Question

Q1) why did you have two different indicators in this expriment rather than just phenolphthalein or just methyl orange?

Q2) Was it necessary to use a dry Erlenmeyer flask to prepare the unknown sample for titration or was it okay if the flask was left slightly wet with distilled water after you washed it and before you placed your sample in it? Explain.

Q3) Using your textbook as a resource, suggest two alternative pH indicators that could be used for this experiment instead of phenolphthalein and methyl orange. Explain your choice.

Q4) If the pH indicator used for the titration in step 1 changed color at a lower pH than phenolphthalein what effect would it have on the mass and what percent of the sodium carbonate determined in this experiment? Explain.

Answer 1

Solution.

Q1) The use of these two indicators allows to catch two equivalence points. The first is at pH = 8.0-8.2 corresponding to the titration of carbonate-ions and the second is at pH = 4.1-4.5 corresponding to the titration of hydrocarbonate-ions.

Q2) It is OK to use a slightly wet flask, because the amount of distilled water will not affect the amount of titrated substance. Therefore, the results of titration will be the same.

Q3) The alternative pH indicators should have the similar pKa values to change a color in the same pH range as the original indicators. For instance, methyl orange (pH = 3.1 - 4.4) can be replaced with congo red (pH = 3.0- 5.0). Phenolphtalein (pH = 8.3-10.0) can be replaced with cresolphtalein (pH = 8.2 - 9.8).

Q4) If the indicator changed its color at pH 6, for example, it would mean that the solution has been over titrated. So, the calculated amount of carbonat-ions will be greater, as the volume of the titrant is greater as well. The percent fraction of sodium carbonate will be greater correspondingly.