Ph indicator-----(Pharmaceutics)

Ph indicator-

1. pH INDICATOR

2. “The compounds (mostly organic) which change their color when pH of the solution changes” Indicators are used to; 1) Check the pH of the solution. 2) Indicate the end-point i.e. completion of the reaction.

3. At 25 °C, considered the standard temperature, the pH value of a neutral solution is 7.0. Solutions with a pH value below 7.0 are considered acidic, whereas solutions with pH value above 7.0 are basic (alkaline). As most naturally occurring organic compounds are weak carboxylic acids and amines , pH indicators find many applications in biology and analytical chemistry.

4. Types of indicators The choice of the indicator depends upon the nature of the reaction. There are three basic types of indicators . 1) Acid- base indicator 2) Redox indicators 3) Precipitation indicators For the quantitative analysis of metal cations , the use of complexometric indicators is preferred.

5. Acid – Base indicators: These are the organic compounds which are used in acid base titrations. A cid - base indicators , respond to a change in the hydrogen ion concentration. Most of the indicators are themselves weak acids other are weak bases which responded to change hydroxyl ion concentration.  Color  Blue Litmus  Red Litmus  Acid  turns red  stays same  Base  stays same turns blue The most common indicator is found on "litmus" paper. It is red below pH 4.5 and blue above pH 8.2.

6. An indicator is usually some weak organic acid or base dye that changes colors at definite pH values. The unionized form of weak acid form ( HIn ) will have one color and the weak acid negative ion (In - ) will have a different color. The weak acid equilibrium is: HIn --> H+ + In - For phenolphthalein: pH 8.2 = colorless; pH 10 = red For bromophenol blue: pH 3 = yellow; pH 4.6 = blue

7. Magic Pitcher Demonstration: Phenolphthalein is an indicator of acids (colorless) and bases (pink). Sodium hydroxide is a base, and it was in the pitcher at the beginning, so when added to the phenolphthalein in beakers 2 and 4, it turned pink (top half of the graphic ). Explanation: Equilibrium: HIn --> H + + In - colorless pink The equilibrium shifts right, HIn decreases, and In - increases. As the pH increase between 8.2 to 10.0 the color becomes red because of the equilibrium shifts to form mostly In - ions.

8. The third beaker has only the NaOH but no phenolphthalein, so it remained colorless. The first beaker contain acetic acid and is skipped over at first. After pouring beakers 2, 3, 4 back into the pitcher it give a pink solution. Bottom half of the graphic: When the pitcher is then poured back into beakers 2, 3, 4 it is a pink solution. In the first beaker, a strange thing happens in that the pink solution coming out of the pitcher now changes to colorless. This happens because the first beaker contains some vinegar or acetic acid which neutralizes the NaOH , and changes the solution from basic to acidic. Under acidic conditions, the phenolphthalein indicator is colorless. Neutralization: HC 2 H 3 O 2 + NaOH --> Na(C 2 H 3 O 2 ) + HOH

9. The simplified reaction is: H + + OH - --> HOH As OH - ions are added, they are consumed by the excess of acid already in the beaker as expressed in the above equation. The hydroxide ions keep decreasing and the hydrogen ions increase, pH decreases. See lower equation: The indicator equilibrium shifts left, In - ions decrease. Below pH 8.2 the indicator is colorless. As H + ions are further increased and pH decreases to pH 4-5, the indicator equilibrium is effected and changes to the colorless HIn form. Equilibrium: HIn --> H + + In - colorless red

10. pH range of indicator INDICATOR COLOR CHANGE pH RANGE PHENOLPHTHALEIN COLOR LESS to PINK 8.3 – 10.0 METHYL ORANGE RED to ORANGE 3.1- 4.4 METHYL RED RED to YELLOW 4.4 – 6.0 BROMOTHYMOL BLUE YELLOW to BLUE 6.0 – 7.6 LITMUS RED to BLUE 5.0 – 8.0

11. What is End Point? In an acid- base titration, the base solution is gradually added from a burette into an acidic solution in a titration flask. When the amount of base added neutralize to the amount of base added in the flask “equivalence point or End point” will reached. The end point of the titration is shown by color changes of an indicator , previously added to the acidic solution in the titration flask.

12. pH – CURVE ( coming soon…..)