☰ Year plan · Chemistry

Cations, and the idea of a titration

Week 8 · Chapter 2 (C): detecting cations (six analytical groups: copper(II), aluminium, iron(II), iron(III), calcium) and the start of quantitative analysis: titration, standard solutions, end point and indicators · 3 lessons
3

Quantitative analysis: titration and indicators

Chemistry Sec 3 Term 1, Chapter 2: pp. 39–43
GoalDescribe a titration in words (standard solution, end point, indicator) and use the colours of common indicators.
New words
titration · adding a solution of known concentration to another until the reaction is just completestandard solution · a solution whose concentration is known exactlyend point · the moment when the indicator changes colour and the reaction is just completeindicator · a substance that changes colour between acid and alkali
Explanation

Volumetric analysis finds a concentration from the volume of a standard solution that reacts exactly with the sample. The method is called titration, and the reaction can be a neutralisation, a redox reaction or a precipitation. In an acid and alkali titration the book describes this set-up: a measured volume of the alkali is put in a conical flask with two drops of an indicator, and a burette of standard hydrochloric acid, 0.1 M, adds acid little by little until the colour changes. That moment is the end point, and the reaction is HCl + NaOH → NaCl + H₂O. A pipette, a burette and a conical flask are laboratory glassware, so the set-up stays on paper. An indicator is chosen for its colours: methyl orange is red in acid and yellow in alkali, phenolphthalein is colourless in acid and red-pink in alkali, litmus is red and blue, and bromothymol blue is yellow and blue. In the kitchen, red cabbage juice is a stand-in indicator, but it is not a replacement for the book’s table.

IndicatorIn acidIn alkali
methyl orangeredyellow
phenolphthaleincolourlessred-pink
litmusredblue
bromothymol blueyellowblue
red cabbage juice (kitchen)pink-redblue-green
Worked examples
1020304050mL
Name the parts. In a titration, 25 mL of potassium hydroxide solution of unknown concentration is in a conical flask with a drop of phenolphthalein, and 0.10 M hydrochloric acid is added from a burette until the pink colour just disappears. The standard solution is the 0.10 M acid, the sample is the potassium hydroxide, the indicator is phenolphthalein and the end point is the moment the pink goes. A measuring jug here only stands for the volume of the sample.
CupBaking sodaDrops of vinegar (invented)
A1 spoon12
B2 spoons24
A kitchen model with invented numbers. Cup A has one level spoon of baking soda in water with cabbage juice; it needs 12 drops of vinegar to turn from blue-green to purple. Cup B has twice as much baking soda and needs 24 drops. Twice the soda took twice the drops. This is only a model of an end point, and the drop counts are invented to show the idea of proportion, which next week becomes a formula.
Which indicator fits? You add acid to an alkali and want a clear change from coloured to colourless: phenolphthalein goes from red-pink to colourless. With methyl orange the colour goes from yellow towards red and then orange at neutral, and the last drops are harder to judge. So the indicator is chosen for a sharp colour change at the end point.
Class activity

Cabbage-juice indicator, with a grown-up. Squeeze a few red cabbage leaves by hand in a cup of cold water to make a purple juice (no heating, no knife in a child’s hand). Pour a little into two clear cups. Add vinegar, drop by drop, to one and a pinch of baking soda to the other, and see what happens. In the baking-soda cup, count the drops of vinegar needed to bring the colour back to purple, then repeat with more soda. The counts are your own, so call them invented numbers. Do not taste anything, wash the cups, and remember that this juice is a stand-in, not the book’s indicator.

Practice
1
A solution turns litmus from red to blue. Is it an acid or an alkali? What colour would phenolphthalein show?
2
How many moles of HCl are in 20.0 mL of 0.100 M hydrochloric acid? (n = V × C ÷ 1000, V in mL)
3
What volume of 0.250 M NaOH contains 0.00500 mol of NaOH?
4
Why is a pipette, a burette and a flask not used in this course’s activities, and what do we use instead?