Doing a grade 9 experiment comparing the alkalinity of antacid tablets. Using bromothymol blue indicator. Dissolved tablets in both tap water and distilled water and in all situations antacid came out yellow in with bromothymol blue indicator ... so the antacid is an acid? I was intending to have students compare brands by titrating with HCl and comparing the volume of HCl used, but have nowhere to go as the antacid is already an acid? Can anyone explain what is wrong?
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1$\begingroup$ I would definitely check the pH of your water ... tap water is usually not neutral and distilled water may or may not be neutral depending on how well controlled the distillation was. $\endgroup$– DissenterCommented Dec 2, 2014 at 2:50
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$\begingroup$ Which antacid brand are you using? You could use methyl orange (pH red 3.1-4.4 yellow) or bromophenol blue (pH yellow 3.0-4.6 blue) as an indicator instead. $\endgroup$– LDC3Commented Dec 2, 2014 at 2:56
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You can do your experiment. The active ingredient in most antacids is bicarbonate, and bicarbonates are amphoteric: they can be both acids and bases. In most cases, bicarbonate is a weak base, so despite your bromothymol blue outcome, bicarbonate is a much weaker acid than $\ce{HCl}$, and you should be able to do your titration.