Aim: Estimation of amino acids

Requirements:

  • Sodium Hydroxide
  • 40% formalin Solution
  • Phenolphthalein
  • Glycine

Principle:

Amino acids like glycine, alanine, etc. contain one amino group and one carboxylic group as part of their structures. These groups being of opposite nature neutralize one another intramolecularly and form internal salts called zwitter ions or dipolar ions. These ions are held together by electrostatic attraction. They are neutral but in presence of alkalis the dissociation favors formation of acid ion.

The free amino group then undergoes condensation with formaldehyde to form mono and dimethyl derivatives. Thus, the formation of these condensation products greatly reduces the basic character of amino group and the carboxylic group can readily be titrated with standard alkali.

Solution Preparation:

  • Sodium hydroxide solution 0.l M: Dissolve 2 g of sodium hydroxide in a 250 mL volumetric flask and make up to the mark with distilled water.
  • Neutral 40% formalin solution: Take 50 mL of 40% formalin solution in a 250 mL conical flask and add 8-10 drops of phenolphthalein indicator. To it add carefully from a burette a dilute solution of sodium hydroxide (0.1M), till the solution is just faintly pink.
  • Phenolphthalein indicator: Dissolve 1.0 g of phenolphthalein in 100 mL of ethanol and then dilute with 100 mL of Water.

Procedure:

  • Preparation of Standard solution of glycine: Weigh accurately 2 g of glycine and transfer to a 250 mL volumetric flask and make up to the mark with distilled water.
  • Titration with standard solution: Take 25 mL of standard glycine solution in a 250 mL conical flask and add 3-4 drops of phenolphthalein indicator. Add dilute sodium hydroxide solution (0.1 M) taken in burette. drop by drop to it until a pink color is just obtained. Now add 10 mL of neutral formalin solution to it. The pink color of the solution immediately disappears. Continue adding sodium hydroxide slowly till pink color is restored. Note the volume of sodium hydroxide used and repeat the experiment until two concordant readings are obtained. Records the observations in Observation Table I.
  • Titration with unknown glycine solution: Perform the titration as described above for 25 mL unknown glycine solution and note the volume of sodium hydroxide used in this titration. Record the observations in Observation Table II

Results:

  1. The amount of glycine in the given solution: …………………… g
  2. The strength of the unknown glycine solution: …………………… g/lit

Observation:

  1. Weight of glycine W = ______g.
  2. Volume of glycine solution taken =  25 mL

Observation Table I

No. I.B.R. F.B.R. Difference Constant B.R. V1 ml
 01        
 02      

Volume of Sodium Hydroxide solution used with standard glycine solution V1 ___mL

Observation II

No. I.B.R. F.B.R. Difference Constant B.R. V2 ml
 01        
 02      

Volume of Sodium Hydroxide solution used with unknown glycine solution V2 ___mL

Calculations:

V1 mL of NaOH = 25 mL of standard glycine solution = \frac{W}{10}  g of glycine

V2 mL of NaOH = 25 mL of standard glycine solution = \frac{W}{10} \times \frac{V_2}{V_1}  g of glycine

Thus, strength of the unknown glycine solution = \frac{W}{10} \times \frac{V_2}{V_1} \times \frac{1000}{25}    g of glycine

                                                                                          = \frac{w \times v_2 \times \mathbf{4}}{v_1}     g of glycine