A-level Applied Science/Finding out about substances/Titration

Uses of titration

Titration can produce one of two pieces of information:

1. When both reagents are known, their reaction is known but only one concentration is known; titration can calculate the unknown concentration.
2. When both reagents and their concentrations are known, but the reaction is unknown; titration can calculate the unknown reaction.

Sample preparation

Samples for titration must contain a known chemical so that a standard chemical can be chosen which will react with it.

The sample must not contain any impurities which would interfere with the main reaction.

Samples need to be in the form of solutions. To assay gases and solids the substances may be dissolved in a suitable solvent (e.g. water).

In some cases the sample is made to react with a solution and then the remaining reactant is titrated. This is known as a back reaction.

Example: 1.00 g of impure calcium carbonate reacted with 100 cm³ of 0.4 mol dm⁻³ hydrochloric acid. The acid concentration was found by titration to be 0.21 mol dm⁻³, which implies that only 0.0095 mol of carbonate reacted. This amount of carbonate would have a mass of 0.95 g so we can say that the calcium carbonate has 5% impurities.

Standard procedures and equipment

Standard Solution

A standard solution is a solution of known concentration. Units of concentration are moles per cubic decimetre i.e. mol dm⁻³

Primary Standard - This is a chemical which can be obtained in a pure state. It is the basis upon which the concentration of other solutions can be measured and it must possess the following properties:

• it should not effloresce nor be deliquescent or hygroscopic
• it must remain stable in solution for a sufficient length of time
• it must possess a fairly large molar mass to reduce errors in weighing

Passing the mine water sample through an ion exchange resin can replace the metal ions with sodium ions. This process converts all the sulphate salts to sodium sulphate. The acid concentration is in the range 0.6-0.7 mol dm⁻³, but we need to measure it exactly by titration.

Experiment 1: Preparing the primary standard (sodium carbonate 0.5 mol dm⁻³)

1. Place a weighing bottle on the balance and tare to obtain a zero reading. Weigh approximately 5.3g the sodium carbonate.
2. Remove the weighing bottle from the balance, zero the balance, and weigh the full bottle.
3. Tip the contents of the bottle into a clean beaker (100 cm³)- it doesn't have to be dry.
4. Weigh the emptied bottle.
5. Add deionised water to the beaker (about 50 cm³) and stir to dissolve the sodium carbonate. Leave the glass rod in the solution at all times - you will lose some to the bench if you put it down.
6. Now transfer the solution via a funnel into a volumetric flask (100 cm³) and wash all remaining drops of solution into the flask with a wash bottle. Wash also the rod and funnel into the flask.
7. Remove the funnel and make up the solution to the graduation mark i.e. until the bottom of the meniscus is just in line with the graduation mark on the neck of the flask - use a dropping pipette for this. Stopper the flask and mix the solution by inverting the flask at least 10 times (shaking is no good).
8. Carefully top up the solution to 100 cm³ again if necessary.
9. Label the flask immediately with your name, name of solution and its concentration.

Weighing Results

• Mass of bottle with sample: ________ g
• Mass of bottle empty: ________ g
• Mass of sample: ________ g
• Concentration of solution _________ g dm⁻³
• M_r of sodium carbonate: _________
• Molar concentration of solution: _________ mol dm⁻³

Titration

Using standard solution of sodium carbonate (0.5 mol dm⁻³) titrate this against the mine water in order to determine its acid content.

1. Use the pipette to measure 25 cm³ aliquots of your mine water into 4 conical flasks.
2. Add a few drops of methyl red or screened methyl orange indicator.
3. Fill the burette with the sodium carbonate (0.5 mol dm⁻³)
4. Titrate to obtain concordant results.
5. Record the results in the table below

	Rough	1st accurate	2nd accurate	3rd accurate
Final burette reading (cm³)
Initial burette reading (cm³)
Titre (cm³)

Indicator:

Mean titre:

Calculations

The acid contaminating the water is sulphuric acid.

1. Write the balanced equation for the reaction of sodium carbonate with hydrochloric acid.
2. Calculate the concentration of the acid.

