pH AND BUFFERS

PRACTICAL  P^H AND BUFFERS

Introduction: - Practical deals with   pH and buffer solutions. We already know that the pH of the system plays a very vital role in success of many chemical and biochemical reactions. Buffers on the other hand are aqueous solutions that tend to resist any pH change when small amounts of acid or base are added. What is pH? What are buffer solutions? What is the role of buffers? How to prepare buffer solutions? We shall learn about these aspects in this practical.                                                                 

P^H and Buffers: Basic Concepts

In this sections we shall review our knowledge about pH and buffers. We being with pH. 

1.P^H    

You have already studied about pH in your undergraduate years and know what it means. Can you define pH? Write the definition of pH in the space provided. 

Yes mathematically we can define pH as equal the negative log of the hydrogen ion concentration, or pH = -log [H⁺]. In simple terms, Ph is a measure of the acidity or alkalinity of a solution. 

You have studied that the pH scale is a convenient means of expressing the relative acidity or alkalinity of solutions. The pH of the system plays a very vital role in success of many chemical and biochemical reactions. The pH of pure water is 7.0, therefore, the pH of any water sample many given an indication of the types of impurity present in water. 

So now you know what pH means also understand its significance. From your Nutritional chemistry lessons in school and undergraduate years, you should be also determined the pH of solutions using the pH paper. However. pH paper is used when approximate pH values are required. For a more accurate determination of pH, a pH meter is used. We shall learn about the pH meter in section 3.3.

Now let us get to know about buffers. 

2. Buffers and Buffer Solutions

Solutions containing both weak acid and their salts or solutions containing weak hydroxides and their salts are referred to as buffer solutions. They have the capacity of resisting changes in pH when either acid or alkali is added to them. 

Buffers are therefore, aqueous solutions that tend to resist any pH change when small amounts of acid or base are added. A buffer system consists of a weak acid (proton donor) and its conjugate base (proton acceptor) or a weak base and its conjugate acid in comparable concentrations. Because they maintain a constant pH, buffers are good reference solutions. 

A buffer solution can be prepared to have a desired value of pH by controlling the amount of acid and its salt in case of acidic buffer. What do we mean by an acidic and basic buffer? An acidic buffer solution is simply one which has a pH less than 7. Acidic buffer solutions are commonly made from a weak acid and one of its salts-often a sodium salt. An alkaline buffer solution, on the other hand, has a pH greater than 7. Alkaline buffer solutions are commonly made from a weak base and one of its salts. Frequently used example of acidic and basic buffer include 

-          Acidic buffer: a mixture of CH₃COOH (acetic acid) + CH₃COONa (sodium acetate), citric acid and sodium acetate, boric acid and borax.

-          Basic Buffer:   a mixture of NH₄Cl (ammonium chloride) + NH₄OH (ammonium hydroxide), potassium dihydrogen phosphate (KH₂PO₄), and disodium hydrogen phosphate Na₂HPO₄), sodium carbonate and sodium bicarbonate.                

 3. Methods for determining pH

There are two general methods for determining pH of a solution, one is the colorimetric method and the other most common method is the use of pH meter for determining the pH. In this practical we shall focus only on the principle, method involved in the use of pH meter for determining the pH. So let us get started by first getting to know about the pH meter. 

What is a pH meter?

A pH meter, we have already studied in Practical 1, is a device used for measuring. pH of any unknown solution. It is composed of:

a.       A reference electrode

b.      Glass electrode whose potential depends on the pH of the solution surrounding it

c.       An electrometer, a device capable of measuring very small potential difference in a circuit of extremely high resistance.

Principle

Hydrogen ions in solution, like other ionic species, conduct an electric current. When a glass electrode is dipped in a solution containing hydrogen ions, a potential difference develops across a very thin glass membrane separating two solutions of different hydrogen ion concentration – one within and the other outside the glass electrode. A pH meter is an electronic voltmeter that measures this difference of potential and through its internal calibration, converts it to a pH reading which is displayed on a scale. The scale is normally taken extending from 0 to 14 pH units with a last count of 0.05 pH units or better. A pointing needle moves across the graduated scale and the pH of the solution can be read directly on the scale. These days digital pH meters are becoming more popular as compared to scale – needle instruments. 

With a review of the principle and functions of the pH meter, we are now ready to get started with the experiments related to pH and buffer solutions. There are 3 experiments in this practical. So let us carry out these experiments and learn as we go about conducting the experiments.    

******************************End**************************************