Titration – Precipitation

This experiment involves the determination of the number of water molecules of crystallization in hydrated barium chloride using titration and precipitation techniques with silver nitrate and potassium chromate. The procedure involves the reaction of chloride ions with silver ions to form a red precipitate of silver chromate, which indicates the end-point of the reaction. Barium ions also react with chromate ions, and to remove them, sulphate ions are added. The mass of anhydrous barium chloride and water present in the sample is calculated, and the ratio of BaCl₂ to H₂O is determined to find the value of x in the formula BaCl₂•xH₂0. This experiment provides a great opportunity to understand the principles of titration and precipitation methods used to calculate the water of crystallization in compounds.

Aim

The purpose of this experiment is to determine the number of molecules of water of crystallisation in hydrated barium chloride, i.e. to calculate the value of x in the formula BaCl₂•xH₂0.

Introduction

You titrate chloride ions with silver ions, according to the equation:

Ag⁺ _(aq) + Cl^– _(aq) → AgCl_(s)

This provides you with the data necessary to do the calculations. The indicator for the titration is potassium chromate(VI). When all the chloride ions have reacted, any more silver ions react with the indicator producing a red precipitate of silver chromate(VI). This is because silver chloride is less soluble than silver chromate(VI).

2Ag⁺ _(aq) + CrO₄^2- _(aq)  → Ag₂CrO_4 (s)

The end-point in this reaction is when one drop of aqueous silver ions produces a red tinge on the precipitate of silver chloride. Barium ions also react with chromate ions so the barium must be removed by adding sulphate ions:

Ba²⁺ _(aq) + SO₄^2- _(aq)  → BaSO_{4 (S)}

Note. This does not affect the concentration of chloride ions.

Requirements

• safety glasses
• weighing boat
• spatula
• barium chloride crystals (TOXIC)
• access to balance capable of weighing to 0.01 g
• beaker, 250 cm³
• wash-bottle of distilled water
• stirring rod
• volumetric flask, 250 cm³
• filter funnel
• dropping pipette
• burette, 50 cm³
• 2 beakers, 100 cm³
• silver nitrate solution, standardised (TOXIC & CORROSIVE)
• pipette, 10 cm³
• pipette filler
• conical flasks, 250 cm³
• sodium sulphate
• potassium chromate solution (TOXIC & OXIDISING)
• ‘silver residues’ bottle

Procedure

1. Prepare a standard solution of hydrated barium chloride by accurately weighing out between 1.4 g and 1.6 g of the salt. Dissolve this and make up to 250 cm³ in a volumetric flask. Record the exact mass in your copy of the table below.
2. Rinse the burette with some silver nitrate solution and fill. Don’t forget the tip.
3. Rinse the 10.0 cm³ pipette with barium chloride solution, and transfer 10.0 cm³ to a conical flask.
4. Add about 1 g of sodium sulphate crystals to the flask and swirl it.
5. Add 2-3 drops of potassium chromate (VI) indicator. Titrate the solution to the end-point, as shown by the first appearance of a permanent but faint reddish precipitate of silver chromate (VI). Use the first flask for a trial run. Enter your results in your copy of the results table.
6. Repeat steps 2 to 5 three times. Don’t wash the contents of the titration flasks down the sink – pour them into a ‘silver residues’ bottle.

Mass of bottle and contents before transfer, m1	g
Mass of bottle and contents after transfer, m2	g
Mass of sample, m = (m2-m1 )	g
Mass of BaCl2 • xH20 in 10.0 cm3	g

Pipette Solution		mol dm-3	cm3
Burette Solution		mol dm-3
Indicator

		Rough	1	2	3	(4)
Burette Reading	Initial
	Final
Volume used (titre) cm3
Mean titre cm3

Solution	Molar ratio	Moles (mol)	Volume (dm3)	Concentration (mol dm-3)

Table of Results (PDF)
Calculation

1. From the mean titre and concentration of silver nitrate, calculate the amount of chloride ions present in a 10.0 cm³
2. Calculate the mass of anhydrous barium chloride, BaCl₂ present in a sample.
3. Calculate the mass of water present by subtracting the mass of BaCl2 from the mass of BaCl₂xH₂0.
4. Determine the ratio of amount of BaCl₂ to amount of H₂0 and thus the value of x