Objective: The acidic or basic strength of various salts will be estimated by observing what effect 0.10 M solutions have on various different pH indicators. Not only will the concept of hydrolysis of salts be observed, familiarity with a variety of pH indicators will be obtains.

Textbook Reading: Chapter 14.8 Acid-Base Properties of Salts pages 694-700

You would expect solutions made from acids, such as HCl or HC₂H₃O₂, to be acidic, and solutions made from bases, such as NaOH or NH₃, to be basic. However, quite a number of aqueous salt solutions may also turn out to be either acidic or basic. Remember that a salt consists of a cation and an anion combined in such a ratio so that their charges cancel. When a salt dissolves in water, it dissolves into its separate ions. One or both of the ions contained in the salt may affect the pH of a solution making the aqueous salt solution acidic or basic. It is possible that the salt may contain a cation that may react with water to produce an acidic solution. For example, a salt containing the ammonium ion, NH₄⁺, will be acidic because NH₄⁺ reacts with water to produce an acidic solution:

(1)

It is also possible that a salt may contain an anion that may react with water to produce a basic solution. For example, a salt containing the fluoride ion, F¯, will be basic because F¯ reacts with water to produce a basic solution:

(2)

The reaction of ions with water is generally called a hydrolysis reaction. To determine whether a salt produces an acidic or basic solution when dissolved in water, it is necessary to determine whether the cation reacts with water to produce an acidic solution, or whether the anion reacts with water to produce a basic solution.

There are general rules as to when a cation or an anion will affect the pH of a solution. Cations that are the conjugate acid of a weak base will hydrolyze to produce an acidic solution. In the example of the ammonium ion, it is the conjugate acid of the weak base, NH₃. The ammonium ion will hydrolyze as shown in equation (1). The ammonium ion will have a K_a value related to the K_b value of NH₃ by the following relationship:

(3)

Cations of the alkali metals and the alkaline earth metals larger than calcium are very weak acids and are too weak to affect the pH of an aqueous solution. They are generally considered to be neutral in the acid-base sense. However, cations of many other metal cations, especially those with large charges such as +2 or +3, are strong enough acids to produce an acidic solution when dissolved in water. It may be difficult to think of a metal cation as "acidic" considering the Brønsted definition of an acid as a proton donor, but an acidic metal cation in aqueous solution can be considered to be an acid by the following process. When any metal cation dissolves in water, water molecules surround it. This is the process described by hydrolysis. The Fe³⁺ cation actually exists as a Fe(H₂O)₆³⁺ ion in water solution where six water molecules have attached themselves to the metal cation. The high positive charge on the metal cation makes the hydrogen atoms on the attached water molecules highly acidic. The attached water molecules may lose H⁺ ions to function as an acid as shown in the reaction below:

(4)

As for anions, many anions are generally conjugate bases of acids. Remember the strength of the conjugate base of an acid is inversely related to the strength of the acid. Anions that are the conjugate base of a strong acid, such as Cl¯ or NO₃¯, are extremely weak bases that do not affect the pH of an aqueous solution. These anions are considered to be neutral in the acid-base sense. Anions are conjugate bases of weak acids, such as F¯ or C₂H₃O₂¯, are strong enough bases to increase the pH of an aqueous solution. These anions will hydrolyze in water to produce a basic solution. These anions will have K_b values related to the K_a value of its conjugate acid by the following relationship:

(5)

Summing up, determining whether a salt solution will be acidic, basic, or neutral, depends on the nature of the cation and the anion. If both the cation and anion are neutral ions, the salt will not affect the pH and the salt solution will be neutral. If the cation is acidic and the anion is neutral, the salt will dissolve in water to produce an acidic solution. If the cation is neutral and the anion is basic, the salt will dissolve in water to produce a basic solution. If the cation is acidic and the anion is basic, the resulting salt solution may be acidic or basic depending on the relative acid and base strengths of the ions. Summing up the rules to determine the pH of salt solutions:

In today's experiment, you will estimate the acidic and basic properties of various salts by measuring the effect of 0.10 M solutions of those salts on a series of six pH indicator solutions. A pH indicator is a substance that changes color over a particular range of pH. You have used a pH indicator called phenolphthalein in a titration in Chemistry 1054. At low pH's below 8.2, phenolphthalein is colorless. In the pH range from 8.2 to 10.0, phenolphthalein changes color to a red color. Indicator solutions can be used to estimate the pH of a solution. The pH interval over which an indicator changes its color is a property of the particular indicator. The pH intervals for the indicators used in this experiment are shown in the figure below.

The pH intervals for all of these indicators are also diagrammed in your textbook (in color!!) on page 756 (Figure 15.8). You can estimate the pH of a solution by determining the color of each of these indicators when added to the solution. For example, if a solution is yellow when methyl orange is added, yellow when methyl red is added, blue when bromothymol blue is added, red when phenol red is added, colorless when phenolphthalein is added, and yellow when alizarin-R is added, the pH of the solution must be within 8.0 (the high pH interval for phenol red) and 8.2 (the low pH interval for phenolphthalein).

When measuring the pH of solutions exposed to air, you will generally find that the solutions are slightly more acidic than you would calculate them to be. This is due to the absorption of carbon dioxide from the atmosphere. CO₂ dissolves in water to produce H⁺ according to the reaction:

Even distilled water will be somewhat acidic unless it is boiled to remove the dissolved CO₂. You will measure this difference in the lab.

1- You will need to obtain 6 test tubes and a test tube rack to perform this experiment. Obtain a hot plate and boil approximately 450 mL of distilled water for about 10 minutes to remove dissolved CO₂ from the water.

2- While the water is boiling and later cooling to room temperature, add about 5 mL of unboiled distilled water to each test tube. Add 3 drops of a different indicator to each of the six test tubes (one indicator per tube). Record the color of each indicator on your data sheet. From these colors and the pH interval table, estimate the pH of the unboiled water to the nearest pH unit.

3- Empty the test tubes, then rinse the test tubes three times with about 3 mL of boiled distilled water.

4- Add about 5 mL of boiled distilled water to each test tube. Add 3 drops of each of the indicators to each test tube (one indicator per tube). Record the color of each indicator on your data sheet. From these colors and the pH interval table, estimate the pH of the boiled distilled water to the nearest pH unit. Empty the test tubes, then rinse three times with about 3 mL of boiled distilled water.

5- Repeat step 4 to determine the pH of each of the following solutions that are 0.1 M: NaCl, NaC₂H₃O₂, Cu(NO₃)₂, NH₄Cl, ZnCl₂, KAl(SO₄)₂, and Na₂CO₃. Use about 5 mL of each of these solutions to each test tube. Do not forget to rinse the test tubes three times with about 3 mL of boiled distilled water when you go from one solution to the next.

6- From the pH values that you have determined, calculate the hydrogen- and hydroxide-ion concentrations in each of the solutions. Complete the tables on the data sheet and calculate the K_a or K_b values.

 

For each of the following salts, show which ions are formed when the salt dissolves in water. Determine whether each of the ions is acidic or basic. Identify why you think the particular ion affects the pH of solution (for example, it is the conjugate base of a weak acid and identify the weak acid) or why you think the particular ion does not affect the pH of solution. Use the categories summarized in Table 1.

KNO₃

 

CH₃NH₃Cl

 

Na₂CO₃

 

AlCl₃