Acid base titration end point detection

How do we detect end point of the acid-base titration?

Short answer is - when indicator changes its color.

However, color change is not instant (see acid-base indicators). Sometimes it seems like it is - very small drop of the titrant completely changes solution color, but sometimes - depending on the concentration of titrant and titrated substance, as well as on the selected indicator - we have to add even several milliliters of titrant before color change will come to an end.

So, when should we stop the titration?

Longer answer is - we should take into account indicator type and concentrations of acid and titrant, as well as their strength. Could be we have to titrate to the very first change in indicator hue, could be we should wait till color change is complete, could be we should look for completely different indicator, as the one selected doesn't guarantee accuracy.

OK, but how do we know which indicator, which change, and why?

That will require delving into details.

Following titration curves, partially already presented in the general end point detection section, show pH changes during titration and color changes of three popular indicators - methyl red, thymol blue and phenolphthalein:

0.1 M strong monoprotic acid titrated with 0.1 M strong monoprotic base in the presence of the methyl red indicator. Note that while only color change area is marked on the plot, solution is red for lower pH and orange for higher pH. Titration curve calculated with BATE - pH calculator.

0.1 M strong monoprotic acid titrated with 0.1 M strong monoprotic base in the presence of the thymol blue. Note that while only color change area is marked on the plot, solution is yellow for lower pH and blue for higher pH. Titration curve calculated with BATE - pH calculator.

0.1 M strong monoprotic acid titrated with 0.1 M strong monoprotic base in the presence of the phenolphthalein. Note that while only color change area is marked on the plot, solution is pink for higher pH. Titration curve calculated with BATE - pH calculator.

Judging from the plots colors of all indicators change completely while we are on the steep part of the curve. As the horizontal axis reflects volume of the titrant added, it seems logical that fast pH change in response to small volumes of titrant should ensure small error of the titration. But 'small' is not a very exact way of stating titration error. Let's try to find it more precisely. For that we can calculate exact volumes of the titrant that have to be added to the solution to change indicator color. We will use formula derived in the acid-base titration curve calculation section:

1

that for known pH value allows easy calculation of a volume of the titrant (strong base in this case) that was added added to a strong acid.

We have assumed both titrant and titrated substance concentrations to be 0.1 M and initial volume of the acid to be 50 mL. Volumes marked green are less than 0.05 mL from the equivalence volume.

For four indicators (three first and the last) volume of the titrant that have to be added for a complete color change is relatively large - even over 14 mL for alizarin yellow. However, all of the indicators that start to change color above pH 4.4 and end color change below pH 9.6 require addition of less than 0.04 mL of titrant to completely change their color. What is also very important, for these indicators both beginning of the color change and end of the color change happen less then 0.05 mL from the equivalence volume of the titrant. As we have already signalled in end point detection section, 0.05 mL difference is comparable with the smallest amount of titrant that we can add to the solution and is identical to the accuracy of the A class burette. That means that for all those indicators it wont matter whether we will end titration at the first color change or once the change is complete - error will be lower than 0.2% (sum of distance from the equivalence point and burette accuracy, we are assuming the worst case - so (0.05mL+0.05mL)/50mL×100%).

If 0.2% is too large, we can titrate to either first color change, or complete color change - and without need for some more exotic indicators we can be less then than 0.01 mL from the equivalence point, which should ensure 0.1% titration error (assuming 0.05 mL burette accuracy). However, in practice it won't work that way. As the smallest volume we can add is about 0.04-0.05 mL, we will have to be very lucky to not overshoot color change.

For two indicators color changes fall short of the 4.4-9.6 range. In the case of bromocresol green if we end titration once the color change is complete we will be 0.0001 mL (0.1 μL) from the quivalence point, in the case of phenolphthalein if we end titration at first sight of the color change we are 0.0016 mL (1.6 μL) from the equivalence point. For all practical purposes this IS equivalence point - and in practice we will be never able to add exactly required volue.

In the case of more diluted reagents initial plateau shifts up, final plateau shifts down, and steep part of the curve becomes shorter:

0.001 M strong monoprotic acid titrated with 0.001 M strong monoprotic base in the presence of the methyl red indicator. Note that while only color change area is marked on the plot, solution is red for lower pH and orange for higher pH. Titration curve calculated with BATE - pH calculator.

0.001 M strong monoprotic acid titrated with 0.001 M strong monoprotic base in the presence of the thymol blue. Note that while only color change area is marked on the plot, solution is yellow for lower pH and blue for higher pH. Titration curve calculated with BATE - pH calculator.

0.001 M strong monoprotic acid titrated with 0.001 M strong monoprotic base in the presence of the phenolphthalein. Note that while only color change area is marked on the plot, solution is pink for higher pH. Titration curve calculated with BATE - pH calculator.

It is obvious now that color changes either start too early on the titration curve, or end too late. To check details we will prepare similar table as the one shown above, this time for diluted acid (and base).