NJC Planning notes

National Junior College SH1 H2 Chemistry 4 Some Possible Practical/Planning Exercises Involving Titration Types of Expts Some Examples To ensure reliability Remarks (a) Acid-Base (b) Redox: MnO4− + reducing agents like H2O2, Fe2+, NO2− and C2O42−. (c) Iodometric titrations: I2 (formed by analyte) + reducing agent S2O32− (d) Back Titration: HCl is added in excess to react with and dissolve the insoluble carbonate compound. The excess HCl is then titrated against a standard solution of sodium hydroxide • To determine the formula of a compound • To determine the unknown concentration / % by mass of a reactant in a given mixture • To determine the solubility product of sparingly soluble salts. • Dropwise addition of titrant (solution in burette) when approaching the end-point. • Repeat titration until results are consistent • (two titres within ± 0.10 cm3) For (a) – (c): ALL titrations • For a given solid sample or aqueous sample of high concentration, preparation of its bulk / diluted solution in a graduated flask is required so that titration can be repeated to obtain consistent results. For (a): Acid-Base Titrations • only 2–3 drops of indicator (eg. phenolphthalein, thymol blue or methyl orange) are required. For (b): Redox Titrations • Add excess dilute H2SO4 (not HNO3 or HCl) to provide the acidic medium required for MnO4− titration. • No indicator is required as MnO4− is self-indicating. For (c): Iodometric titrations normally involve two steps: (i) An oxidising agent + KI to produce iodine, I2. (ii) The I2 produced is then titrated against sodium thiosulfate(VI), Na2S2O3. 1 cm3 of starch solution serves as the indicator and is only added near the end-point when most of the I2 has been reacted away by Na2S2O3. For (d): Back Titrations • For substances that are insoluble in water. A bulk solution of the sample is prepared through the use of a reaction (usually acid-base reaction). The unreacted excess reagent that is used to dissolve the sample is then titrated against a standard solution.

National Junior College SH1 H2 Chemistry 5 TITRATION 1 Standard Solution • Standard solution: A solution of accurately known concentration. • Primary standard solution: A solution of known concentration prepared by dissolving an accurately known mass of solute in a known volume of solution. Such a solution can only be prepared using a solute which has the following properties: (a) is available in high purity (b) forms a stable solution which is unaffected by air or light (c) does not have the following properties: volatile2, deliquescent3, hygroscopic4 • Secondary standard solution (bulk solution): A solution whose con