The final equivalence point of the carbonate determination occurs just before the bromocresol green begins to turn from blue to blue green to green to yellow. It is necessary therefore to prepare a "blank" for comparison with each of your samples AND to know what volume must be subtracted from each of your end points.
Your blank solution simulates a complete titration of sodium carbonate with all the carbon dioxide removed by boiling so that the predominate ions remaining in solution are sodium and chloride. The photo at the left shows a blank preparation with the bromocresol green indicator added and a simulation of the removal of any remaining carbon dioxide by boiling.
On the right the solution has begun to boil. Allow the boiling process to go on for about a minute in each case. The flask ought not to be placed immediately in the ice bath. Lift it from the heating pad using a folded paper towel as shown and swirl it ever so gently until the bottom of the pyrex equilibrates to the temperature of the solution inside.
Then place it in the ice bath. Don't put too much water in with the ice or the flask may become boyant and tip over.
From left to right below, note the change in color as fractions of drops are added (and washed down with distilled water). Swirl the flask after each addition until you have a color definitely not blue, but not yellow either. Something in between with which you can compare all subsequent titrations will be just fine. Don't forget to read your blank volume.
Cover your blank solution so as not to allow any absorption of carbon dioxide from the atmosphere and place on a white piece of paper for subsequent comparisons.
Each of your carbonate titrations will progress through an intermediate end point using phenolphthalein indicator which changes color at about the pH where all carbonate has been converted to bicarbonate. For a sample initially containing only carbonate, the volume required to achieve the phenolphthalein color change is close to half that required for the subsequent bromocresol green end point. It is not exactly half because the color change and the equivalence point are not exactly the same.
On the left, phenolphthalein is being added to a carbonate sample and on the right the titration with hydrochloric acid has begun.
The titration is continued until the "ghostly pink" (photo at left) described in your lab manual is achieved. Even if you go beyond the ghostly pink (photo at right), not to worry as the phenolphthalein end point is not critical in the determination of carbonate. The volume gives you an indication that the bromocresol green end point will occur at approximately double this volume.
At this time, add bromocresol green to the solution. Note on the left the different colors represented between the blank and the solution to be titrated. Begin titrating the solution with the bromocresol green added (photo at right).
As the titration progresses, the color of the bromocresol green will begin to lighten a bit. You have the blank to make periodic comparisons. Try not to pass beyond the color achieved by the blank titration because you may inadvertenly go irreversibly beyond the end point.
When you have arrived at such a point, place your flask on a heating pad and raise the temperature of the solution to the boiling point. Boiling for about a minute will expel carbon dioxide gas from the solution and raise the pH sufficiently to cause the color to change back to blue. Note in the two photos below, the color of the solution on the left remains about the same just as the solution begins to boil, but the right image, taken a minute later definitely shows a darkening of the blue color. When this darkening occurs, the solution is ready for the final few drops of acid titrant to achieve the final end point.
As you approach the final end point, continue to compare the color of the solution being titrated with the color of the blank. When the two are the same you may reasonably assume that you have achieved the desired end point. Don't forget to subtract the blank volume from the total volume of titrant just used for the sample.
- Oliver Seely (California State University, Dominguez Hills).