General Chemistry Laboratory Experiment: Acid Standardization PDF

Ministry of Higher Education and Scientific Research Al-Ayen University College of Health & Medical Technologies General Chemistry Laboratory (1st year) Experiment 3: Standardization of Hydrochloric acid by titration against standard Sodium Carbonate...

Ministry of Higher Education and Scientific Research Al-Ayen University College of Health & Medical Technologies General Chemistry Laboratory (1st year) Experiment 3: Standardization of Hydrochloric acid by titration against standard Sodium Carbonate 1  Theoretical part Principle: Sodium carbonate reacts with hydrochloric acid according to the following equation: Na2CO3 + 2 HCl → 2 NaCl + CO2 + H2O In other words, to neutralize all the carbonate, two equivalent of HCl should be used and as such the equivalent weight of sodium carbonate = M.wt/2 =53. When one equivalent of HCl is added to the carbonate it is transformed into bicarbonates. Na2CO3 + HCl → NaHCO3 + NaCl And the pH of the solution changes form 11.5 (alkaline) to 8.3. When phenolphthalein is used, it changes to colorless at the end of this stage as its color range falls within the same zone (ph.ph: 8.3-10). When another equivalent of HCl is added to the solution of bicarbonate, complete neutralization takes place and it is transformed into sodium chloride and CO 2 gas is evolved. NaHCO3 + HCl → NaCl + H2O + CO2 The pH of solution changes from 8.3 to 3.8, which is near enough to the color range of Methyl Orange. (3.1- 4.4). If methyl is used at this stage, the color of the solution changes from yellow to red. It thus follows that phenolphthalein is used in the neutralization of HCl with sodium carbonate, the volume of acid used will be equivalent to half of the carbonate, when methyl orange is used in this titration the volume of acid used will be equivalent to all carbonate. Methyl orange is generally used in this as phenolphthalein is sensitive to carbon dioxide. Experimental part: Aim of experiment: To determine the concentration of an unknown solution of HCl by titrating with a 0.2 mol/L solution of Na2CO3. Materials: 2   Standardized Na2CO3 solution (0.2 mol/L)  Unknown HCl solution  Methyl orange indicator solution  Burette  Titration Clamp  Conical flask  Distilled water  10 mL pipette  Measuring cylinder Preparation of standard solution of Na2CO3 (0.2 N): 1- Weigh out accurately of A.R. Na2CO3. 2- Dissolve in small quantity of distilled water and transfer quantitatively to 250 ml measuring flask. 3- Complete to the mark and shake well. 4- Calculate the exact normality of Na2CO3 solution. Weight required = Normality x eq.wt. x volume in liter. Procedure:  Wash and Fill the burette with the Na2CO3 solution and put it above a conical flask (Note: be sure there are no air bubbles in the tip of the burette).  Add 10 ml of Hydrochloric acid solution to a conical flask using a measuring cylinder and add two drops of Methyl Orange indicator solution into the flask.  Record the initial volume of Na 2CO3 in the burette before you begin the titration. 3   Begin the titration, the solution in the conical flask as you add Na 2CO3 in a drop-wise slowly, and continue until the solution turns from Reddish to Yellow color.  Record the final volume of Na2CO3 in the burette when the solution in the conical flask remains Yellow color for about 10 seconds. Calculation: Suppose that the volume of HCl is V1 and its normality is N1 while V2 is the volume taken from sodium carbonate and N2 is its normality. The volume of HCl (from burette) ≡ all carbonate = V1 (N1 x V1 ) HCl = (N2 x V2) Na2CO3 N1 HCl = (N2 x V2) Na2CO3 =N V1 HCl Strength in g/L = normality x eq. wt. Strength of HCI = N1 x 36.5 g/L Questions: 1. Why sodium carbonate is used to standardize HCl? 2. What happens when you add hydrochloric acid to sodium carbonate? 3. What happens if we use phenolphthalein for the standardization of HCl with sodium carbonate? 4. How do you standardize 2.5N HCl? 5. What is the difference between primary and secondary solutions? 6. Why the standard solution should be colorless? 4