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Volumetric Analysis

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1. Volumetric Analysis Equipment and Terminology

2. Carry out an acid-base titration An acid-base titration is a procedure where a reaction between an acid and a base is used to determine the concentration of the acid or the base (the concentration of either the acid or base is known). It depends on accurately measuring the volumes of reactants used — hence the termvolumetric analysis.

3. Acid-base titrations involve: • Knowledge of the reaction occurring so that mole ratios can be determined. • Preparation of standard solutions. • Using specialized equipment.

5. Standard solutions A standard solution is one for which the concentration is accurately known. A distinction is made between primary and secondary standard solutions.

6. Primary standard Its concentration is determined by directly weighing a reactant and dissolving it to give an accurately known volume of solution. For a substance to be a primary standard it must: • Be readily available in pure form. • Resist absorbing water. • Be stable (unreactive) when stored. • Have a high molar mass so weighing errors are minimized.

7. Secondary standard Its concentration is determined by experiment,rather than by accurately weighing out the standard. This is achieved by: • Making up a solution to the approximate concentration required. • Finding its concentration exactly by titrating against a primary standard. This is called standardizing the solution.

8. For both primary and standard solutions, the concentration is calculated using the equation: c = n/V where: n is the amount of dissolved solute (moles), c is the concentration of the standard solution (in mol L-1), V is the volume of the standard solution (in L).

9. Titration terminology • titrate: adding liquid from a burette. • titre: the volume of liquid added. • titration: the name of the procedure using a burette. • aliquot: a precisely measured volume of liquid using a pipette.

10. Titrations — some terminology indicator equivalence point end-point rough titration concordant titres meniscus

11. The ‘mystical’ meniscus

12. Titration techniques and tips • The pipette and burette must be clean and rinsed with the solution they will contain. • The flask must be clean but may be wet. Do not rinse the flask with the solution it will contain. • Fill the pipette past the mark, then run the liquid back until the bottom of the meniscus is level with the mark. • Allow the pipette to drain naturally, then touch the tip of the pipette to the side of the flask. Do not blow out the last drop. • Make sure there are no air bubbles in the burette, especially between the tip and tap. Check the tap doesn’t leak. • During titration, swirl the flask constantly — practise this.

13. A note about accuracy and significant figures • Titrations use glassware with high accuracy. • Make sure you keep this accuracy as you do the calculation. • Your data will be accurate to at least 3 significant figures. • During the calculation, record your data to 1 more significant figure than this to reduce rounding errors. • Your final answer should be stated to the same accuracy as the data.

14. A titration calculation • Write a balanced equation for the reaction. • Find the number of moles of the known substance. • Use the balanced equation to find the number of moles of the unknown substance. • Work out the mass, concentration or volume of the unknown.

15. DETERMINATION OF TOTAL TITRATABLE ACIDITY IN WINE Reagents: Standard sodium hydroxide solution 0.10 mol L-1; phenolphthalein indicator; distilled water; wine diluted 1:1 with distilled water. Method: Pipette 20 mL of diluted wine into a 250 mL conical flask. Add about 50 mL of distilled water. Add a squirt of phenolphthalein indicator. Titrate against 0.10 mol L-1 NaOH to a faint pink colour (pH 8.4). Repeat until concordant titres are obtained. Calculations: (a) Calculate the concentration of total titratable acid (in molL-1) in the wine assuming a 1:1 mole ration in the reaction of titratable acid with NaOH. (b) Calculate the total titratable acid in g L-1, expressed as tartaric acid, given that 1 mL of 0.1 mol L-1 sodium hydroxide will react with 0.0075 g of tartaric acid. In both NZ and Australia, the wine industry expresses total titratable acidity as grams of tartaric acid per litre of wine.