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UNIT 2

TITRATION

ADNAN CHOWDHURYCHEMISTRY TEACHER

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UNIT 2: GROUP 2 METALS

ADNAN CHOWDHURY 1

Acid Base Titration:

Preparation of a Standard Solution:

In any titration, the concentration of one of the solutions must be accurately known. The method is asfollows:

The mass of the solid needed to make a solution of the required concentration is calculated.

A weighing bottle is placed on a toppan balance. The tare button is pressed, so that the scale readszero.

The solid is added to the weighing bottle until the required mass is reached.

The best way to do this is to remove the bottle from the pan and then the solid is added, checking themass until the correct amount has been added. This prevents errors caused by spilling solid onto thepan of the balance.

The contents of the weighing bottle is transferred into a beaker. Any remaining solid is washed fromthe bottle into the beaker.

Some distilled water is added to the beaker containing the solid. Using a glass rod, the solution isstirred until all the solid has dissolved. In order to dissolve the solid completely, it may be necessaryto heat the beaker.

The solution is transferred through a funnel into a volumetric flask (250 cm3 or 100 cm3). The stirringrod and the beaker is washed, making sure that all the washings go through the funnel into thevolumetric flask.

More distilled water is added to the solution until the bottom of the meniscus is level with the markon the standard flask.

The stopper is placed on the flask and mixed thoroughly by inverting and shaking several times.

Performing a Titration:

Apparatus required:

Burette

Pipette (25cm3or 10cm3)

Conical Flask

Chemicals required:

Standard solution

The solution of unknown concentration

Suitable indicator

Procedure:

A small amount of one of solution (normally acid) is drawn into a pipette using a pipette filler and it isrinsed with the solution. The rinsings are then discarded.

Using a pipette filler, the pipette is filled so that the bottom of the meniscus is on the mark.

The pipette is allowed to discharge into a washed conical flask. When the pipette has emptied, thesurface of the liquid is touched in the flask with the tip of the pipette.

Making sure that the tap is shut, a burette is rinsed out with a small amount of the other solution(normally alkali) and the rinsings are discarded.

Using a funnel, the burette is filled to above the zero mark and the liquid is run out until the meniscusis on the scale. It is checked that the burette below the tap is filled with liquid and that there are noair bubbles. The funnel is then removed.

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ADNAN CHOWDHURY 2

The initial volume is recorded by looking at where the bottom of the meniscus is on the burette scale.

The liquid is run slowly from the burette into the conical flask, continually mixing the solutions byswirling the liquid in the flask. The liquid is added dropwise as the end point is neared and stoppedwhen the indicator shows the end point colour. The burette reading is recorded to the nearest0.05cm3.

The titration is repeated until three concordant (it means that the difference between the highest and

the lowest titre is not more than 0.2 cm3) are obtained.

Any non-concordant titres are ignored and average of the concordant values is calculated, to get themean titre.

Example 1:

23.67cm3is incorrect because the burette cannot be read to that level of accuracy.

23.62cm3should not have been used to calculate the mean titre because it is not in the range ofaccuracy of the other two values. Two titres are required that are the same or 0.20cm3of eachother.

Too many significant figures in the mean titre answer.

Example 2:

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ADNAN CHOWDHURY 3

Example 3:

Example 4:

Example 5:

Example 6:

Example 7:

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ADNAN CHOWDHURY 4

Example 8:

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ADNAN CHOWDHURY 5

Example 9:

Example 10:

Redox Titrations:

A substance which is found at a high state of purity is known as a primary standard substance.

The crystals of a primary standard substance are neither deliquescent (it does not become a liquid byabsorbing moisture from air) nor efflorescent (it does not loose water of crystallisation, if any, toatmosphere).

The water of crystallisation in a primary standard substance is fixed.

The molarity (concentration) of a primary standard substance can be calculated accurately if the massor number of moles and the volume of the solution is known.

Substances which are not primary standard are usually standardised by titrating them against aprimary standard substance. These titrations are normally redox titrations.

Iodometric Titration:

Sodium thiosulfate, Na2S2O3.nH2O is not a primary standard substance as the water of crystallisation isvariable (maximum value of n = 5).

