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Titration Curves of Acids and Bases

Acid-base titrations are reactions by which we can determine the amount of acid or base present in a solution. This is done by reacting the solution with a base or acid (of known concentration), and by measuring the volume of the known acid or base used up in the process. The reaction data is usually plotted with the volume of the substance (concentration known) in the x-axis, and the pH of the solution in the y-axis. The graph is called a titration curve.
Take a look at the first titration plot (Figure 9-1) given. You will see a steep region in the plot. The center of this steep region is called the equivalence point. The equivalence point denotes the point at which equivalent amounts of acid and base have reacted. To know the equivalence point, we usually add an indicator which will change its color close to the equivalence point. We can use the following relation to equate the amount of acid or base present in a solution against the volume of acid or base added whose concentration is known.

N1V1 = N2V2


where N1 is the normality of the unknown acid or base,


N2 is the normality of the known base or acid,


V1 is the volume of unknown acid or base, and


V2 is the volume of the known base or acid.



An indicator is usually used to detect the equivalence point in an acid-base reaction or titration. The most common indicators used are weak organic acids or bases that change color in response to a change from acidic to basic medium or vice versa. The pH at which the color change occurs is characteristic of each indicator. For an acid-base reaction, the indicator is chosen based on the pH at which the equivalence point is expected to occur. Consider the hypothetical dissociation reaction of an indicator represented by HIn.

image\Ch 9 Indicators.png

Let's consider a scenario in which the indicator is in an acidic solution. Acidic solution means there is excess H+. So the equilibrium will shift to the left (LeChatelier's principle), and the predominant species will be HIn making the indicator show yellow color. On the other hand, in a basic solution, the equilibrium will shift favoring the forward reaction and the predominant species will be In (blue color). Because HIn (acid form) and In have different colors, this can indicate the equivalence point of acid-base reactions. You might be thinking that the dissociation of the indicator used itself might change or affect the pH of the tested solution. But this is negligible because only a tiny amount of the indicator is needed to show a visible color change.

Table: Some Common Indicators

image\Ch 9 table 9-2.png

Titration of a Strong Acid with a Strong Base

Consider the titration reaction involving HCl against NaOH. Let's say that we have 50 ml of a 0.1 M solution of HCl, titrated with 0.1 M solution of NaOH. As we add NaOH into the solution of HCl, initially the pH increase will not be very drastic. The pH increases slowly. When certain amount of NaOH is added, the change in pH becomes more drastic and the plot becomes very steep, as indicated in the plot (Figure 9-1). At this steep region, the equivalence point is reached. Can you guess the pH at the equivalence point? If you guessed 7, you are right. At the equivalence point of this titration you have a salt which is neutral, and hence the pH is 7.

image\25088 ch 9.png
Figure 9-1

Titration of a Weak Acid with a Strong Base

The curve of a weak acid-strong base titration is different from that of the plot we learned for the strong acid-strong base titration. Unlike the previous curve, this plot will have a region which has buffering properties. Besides that, the equivalence point will be above the pH value 7. The general shape of the curve is shown in Figure 9-2.

image\25097 ch 9.png

Figure 9-2

Titration of a Weak Base with a Strong Acid

The trend seen in the weak base-strong acid titration curve is somewhat similar to that of the weak acid-strong base curve. At first, as we add the acid, the pH slowly decreases. Then the decrease in pH becomes drastic, as you can see in Figure 9-3.

image\25107 ch 9.png
Figure 9-3

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