Question

Titration with an indicator is a common technique that chemists use when counting moles.

a ) Would this technique be useful for determining the pH of a water sample from a lake or stream?

b) Can this technique determine if a lake or stream is affected by acid rain?

Answer 1

a) Titration with an indicator techniqe can be used to determine pH of a water samole from a lake or stream.

Titration technique to determine Alkalinity:

Collect a known volume of water say 50 mL add 3 drops of phenlphthaein indicator and titrate against 0,02N H2SO4 and estimate phenolphthalen alkalinity. Phenlphthaein will change clor from pink to colorless.

Titration technique to determine Acidity

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

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. From the volume of titrant
consumed calculate the methyl orange acidity .

b) Lake water is complex; it is full of chemical "shock absorbers" that prevent major changes in pH. Small or localized changes in pH are quickly modified by various chemical reactions, so little or no change may be measured. This ability to resist change in pH is called buffering capacity. The pH of natural waters hovers between 6.5 and 8.5.

An important exception to the buffering of pH changes in lakes is the case of lakes affected by acid rain. Lakes that have received too much rain with a low pH (acid rain), lose their buffering capacity. At a certain point, it takes only a small bit of rain or snowmelt runoff for the pH to change. After that point, change occurs relatively quickly.

The titration technique can determine if a lake or stream is affected by acid rain.If the pH is found to be much than 6.5 , the lake is definitely affected by acid rain.