Preparation and standardization of various molar and normal solutions
The process of determining the exact concentration (molarity) of a solution is known as standardization. Titration is a type of analytical procedure that is frequently used in standardization. A titration involves reacting an exact volume of one substance with a known amount of another substance.
1.Oxalic acid (COOH)2
Oxalic acid is available in pure state and its standard solutions can, therefore, be prepared by the direct method. Eq. wt. of hydrated oxalic acid (C2H2O4.2H2O), being 63 its N/10 solution would contain 6.3 gm/litre, and N/20 solution would contain 3.15 gm/litre. These standard solutions are employed to find the strength of solutions of alkalis (NaOH and KOH) whose standard solutions cannot be prepared by the direct method.
Preparation of oxalic acid solution
Standard solutions are prepared by using standard substances. Here a known quantity of standard substances depending upon the requirement is dissolved in a known amount of water and desired volume is made. Since, these substances have a constant weight, high purity, non- hygroscopic property, so the solution obtained is of known and definite concentration. The examples of such solutions are as follows.
Standardization N/10 oxalic acid solution (Primary standard)
To prepare N/10 solution of oxalic acid, weigh 6.3 gm of oxalic acid & dissolve in distilled water & finally make up the volume to one litre in a volumetric flask. The standard solution of oxalic acid (Primary standard) is used to find the strength of solutions of alkalis like NaOH, KOH (Secondary standards) whose standard solutions cannot be made by direct weighing. Secondary standard substances. Those substances or reagents which cannot be obtained in a sufficient pure state, e.g. NaOH, KOH, HCl, H2SO4 are called secondary standard substances.
2.Preparation and standardization of Sodium Hydroxide (NaOH) solutions N/10 NaOH
Prepare concentrated stock solution (say 50%) of NaOH by dissolving equal parts of NaOH pellets (50 gm) & water (50 gm) in a flask. Keep it tightly stoppered for 3-4 days. Use the clean, supernatant liquid for preparing N/10 solution. Approximately 8 ml of this stock solution (50%) is required per litre of distilled water. This will give approximate solution. Now take 10 ml of standard N/10 oxalic acid (primary standard) solution in conical flask and add 2- 3 drops of phenolphthalein indicator. Take unknown solution i.e. approximate N/10 NaOH solution in burette and add to the conical flask containing standard oxalic acid solution by continuous mixing by swirling the flask till the appearance of pink colour. NaOH is taken in burette and standard oxalic acid in conical flask. Note down the volume of approximate N/10 NaOH solution used in the titration of 10 ml of standard oxalic acid. Calculate the normality of the unknown sodium hydroxide solution by using following equation.
N1V1 = N2V2
(Base) = (Acid)
N1 = Normality of NaOH solution. (ml)
V1 = Volume of NaOH solution used (ml)
N2 = Normality of standard oxalic acid solution (0.1 N)
V2 = Volume of standard oxalic acid solution (10 ml)
If the volume of approximate NaOH used in the titration is less than 10 ml, means the solution is strong and its normality is not N/10, so dilute the basic solution and again standardize with standard oxalic acid solution till normality of approximate solution is same as that of standard solution.
3.Preparation of Hydrochloric Concentrated acids (HCl)
Prepare approximately 0.1 N solutions on the basis of strength diluting it 120 times with distilled water. Then standardize it against standard N/10 Na2CO3 using methyl orange as an indicator.
Standardization of Concentrated acids (HCl)
Prepare approximately 0.1N solution on the basis of the strength given on the label by diluting it 120 times with distilled water. Then standardize it against standard N/10 NaOH which is already Standardized against N/10 oxalic acid using phenolphthalein indicators.
4.Preparation of sulphuric acid (H2SO4)
Concentrated H2So4 is very corrosive in nature; therefore, it should be handled carefully. And always remember add acid to water under cold condition this is done to avoid pumping due to the heat generated.
Standardization of Sulphuric Acid H2SO4
For the preparation of N/10 H2SO4, take 10 ml of concentrated H2SO4 (usually about 36 N), dilute 36 times by adding acid in small quantity to distilled water in a cold-water bath, to make it 1N and then dilute this 1N solution further 10 times to make it N/10. Then standardize against standard N/10 NaOH or N/10 KOH using phenolphthalein indicator.
