(f) Acids, alkalis and titrations

Learning objectives

2.28Acid–base indicatorsDescribe the use of litmus, phenolphthalein and methyl orange to distinguish between acidic and alkaline solutions 2.29The pH scaleUnderstand how to use the pH scale, from 0-14, can be used to classify solutions as strongly acidic (0-3), weakly acidic (4-6), neutral (7), weakly alkaline (8-10) and strongly alkaline (11-14)2.30Universal indicatorDescribe the use of universal indicator to measure the approximate pH value of an aqueous solution 2.31Acids and alkalis in waterKnow that acids in aqueous solution are a source of hydrogen ions and alkalis in a aqueous solution are a source of hydroxide ions 2.32NeutralisationKnow that alkalis can neutralise acids 233C Carry out an acid-alkali titrationDescribe how to carry out an acid-alkali titration

An acid–base indicator changes colour according to whether a solution is acidic or alkaline. Add only a few drops to a small sample and identify the solution from the resulting colour.

Indicator	In an acidic solution	In an alkaline solution
litmus solution	red	blue
phenolphthalein	colourless	pink
methyl orange	red	yellow

When acid is added to alkaline phenolphthalein until the acid is in excess, the colour changes from pink to colourless. The direction matters because the starting solution determines the first colour.

These indicators distinguish acidic from alkaline solutions but do not give a numerical pH. Use the named indicator's own colour pair—methyl orange is not blue in alkali, and phenolphthalein is not pink in acid.

The pH scale from 0 to 14 classifies aqueous solutions. Values below 7 are acidic, 7 is neutral and values above 7 are alkaline.

pH range	Classification
0–3	strongly acidic
4–6	weakly acidic
7	neutral
8–10	weakly alkaline
11–14	strongly alkaline

As pH decreases below 7, a solution is more acidic; as pH increases above 7, it is more alkaline. Thus pH 2 is strongly acidic, pH 5 weakly acidic, pH 9 weakly alkaline and pH 13 strongly alkaline.

Do not reverse the scale: a high pH is alkaline, not strongly acidic. The classification describes the solution at that pH; it does not by itself identify which acid or alkali is present.

Universal indicator gives an approximate pH because it produces a sequence of colours across the pH scale rather than one colour change.

Stage	Action
1	Place a small sample of the aqueous solution in a clean container, or use universal-indicator paper.
2	Add a few drops of universal indicator, or touch the paper with the solution.
3	Compare the final colour immediately with the supplied colour chart.
4	Report the closest pH value or range as an approximation.

Typical progression is red/orange for acidic values, green near neutral and blue/purple for alkaline values. The exact pH must be read from the chart supplied with that indicator.

Universal indicator estimates pH; it does not provide the precision of a calibrated pH meter. It is also unsuitable for an accurate titration end-point because its gradual sequence of colours does not give one sharp change.

In aqueous solution, acids are sources of hydrogen ions, $\ce{H+}$ , while alkalis are sources of hydroxide ions, $\ce{OH-}$ .

Solution	Ions formed in water	Ion responsible for behaviour
hydrochloric acid	$\ce{H+}$ and $\ce{Cl-}$	$\ce{H+}$ makes it acidic
sodium hydroxide solution	$\ce{Na+}$ and $\ce{OH-}$	$\ce{OH-}$ makes it alkaline

\ce{HCl(aq) -> H+(aq) + Cl-(aq)}\qquad\ce{NaOH(aq) -> Na+(aq) + OH-(aq)}

The aqueous condition matters. Dry hydrogen chloride consists of covalent molecules and does not show acidic behaviour without water; hydrogen chloride dissolved in water supplies mobile ions. Write hydroxide as $\ce{OH-}$ , including its charge.

Neutralisation occurs when an acid reacts with an alkali. Hydrogen ions from the acid combine with hydroxide ions from the alkali to form water.

\ce{H+(aq) + OH-(aq) -> H2O(l)}

The complete reaction forms a salt and water. For example, $\ce{HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l)}$ : sodium and chloride ions remain in solution as the salt while $\ce{H+}$ and $\ce{OH-}$ form water.

Mixing an acid and alkali does not guarantee a neutral final solution: if either reactant is in excess, the mixture remains acidic or alkaline. Neutralisation describes the reaction, while exact neutral conditions require suitable reacting amounts.

A titration measures the volume of one solution needed to react exactly with a fixed volume of another. A pipette delivers the fixed volume; a burette measures the variable volume accurately.

Stage	Accurate action
1	Rinse the pipette with the solution it will deliver, then transfer $25.0\text{ cm}^3$ to a conical flask. Rinse the flask only with distilled water.
2	Add a few drops of a suitable indicator such as methyl orange or phenolphthalein; place the flask on a white tile.
3	Rinse and fill the burette with the other solution, fill the jet, remove the funnel and record the initial reading at eye level from the bottom of the meniscus.
4	Add from the burette while swirling. Near the end-point, add dropwise until one permanent indicator colour change occurs; record the final reading.
5	Calculate titre = final − initial reading. Repeat with fresh portions until concordant titres within $0.20\text{ cm}^3$ are obtained, then average only concordant accurate titres.

Use a rough titration to locate the end-point, then approach it slowly in accurate repeats. A few drops of indicator do not materially change the volume; excess indicator or universal indicator makes the end-point less reliable.

Wear eye protection, clamp the burette vertically and fill it below eye level. Remove air bubbles from the jet and never read the scale from above or below, because parallax changes the measured titre.

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—(g) Acids, bases and salt preparationsRead

(f) Acids, alkalis and titrations

Learning objectives

Distinguish acids and alkalis with three indicators

Classify solutions on the pH scale

Estimate pH with universal indicator

Identify the ions supplied by acids and alkalis

Explain acid–alkali neutralisation

Carry out an accurate acid–alkali titration

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