Equilibrium in solutions
Hydrofluoric acid is a weak acid and the most dangerous reagent in an electronics laboratory. The two facts do not contradict each other: the strength of an acid measures its dissociation equilibrium, not its toxicity.
01Water ionizes itself
Two water molecules exchange a proton, and everything else follows from that equilibrium. Its constant is small but not zero, and that is enough to fix the scale on which acidity is measured:
And from this also follows the resistivity limit of ultrapure water that appeared in the unit on solutions: those 10⁻⁷ moles per liter of each ion are what set the floor of 18.2 MΩ·cm. No purification can remove them, because the water itself makes them.
\( K_w \) grows with temperature: at 60 °C it is \( 9.6\times10^{-14} \) and pure water has a pH of 6.5. It is still neutral —there are as many protons as hydroxide ions— but the number is no longer 7. Neutral means \( [\mathrm{H^+}] = [\mathrm{OH^-}] \), not “pH equal to seven.”
02Strong, weak and the constant that separates them
A strong acid dissociates completely; a weak one reaches an equilibrium, and how much it dissociates is given by its \( K_a \). The scale is handled better in logarithms:
| Acid | \( K_a \) | \( \mathrm{p}K_a \) | In electronics |
|---|---|---|---|
| Hydrochloric, HCl | Strong | — | Cleaning of metal oxides, pickling |
| Hydrofluoric, HF | \( 6.8\times10^{-4} \) | 3.17 | The only one that attacks SiO₂; weak, and yet extremely dangerous |
| Acetic | \( 1.8\times10^{-5} \) | 4.74 | Aluminum etchant mixtures |
| Ammonium ion, NH₄⁺ | \( 5.6\times10^{-10} \) | 9.25 | The ammonia pair in BOE |
Hydrofluoric acid is a weak acid and the most dangerous reagent in an electronics laboratory. Precisely because it is weak it circulates undissociated, passes through the skin without warning —at first it hardly hurts— and once inside the fluoride binds to the calcium of the bones. A splash on a small area can be fatal. The strength of an acid measures its dissociation equilibrium, not its toxicity.
03Titration and buffer solutions
Adding base to an acid a little at a time and following the pH draws a curve with three regions: a plateau at the beginning —the buffer region, where the pH hardly moves—, a sharp jump at the equivalence point and another plateau at the end. In a buffer solution the pH is set by the Henderson-Hasselbalch equation:
25 mL of 0.1 M acid titrated with 0.1 M sodium hydroxide. The pH does not come from an approximate formula: the exact charge balance is solved for each point. Compare the jump of hydrochloric acid with that of ammonium.
The buffered oxide etch —hydrofluoric acid with ammonium fluoride— is the standard etch of silicon oxide. Ammonium fluoride is the conjugate base of HF, so the mixture is exactly a buffer pair: it keeps constant the concentration of the etching species as it is consumed, and that is why the etch rate does not keep falling during the process. Unbuffered HF etches more and more slowly and the thickness removed stops being predictable.
04Solubility product
A sparingly soluble salt in contact with its saturated solution is also in equilibrium, and its constant is called the solubility product:
Comparing the product of the concentrations with \( K_{sp} \) tells you whether it will precipitate or not, just like \( Q \) against \( K \). And from this follows the common-ion effect: adding one of the two ions reduces the solubility of the other, which is the principle by which salts are purified by recrystallization.
| Salt | \( K_{sp} \) | Where it appears |
|---|---|---|
| AgCl | \( 1.8\times10^{-10} \) | Ag/AgCl reference electrodes |
| CaF₂ | \( 3.9\times10^{-11} \) | What forms when HF waste is neutralized with lime |
| Cu(OH)₂ | \( 2.2\times10^{-20} \) | Treatment of electroplating effluents |
| PbSO₄ | \( 1.8\times10^{-8} \) | What is deposited on the plates of a discharged battery |
The second row is the safety procedure for a hydrofluoric acid spill: it is neutralized with calcium carbonate or hydroxide, and the fluoride precipitates as calcium fluoride, which is insoluble and harmless. The last one explains why a lead battery that was left discharged for a long time does not recover: the precipitated sulfate crystallizes and stops responding.
05In the lab
Titrate 25 mL of 0.1 M acetic acid with 0.1 M sodium hydroxide, measuring the pH every milliliter. Plot it, locate the equivalence point by the derivative and read the \( \mathrm{p}K_a \) at the half-equivalence point. Compare with the tabulated value.
Prepare an acetate/acetic acid buffer and measure how much its pH changes when a drop of strong acid is added. Repeat on distilled water and compare the two shifts.
Form silver chloride by mixing dilute silver nitrate with sodium chloride. Then add excess chloride and observe. Calculate the solubility before and after with \( K_{sp} \), and compare with what you see.
06Common mistakes
- Believing that pH 7 is always neutral. It is at 25 °C; neutral means \( [\mathrm{H^+}] = [\mathrm{OH^-}] \).
- Confusing a weak acid with a dilute acid. Strength is the constant; concentration is how much there is.
- Assuming that the pH is 7 at the equivalence point. Only if the acid and the base are both strong; with a weak acid the salt hydrolyzes and the solution ends up basic.
- Believing that a weak acid is less dangerous. Hydrofluoric acid disproves that in the worst way.
- Using Henderson-Hasselbalch outside the buffer region. It is an approximation valid near \( \mathrm{p}K_a \).
- Comparing \( K_{sp} \) between salts of different stoichiometry. The exponents are not the same and the numbers are not directly comparable.
07Self-assessment
What is the pH at the half-equivalence point?
Exactly \( \mathrm{p}K_a \): there is as much acid form as basic form and the Henderson-Hasselbalch logarithm vanishes.
Why does the equivalence point of acetic acid with sodium hydroxide give a basic pH?
Because what is left is sodium acetate, and acetate is a weak base that hydrolyzes.
What is a buffer solution?
A mixture of a weak acid with its conjugate base, which resists changes in pH when acid or base is added.
What is ammonium fluoride for in BOE?
It is the conjugate base of HF: it buffers the pH and keeps the oxide etch rate constant as the acid is consumed.
How is a hydrofluoric acid spill neutralized?
With a calcium compound: the fluoride precipitates as CaF₂, which is insoluble —\( K_{sp} = 3.9\times10^{-11} \)— and harmless.
What is the common-ion effect?
That adding one of the ions of a sparingly soluble salt shifts the equilibrium and reduces its solubility.
08Further reading
- Raymond Chang and Kenneth A. Goldsby. Chemistry. 12th ed. (Spanish edition, “Química”), McGraw-Hill, 2017. Chapters 15 and 16: acids and bases, pH, hydrolysis, buffers and solubility product.
- Daniel C. Harris. Quantitative Chemical Analysis. 3rd ed. (Spanish edition, “Análisis químico cuantitativo”), Reverté, 2007. Titrations worked out with the exact charge balance, which is how the lab on this page calculates them.
- Werner Kern (ed.). Handbook of Semiconductor Wafer Cleaning Technology. Noyes Publications, 1993. BOE, etchant mixtures and the safe handling of hydrofluoric acid in industry.