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We just had chemistry practical few days a go. The topic was strong acid-base titration, in which we had to find the normality and concentration of $\ce{HCl}$.

My teacher instructed me to add 2 drops of Phenolphthalein in 10 ml $\ce{HCl}$, and then add $\ce{NaOH}$ from buret and take a reading.

We needed to repeat the process 4 times and then find the constant reading, but in that I never used to take 2 drops of Phenolphthalein, instead I used to take 5-6 drops and then also used to get almost same reading as of pilot reading.

So my question is whether it is compulsory to take 2 drops of Phenolphthalein or it was just a instruction so students don't waste it?

In this same instruction given. (Procedure point 2)

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Using a little extra phenolphthalein won't matter. You just need enough so that the color is visible once the pH becomes basic enough. If you were titrating a large volume, you might need more than a couple of drops, but 2 drops is enough to be sure that the color will be visible in a typical lab-scale titration vessel. One drop would probably even work. The reason they say "two drops" and not "at least two drops" is that they don't want you to be wasteful.

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  • $\begingroup$ Actually, using the whole bottle should give a significant difference. But nobody would be that naive. $\endgroup$ – LDC3 Aug 20 '14 at 1:08
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    $\begingroup$ I slightly disagree. There are several reasons, why one should not use too much indicator. Indicators are usually acids and bases and the measurement will be affected when dealing with significantly diluted systems. Also for a colour change you need approximately a 1:10 ratio of protonated vs unprotonated indicator. Using too much indicator will result in adding too much acid and base. The human eye is not the most reliable measuring instrument so the difference in tiny quantities should not matter too much. Accurate titrations can be done by conductometric measurements. $\endgroup$ – Martin - マーチン♦ Aug 20 '14 at 2:58
  • $\begingroup$ So we can conclude from answer and comments that I should not use more than 2 drops $\endgroup$ – Freddy Aug 20 '14 at 4:16
  • $\begingroup$ @LDC3 - never underestimate the capacity of people to do stupid things! $\endgroup$ – thomij Aug 20 '14 at 15:51
  • $\begingroup$ @Martin - yes, this is true. I was assuming we were talking about a range of 1-5 drops in maybe 30-100 mL of solution, which is typical for undergraduate labs. $\endgroup$ – thomij Aug 20 '14 at 15:52
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Phenolphtalein is slightly acidic. It does affect titration because if you add too much, then it will alter the concentration of whichever solution you are adding it in. If you added it to the acid, it will cause that acid to be more acidic than you may have calculated. The effect of that on the titration is that afterwards when you are adding the base in the acid to find the end-point is that the end point will not be the true end-point since the pH of the acid is lower because of the indicator. The amount of base needed to add to the acid will be more so it can neutralize it and this while ruin your entire experiment since it will not be accurate. It will not have a large difference but it's still different.

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  • $\begingroup$ Welcome to Chemistry.SE! Take the tour to get familiar with this site. Mathematical expressions and equations can be formatted using $\LaTeX$ syntax. For more information in general have a look at the help center. $\endgroup$ – Martin - マーチン♦ Jun 8 '16 at 5:07

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