Why titration curve shape

If the pH of an acid solution is plotted against the amount of base added during a titration, the shape of the graph is called a titration curve. All acid titration curves follow the same basic shapes. In the beginning, the solution has a low pH and climbs as the strong base is added.

The first curve shows a strong acid being titrated by a strong base. There is the initial slow rise in pH until the reaction nears the point where just enough base is added to neutralize all the initial acid.

This point is called the equivalence point. As the solution passes the equivalence point, the pH slows its increase where the solution approaches the pH of the titration solution. A weak acid only partially dissociates from its salt.

The pH will rise normally at first, but as it reaches a zone where the solution seems to be buffered, the slope levels out. There are two main points to notice about this curve. The first is the half-equivalence point. The half-equivalence point is when just enough base is added for half of the acid to be converted to the conjugate base. The second point is the higher equivalence point.

When a weak acid is neutralized, the solution that remains is basic because of the acid's conjugate base remains in solution. These acids are called polyprotic acids. Previous Video When titrating an acid with a base, the titrant is gradually added to the acid, raising the pH. The pH change is plotted against the volume of the base to create a titration or pH curve. The titration curves are S-shaped, though differences exist depending on whether the acid is strong or weak. The pH at the start of the titration will be acidic but higher for a weak acid, assuming equal initial concentrations of acid.

The equivalence point is the point on the titration curve at which the moles of base equals the moles of acid in solution. In the case of a strong acid, the equivalence point is reached when a strong base neutralizes all the hydronium ions produced by the strong acid; therefore, it always occurs at pH 7. Near the equivalence point, there is a sudden jump in the pH of the solution.

However, in a weak acid titration, this rapid pH shift is not as steep as in a strong acid titration. The pH after the equivalence point on a titration curve for both weak and strong acids gradually increases due to excess strong base. Other features of a weak acid titration curve, but not a strong one, are the buffer region and the half-equivalence point. When a strong base is added, a weak acid produces its conjugate base creating a buffer.

The half-equivalence point lies within this buffer region. This point is when half of the volume of base needed to reach the equivalence point has been added. Here, the concentration of weak acid equals its conjugate base, and the pH of the solution equals the p K a. The titration curves of a strong or a weak base with a strong acid as the titrant is an inverted S-shaped, where the pH starts high and decreases with the addition of the strong acid.

A titration curve is a plot of some solution property versus the amount of added titrant. Acid-base titration can be performed with a strong acid and a strong base, a strong acid and a weak base, or a strong base and a weak acid. For a titration carried out for Figure 1 a The titration curve for the titration of The titration of a strong or weak base with a strong acid has a similar S-shaped curve; however, the curve is inverted as the pH will start in the basic region and decrease with the addition of the strong acid.

The shape of the curve provides important information about what is occurring in solution during the titration. The pH of the sample in the flask is initially 7. Eventually the pH becomes constant at 0. In contrast, when 0.

As you can see from these plots, the titration curve for adding a base is the mirror image of the curve for adding an acid. Suppose that we now add 0. Thus the pH of the solution increases gradually.

At the equivalence point when The pH is initially As the equivalence point is approached, the pH drops rapidly before leveling off at a value of about 0.

The titration of either a strong acid with a strong base or a strong base with a strong acid produces an S-shaped curve. The curve is somewhat asymmetrical because the steady increase in the volume of the solution during the titration causes the solution to become more dilute.

Due to the leveling effect, the shape of the curve for a titration involving a strong acid and a strong base depends on only the concentrations of the acid and base, not their identities. The shape of the titration curve involving a strong acid and a strong base depends only on their concentrations, not their identities. Calculate the pH of the solution after A Because 0. To completely neutralize the acid requires the addition of 5.

Because only 4. B The final volume of the solution is As we shall see, the pH also changes much more gradually around the equivalence point in the titration of a weak acid or a weak base. Thus the pH of a solution of a weak acid is greater than the pH of a solution of a strong acid of the same concentration. Below the equivalence point, the two curves are very different. Note also that the pH of the acetic acid solution at the equivalence point is greater than 7.

That is, at the equivalence point, the solution is basic. Above the equivalence point, however, the two curves are identical. In this situation, the initial concentration of acetic acid is 0. However, you should use Equation Inserting the expressions for the final concentrations into the equilibrium equation and using approximations ,.

Thus the pH of a 0. Now consider what happens when we add 5. Suppose you start with 25 cm 3 of sodium carbonate solution, and that both solutions have the same concentration of 1 mol dm That means that you would expect the steep drop in the titration curve to come after you had added 50 cm 3 of acid.

The graph is more complicated than you might think - and curious things happen during the titration. You expect carbonates to produce carbon dioxide when you add acids to them, but in the early stages of this titration, no carbon dioxide is given off at all.

Then - as soon as you get past the half-way point in the titration - lots of carbon dioxide is suddenly released. The graph is showing two end points - one at a pH of 8. The reaction is obviously happening in two distinct parts. In the first part, complete at A in the diagram, the sodium carbonate is reacting with the acid to produce sodium hydrogencarbonate:.

In the second part, the sodium hydrogencarbonate produced goes on to react with more acid - giving off lots of CO 2. It is possible to pick up both of these end points by careful choice of indicator. That is explained on the separate page on indicators. Ethanedioic acid was previously known as oxalic acid. It is a diprotic acid , which means that it can give away 2 protons hydrogen ions to a base. Something which can only give away one like HCl is known as a monoprotic acid.

The reaction with sodium hydroxide takes place in two stages because one of the hydrogens is easier to remove than the other. The two successive reactions are:. If you run sodium hydroxide solution into ethanedioic acid solution, the pH curve shows the end points for both of these reactions. The curve is for the reaction between sodium hydroxide and ethanedioic acid solutions of equal concentrations. If this is the first set of questions you have done, please read the introductory page before you start.

The equivalence point of a titration Sorting out some confusing terms When you carry out a simple acid-base titration, you use an indicator to tell you when you have the acid and alkali mixed in exactly the right proportions to "neutralise" each other. But that isn't necessarily true of all the salts you might get formed. To summarise: The term "neutral point" is best avoided. Simple pH curves All the following titration curves are based on both acid and alkali having a concentration of 1 mol dm Titration curves for strong acid v strong base We'll take hydrochloric acid and sodium hydroxide as typical of a strong acid and a strong base.

Running acid into the alkali You can see that the pH only falls a very small amount until quite near the equivalence point. Running alkali into the acid This is very similar to the previous curve except, of course, that the pH starts off low and increases as you add more sodium hydroxide solution.