Notes for Teachers

Estimation of metal cations present in an antacid using complexometric titration

Notes for teachers

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 3 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

In the experiment, we have used crushed 4-5 tablets which contain Al and Mg as hydroxides. The From this finely crushed powder 1.00 g sample was given to each student. The sample preparation also involves heating with acid, this is done to completely dissolve the tablet and bring the metal ions in solution form. The solution then needs to be filtered off as usually the tablets contain binders, which may interfere in the titration and obscure the end point detection.

 

Precautions:

In Part A of the experiment,

• The experiment involves heating, it is necessary to ensure this to get consistent readings. (The solution should be boiled for 5 mins as mentioned in the procedure and not just heated as the reaction is kinetically slow.
• While heating NH₃ gas is released. So care should be taken in maintain the pH.
• During the course of titration, the hot condition and pH=10 should be maintained
• 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.

In Part B of the experiment,

• The colour change in this titration does have intermediate colours also so please see to it that the students understand the blue colour. (wine red – purple – Blue)

 

 

 

 

 

 

 

 

 

Sr Sr No No	Volume of ZnSO4  required in estimation of total ions (mL)		Volume of EDTA required only for Mg (mL)		mmoles of Mg in 10ml sample solution (mmol)		Amount of Mg in (g)		% Error in Mg (%)	mmoles of Al in 10ml sample solution (mmol)		Amount of Al in (g)		% Error in Al (%)
	Exp	Obs	Exp	Obs	Exp	Obs	Exp	Obs		Exp	Obs	Exp	Obs
1	12.3	13.1	15.8	15.9	0.1530	0.1538	0.00372	0.00374	0.53	0.1156	0.1267	0.00312	0.00342	9.6
2		12.2		15.8		0.1530		0.00372	0		0.1182		0.00319	2.24
3		13.3		15.9		0.1538		0.00374	0.53		0.1289		0.00348	11.5
4		12.9		15.7		0.1522		0.00370	0.53		0.1248		0.00337	8.0
5		12.2		15.2		0.1472		0.00358	3.76		0.1182		0.00319	2.24

Typical Results

 

  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Typically the % error in estimation of Mg is negligible. Error % in case of Al is more as this is a dependent calculation.

It depends on how the error the first titration reading has introduced.

 

Post activity Discussion

 

In complexometric  titrations, the  metal ion present forms a soluble co-ordination complex with the titratnts. These titrants are commonly known as complexing agents.  EDTA is the most widely used titrant for complexometric titrations is Ethylenediaminetetraacetic acid (EDTA) and that has six donor groups. The structure of EDTA is given below.  

 

EDTA has low solubility in water and thus, for complexometric titration, disodium salt of Ethylenediaminetetraacetic acid (abbreviation Na₂EDTA or Na₂H₂Y) is generally used for the titration. The greatest advantage of EDTA (or Na₂EDTA) is that it complexes with metal ions in 1:1 proportion irrespective of the charge on the metal ion.  

Complexometric titartions also depend on the formation constants of the complexes that are getting formed in the titration. The graph given below is the graph of log of the formation constants of the complexes (symbol- formation constant) of the metal ions with Na₂EDTA, as a function of pH The metal ions you will be estimating are indicated on the graph.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

EDTA is slightly soluble in water so its disodium salt is commonly used instead as a titrant.

  

 

 

 

Reagent Preparation

Sample: Obtain antacid tablets from the medicine shop. Note down the composition of the tablet carefully. Take at-least 4-5 tablets and grind the tablets in clean mortar and pestle to make a fine. From this weigh 1.00 g of sample to be given to each student. Transfer the sample to a clean 250 mL conical flask, add 100 ml of water and about 6 mL of 6 M. hydrochloric acid to the flask. Gently and carefully, boil the mixture for about 20 minutes on a hot plate. After the heating time is over let the mixture cool for some time and then filter it using a pleated filter paper in a250 mL standard flask. At the end of filtration dilute the sample to the mark.

After this cork the flask and mix it well to homogenize the solution.

 

EDTA standard solution: Accurately weigh 1.861 g of reagent grade EDTA dihydrate, (molecular weight = 372.25) and transfer it of a clean 1000 mL standard flask, add distilled water to fill half of the flask. Shake the flask thoroughly to dissolve the EDTA, then dilute it upto the mark. After this cork the flask and mix it well to homogenize the solution.

Standardized zinc sulfate solution: Accurately weigh out approximately 0.718 g of reagent grade zinc sulfate heptahydrate, (formula weight = 287.53). Transfer it quantitatively into a 250 mL volumetric flask, add deionized water in small portions to dissolve it, and then dilute to the mark. After this cork the flask and mix it well to homogenize the solution.

Ammonium Chloride-Ammonia buffer solution: Dissolve 35.0 g of ammonium chloride,

(molecular weight = 136.08), in a 500-mL beaker containing about 288.5 mL of  ammonia solution. Dissolve the ammonium chloride in this solution, and then add dilute it with deionized water to the 500-mL mark with intermittent stirring. This buffer solution should have a pH of 10.00

Calmagite indicator: Dissolve 0.25 g of Calmagite with 50 mL of absolute ethanol. The blue indicator is suitable for analyzing solutions at pH = 10.0. If the indicator appears purple in color, dropwise a pH 10 buffer solution until it changes back to blue in color.

 

 

Good Practices

It is necessary that distilled water should be used for solutions preparation and rinsing all glassware.

In complexometric titration, the order for the additions of solutions and indicators, heating and back-titration should be carefully carried out to get precise results.

Disposal

The remaining zinc solution can be retained and stored. Triethanolamine should be disposed off as per safety rules.

Unused solutions containing EDTA, buffer, and sample should be disposed by neutralization, or diluting it first with water, and then flushing down the drain.

 

Extension

The experiment can be carried with various brands of antacids, which contain the cations in hydroxide form.

The experiment can be extended by using antacids which contain the cations in different form such as carbonate, bicarbonate with appropriate modifications etc.