Risk Assessment

Chemical/Procedure	Hazard	Source of advice	How to reduce risk	Disposal
Sodium Carbonate	Irritant	CLEAPPS Hazcards	Wear lab coat and goggles when using	For spillages rinse with water, dispose of clothes which encounter the substance
Sulphuric Acid 0.05 – 0.15 mol dm-3	Irritant as it is of low concentration	CLEAPPS Hazcards	Wear lab coat and goggles at all times	Rinse with water and dispose of contaminated clothes
Methyl Orange	Stains/Minimal Hazard		Do not swallow, wear lab coat to protect clothing	If swallowed rinse mouth out thoroughly with water
Running burette tap	Overshoots	Tutor	Always ensure your turn the tap with the left hand and swirl with the right	If overshoot occurs, repeat procedure
Fixing pipette into pipette filler	Breaking/stabbing	Common sense	Hold pipette at the top, near the opening and put on with applying too much pressure	If broken, do not touch, clean up with dustpan and brush
Glassware	Breaking/stabbing	Common sense	Use proper procedure	If broken, do not touch, clean up with dustpan and brush
Weighing balance	Electrical Shock	Student safety sheets	Do not touch exposed conductors	If shocked, seek medical advice

Observations and measurements

phenopthalein:(indicator)

is added in the base solution

burette:

>Take acid in the burette as 50 ml.

>measuring cylinder take 25 ml base.

>now take five readings

>the readings must be in between 11 to 15 ml if these does not comes then there must be wrong in some titration reading.

Interpretation, calculation and evaluation of results

Exercise based on minewater scenario

The mine water sample can be purified by a cation-exchange resin so that it contains only sulphuric acid and sodium sulphate, which does not interfere with acid-base titrations.

The acid concentration is very high, so before titration it is diluted by pipetting 25 cm3 into a volumetric flask and making the solution up to 250 cm3.

The following data were obtained by six students:

A. 2.240 g of sodium carbonate decahydrate were dissolved to make 250 cm3 of solution. When this solution was titrated against 25 cm3 of mine water residue, 49.50 cm3 were required to neutralise the acid. What is the acid concentration?

B. 2.426 g of sodium carbonate decahydrate were dissolved to make 250 cm3 of solution. When this solution was titrated against 25 cm3 of mine water residue, 44.85 cm3 were required to neutralise the acid. What is the acid concentration?

C. 3.205 g of sodium carbonate decahydrate were dissolved to make 250 cm3 of solution. When this solution was titrated against 25 cm3 of mine water residue, 34.60 cm3 were required to neutralise the acid. What is the acid concentration?

D. 4.730 g of sodium carbonate decahydrate were dissolved to make 250 cm3 of solution. When this solution was titrated against 25 cm3 of mine water residue, 24.25 cm3 were required to neutralise the acid. What is the acid concentration?

E. 4.95 g of sodium carbonate decahydrate were dissolved to make 250 cm3 of solution. When this solution was titrated against 25 cm3 of mine water residue, 32.70 cm3 were required to neutralise the acid. What is the acid concentration?

F. 5.700 g of sodium carbonate decahydrate were dissolved to make 250 cm3 of solution. When this solution was titrated against 25 cm3 of mine water residue, 19.15 cm3 were required to neutralise the acid. What is the acid concentration?

Plot a graph of the mass of carbonate in 250 cm3 against the volume of carbonate used in the titration.

From the graph, identify which, if any, results is/are unreliable.

From the graph, estimate the average mass of carbonate and the average volume of carbonate required.

From your estimates, calculate the concentration of sulphuric acid.

Find out what percentage errors are associated with the apparatus used in the experiment.

Apparatus:

• Burette error: 0.1 cm3
• Pipette error: 0.06 cm3 out of 25 cm3.
• Volumetric flask: 0.3 cm3 out of 250 cm3.
• Top-pan balance error: 0.001 g

What is the percentage variation in the students’ estimates of the acid concentration?

Estimate the total error for this experiment.

Experimental work for Scenario 1

Analysis of the acid content of the mine water sample.

Passing the mine water sample through an ion exchange resin can replace the metal ions with sodium ions. This process converts all the sulphate salts to sodium sulphate. The acid concentration is in the range 0.6 - 0.7 mol dm⁻³, but we need to measure it exactly by titration.

Experiment 1: Preparing the primary standard

(sodium carbonate 0.064 mol dm-3)

Notes:

Standard Solution - This is a solution of known concentration. Units of concentration are moles per cubic decimetre i.e. mol dm-3

Primary Standard - This is a chemical which can be obtained in a pure state. It is the basis upon which the concentration of other solutions can be measured and it must possess the following properties

• it should not effloresce nor be deliquescent or hygroscopic
• it must remain stable in solution for a sufficient length of time
• it must possess a fairly large molar mass to reduce errors in weighing

Method For Preparing A Standard Solution

1. Place a weighing bottle on the balance and tare to obtain a zero reading. Weigh approximately 4.55 g of the sodium carbonate.

2. Remove the weighing bottle from the balance, zero the balance, and weigh the full bottle.

3. Tip the contents of the bottle into a clean beaker (100 cm3)- it doesn't have to be dry.

4. Weigh the emptied bottle.

5. Add deionised water to the beaker (about 50cm3) and stir to dissolve the sodium carbonate. Leave the glass rod in the solution at all times - you will lose some to the bench if you put it down.

