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1.ACID-BASE INDICATORSAn acid-base indicator is a weak acid or a weak base.Examples of indictors used in acid base reactions-Litmus-Phenolphthalein-Methyl orangethymol blue, tropeolin OO, methyl yellow, methyl orange, bromphenolblue, bromcresol green, methyl red, bromthymol blue, phenol red,neutral red, phenolphthalein, thymolphthalein, alizarin yellow,tropeolin O, nitramine, and trinitrobenzoic acid.

2.The pH range of indictorsIndictors dose not change colour sharply at oneparticular pH, they change over a narrow range of pHindctorspK ndpHlitmus6.55-8methylorange3.73.1-4.4phenophthaline9.38.3-10.0i

3.Litmus• Indicators as weak acids Litmus• Litmus is a weak acid. It has a seriouslycomplicated molecule which we willsimplify to HLit. The "H" is the protonwhich can be given away to somethingelse. The "Lit" is the rest of the weak acidmolecule.The un-ionised litmus is redThe ionised litmus is blue

4.What happened when• Adding hydrogen ions

5.• Adding hydroxide ion

6.If the concentrations of HLit and Lit- are equal:•At some point during the movement of the position of equilibrium,the concentrations of the two colours will become equal. The colouryou see will be a mixture of the two.•redbluebrown

7.Phenolphthalein•is a chemical compound with the formula C20H14O4 (often written as "HIn" or"phph"). Often used in titrations, it turns colorless in acidic solutions and pink in basicsolutionsCollarless acidic

8.Pkind valueindictorPkindlitmus6.5methylorange3.7phenolphthaline9.3

9.• The importance of pKind•Think of what happened through the color this point theconcentration of the acid and its ion are equal••pH pkindThis means that the end point for the indictor depends entiry onwhat pKind value is

10.Methyl orangeWhat is methyl orange? How is it made?Methyl orange is an intensely colored compound used in dyeing and printingtextiles. It is also known as C.I. Acid Orange 52, C.I. 13025, helianthine B,Orange III, Gold orange, and Tropaeolin D. Chemists use methyl orange as anindicator in the titration of weak bases with strong acids. It changes from red(at pH 3.1) to orange-yellow (at pH 4.4):pH-related color changes result from changes in the way electrons areconfined in a molecule when hydrogen ions are attached or detached. Methylorange in acidic solution.Methyl orange in basic solution.Methyl orange in acidic solution.

11.Titration curves for strong base with strong acidindctorspK ndpHmethylorange3.73.1-4.4phenophthaline9.38.3-10.0iBoth of and M.O. are useful

12.Titration curves for strong acid v weak base• This time we are going to use hydrochloricacid as the strong acid and ammoniasolution as the weak base.indctorspK ndpHmethylorange3.73.1-4.4phenophthaline9.38.3-10.0iMO is useful and is useless

13.Titration of weak base with strong acidindictorMethyl orangepKind pH3.7 3.1-4.4Phenolphaline9.38.3-10.0M.O is useful phph is useless

14.Choosing indictor for titrationTitration of weak base withstrong acidindictorMethyl orangepKind pH3.7 3.1-4.4Phenolphaline9.38.3-10.0Both phph and M.O are useful

15.Titration curves for weak base v strong acidindctorspK is useful M.O is useless

16.Titration curves for weak base with weekacidindctorspK ndpHmethylorange3.73.1-4.4iphenophthaline 9.38.3-10.0Both phph and MO are useless

17.Titration of sodium carbonate withHClPhph useful for detect firstendpointMO is useful for detect thesecond end point

18.A summary of the importantcurvesThe way you normally carry outa titration involves adding theacid to the alkali. Here arereduced versions of the graphsdescribed above so that youcan see them all together


Methyl orange is an intensely colored compound used in dyeing and printing textiles. It is also known as C.I. Acid Orange 52, C.I. 13025, helianthine B, Orange III, Gold orange, and Tropaeolin D. Chemists use methyl orange as an indicator in the titration of weak bases with strong acids. It changes from red (at pH 3.1) to orange-yellow (at pH 4 ...

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