8. theory of indicators

8. theory of indicators

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  THEORY OF INDICATORS Preparedby G. Nikitha, M.Pharmacy Assistant Professor Department of Pharmaceutical Chemistry Sree Dattha Institute of Pharmacy Hyderabad
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  CONTENTS  Introduction  Theoryof Indicators  Refference
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  INTRODUCTION  An indicatoris a substance which is used to determine the end point in a titration.  In acidbase titrations, organic substances (weak acids or weak bases) are generally used as indicators.  They change their colour within a certain pH range.
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  The colour changeand the pH range of some common indicators are tabulated below Indicator pH range Colour in acid Colour in base Phenolphthalein 8.3-10.5 Colourless Red Litmus 5.5-7.4 Red Red Methyl red 4.5-6.5 Red Yellow Methyl Orange 3.2-4.5 Pink Yellow
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  THEORY OF ACID-BASEINDICATORS Two theories have been proposed to explain the change of colour of acid-base indicators with change in pH. 1) Ostwald's theory 2) Quinonoid theory
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  OSTWALD'S THEORY According tothis theory: (a) The colour change is due to ionisation of the acid- base indicator. The unionised form has different colour than the ionised form. (b) The ionisation of the indicator is largely affected in acids and bases as it is either a weak acid or a weak base. In case, the indicator is a weak acid, its ionisation is very much low in acids due to common H+ ions while it is fairly ionised in alkalies. Similarly if the indicator is a weak base, its ionisation is large in acids and low in alkalies due to common OH- ions.
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   Ostwald's theoryis based on Colour change 1. Phenolphthalein 2. Methyl orange Weak acid Indicator Weak Base Indicator Phenolphthalein(weak acid)
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  Weak acid IndicatorWeak Base Indicator Methyl orange
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  QUINONOID THEORY  Accordingto quinonoid theory, an acid-base indicators exist in two tautomeric forms having different structures which are in equilibrium.  One form is termed benzenoid form and the other quinonoid form.  The two forms have different colors. The color change is due to the interconversation of one tautomeric form into other. One form mainly exists in acidic medium and the other in alkaline medium.  Thus, during titration the medium changes from acidic to alkaline or vice-versa. The change in pH converts one tautomeric form into other and thus, the colour change occurs.
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  Quinonoid theory isbased on structural change 1. Benzenoid form 2. Quinonoid Form
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  REFERENCE  Pharmaceutical Chemistry-Inorganic Volume-1 by G. R. Chatwal.  Essentials of Inorganic Chemistry by Katja A. Strohfeldt.  Indian Pharmacopoeia.  M.L Schroff, Inorganic Pharmaceutical Chemistry.  P. GunduRao, Inorganic Pharmaceutical Chemistry, 3rd Edition  A.I. Vogel, Text Book of Quantitative Inorganic analysis.  Bentley and Driver's Textbook of Pharmaceutical Chemistry
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