Please help me out with this topic,I just can't get around to understanding it properly--Its about O Hehner's methos to determine the alkalinity of water.

Assuming that alkalinity of water is caused exclusively due to carbonate ,hydroxyl and bicarbonate ions.......I want to know why only half of the carbonates are detected by phenopthalein and half by methyl orange ,whereas hydroxyl ions are only detected by phenopthalein and bicarbonates are detected exclusively by methyl orange.
Also, what are the species formed during reaction with phenopthalein and methyl orange that cause the colour changes.

When i did determination of alkalinity by titration, i simply used a pH meter to titrate with acid (0.1M) to a certain pH end point to then calcualte the alkalinity.

My guess/understanding is the reason that the different indicators are used (that is methyl orange and phenolphtalein) is beacuse they react different at different pH end points. See titration curves. The pH end point of phenolphtalein is different then that off methyl-orange. The choice of indicator depends on the pH end point of the reaction.

Therefore for the total alkalinity of water there are several 'inflections' in the titration curve as the HCO3- and bicarbonate react with the added acid. So you either need to add 2 indicators, or simply usea pH meter to observe the end points.

Thanks,but my doubt was as to how phenopthalein knows that it has to neutralise exactly half of the total carbonates and the rest should get neutralised by methyl orange. On the other hand,all bicarbonates and hydroxides get neutralised by methul orange and phenopthalein respectively.

Not sure if your understanding this correctly, but the carbonates and hydroxides get neutrilized by the acid titrant, to give salts. As soon as all the carbonate/hydroxide molecules get consumed by the acid in the reaction, there is then an excess of acid in the solution/ This excess of H+ in solution then either protonates or deprotonates the indicator to cause the color change, depending on the pH range and hence the end point is determined.

I understood about the fact that the hydroxides and carbonates get neutralised by the acid and when they are all consumed,the excess acid that spills into the solution causes colour change.

However,there is a slight complication-the carbonate gets neutralised in 2 steps-
1. carbonate to bicarbonate
2 bicarbonate to caorbon dioxide and water(via H2CO3 which doesn't last long).

Now,it says in my book that only the end of neutralisation of half of the total quantity of carbonates present in the solution via equation(1.) is detected by phenopthalein and the rest half of the quantity of carbonates are said to be neutralised to bicarbonates only when methyl orange changes colour.

This is what I don't understand.

Your guestions are simple.


CO3 (2-) carbonate ( i cant make the silly subscirpts) plus H+ (acid) gives HCO3-

then when the pH drops,

HCO3- plus H+(more acid) gives H2CO3 (carbonic acid) which then dissolutes into

H2O (water) + CO2

also, the amount of carbonic acid formed (that stays in solution) depends on the hardness of the water (just to complicate things a bit) '

I don't know why,but you are really repeating what I am saying --I've probably been bad at putting my question.ANyway,it was fun