Titrations and Preparations: Quick Revision

A brief note on Titrations and Preparations. Read it before a test. Try each question at the end before you open its answer.

Volumetric analysis

• A primary standard is pure, stable and heavy: oxalic acid dihydrate ($126\ \text{g/mol}$), sodium carbonate, Mohr's salt.
• Make a standard solution in a volumetric flask: $c = m / (M V)$.
• Rinse the burette and pipette with their own solutions and the flask with distilled water.
• Read the bottom of the meniscus. Use the mean of concordant titres, within $0.10\ \text{mL}$.
• Ratio rule: $c_A V_A / a = c_B V_B / b$.
• Phenolphthalein for a weak acid with a strong base. Methyl orange or methyl red for a strong acid with a weak base.

Permanganate titrations

• Permanganate goes in the burette. It is a self indicator: the end point is a faint pink that lasts half a minute. Read the top of the meniscus.
• Acidify with dilute sulphuric acid. Never hydrochloric or nitric acid.
• $2\text{MnO}_{4}^{-}\ +\ 5\text{C}_{2}\text{O}_{4}^{2-}\ +\ 16\text{H}^{+}\ \rightarrow \ 2\text{Mn}^{2+}\ +\ 10\text{CO}_{2}\ +\ 8\text{H}_{2}\text{O}$. Warm the oxalic acid to about 335 K.
• $\text{MnO}_{4}^{-}\ +\ 5\text{Fe}^{2+}\ +\ 8\text{H}^{+}\ \rightarrow \ \text{Mn}^{2+}\ +\ 5\text{Fe}^{3+}\ +\ 4\text{H}_{2}\text{O}$. Do not warm Mohr's salt.

Preparations

• Mohr's salt: equimolar iron(II) sulphate and ammonium sulphate with dilute acid. Light green crystals.
• Potash alum: aluminium and potassium hydroxide, then sulphuric acid. Colourless octahedral crystals.
• Acetanilide: aniline with acetic anhydride, with zinc dust. It is the protected form of aniline.
• p-Nitroacetanilide: nitrate acetanilide below about 293 K. The para isomer is the main product.
• Aniline yellow: diazotise aniline at 273 to 278 K, couple with aniline to give p-aminoazobenzene.
• Iodoform: propanone or ethanol with iodine and sodium hydroxide. Yellow, medicinal smell.

A worked example

A 25.0 mL portion of sodium hydroxide needs 22.5 mL of hydrochloric acid of concentration $0.0500\ \text{mol/L}$. What is the concentration of the sodium hydroxide?

1. Moles of acid. $0.0500 \times 22.5 = 1.125\ \text{mmol}$.
2. Moles of base. They react 1 : 1, so also $1.125\ \text{mmol}$.
3. Concentration. $\dfrac{1.125\ \text{mmol}}{25.0\ \text{mL}} = 0.0450\ \text{mol/L}$.

Practice with new numbers

A program to try

The program finds the molarity of potassium permanganate from an oxalic acid titration.

Mistakes to avoid

• Rinsing the flask with the solution. Rinse it with water.
• Hydrochloric acid in a permanganate titration. Use sulphuric acid.
• Warming Mohr's salt. Only the oxalic acid is warmed.
• Nitrating aniline directly. Acetylate it first.

Check yourself

1. Why is permanganate called a self indicator?

2. How many moles of permanganate react with 5.00 mmol of iron(II)?

3. Which indicator is used for a weak acid with a strong base?

4. What is the temperature range for the diazotisation of aniline?