6. Now transfer the solution via a funnel into a volumetric flask (250 cm3) and wash all remaining drops of solution into the flask with a wash bottle. Wash also the rod and funnel into the flask.

7. Remove the funnel and make up the solution to the graduation mark i.e. until the bottom of the meniscus is just in line with the graduation mark on the neck of the flask - use a dropping pipette for this. Stopper the flask and mix the solution by inverting the flask at least 10 times (shaking is no good).

8. Carefully top up the solution to 250 cm3 again if necessary.

9. Label the flask immediately with your name, name of solution and its concentration.

Weighing Results

• Mass of bottle with sample ________ g
• Mass of bottle empty ________ g
• Mass of sample ________ g
• Concentration of solution _________ g per 100 cm3
• _________ g dm-3
• Mr of sodium carbonate _________
• Molar concentration of solution _________ mol dm-3

Titration

Using standard solution of sodium carbonate (approx 0.064 mol dm-3) titrate this against the mine water in order to determine its acid content.

Procedure

Dilution

1. Use the pipette to measure a 25cm3 sample of your mine water into a clean 250 cm3 volumetric flask.

2. Add water to make the solution up to 250 cm3

Titration

• Solution in burette: (give concentration)
• Solution in pipette:
• Volume of pipette:
• Indicator:
• Colour change:
• Other details:

3. Use the pipette to measure 25cm3 samples of your diluted mine water into 4 conical flasks.

4. Add a few drops of methyl red or screened methyl orange indicator.

5. Fill the burette with the sodium carbonate (0.064 mol dm-3)

6. Titrate to obtain concordant results.

7. Record the results in the table below

	Rough	1st Accurate	2nd Accurate	3rd Accurate
2nd Burette reading
1st Burette reading
Titre

Calculations

The acid contaminating the water is sulphuric acid.

In your report of this practical:

1. Write the balanced equation for the reaction of sodium carbonate with hydrochloric acid.

2. Calculate the concentration of the acid from the mean titre.

Mean titre = cm3

Percentage error: (show calculation)

Your report should include:

• A description of the sample and its preparation for the titration (ion-exchange resin, dilution).
• An account of the methods you used to estimate the acid concentration
• Calculation of the concentration of acid and an evaluation of the method and results.

Errors

The equipment we use in titration has the following tolerances:

• Burette 0.1 cm3
• 25 cm3 pipette 0.06 cm3
• 250 cm3 volumetric flask 0.3 cm3
• 3 decimal place balances: 0.001 g

Each item of equipment will introduce a random error to the result. The only way to reduce this error is to repeat the experiment or use more accurate equipment – and we do not have the time or the money.

To calculate the total error we can add the errors from each item of equipment. First the errors must be written as percentages:

23.2 cm3 reading on burette – error is 0.1 cm3, which is _________ %

25 cm3 pipette with a tolerance of 0.06 cm3; __________ %

1.372 g reading on balance with accuracy of ± 0.001 g; _________ %

250 cm3 volumetric flask used to make up solution; __________ %

So, the answer to question (3) has a total error of ___________ %

Errors in the way you perform the experiment are less easy to spot, but if you take several titration readings, you can estimate the random error you have caused by the way you titrate:

The simplest way of considering the uncertainty in the final results is to look at the deviation of the titrations from the average value, so that, for example, a deviation of 0.15 cm3 in 25.15 cm3 leads to an uncertainty of 0.6%.

Readings used to calculate mean titre:

25.0 25.1 25.2 25.3

Precision is best given as the ‘spread’ of possible values e.g. 25.15 ± 0.15 or 25.15 ± 0.6%. The ‘spread’ is usually the ‘standard deviation’, which can be estimated from the range: Divide the range by a conversion factor from the following table:

Sample Size	Conversion factor
2	1.5
3-6	2.5
7-12	3
13-30	4
31-150	5
151-500	6	501+	6.5

[1]

So, for our four values: 25.0 25.1 25.2 25.3

The range is 0.3, so the standard deviation is 0.3/2.5 = 0.12 cm3 or 0.48%

Result: 25.15 ± 0.12 cm3 or 25.15 cm3 ± 0.48%

What is the standard deviation for these values:

23.1 23.3

What is the standard deviation for these values:

23.1 23.3 23.2 23.1 23.3 23.2 23.1 23.3 23.2

Notice how, with more readings, the standard deviation decreases. Notice also that the error calculated in this way can be added to the equipment errors to estimate the total error.