pH, Acid Concentration and Strength

What pH tells us about a solution

An aqueous solution is a solution in which water is the solvent. Acids produce hydrogen ions, $\mathrm{H^+}$, in aqueous solution. Alkalis are soluble bases and produce hydroxide ions, $\mathrm{OH^-}$, in aqueous solution.

The pH scale measures how acidic or alkaline a solution is. The scale normally used at GCSE runs from 0 to 14:

• Below pH 7: acidic. The lower the pH, the more acidic the solution.
• pH 7: neutral.
• Above pH 7: alkaline. The higher the pH, the more alkaline the solution.

For example, a solution at pH 2 is more acidic than one at pH 5. A solution at pH 12 is more alkaline than one at pH 9. pH describes the solution, rather than simply naming the substance dissolved in it.

Measuring pH with an indicator or a meter

Universal indicator is a mixture of indicators that gives different colours at different pH values. It is typically red in very acidic solutions, orange or yellow in less acidic solutions, green at neutral pH, and blue or purple in alkaline solutions.

To use universal indicator solution, place a small sample of the liquid in a clean test tube or spotting-tile well. Add a few drops of indicator, mix and compare the colour with the supplied pH chart, preferably against a white background. The chart gives an estimate of pH. Universal indicator paper can also be used: transfer a drop of the sample onto the paper with a clean glass rod and compare the developed colour with its chart.

A pH meter uses a probe to give a numerical reading, avoiding the need to judge a colour. If necessary, calibrate it using a buffer solution of known pH. Rinse the probe with distilled water, place its sensing end in the sample and record the reading once it has stabilised. Rinse the probe between samples so that one solution does not contaminate the next.

Wear eye protection when handling acids and alkalis, use small samples and avoid skin contact. Neither indicator testing nor meter testing requires heating or tasting a sample.

Concentration: how much acid is present? — Higher tier

Concentration describes the amount of acid in a given volume of solution. A dilute acid contains relatively little acid in that volume; a concentrated acid contains relatively much more.

The volume matters. A large bottle of dilute acid may contain more acid in total than a small bottle of concentrated acid. To compare concentrations, compare the amounts of acid in equal volumes of solution.

Adding water spreads the same amount of acid through a larger volume, so the solution becomes more dilute. This changes its concentration, but does not change the identity of the acid.

Strength: how fully does the acid ionise? — Higher tier

Ionisation is the formation of ions. When an acid ionises in water, it releases hydrogen ions.

A strong acid is completely ionised in aqueous solution. Hydrochloric, nitric and sulfuric acids are the common GCSE examples. For hydrochloric acid, the ionisation can be represented as:

$$\mathrm{HCl(aq) \rightarrow H^+(aq) + Cl^-(aq)}$$

Here, $(aq)$ means dissolved in water.

A weak acid is only partially ionised: only some of its molecules release hydrogen ions, while others remain as acid molecules. Ethanoic acid, found in vinegar, is a weak acid. Citric acid and carbonic acid are further examples.

Strength and concentration therefore answer different questions. Concentration asks how much acid is present per unit volume; strength asks how fully it ionises. Hydrochloric acid can be dilute and still be strong. Ethanoic acid can be concentrated and still be weak. Diluting a strong acid does not turn it into a weak acid.

How strength affects pH and reactions — Higher tier

Compare hydrochloric acid and ethanoic acid at the same acid concentration. Hydrochloric acid is completely ionised, whereas ethanoic acid is only partially ionised. The hydrochloric acid therefore has a higher hydrogen ion concentration and a lower pH.

Hydrogen ions also take part in reactions with suitable metals and carbonates. Under otherwise identical conditions, the strong acid generally reacts faster because more hydrogen ions are available to react.

For example, equal pieces of magnesium ribbon fizz more vigorously initially in hydrochloric acid than in ethanoic acid of the same concentration. The gas is hydrogen. With equal pieces of calcium carbonate, the hydrochloric acid produces carbon dioxide bubbles faster. These comparisons concern reaction rate, not a rule that strong acids must always produce more gas overall.

A pH reading alone does not identify whether an acid is strong or weak: hydrogen ion concentration depends on both the acid's concentration and its degree of ionisation.

Hydrogen ion concentration and whole-number pH changes — Higher tier

pH does not change in direct proportion to hydrogen ion concentration. Instead, each decrease of one pH unit means a tenfold increase in hydrogen ion concentration:

$$\text{hydrogen ion concentration} \times 10 \quad\Rightarrow\quad \text{pH decreases by 1}$$

A solution at pH 3 has ten times the hydrogen ion concentration of a solution at pH 4. A solution at pH 2 has $10 \times 10 = 100$ times the hydrogen ion concentration of a solution at pH 4.

The relationship also works backwards: a tenfold decrease in hydrogen ion concentration raises pH by one. Thus, a hundredfold decrease raises pH by two. Apply this rule to changes in hydrogen ion concentration, rather than assuming it describes every change in the total concentration of any acid.

Neutrality and alkalinity in terms of ions — Higher tier

Neutral does not mean that hydrogen ions and hydroxide ions are absent. Both are present, but their concentrations are equal. This corresponds to pH 7 under the usual school conditions.

An acidic solution has a higher concentration of hydrogen ions than hydroxide ions, and its pH is below 7. An alkaline solution has a higher concentration of hydroxide ions than hydrogen ions, and its pH is above 7.

Alkaline solutions still contain hydrogen ions, but at a lower concentration than a neutral solution. Increasing hydrogen ion concentration lowers pH; decreasing it raises pH. This connects the whole scale, from acidic through neutral to alkaline.

pH and measurement

• pH < 7: acidic; pH = 7: neutral; pH > 7: alkaline.
• Lower pH means more acidic; higher pH means more alkaline.
• Universal indicator: compare its colour with a chart to estimate pH.
• pH meter: rinse the probe, place it in the solution and record the stable numerical reading.

Concentration and strength — Higher tier

• Dilute/concentrated: relatively little/much in a given volume of solution.