THEORY OF INDICATORS

Transcription

1 THEORY OF INDICATORS

2 Methods to determine the end point Visual indicators: Colour change: In some reactions, the solution changes colour without any added indicator. This is often seen in redox titrations, for instance, when the different oxidation states of the product and reactant produce different colours. Precipitation: If the reaction forms a solid, then a precipitate will form during the titration. A classic example is the reaction between Ag + and Cl - to form the very insoluble salt AgCl. Physical and chemical methods with the subsequent analysis of curves of the titration showing changes which occur in the course of titration (change of physical and chemical parameters standard solutions)

3 An indicator is a substance which is used to determine the end point in a titration.

4 Chemists often use a chemical dye rather than a ph meter to detect the equivalence point of an acid-base titration. Chemical dyes whose colors are affected by acidic and basic solutions are called acid-base indicators. Many natural substances act as indicators. If you use lemon juice in your tea, you might have noticed that the brown color of tea gets lighter when lemon juice is added. Tea contains compounds called polyphenols that have slightly ionizable hydrogen atoms and therefore are weak acids. Adding acid in the form of lemon juice to a cup of tea lessens the degree of ionization, and the color of the un-ionized polyphenols becomes more apparent. Chemists have several choices in selecting indicators. Bromthymol blue is a good choice for the titration of a strong acid with a strong base, and phenolphthalein changes color at the equivalence point of a titration of a weak acid with a strong base. 4

5 Acid base indicators In an acid-base titration, addition of titrant near the equivalence point causes the solution ph to change drastically. This ph change is detectable with indicators that change color as a function of ph. Remember that the role of the indicator is to indicate to you, by means of a color change, that just enough of the titrating solution has been added to neutralize the unknown solution.

6 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. Indicators are weak acids or bases that change color when they gain or lose their acidic proton(s). Acid-type indicators: donor HInd of proton H Ind Base-type indicators: IndOH acceptor of proton Ind OH

7 Requirements to indicators used in titrimetric methods of analysis 1. Indicator colour at near values рн should differ drastically 2. Change of an indicator colour should occur sharply in a small interval of рн 3. Indicator colour should be intensive as more as it is possible 4. The quantity of base or acid, necessary for change of colour of the indicator, should be very small 5. Change of colour of the indicator must to be reversible

8 Indicator ph range Basic colour Acidic colour Phenolphthalein o-cresolphthalein Thymolphthalein Colourless Colourless Red Pink Blue Yellow Thymol blue (acid range) Thymol blue (basic range) Bromophenol blue Bromophenol red Bromothymol blue Phenol red Cresol red Red Yellow Yellow Yellow Yellow Yellow Yellow Yellow Blue Blue-violet Red Blue Red Red Methyl yellow Methyl orange Methyl red Alizarin yellow R Yellow Yellow Yellow Violet Red Red Red Yellow

9 Theory of acid-base indicators: Three theories have been suggested to explain the change of colour of acid-base indicators with change in ph.

10 1. Ostwald Theory (Ionic Theory): The colour change is due to ionisation of the acid-base indicator. The unionised form has different colour than the ionised form. HInd acidic form H basic Ind form 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 alkalis. Similarly if the indicator is a weak base, its ionisation is large in acids and low in alkalis due to common OH - ions.

11 Considering two important indicators phenolphthalein (a weak acid) and methyl orange (a weak base), Ostwald theory can be illustrated as follows:

12 Phenolphthalein is a chemical compound with the formula C 20 H 14 O 4 (often written as "HIn" or "phph"). Often used in titrations, it turns colorless in acidic solutions and pink in basic solutions Colorless acidic

13 Phenolphthalein: It can be represented as HIn. It ionizes in solution to a small extent as: HIn uncolored form H In pink form The undissociated molecules of phenolphthalein are colourless while In - ions are pink in colour. In presence of an acid the ionisation of HIn is practically negligible as the equilibrium shifts to left hand side due to high concentration of H + ions. Thus, the solution would remain colourless. On addition of alkali, hydrogen ions are removed by OH - ions in the form of water molecules and the equilibrium shifts to right hand side. Thus, the concentration of Ph - ions increases in solution and they impart pink colour to the solution

14 Methyl orange 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 , 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 4.4). ph-related color changes result from changes in the way electrons are confined in a molecule when hydrogen ions are attached or detached.