So it is standardised against a solution of iodine, I2or potassium iodate(V), KIO3or potassiumdichromate(VI), K2Cr2O7.

Thiosulfate reduces iodine to iodide ions, I-and forms tetrathionate, S4O62-.

2S2O32-(aq) S4O6

2-(aq) + 2e-

I2(aq) + 2e-2I-(aq)

2S2O32-(aq) + I2(aq) S4O6

2-(aq) + 2I-(aq)

The standardised sodium thiosulfate solution can then be used to determine the percentage purity ofcopper in a substance, the percentage purity or concentration of KIO3, H2O2, K2Cr2O7, etc.

The substance to be analysed must be an oxidising agent and will produces iodine in another reaction

by oxidation of iodide ions.

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ADNAN CHOWDHURY 6

Procedure:

A known volume of the solution of the oxidising agent is transferred into a conical flask using apipette.

Dilute sulfuric acid is then added to the conical flask using a measuring cylinder (as it is in excess).

Iodine is liberated and the solution becomes brown. It must be ensured that if any solid particles

produced must be completely dissolved.

The liberated iodine is then titrated against standardised sodium thiosulfate solution added from abeaker.

The brown colour fades to a pale yellow or pale straw colour.

At this point, freshly prepared starch solution is added and the solution becomes blue black. If thestarch solution is not freshly prepared, then blue black colour may not appear. Starch solution shouldnot be added too early because enough iodine may not be liberated nor too late because at thatevent, the end point will be missing. Without starch solution, the colour would gradually fade awayand no sharp end point will be obtained. The blue black complex is formed because the remaining

unreacted iodine will react with the starch reversibly.

The thiosulfate solution is continued adding drop by drop until the blue black colour disappears andthe solution becomes colourless.

The procedure is repeated until at least two concordant titres are obtained.

Determination of Copper:

Points 1, 2 and 3 are as before.

Iodine is liberated and solution becomes milky brown.

2Cu2+(aq) + 4I- (aq) 2CuI (s) + I2(aq)

white brown

Point 5 as before.

The milky brown or white brown fades to a whitish yellow colour.

Point 6 as before (except for the fact that the solution now becomes whitish or milky blue black).

Point 7 as before (except for the fact that the solution changes colour from milky blue black to

colourless.)

Point 8 as before.

Example 10:

3.22g of iodine and 7g of potassium iodide are dissolved in distilled water and made up to 250 cm3. A 25 cm3portion of this solution required 19.0cm3of sodium thiosulfate solution in a titration. What is theconcentration of the sodium thiosulfate solution?

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ADNAN CHOWDHURY 7

Example 11:

5.65g of a copper (II) salt is dissolved in water and made-up to 250 cm3. A 25.0 cm3sample of solution is

added to an excess of potassium iodide, KI. The iodine formed by the reaction required 21.0cm3of a 0.10

mol dm-3 solution of sodium thiosulfate for its reduction. What is the percentage by mass of copper in the

salt?

Example 12:

Example 13:

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ADNAN CHOWDHURY 8

Example 14:

Example 15:

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ADNAN CHOWDHURY 9

Example 16:

Example 11:

Example 12:

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ADNAN CHOWDHURY 10

Example 13:

Example 14:

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ADNAN CHOWDHURY 11

Problem 1:

Problem 2:

Problem 3:

Problem 4:

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Problem 5:

Problem 6:

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Problem 7:

Problem 8:

Problem 9:

Problem 10:

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Problem 11:

Problem 12:

Problem 13:

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Problem 14:

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ADNAN CHOWDHURY 16

Problem 15:

Problem 16:

Problem 17:

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Problem 18:

Problem 19:

Problem 20:

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Problem 21:

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ADNAN CHOWDHURY 19

Problem 22:

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Problem 23:

Problem 24:

Problem 25:

Problem 26:

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Problem 27:

Problem 28:

Problem 29:

Problem 30:

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ADNAN CHOWDHURY 23

Problem 31:

Problem 32:

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Problem 33:

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Problem 34:

Problem 35:

05. titration

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