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I had a chemistry lab where we had to titrate three solutions with $\ce{HCl}$: a buffer, a diluted buffer and deionized water. My partner and I didn't label the charts we made, so now I have to figure out which one is which.

The way I see is that from the Henderson-Hasselbalch equation: $\mathrm{p}\ce{H} = \mathrm{p}K_a + \log\left(\ce{[A^{-}]/[HA]}\right)$, the diluted buffer should have a larger slope at the beginning then the original buffer solution.

My question is, is my logic correct, and how do I differentiate between the original buffer and the deionized water?

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  • $\begingroup$ What is the expected pH of deionized water and what would the pH be after adding a few drops of acid? For both buffer solutions, there is an equivalence point, how would you use that to tell them apart? $\endgroup$
    – LDC3
    Jul 6, 2014 at 17:56
  • $\begingroup$ pH of water is expected to be close to be near 7 however two my initial pH's were around 4.6 and the other was around 4.2. I would expect the equivalence point to occur sooner for the diluted buffer $\endgroup$
    – Sahar Rabinoviz
    Jul 6, 2014 at 18:04
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    $\begingroup$ Usually deionized water is slightly acidic. I kind of mentioned that water doesn't have an equivalence point. Is there a graph without one? $\endgroup$
    – LDC3
    Jul 6, 2014 at 18:09
  • $\begingroup$ One obviously has an equivalence point, while the other two appear to drop rapidly after adding one drop of HCl, although one drops much more quickly then the other. $\endgroup$
    – Sahar Rabinoviz
    Jul 6, 2014 at 18:12
  • $\begingroup$ I guess your diluted buffer is more diluted than I thought. What is the purpose of a buffer? A diluted buffer would have the same characteristic, but in a smaller amount. Water would be missing this characteristic. You should be able to figure out which graph is which now. $\endgroup$
    – LDC3
    Jul 6, 2014 at 18:16

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What is the expected pH of deionized water and what would the pH be after adding a few drops of acid? For both buffer solutions, there is an equivalence point, how would you use that to tell them apart? Usually deionized water is slightly acidic. I kind of mentioned that water doesn't have an equivalence point. Is there a graph without one?

What is the purpose of a buffer? A diluted buffer would have the same characteristic, but in a smaller amount. Water would be missing this characteristic. You should be able to figure out which graph is which now.

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