In solutions with pH less than 8. Phenolphthalein is colorless in weak alkaline solution and pink in strong alkaline solution. It is clear that it is not necessary that the color of an acid base indicator is the same in all alkaline solutions.
Likewise, it is also not necessary. That the color of an acid base indicator be the same in all acidic solutions. The pH ranges of some key indicators are given in the following table: —. Two theories have been given to explain the behaviour of acid base indicators. According to this theory, acid base indicator is either a weak carbanic acid and or a weak base. The unionized molecules of the indicators in the solution are of one color and the ions obtained by their separation are of another color.
Due to weak electrolytes, their volume is reduced. Acidic indicator gives colorless anion and alkaline indicator colorless cation. Example: phenolphthalein is a weak acid.
It is colorless. For convenience, we display it from HPh. Water is a weak electrolyte and its ionization is very small. The amount of ionization of HPh increases to restore equilibrium.
Thus the concentration of Ph — ions in the solution increases and the color of the solution becomes pink. This theory also suggests that phenolphthalein is not a good indicator for the titration of strong acids and weak bases. In the final step of the titration, small amounts of OH — ion are obtained from the weak ionization of the weak base that they are not sufficient to displace the equilibrium reaction to the right.
Hence the pink color is not obtained in the solution at the end point of the reaction. In order to give the solution a pink color, an excess of weak base has to be added to the solution. In other words, the titration process is flawed. Thus the color change of methyl orange can also be explained. Methyl orange is a weak base and thus dissociates —.
This causes a decrease in the concentration of OH — ions and the equilibrium of the above equation is disturbed. The amount of ionization of MeOH increases to restore equilibrium. Therefore, red color is not obtained in the solution at the end point of the reaction. To give the solution a red color, a high amount of weak acid has to be added to the solution. In other words the action of titration is flawed.
This theory is a modern theory to explain the working of indicators. According to this theory, indicator aromatic carbonic compounds used in acid base titrations are.
They are a mixture of at least two movable forms. Among these, one form is in acidic medium and the other form in alkaline medium in greater proportion. These two forms are called benzenoid and quinonoid forms. Both these forms have different colors.
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Please check your email for login details. Studying in Grade 6th to 12th? Registration done! Sit and relax as our customer representative will contact you within 1 business day Continue. Book a free demo of live class. Phenolphthalein It is represented as HPh. The neutralisation reactions are of the following four types: A strong acid versus a strong base. A weak acid versus a strong base. A strong acid versus a weak base.
A weak acid versus a weak base. Methyle orange b. Curcumin c. Indego d. Beet Question 2: Which of the following acid-base indicators give red colour in basic medium?
Methyl Red b. Alizarin Yellow c. Phenolphthalein d. Bromothymol Blue Question 4: pH range for litmus is.. Methyl Yellow b. Bromothymol Blue Q. Full Name. Email ID. Phone no. Select Grade --Please Select Grade-- 6 7 8 9 10 11 12 12th pass. Course Features. Tap to close. View courses by askIITians. Click Here Know More.
You will get reply from our expert in sometime. We will notify you when Our expert answers your question. To View your Question Click Here. Buffer Solutions Buffer Solution For several purposes, we need Latest articles from Blog. See More. 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 Quinonoid theory: According to this theory: a 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 color change in due to the interconversation of one tautomeric form into other. 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. Phenolphthalein has benziod form in acidic medium and thus, it is colourless while it has quinonoid form in alkaline medium which has pink colour. Methyl orange has quinonoid form in acidic solution and benzenoid form in alkaline solution. The color of benzenoid form is yellow while that of quinoniod form is red.
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