Advanced Projects in Chemistry - Solubility and Solubility Product Determination

All electrolytes dissolve in water. Salts like calcium hydroxide, barium sulfate, lead sulfate are sparingly soluble salts. A saturated solution of a sparingly soluble salt contains a very small amount of its ions dissociated in solution. This solubility is important to determine the solubility product . Ksp or the solubility product of a salt helps predict precipitation reactions which are a part of many commercially important industrial and biological processes. Precipitation reactions are important in industry, medicine as well as in everyday life. The preparation of important industrial chemicals like sodium carbonate makes use of precipitation. Dissolution of hydroxyapatite the ingredient of tooth enamel, in an acidic medium leading to tooth decay is an example of solubility. Kidney stones are the precipitation of calcium oxalate crystals in kidneys. In medicine the use of a sparingly soluble salt such as Barium Sulfate is made. Barium sulfate is insoluble in the digestive tract as well as opaque to X rays. Hence it is used in the barium meal test to diagnose ailments of the digestive tract. Stalactites and stalagmites which naturally adorn caverns, are a result of the precipitation of calcium carbonate. Thus, solubility, solubility product and precipitation assumes a sizeable portion of Chemical Equilibrium studied in grades K-11through 12.

A project which incorporates these concepts and which would enhance the student’s understanding of these concepts would be the study of a sparingly soluble salt such as calcium hydroxide. Salts with solubility of 0.1M soluble or greater are considered soluble. Salts that have a solubility of less than 0.01M are sparingly soluble salts. Calcium hydroxide is a salt which is sparingly soluble in water. Calcium hydroxide is particularly important in its use in dentistry as a dressing in paste form during a root canal filling. It is added to sea water to reduce acidity caused by dissolved carbon dioxide in the fight against the greenhouse effect. In the production of metals lime water is injected into the waste gas stream to neutralize acids such as fluorides, chlorides and other noxious gases prior to their release into the atmosphere. When such a sparingly soluble salt is made soluble in water, a small amount of it dissolves and the rest remains undissolved. An equilibrium is set up between the undissolved salt and the ions of the salt in solution.

This equilibrium can represented as
Ca(OH)2↔Ca2+ +2OH-

Using the law of chemical equilibrium, the solubility product of calcium hydroxide can be represented as Ksp= [Ca2+] [OH-]2

The solubility product of a sparingly soluble salt at a given temperature can be defined as the product of the concentration of its ions in a saturated solution with each concentration term raised to the power equal to the number of times the ion occurs in the equation representing the dissociation of the electrolyte. Solubility is defined as the molar concentration of a solute in its saturated solution at a given temperature. Molar solubility is the number of moles of solute in one liter of a saturated solution (mol/L).

Solubility depends on three factors - temperature, pH and common ion.
Temperature- As the temperature of the solution in which the sparingly soluble salt is made soluble increases solubility increases.

pH- When the pH decreases the solubility of salts of weak acids increase due to Le Chatelier’s Principle.

Common ion Effect-when a salt with a common ion is added to the solution of a sparingly soluble salt the presence of the common ion suppresses the solubility of the salt and thus decreases its solubility in solution.

When a sparingly soluble salt is added to water, there exists a dynamic equilibrium between the undissolved solid salt and the ions which it furnishes in solution.

In general, for any electrolyte AxBy, this dynamic equilibrium may be represented as AxBy↔ xAy++ yBx-

Thus, the solubility product for AxBy may be written as
Ksp=[Ay+]x+[Bx-]y

Where x and y represent the number of ions in the formula of the electrolyte. Thus, solubility product of an electrolyte at a specified temperature may be defined as the product of the molar concentration of its ions in a saturated solution, each concentration raised to the power equal to the number of ions produced on dissociation of one molecule of the electrolyte. The solubility product of a salt is constant at constant temperature. The solubility product of a sparingly soluble salt at a particular temperature can be calculated from the knowledge of the solubility of the salt in water at that particular temperature.

A good learning of the concepts of solubility and solubility product would include determination of the solubility of a sparingly soluble salt like calcium hydroxide. The solubility is determined via a simple acid base titration. The solubility of the salt can then be determined at a temperature higher and lower than the ambient temperature. Here students learn the temperature relationship of solubility for sparingly soluble salts. Calcium hydroxide is a ternary salt whose solubility product is calculated as 4S3. Using this relationship the Ksp for calcium hydroxide can be calculated at room temperature. The student can then compare his experimentally determined value to the standard values @ 298K. The addition of a salt like calcium chloride and its effect in decreasing the dissociation of calcium hydroxide due to the increase in Ca2+ ions would decrease the solubility of the calcium hydroxide. The understanding of the importance of the value of Ksp becomes clear when precipitation reactions are studied. Calcium hydroxide has a Ksp value in the range of 4.68 X10-6.

If its precipitation is to be studied in the presence of sodium hydroxide, then the ionic product of the [OH-] and [Ca2+] ions must exceed the Ksp value for Calcium hydroxide.

Experimental determination and hands on work in the lab reiterate these concepts which otherwise would remain drab when learnt only as theory.

Acknowledgements: The experimental work for the solubility and solubility product determinations of calcium hydroxide were diligently carried out by Soumya Asthana and Nishant Seth, Grade XII of R.N. Podar, CBSE Sr. Secondary High School, Santacruz (West).