Potassium

Preparation of primary standard solution of sodium thiosulfate (Na₂S₂O₃), Oxalic acid [ ( COOH )₂.2H₂O ], Potassium dichromate (K₂Cr₂O₇).

Preparation of  secondary standard  solution of  Sulfuric acid (H₂SO₄), Hydrochloric acid (HCl), Acetic acid (CH₃COOH).                        

PRINCIPLE :

          STANDARD SOLUTION : In the case of  volumetric analysis,  the solution whose strength is known is called standard solution. Standard solution can be prepared directly from primary standard substance by taking exact weight and then dissolving in distilled water .

         (A) PRIMARY STANDARD SUBSTANCE : Primary standard substance are substance from  which standard solution can be prepared directly by taking weight at a electrical balance and then dissolving that substance in distilled water or any other  suitable solvent . Examples are-

                      Na₂CO₃, COOH.2H₂O, COONa.2H₂O, K₂Cr₂O₇  etc.

                                  |                       |            

                                 COOH            COONa

CRITERIA OF PRIMARY STANDARD SUBSTANCE : 

  

                (I)  They are chemically pure substance .   

             (II) The molecular weight of the substance corresponds to the chemical  

                 formula .

             (III)  The substance are stable both in solid or liquid state .

             (IV)  The gram equivalent weight is large .

          (B) SECONDARY STANDARD SUBSTANCE : Secondary standard substance are the substance from which standard solution cannot be prepared by directly weighting and then dissolving in suitable  solvent.

               Examples are :- NaOH, KOH, H₂SO₄, HCl, CH₃COOH, KMnO₄ etc .

                                                                                                               

In this case , the appropriate weight of the substance under test is taken and then dissolved in distilled water and dilute solution is prepared. This solution is then standardized with primary standard solution and then its strength is determined by using the following equation --

                   V₁S₁ = V₂S₂    

                                   S₁ = Strength of Primary standard solution

                                   V₁ = Volume of Primary standard solution

                                   S₂ = Strength of Secondary standard solution (?)

                                   V₂ = Volume of Secondary standard solution

   CONCENTRATION TERM : Normal solution   

REAGENT :-

          

                 Primary standard substance :

                                     I) Sodium carbonate (Na₂CO₃) = 106 ÷ 2 = 53                                         

                                     II) Oxalic acid (HOOC-COOH).2H₂O = 126 ÷ 2 = 63

                                     III) Sodium oxalate(NaOOC-COONa).2H₂O=170÷ 2=85

                                     IV) Potassium dichromate (K₂Cr₂O₇) =149 ÷ 6= 24

                                     V) Distilled water (H₂O)

              Secondary standard substance :

                                     I) Sulfuric acid (H₂SO₄)

                                     II) Hydrochloric acid (HCl)

                                     III) Nitric acid (HNO₃)

                                     IV) Acetic acid (CH₃COOH)

                                     V) Sodium hydroxide (NaOH)

                                     VI) Distilled water (H₂O)

APPARATUS :

                i) Balance

                ii) Conical flask

                iii) Measuring cylinder

                iv) Funnel

                v) Volumetric flask.

PROCEDURE :

               A) Primary standard substance :

                          (i) Preparation of  standard  0.1(N) Na₂CO₃  solution : The gram equivalent weight of  Na₂CO₃ is 53.50. 53 ÷ 100= 0.53 gm. of Na₂CO₃ is needed to prepare 0.1(N) 100ml solution of Na₂CO₃ .

                                      Now 0.53gm. of  Na₂CO₃  was weighted in the electrical balance and dissolved in 50ml of distilled water in 100ml volumetric flask and shake gently. Then make the volume up to the mark with distilled water.

                         (ii) Preparation of standard 0.1(N) Oxalic acid solution: The gram equivalent weight  of oxalic acid is 126 ÷ 2= 63 .

                                    So, to prepare 100ml of  0.1(N) solution of oxalic acid we need (63 ÷ 100)= 0.63gm of oxalic acid .

                                        Now weight out 0.63gm of oxalic acid in electrical balance and dissolved in 50ml of  distilled water in a 100ml volumetric flask and shake gently. Then make the volume up to the mark with distilled water.

                 (iii) Preparation of  standard  0.1(N) Sodium Oxalate solution: The gram equivalent weight  of this substance is 170 ÷ 2= 85.

                                So , to prepare 100ml of  0.1 (N) solution of sodium Oxalate, we need (85 ÷ 100)= 0.85gm of sodium Oxalate.

                                Now weight out 0.85gm of sodium Oxalate in electrical balance and dissolved it in 50ml of  distilled  water in a 100ml volumetric flask and shake gently. Then make the volume up to the mark with distilled water.

                  (iv) Preparation of  standard  0.1(N) Potassium dichromate (K₂Cr₂O₇) solution : The gram equivalent weight of Potassium dichromate is 149 ÷ 6= 49  .

                               So , to prepare 100ml of  0.1 (N) Potassium dichromate solution we need (49 ÷ 100)= 0.49gm of  K₂Cr₂O₇ .

