Environmental Biotechnology

Experiment 3

Aim of the Experiment

Measure mineral and phenolphthalein acidity.

Introduction:

Acids contribute to corrosiveness and influence chemical reaction rates, chemical speciation and biological processes. Acidity of water is its quantitative capacity to react with a strong base to a designated pH. The measured value may vary significantly with the end point pH used in the determination. When the chemical composition of the sample is known study mineral acids, weak acids such as carbonic and acetic and hydrolyzing salts such as iron or aluminum sulfate may contribute to the measured acidity according to the method of determination.

Mineral acidity: It is measured by titration to a pH of about 3.5, the methyl orange end point (also known as methyl orange acidity). Total acidity: Titration of a sample to the phenolphthalein end point of pH 8.3 measures mineral acidity plus acidity due to weak acids, thus this is called as total acidity (or phenolphthalein acidity). In water analysis, this test does not bear significant importance because methyl orange acidity invariably remains absent in the raw water and even phenolphthalein acidity (that too principally due to the excessive-prevalence of dissolved carbon dioxide and carbonic acids) normally does not exist to a significant extent in the raw water.

Importance: As for as water analysis is concerned, acidity test does not bear significant importance because methyl orange acidity invariably remains absent in the raw water and even phenolphthalein acidity (that too principally due to the excessive-prevalence of dissolved carbon dioxide and carbonic acids) normally does not exist to a significant extent in the raw water.

Materials required

• pH meter; Reagents: Sodium hydroxide titrant (0.02 N); Phenolphthalein Indicator; Methyl Orange Indicator.

Procedure:

1. Take 50 ml sample in a conical flask and add 2-3 drops of methyl orange indicator solution.

2. Fill the burette with 0.02 N NaOH solution and titrate till the colour of solution just changes to faint orange colour, indicating the end point. Record the volume of titrant consumed as V1 in ml.

• Calculate the methyl orange acidity using Eq (1a):

Methyl orange acidity (or Mineral Acidity) = (V1×1000)/(Sample volume) --------------------- (1a).

• When the 0.02 N NaOH solution, used in titration is not standardized, mineral acidity is calculated using following Eq (1b):

Methyl orange acidity= (V1×N×50×1000)/ (Sample vol.) -------------------------(1b).

• For phenolphthalein acidity test, add 2-3 drops of phenolphthalein indicator solution to water sample from step 2 and continue the titration till the faint pink colour develops in the solution (i.e., the end point of titration). Record the volume of titration consumed as V2 (mL) and calculate total acidity or phenolphthalein acidity using Eq.(2):

Total acidity (or Phenolphthalein Acidity)= (V2×N×50×1000)/ (Sample vol.)------------------ (2)

Observation

• Calculate the above total acidity by using the above formula.

Reference Material:

• Standard Methods for the Examination of Water and Wastewater.

• Chemistry for Environmental Engineering 4th Edition.