Challenge Problem:
(a) Why are indicators used only in the form of dilute solutions?
(b) Suppose that 0.1% methyl red (molar mass 269 g/mol) is used as the indicator in a titration to
determine the acid neutralizing capacity of an Ohio lake. Five drops (0.25 mL) of methyl red solution are
added to a 100-mL sample of water, and 4.74 mL of 0.01072 M hydrochloric acid is required to change
the indicator to the midpoint in its transition range. Assuming that there is no indicator error, what is
the acid neutralizing capacity of the lake expressed as milligrams of calcium bicarbonate per liter in the
sample?
(c) If the indicator was initially in its acid form, what is the indicator error expressed as a percentage of
the acid neutralizing capacity?
(d) What is the correct value for the acid neutralizing capacity?
(e) List four species other than carbonate or bicarbonate that may contribute to acid neutralizing
capacity.
(f) It is normally assumed that species other than carbonate or bicarbonate do not contribute
appreciably to acid neutralizing capacity. Suggest circumstances under which this assumption may not
be valid.
(g) Particulate matter may make a significant contribution to acid neutralizing capacity. Explain how you
would deal with this problem.
(h) Explain how you would determine separately the contribution to the acid neutralizing capacity from
particulate matter and the contribution from soluble species.
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