back titration
The reason is the presence of carbonate. As you observed, CO2 is generate during the "direct" titration.
Because of that, is difficult to identify precisely the equivalence point (partial solubilised into the water). Second, carbonic acid is weak and for precise dosage would be hard to choose the appropriate mixture of pH indicators.
Then, this method supersedes all the inconvenience using a titration of a hard acid (hydrocloric) with a hard base.
If you really love Chem, you'll easelyfind some theoretic description of acid-base titration curves. You'll find useful.
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