why is phenolphthalein the indicator of choice when titrating calcium carbonate with hydrochloric acid?

Answer 1

Well, technically speaking phenolphthalein is not necessarily the best indicator for a strong acid and weak base. The equivalence point will be slightly below pH of 7, yet phenolphthalein does an indicator change from around 8 to 9 pH.

In other words, it changes color AFTER the point of equivalence is passed, if you are adding HCl to CaCO3 solution. If you are adding a CaCO3 solution to a HCl solution, it will change to clear BEFORE it is completely reacted.

The point of equivalence and the point of indicator change are not the same point.

But it is the most commonly used indicator and it is easy to see when it changes if the solution is clear. Sometimes it is the only indicator available in the school lab.

Using a pH meter is much more accurate.

Answer 2

Phenolphthalein is good to see the equivalence point in an acid-base titration. It is clear in an acid and turns pink in a base. It is a dramatic color change so you have not doubt that you have reached the equivalence point.

Answer 3

In irrigation water analysis, the estimation of carbonates, phenolphthaline idicator is used. The titration of hydrochloric acid or sulphuric acid with the carbonate (CO3) solution results in the formation of HCO3 ions. This HCO3 ions present in the irrigation water will make the colourless phenolphathaline indicator into red colour. The addition of one drop of excess acid to this irrigation water makes colourless at the end point. Methyl orange indicator is useful for estimating the HCO3 ions.

Answer 4

it is because the pka of phenolphthalein or the point in which phenolphthalein's color changes, is nearly close to the equivalence point of solution (end point or the point when the solution's color changes while titrating). this way u understand the ph of solution more accurately

Answer 5

K.J. is right.

CO3(-2) is a divalent weak base. The conjugate acid dissociation reactions are:

H2CO3 <=> HCO3(-) + H(+) with pKa1=6.37
HCO3(-) <=> CO3(-2) + H(+) with pKa2=10.25

Thus you can use phenolphthalein to determine only the first equivalence point of the base CO3(-2).
(respective reaction is CO3(-) + H(+) <=> HCO3(-))

Answer 6

It probably creates a colorant complex of some sort which works selectively for determining pH or conc for the given rxn

Answer 7

Because tequila wouldn't turn pink in an acid.

Answer 8

huh?