Acid-Base Titration Curves

Acid-Base Titration Curves

Krishel Eunice O. Ngo, Camille Lindley O. Tiu

Submitted on July 8, 2016

Abstract

    This experiment deals with the construction of titration curves from data gathered through potentiometric titrations. A titration curve is a plot of pH as a function of the titrant added. The uses of titration curves include the determination of the feasibility of the reaction for titration and the evaluation of a suitable indicator to be used in the titration. For this experiment, different types of acid-base titrations were used, including the titration of strong and weak monoprotic acids with strong bases and the titration of strong and weak polyprotic acids with strong bases. Using the results obtained, the differences among the acids and bases were compared and evaluated. The values obtained for the equivalence point and half - equivalence points (pKa/pKb), however, are visual estimations. Nevertheless, titration curves may still be used to determine the acidity or basicity of an unknown substance, using the values of the equivalence point and dissociation constants as basis.

Keywords: titration curves, acids, bases, equivalence point, ionization constant

I. Introduction

Titration is a common technique used in determining the concentration of unknown solution by the addition of another solution of known concentration. Acid-base titrations take advantage of the differences in pH of the reagents to determine the concentration. Usually an indicator is used to signal the endpoint or represent the equivalence point in an experimental set-up (Petrucci et al, 2011).

The equivalence point is the point where the concentrations of the two solutions are equal. This is also where both the acid and the base are consumed in the neutralization reaction. The endpoint is simply the physical manifestation of the equivalence point. This is signaled by the change in color of the solution due to the addition of an indicator. Because of this, there is a discrepancy in the values of the endpoint and of the equivalence point or the titration error (Harvey, 2009).

Titration curves are used to show the process of titration graphically. The data is plotted pH of the analyte against volume of the titrant added. When plotted on a graph, the points resemble a curve. From this graph, several things can be deduced: the pH of the substance of unknown concentration or the initial pH, the pKa or pKb, the dissociation constants of acids and bases, and the equivalence point or the endpoint.  

[pic 1]

Above is an example of a titration curve of a monoprotic acid. Monoprotic acids/bases are substances that donate or accept one H+ (Chang, 2010). The initial pH is the starting point of the graph. Visually, the equivalence point can be estimated to be the point found in the middle of the slope, while the pKa or pKb values can be represented by the midpoint of the horizontal curves of the graph. This is also true for polyprotic acids as shown below.

[pic 2]

Polyprotic acids/bases are substances that donate or accept more than one H+. These substances will have multiple slopes, equivalence points, and pKa/pKb values as shown in the example above. The number of slopes or equivalence points will depend on the number of H+ donated or received. The one above for example is a graph of a triprotic acid because it has three slopes or three equivalence points (Chang, 2010).

Titration is important in the field of medicine. Pharmacists use titration to mix the desired substances to form drugs. This can also be in monitoring glucose levels of the blood. It is also

important in the food industry. Wine and cheese artisans use this to check the quality of their products. Titration is also used in maintaining marine life in aquariums. These are just some of the examples of the uses of titration. Thus, titration curves are useful because they give a visual representation of the process.

II. Experimental

The analyte was measured to 25mL and transferred to a 100mL Erlenmeyer flask. The buret was filled with the appropriate titrant, making sure there were no air bubbles on the tip. Titration was done in increments of 0.5mL. After the addition of 0.1mL of titrant, the pH of the solution was recorded with a pH meter. When pH abruptly changed, the increments were lessened to 0.1mL. This was continued until the pH was 5-6 units past neutral (pH = 7).

The concentration and volumes of the substances were known. The following reagents were used as analytes: NaOH, HCl, HOAc, H2SO4, H3PO4, NaOAc, Na2SO4, Na3PO4. If the assigned analyte was acidic, the titrant used was NaOH. If the analyte was basic, the titrant used was HCl.

The data gathered was used in constructing the titration curves. The pH of the analyte was plotted against the volume of the titrant added. The equivalence point and pKa/pKb were determined visually.

III. Results

1. Titrations of Monoprotic Acids

1. Strong Acid - Strong Base

    Table 1. Titration of 10mL 0.2M HCl with 0.1M NaOH

Figure 1. Titration curve of 10mL 0.2M HCl with 0.1M NaOH

[pic 3]

Table 2. Titration of 10 mL 0.1M NaOH with 0.1M HCl

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