Starter Question

After completing this lesson you should be able to : Chemical analysis - Volumetric analysis. Standard solution A solution of accurately known concentration is known as a standard solution. Redox Titration Volumetric analysis involves using a solution of accurately known concentration in a quantitative reaction to determine the concentration of another substance. Redox titrations are based on redox reactions. Substances such as potassium manganate(VII), which can act as their own indicators, are very useful reactants in redox titrations. The volume (usually measured to one decimal place) of reactant solution required to complete the reaction is determined by titration. Data from concordant titres is used to calculate an average volume used and hence the concentration of a solution. Volumes within 0·2 cm 3 are considered to be concordant at Higher. The end-point is the point at which the reaction is just complete. An indicator is a substance which changes colour at the end-point. The concentration of a substance can be calculated from experimental results by use of a balanced equation.

Experimental work could include any acid/base or redox titration. Interesting examples could include the determination of:  the purity of aspirin  the purity of vitamin C tablets  the concentration of ethanoic acid in vinegars (by titration with sodium hydroxide using phenolphthalein indicator)  the mass of citric acid in chewing gum (by titration with sodium hydroxide solution using phenolphthalein indicator)  the calcium carbonate content of antacid tablets (back titration in which tablet is dissolved in standard hydrochloric acid, and the excess acid determined by titration with sodium hydroxide solution using

Revision of practical skill Standard solution - In analytical chemistry, a standard solution is a solution containing a precisely known concentration of an element or a substance, a known weight of solute is dissolved to make a specific volume. It is prepared using a standard substance, such as a primary standard. Distilled water is water that has many of its impurities removed through distillation. Distillation involves boiling the water and then condensing the steam into a clean container. Water purification techniques Distilled vs de-ionised De-ionised Water from which ionic salts have been removed by ion-exchange. This is preparing a standard solution of sodium carbonate but the principles can be applied to other standard solutions. Water purification COPY

Calculation The chemist in the previous video measured 2.58g of sodium carbonate (Na 2 CO 3 = GFM 106g) and this was then washed into a 250 cm 3 standard flask and made up to the water mark. Using the direct proportion method you can calculate the concentration (mol l -1 ) 1 mol of Na 2 CO 3 106g 2.58gX mole mol 0.25 L 1 L mol mol l -1 COPY

Elga CD 100 water deioniser

Revision of practical skill This is an acid base titration but the skills of titration can be applied to the practical today and of course your indicator and reactants will be different but principle is the same.

1 mark – Burette is rinsed with de-ionised water and then the solution to be put in it. 2 marks – for any of the following: Fill the burette above the scale with solution being added to it. Filter funnel used should be removed. Tap opened/ some solution drained to ensure no air bubbles. Scale reading should be made from bottom of meniscus. Zinc ions/impurities/metal ions/ salts may be present in tap water Pipette COPY

Practical Task Vitamin C titration Collect a workcard