared in the Laboratory in such a way that the amounts of copper and nickel were calculated to be similar to be present in a cupro-nickel coins used in the olden days.
1) 0.5grams of CuSO4.5H2O + 1-2 ml of conc. HCl diluted to 100 ml distilled water.
2) 0.5 grams of NiSO4.6H2O + 1-2 ml of conc. HCl diluted to 100 ml distilled water.
5 ml from each of the above solution is taken for the titration in both total and Ni estimation.
5 ml of diluted CuSO4.5H2O sample contains 0.00635g of Cu and 5 ml of diluted NiSO4.6H2O sample contains 0.0056g of Ni
In Part A of the experiment,
- The first titration is straightforward, maintaining the proper pH conditions is the important part during this titration.
In Part B of the experiment
- The second titration involves masking of one of the element present in the mixture at proper pH.
- In the second titration, you need to sensitize students about the endpoint, as detection of endpoint in this part is difficult as the colour change is not very distinct.
- While doing so the students should be asked to observe the centre of the conical flask to observe the change in color. The flask should be shaken thoroughly after each addition and the addition also should be carried out as slow as possible.
- The colour change in this titration does have intermediate colours also so please see to it that the students understand the pink colour. (yellow-orange-pink)
Post activity Discussion: Answers to Questions
Ph plays a very important role in this titration.
Masking of copper is a very important step in the experiment so as to minimise the errors.
The experiment can be extend to using actual alloy samples available in the market, which will also involve another skill of opening an alloy.
|Total reading||Total Millimoles||Only Ni reading||Millimoles of Ni||Amt of Ni||% Error||Only Cu reading||Amt of Cu||% Error|
|19.4 ml||0.194||9.7 ml||0.097||0.00569||-1.6||9.7||0.00616||2.9|
|19.3 ml||0.193||9.7 ml||0.097||0.00569||-1.6||9.6||0.00609||4.2|
|19.4 ml||0.194||9.6 ml||0.096||0.00563||-0.53||9.8||0.00622||2.04|
|19.2 ml||0.192||9.7 ml||0.097||0.00569||-1.6||9.5||0.00603||5.03|