Lab # 2: pH Measurement and Preparations of Standard Solutions
Part A: pH Measurement
Introduction:
Measurement of pH is one of the most important and frequently used tests in water
chemistry. Practically every phase of water supply and wastewater treatment (acid base
neutralization, water softening, precipitation, coagulation, disinfection, corrosion control,
etc) is pH dependent. At a given temperature, the intensity of the acidity or basic
character of a solution is indicated by pH or hydrogen ion activity. Pure water dissociates
to give equal amounts of hydrogen ion (H+) and hydroxide ion (OH-), i.e. [H+] = 107
moles/l, [OH-] = 10-7moles/l. The fixed relationship between [H+] and [OH-] is :
[H+] [OH-] = 10-14 = Kw
The environmental impacts of pH are that water of high or low pH may cause corrosion
of sewers and plumbing materials. Furthermore, pH influences biological processes of
wastewater treatment. Low pH causes foul smell due to production of H2S. Very high and
very low pH is detrimental to marine life.
Objectives:
The objective of the experiment is to determine the activity of the hydrogen ions by
potentiometric measurement using a pH meter.
Materials: pH meter, buffer solutions of pH 4, pH 7, and pH 10; beaker
Procedure:
1.
2.
3.
4.
5.
6.
Calibrate the pH meter with the buffer solutions.
Establish equilibrium between electrodes and sample by stirring sample to insure
homogeneity.
Stir gently to minimize carbon dioxide entrainment.
Condition electrodes after cleaning by dipping them into sample for 1 min.
Immerse in a fresh portion of the same sample and read the pH.
After finishing the readings store the electrodes in pH 7.0 buffer.
Part B: Preparation of Standard Solutions
Introduction:
For the preparation of standard solutions the purest reagents available should be used.
The analytical reagent quality (AnalaR) is generally employed. If there is any doubt as to
the purity of the reagents used, they should be tested by standard methods for the
impurities that might cause errors in the determinations. In any analytical laboratory it is
essential to maintain stocks of solutions of various reagents: some of these will be of
accurately known concentration (i.e. standard solutions) and correct storage of such
solutions is imperative. Primary standards are usually salts or acid salts of high purity
that can be dried at some convenient temperature without decomposing and that can be
weighed both at high degree of accuracy. Secondary standards are solutions that have
been standardized against primary standards. For reagent solutions, it is sufficient to
weigh out approximately the amount of material required, using a watch glass or a plastic
weighing container, and then to add this to the required volume of solvent (distilled
water) which has been measured with a measuring cylinder. To prepare a standard
solution, a funnel is inserted into the neck of a graduated flask of the appropriate size. A
suitable amount of the chemical is placed in a weighing bottle which is weighed, and then
the required amount of substance is transferred from the weighing bottle to the funnel,
taking care that no particles are lost. The funnel is thoroughly washed, inside and out, and
the removed from the flask. The contents of the flask are dissolved, if necessary, by
shaking the liquid, and then made up to the mark. For the final adjustment of volume, a
very fine jet is employed. A normal solution is defined as one that contains one
equivalent mass of a substance per liter of solution.
Objectives:
To prepare primary and secondary standards and to understand the principles involved in
their preparation.
Materials:
Analytical balance, 250-ml Erlynmer, sodium carbonate, pH meter, methyl orange
indicator, magnetic stirrer, standard buffers, sulfuric acid, volumetric flasks, and beakers.
Procedure:
1. Prepare one liter of standard 0.02N Na2CO3 by dissolving 1.06g anhydrous reagent
grade Na2CO3 , dried at1030C, in distilled water.
2. Dissolve…… ml concentrated sulfuric acid in distilled water and make upto 1000 ml.
3. Mount a 25 ml burette and fill it to the mark with the acid solution.
4. Take 50 ml of Na2CO3 solution in an Erlynmer flask, add 5 drops of methy orange
indicator and place on a magnetic stirrer.
5. Add acid slowly while stirring till orange colour turns to pink
6. Check the pH of the solution after titration is completed which should be
approximately 4.3
7. Record the volume of acid used.
8. Repeat titration two more times and calculate average volume of acid used.
Calculations:
Calculate the normality of the sulfuric acid (H2SO4) using the relationship:
Na  Va = Nb  Vb
Where
N = normality
V = volume
a = acid
b = base