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Determination Of PH
pH of a solution can be determined by using proper indicator or with the help of a pH meter. The latter is a device which gives accurate value of pH.
Indicators are generally organic compounds which changes color according to the pH of the solution in which they are dissolved . They are chemicals that visually indicate the end point of chemical processes . They achieve these by change of color, fluorescence or by formation of precipitates . They work over some ranges normally between 2-3 unit. These therefore means a different indicator must be used according to the range being examined
But to avoid having to use several indicators , a standard mixture showing gradual and continuous color changes over a wide range of pH have been developed .these mixture is known as a universal indicator.
To determine the pH value of a solution of unknown hydrogen concentration , Only a few drops of the appropriate indicator are added to it .the resulting color is then compared to standard colors on a pH chart which reveals the pH of the solution .alternatively ,a slip of paper saturated with an indicator is dipped into the test solution , the color of the paper is then compared on the chart
Indicators are mostly used in titration experiments to test or determine neutralization ,precipitation or redox reactions have reached their end point . Indicators can be classified into the following categories .
- Acid-base indicators
- Oxidation – reduction indicators
- Precipitation indicators
Acid-Base indicators
They are normally used in neutralization reaction .they are generally organic substances which changes in H+ ion concentration or pH causes alteration in their molecular structure which result in change of color in the substance itself.
As a general rule , acid base indicators are generally weak acids or bases that dissociate when immersed in water . Indicators consist of the acid and its conjugate base i.e. Hin .to be an effective indicator ,Hin and its conjugate base In – must have distinct colors in acidic and basic solutions . The acid ionizes to a small extend
Hin(ag ⇌ H+ (ag) + In–(ag)
If the indicator is in a sufficient acidic medium , the equilibrium according to Le chatelier,s principle will shift to the left and the predominant color of the indicator is that of the unionized form (HIn). On the other hand ,in a basic medium , the equilibrium shifts to the right and the color of the solution will be due to that of the conjugate base (In–). It then follows that the color of the indicator is directly dependent on the pH of the surrounding solution .The constant of these equilibrium is
- K= (H+)(In-)
- (HIn) Which gives, HIn = (H+)
- (In) K
Oxidation – Reduction indicator,
These are indicators whose color in oxidized states is different from that in the reduced state .the end point is reached when the concentration of the oxidized form is equal to that of the reduced form
Some oxidation –reduction reaction do not require use of indicators . This is especially the case when one of the reagent itself changes color according to whether it is reduced or oxidized eg KMnO4 and iodine . The end point of titration in the case of iodine can be brought out even more by adding starch which gives a dip blue color
Indicators of precipitation reaction
Such indicators are used to determine endpoint of precipitation reaction .the most common ones are ;
- Adsorption indicators. They are adsorbed on the surface of the precipitate where it colors it deeply eg silver nitrate in the titration of the chlorides in presence of fluorescent . These gives a pink complex with the fluorescent coloring the complex .
- Indicators that produce colored precipitates .such indicators react with an excess titrant to produce soluble colored complex .e.g. titration off silver ions with thiocyanate ion to produce a white precipitate of silver thiocyanate in presence of ferric alum .another example is titration of a chloride with silver nitrate which can be done in presence of potassium dichromate forming a deep red silver chromate.
Universal Indicator/pH paper.
It is a mixture of a number of indicators. It shows a specific colour at a given pH. A colour guides is provided with the bottle of the indicator or the strips of paper impregnated with it which are called pH paper strips. The test solution is tested with a drop of the universal indicator, or a drop of the test solution is put on pH paper. The colour of the solution on the pH paper is compared with the colour chart/guard and pH is read from it. The pH values thus obtained are only approximate values.
PH Scale
The pH scale ranges from 0 to 14 on this scale. pH 7 is considered neutral, below 7 acidic and above 7 basic. Farther from 7, more acidic or basic the solution is.
We have learnt earlier that the sum of pH and pOH of any aqueous solution remains constant. Therefore, when one increases the other decreases. This relationship is shown below
pH + pOH = 14
Importance of pH in everyday life
pH plays a very important role in our everyday life. Some such examples are described here.
(a) pH in humans and animals
Most of the biochemical reactions taking place in our body are in a narrow pH range of 7.0 to 7.8. Even a small change in pH disturbs these processes.
- b) Acid Rain
When the pH of rain water falls below 5.6, it is called acid rain. When acid rain flows into rivers, the pH of the river water also falls and it become acidic. As a result, the survival of aquatic life become difficult.
(c) pH in plants
Plants have a healthy growth only when the soil has a specific pH range which should be neither highly alkaline nor highly acidic.
(d) In digestive system
Our stomach produce hydrochloric acid which helps in digestion of food. When we eat spicy food, stomach produces too much of acid which causes ‘acidity’ i.e. irritation and sometimes pain too. To get rid of this we use ‘antacids’ which are bases like ‘milk of magnesia’ (suspension of magnesium hydroxide in water).
(e) Self defence of animals and plants
Bee sting causes severe pain and burning sensation. It is due to the presence of methanoic acid in it. Use of a mild base like baking soda can provides relief from pain.
Some plants like ‘nettle plant’ have fine stinging hair which inject methanoic acid into the body of any animal or human being that comes in its contact. This causes severe pain and burning sensation. The leaves of dock plant that grows near the nettle plant when rubbed on the affected area provides relief.
(f) Tooth decay
Tooth enamel is made of calcium phosphate which is the hardest substance in our body and can withstand the effect of various food articles that we eat. If mouth is not washed properly after every meal, the food particles and sugar remaining in the mouth undergoes degradation due to the bacterial present in the mouth. This process produces acids and the pH goes below 5.5. The acidic condition thus created corrode the tooth enamel and in the long run can result in tooth decay.