5.Preparation of 0.1N sodium thiosulphate solution (Na2SO3.5H2O)
Dissolve approximately 24.8 gm of sodium thiosulphate crystals in previously boiled and cooled distilled water and make the volume to 1000 ml. Store the solution in a cool place in a dark coloured bottle. After storing the solution for about two weeks, filter if necessary.
Standardization of Sodium thiosulphate solution (Na2SO3.5H2O)
Weigh accurately about 5.0 gm of finely ground potassium dichromate which has been previously dried to a constant weight at 105 ± 2° in to a clean 1.0 litre volumetric flask. Dissolve in water make up to the mark; shake thoroughly and keep the solution in dark place. Pipette 25.0 ml of this solution into a clean glass stoppered 250 ml conical flask. Add 5.0 ml of concentrated hydrochloric acid and 15.0 ml of 10% potassium iodide solution. Allow to stand in dark for 5 minutes and titrate the mixture with the solution of sodium thiosulphate using starch solution as an indicator towards the end. The end point is taken when blue colour changes to green. Calculate the normality (N) of the sodium thiosulphate as follows:
N = 25W / 49.03 V
W= Weight in g of the potassium dichromate
V= Volume in ml of solution thiosulphate solution required for the titration.
6.Preparation of 0.1N ceric ammonium sulphate (NH₄)₄Ce(SO₄)₄2 H₂O
66gm of ceric ammonium sulphate was dissolved with gentle heat in a mixture of 30 ml of sulphuric acid and 500 ml of water. The mixture was cooled and filtered. The resulting solution was diluted to 1000ml with water.
Standardisation of 0.1 N Ceric Ammonium Sulphate
- About 0.2 gm of Arsenic trioxide which was previously dried for about an hour was accurately weighed and transferred into a 500 ml conical flask.
- The inner walls of the flask were washed with 100 ml of water and mixed thoroughly.
- Then 300 ml of dil. sulphuric acid, 0.15 ml of osmic acid, 0.1 ml of ferroin sulphate indicator were added.
- Titration was carried out until pink colour of solution changed to pale blue or yellowish green colour
- Each ml of 0.1 N ceric ammonium sulphate ~ 0.6326 gm of ceric ammonium sulphate ~ 4.946 grams of arsenic trioxide
7.Preparation of Potassium Permanganate Potassium Permanganate 0.1 N
Dissolve 3.3 g of reagent grade potassium permanganate (KMnO4) in 1 L of purified water and heat on a steam bath for two hrs. Cover and allow to stand for 24 hrs. Filter through a fine porosity sintered glass crucible, discarding the first 25 mL. Store in a glass-stoppered, amber-coloured bottle. Avoid exposure to direct sunlight; cover the neck of the bottle with a small beaker as a protection against dust. If manganese dioxide precipitates on standing, refilter and restandardize before use.
Standardization of Potassium Permanganate Potassium Permanganate 0.1 N
Weigh accurately 0.2-0.3 g sodium oxalate (Na2C2O4) (dried 2 hrs., 105-110 °C) National Institutes of Science and Technology, U. S. Department of Commerce. Cool in a desiccator and transfer quantitatively to a 600 mL beaker. Add 250 mL of purified water (freshly boiled and cooled) and 10 mL sulfuric acid (96% H2SO4, sp g 1.84). Add rapidly from a burette about 95% of the theoretical quantity of potassium permanganate solution needed; stir until the solution is clear. Heat the solution to 55-60 °C (Maintain temperature range during titration.) and complete the titration by slow dropwise addition until the appearance of a pink colour which persists for 30 secs. Determine and subtract a blank titration run at 55-60 °C on a mixture of 250 mL of purified water (freshly boiled and cooled) and 10 mL of concentrated sulfuric acid.
Conclusion
Standardization is the process of determining the exact concentration (molarity) of a solution. Titration is one type of analytical procedures often used in standardization. In a titration, an exact volume of one substance is reacted with a known amount of another substance. We also learn about the various standard solutions and their preparation and standardization like oxalic acid, sodium hydroxide, sulphuric acid and etc.
Frequently asked questions
What is standardization?
The process of determining the exact concentration (molarity) of a solution is known as standardization.
Why does NaOH need to be standardized?
NaOH is highly hygroscopic, meaning it absorbs water from the air, as time goes the titrant will become more dilute as it absorbs water hence it is necessary to standardized the solution.
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