How does buffer solutions work

A buffer solution is produced by combining a weak acid with its conjugate base, or a weak base with its conjugate acid. What are conjugates? Over to Wikipedia …. A conjugate base is what remains after an acid has provided a proton during a chemical reaction. The pH is the ratio between these two types of ion. Buffer solutions do have thresholds.

If enough strong acid or base is added to the solution, the pH will ultimately change. This is known as the buffer capacity. What is a buffer solution? Definition A buffer solution is one which resists changes in pH when small quantities of an acid or an alkali are added to it.

Acidic buffer solutions An acidic buffer solution is simply one which has a pH less than 7. Alkaline buffer solutions An alkaline buffer solution has a pH greater than 7.

How do buffer solutions work? Acidic buffer solutions We'll take a mixture of ethanoic acid and sodium ethanoate as typical. Ethanoic acid is a weak acid, and the position of this equilibrium will be well to the left: Adding sodium ethanoate to this adds lots of extra ethanoate ions.

Use the BACK button on your browser to return to this page. The solution will therefore contain these important things: lots of un-ionised ethanoic acid; lots of ethanoate ions from the sodium ethanoate; enough hydrogen ions to make the solution acidic. Other things like water and sodium ions which are present aren't important to the argument. Adding an acid to this buffer solution The buffer solution must remove most of the new hydrogen ions otherwise the pH would drop markedly. Adding an alkali to this buffer solution Alkaline solutions contain hydroxide ions and the buffer solution removes most of these.

Removal by reacting with ethanoic acid The most likely acidic substance which a hydroxide ion is going to collide with is an ethanoic acid molecule. Because most of the new hydroxide ions are removed, the pH doesn't increase very much. Removal of the hydroxide ions by reacting with hydrogen ions Remember that there are some hydrogen ions present from the ionisation of the ethanoic acid.

Alkaline buffer solutions We'll take a mixture of ammonia and ammonium chloride solutions as typical. Ammonia is a weak base, and the position of this equilibrium will be well to the left: Adding ammonium chloride to this adds lots of extra ammonium ions.

The solution will therefore contain these important things: lots of unreacted ammonia; lots of ammonium ions from the ammonium chloride; enough hydroxide ions to make the solution alkaline.

Other things like water and chloride ions which are present aren't important to the argument. Adding an acid to this buffer solution There are two processes which can remove the hydrogen ions that you are adding. Removal by reacting with ammonia The most likely basic substance which a hydrogen ion is going to collide with is an ammonia molecule.

Removal of the hydrogen ions by reacting with hydroxide ions Remember that there are some hydroxide ions present from the reaction between the ammonia and the water. Adding an alkali to this buffer solution The hydroxide ions from the alkali are removed by a simple reaction with ammonium ions. Calculations involving buffer solutions This is only a brief introduction. Acidic buffer solutions This is easier to see with a specific example.

So for ethanoic acid, you have the equilibrium: The presence of the ethanoate ions from the sodium ethanoate will have moved the equilibrium to the left, but the equilibrium still exists. That means that you can write the equilibrium constant, K a , for it: Where you have done calculations using this equation previously with a weak acid, you will have assumed that the concentrations of the hydrogen ions and ethanoate ions were the same.

That's no longer true for a buffer solution: If the equilibrium has been pushed even further to the left, the number of ethanoate ions coming from the ethanoic acid will be completely negligible compared to those from the sodium ethanoate. So the assumptions we make for a buffer solution are: Now, if we know the value for K a , we can calculate the hydrogen ion concentration and therefore the pH. K a for ethanoic acid is 1. Feed that into the K a expression.

In other words, the concentration of the ethanoate has to be half that of the ethanoic acid. Alkaline buffer solutions We are talking here about a mixture of a weak base and one of its salts - for example, a solution containing ammonia and ammonium chloride. You can change the pH of the buffer solution by changing the ratio of acid to salt, or by choosing a different acid and one of its salts. Alkaline buffer solutions have a pH greater than 7 and are made from a weak base and one of its salts.

A very commonly used example of an alkaline buffer solution is a mixture of ammonia and ammonium chloride solution. If these were mixed in equal molar proportions, the solution would have a pH of 9. Similarly when NaOH strong base is added to this buffer system, the ammonium ion donates a proton to the base to become ammonia and water thus neutralizing the base without any significant pH change.

Home About Teachers Help Feedback. See home page click here for information on coronovirus Covid