Acid-Base Titration Lab Report

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Indicators An indicator is a substance which is used to determine the end point in a titration. In acid-base titrations, organic substances (weak acids or weak bases) are generally used as indicators. They change their colour within a certain pH range. The colour change and the pH range of some common indica­tors are tabulated below: Indicator pH Range Over Which Colour Change Occurs Colour of Acidi Form Colour of Base Form Methyl Orange 3.2-4.5 Orange Yellow Methyl Red 4.4 – 6.5 Red Yellow Bromothymol blue 6.0 -7.8 Yellow Blue Phenolphthalein 8.3- 10.0 Colourless Pink Alizarin Yellow 10.1 – 12.1 Yellow Red Litmus 5.5-7.5 Red Blue Phenol red 6.8-8.4 Yellow Red Refer to the following video for acid-base indicators IFrame Two theories…show more content…
So phenolphthalein is the suitable indicator for such a titration. weak-acid-vs-weak-base (iii) Strong acid vs. weak base: pH curve of strong acid (say HCl or H2SO4 or HNO3) with a weak base (say NH4OH) is vertical over the pH range of 4 to 7. So the indicators methyl red and methyl orange are suitable for such a titration. strong-acid-vs-weak-base (iii) Weak acid vs. weak base: pH curve of weak acid and weak base indicates that there is no vertical part and hence, no suitable indicator can be used for such a titration. Titration of soluble carbonate with strong acid. pH curve of sodium carbonate with HCI shows two inflec­tion points (Fig. 10.5). First inflection point (pH 8.5) indicates conversion of carbonate into bicarbonate. Na2CO3 + HCI → NaHCO3 + NaCl As the inflection point lies in the pH range 8 to 10, phenolphthalein can be used to indicate the above conversion. The second inflection point (pH 4.3) indicates the following reaction: NaHCO3 + HCI → NaCl + CO2 + H2O As the point lies between 3 to 5, methyl orange can be used. You can also refer JEE Physical Chemistry Syllabus, Reference books of

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