[Dazalincho]

Hi, I recently did a titration at school and I was told that washing down the sides of a conical flask will not alter the results.

This conical flask was holding NaOH which was added using a pipette, and the sides of the flask were washed down with distilled water to ensure that no NaOH was left on the sides. A few drops of an indicator were added. HCl was added after, using a burette.

I agree that the concentration will be the same after adding the same amount of HCl, but the colour will not be as vibrant if distilled water is added. Therefore it will be difficult to judge whether the same spot (i.e. pH/ratio of solutions/what affects the colour) has been reached.

So I'm just asking... In this way does the added distilled water used to wash the NaOH (or whatever solution) have potential to affect the results of the practical?

[fledarmus]

1) How many moles of NaOH do you have in your conical flask?

2) Does the number of moles change when you add distilled water to the flask?

3) How many moles of HCl will it take to neutralize all of the NaOH?

4) How many mL of your standardized HCl solution will be required to deliver that number of moles?

5) Does this number change when you add distilled water to the flask?

Answering those four questions should give you the effective answer to your question.



Your question about the color of the solution is actually considerably more complicated. How carefully are you measuring out your indicator solution? In most of the labs I've seen, it's "a few drops", or "1 or 2 mLs", or something of that sort. However, I did have an analytical instructor once that directed us (in the form of a homework set) through the equilibrium calculations for the phenolphthalein color change (which is also an acid-base equilibrium), a series of experiments to determine the minimum detectable (by eye) concentration of the colored species in solution, some error calculations based on the concentration/absorbtion measurements for the endpoint, changes in the effective endpoint based on concentration of indicator, and a few other possible sources of error. It turns out that, especially with such strong acids and bases as NaOH and HCl, any possible sources of error involving identifying the exact point of the color change were negligible. Errors in the weighing of NaOH and in determining the concentration of the HCl standard solution were far more significant.

[Dazalincho]

1) How many moles of NaOH do you have in your conical flask?

2) Does the number of moles change when you add distilled water to the flask?

3) How many moles of HCl will it take to neutralize all of the NaOH?

4) How many mL of your standardized HCl solution will be required to deliver that number of moles?

5) Does this number change when you add distilled water to the flask?

Answering those four questions should give you the effective answer to your question.

Thanks for the reply. I'll answer the questions...

1) 2.2 x 10-3 mol

2) No

3) 2.2 x 10-3 mol

4) 19.0 mL

5) No

I am adding 4 drops each time of indicator, since we were using methyl orange and it is a more difficult one to see a change from. Although adding distilled water to it will not change the volume needed, moles etc... it should change the visibility of the colour of the indicator, no? A lot of people have told me that it won't matter, but if you need to compare the colour of the titration against the colour of another titration to find out when you have reached the same point, then adding a random amount of distilled water to wash down the sides of the conical flask would cause this judgment to be inaccurate... i.e. if you add 4 drops of indicator and then wash it down with a litre of distilled water, then the concentration of the indicator in the solution will not be the same, and will not stand out as much as if you add 10 mL of distilled water to wash it down...

Could this be correct? The solutions were only approximately 0.11 mol/L, so they aren't extremely reactive or anything.

[Arkcon]

You've got a lot of things to work with in this problem, and fledarmus : has highlighted them for you.  You seem to be obsessed with your indicator, and you shouldn't be.  An indicator is present in vanishingly small amounts, so long as it is visible, it will work properly.  And its always visible, because if it weren't, it wouldn't be used as a common indicator.  If you had the free time and resources, you could play with 1, 2, or 3 drops of indicator in 100 or 200 or 500 mls of dilute solution and see, the difference is minimal.  If you added, say, 10 mls of indicator, that would ruin the titration -- indicators are large organic acids and bases that undergo a color change when protonated, too much of them will ruin a titration.

[Borek]

Take a look here: http://www.titrations.info/acid-base-titration-indicators