15 HInd red form H Ind yellow form Methyl orange in acidic solution. Methyl orange in basic solution.

16 Disadvantages of ionic theory of indicators: Ascertaining of different colour of acidic and basic forms, but there is not an explanation of why a colour change occurs. The structure and colouring are not connected. Colouring change is ionic process but why it often is long in time? Advantages of the ionic theory: possibility of quantitative interpretation of results of change of colour.

17 2. Chromophore Theory: The acid-base indicators exist in two tautomeric forms having different structures. Two forms are in equilibrium. One form is termed benzenoid form and the other quinonoid form. The two forms have different colors. The color change in due to the interconversation of one tautomeric form into other. One form mainly exists in acidic medium and the other in alkaline medium.

18 Colour change depends on the presence of unsaturated chromophoric group in the indicator molecule. Auxochromes (OH or NH 2 ) with chromophore increase colour intensity. Nitro group O N O HO N O Azo group N N HN N Quinone group

19 Indicator p-nitrophenol O N O O N OH OH O colorless (benzenoid form ) yellow (quinonoid form) tautomers

20 Disadvantages of chromophore theory of indicators Does not give an explanation why tautomeric transformations and change of colour of a solution of indicators occurs at change рн a solution. Colour changes instant, where as intramolecular transformations generally long processes is frequent. Does not give a quantitative estimation of connection of colouring change with change рн.

21 3. Ionic - Chromophore Theory: The acid-base indicators are weak acids and the bases, and the neutral molecule of the indicator and its ionised form contain different chromophore groups N O O O N-O-H O N-O - + H + OH O O colourless yellow yellow

22 HInd K1 K2 ( HInd ) tautomer H Ind Let s apply the Low of mass action to both equilibriums. K 1 c( HInd ) t c( HInd ) c( HInd ) K1 c( HInd ) t K 2 c( H ) c( Ind c( HInd ) t ) c( H ) c( Ind c( HInd ) K 1 ) K 1 K 2 K ionization с( Н ) с( Ind c( HInd ) ) с( Н ) K ionization с( Ind ) c( HInd )

23 с( Н ) K ionization с( Ind ) c( HInd ) Taking logarithms and reversing the sign throughout ph pk in lg с( Ind ) c( HInd ) pk in -ionization constant of the indicator

24 Human visual only responds to dramatic color changes. For indicator color change to be detectable by normal eyesight, a rough tenfold excess of one or the other form of the indicator must be present. This corresponds to a ph range of 1 ph unit about the pk of the indicator To see the In - color: ph с( Ind ) c( HInd ) pk in ph 10 [10] log [1] pk in lg -To see the HIn color: с( Ind ) c( HInd ) ph с( Ind ) c( HInd ) pk in 0,1 log [1] [10] ph pk in 1 Only the color of unionized form is seen ph pk in 1 Only the color of ionized form is seen

25 ph pk in 1 Acid-base indicators (like any ionizable molecule) are 50% ionized at the pk a At 1 ph unit above the pk a, 90% of the ionizable indicator is in its basic form. At 1 ph unit below the pk a, 90% of the ionizable indicator is in its acid form. Thus, indicators show a full color transition +/- 1 ph unit of the pk a, and indicators are generally selected based upon the closeness of their pk a to the endpoint ph.

26 Ex, methyl orange рк in = 3.5 phenolphthalein рк in = 8.7 ph ph ph orange rose yellow ph ph ph rose uncolored pink Each indicator has its own particular ph or ph ranges over which it changes color. During the transition the observed color is a mixture of the two colors Midway of the transition the concentration of the two forms are equal pka of indicator should be close to the ph of the equivalence point

27 pt of the indicator is a value of рн at which colour of the indicator sharply changes and indicates the moment when we should stop to add titrant (there is end point of titration) рт of most often used indicators in the acid-base titration: Мethyl orange рт = 4,0 Methyl red рт = 5,5 Litmus рт = 7,0 Phenolphthalein рт = 9,0

28 Factors which influence the indicator indication. At increase tо the temperature indicator becomes less sensitive to Н + - ions for indicators-bases Presence of organic solvent (alcohol, acetone), albuminous molecules, salts changes рк of the indicator It is necessary to define titre a working solution in the same conditions at which the test analysis is conducted it isn t recommended to take a lot quantity of indicator