                               Now weight out 0.49gm of  K₂Cr₂O₇  in electrical balance and dissolved it in 50ml of  distilled water and shake gently. Then make the volume up to the mark with distilled water.

                     (v) Preparation  of   standard  0.1(N) Sodium  thiosulfate solution: The gram equivalent weight  of Na₂S₂O₃  is 158 .

              

                                So, to prepare 100ml of  0.1 (N) solution of sodium Oxalate, we need (158 ÷ 100)= 1.58gm of Na₂S₂O₃.

                     Now weight out 1.58gm of Na₂S₂O₃ in electrical balance and dissolved it in 50ml of  distilled  water in a 100ml volumetric flask and shake gently. Then make the volume up to the mark with distilled water.

      (B) SECONDARY STANDARD SUBSTANCE :

                           (i) Preparation of standard 0.1(N) Sulfuric acid (H₂SO₄) solution: The gram  equivalent  weight  of H₂SO₄ is (98 ÷ 2)= 49 .So , to prepare 100ml of  0.1 (N) H₂SO₄ solution 49 ÷ 100=0.49gm of 100% H₂SO₄ acid is needed. Since H₂SO₄ is not 100% pure,it is 98% pure. So to prepare 100ml of 0.1(N) H₂SO₄ solution [ (100 x 49) ÷ 98 ] = 0.5gm H₂SO₄ acid is needed. But H₂SO₄ is liquid and Secondary standard substance. So its volumetric measurement may be obtained by the following  equation.

                             

                                       mass = volume x density.

                      Thus density of H₂SO₄ is 1.89 so to prepare 100ml 0.1(N) H₂SO₄ the needed volume of 98% H₂SO₄ is 0.5 ÷ 1.89=0.282ml=0.3ml.

          

                      Now take 100ml volumetric flask, which contains 50ml of distilled water and 0.282ml of H₂SO₄ is added slowly. Then make the volume up to the mark with distilled water.

    

                       (ii) Preparation of standard 0.1(N) HCl solution:-                       The gram equivalent weight of HCl is 36.5 .So, to prepare 100ml of  0.1 (N) HCl solution 36.5 ÷ 100 = 0.36gm of 100% HCl acid is needed .Since HCl  is not 100 pure,it is 32% pure.So to prepare 100ml of 0.1(N) HCl solution of HCl [(100 x 0.365) ÷ 32] = 1.4gm HC acid is needed. But HCl is liquid and Secondary standard substance. So volumetric measurement is needed which may be obtained by the following  equation.

                             mass = volume x density.

            Thus density of HCl is 1.18 so to prepare 100ml 0.1(N) solution of HCl the needed volume of 32% HCl is (1.140 ÷ 1.18)= 0.966ml= 1ml .

            Now  take 100ml volumetric flask, which contains 50ml of distilled water and 1ml of HCl is added slowly. Then make the volume up to the mark with distilled water.

                  (iii) Preparation of  standard  0.1(N) Acetic acid (CH₃COOH) solution: The gram equivalent weight  of CH₃COOH  is 60. So , to prepare 100ml of  0.1 (N) Acetic acid  solution we need (60 ÷ 100)= 0.6gm of CH₃COOH which is 100% pure . But it  is liquid and Secondary standard substance. So volumetric measurement is needed which may be obtained by the following  equation.

                             mass = volume x density.

                        Thus density of CH₃COOH  is 1.055 .So to prepare 100ml 0.1(N) CH₃COOH is (0.6 ÷ 1.055)= 0.56ml= 0.6ml .

          

                        Now  take 100ml volumetric flask, which contains 50ml of distilled water and 0.6ml of CH₃COOH is added slowly. Then make the volume up to the mark with distilled water.

                  (iv) Preparation of  standard  0.1(N) Sodium hydroxide (NaOH) solution : The gram equivalent weight  of  NaOH is 40 . So , to prepare 100ml of  0.1 (N) NaOH solution we need (40 ÷ 100)= 0.4gm of NaOH .

                      Now  take 50ml of distilled water in a conical flask (volume is 100ml) and Shake gently until the NaOH granules dissolved completely. Then make the volume up to the mark with distilled water.

                   (v) Preparation of  standard  0.1(N) Potassium hydroxide (KOH) solution : The gram equivalent weight  of  KOH is 56 . So , to prepare 100ml of  0.1 (N) KOH solution we need (56 ÷ 100)= 0.56gm of  KOH .

                       Now  take 50ml of distilled water in a conical flask (volume is 100ml) and Shake gently until the KOH granules dissolved completely. Then Pour this solution to 100ml volumetric flask and rinse the conical flask into the volumetric flask and make the volume up to the mark with distilled water.

PRECAUTION : -

               i) Acid should be poured very slowly to the water. If acid is poured to water very rapidly then the flask will be heated and may burst.

               ii) Concentrated acid should be measured with measuring cylinder pipette as accurately as possible.

               iii) The surface of the funnel should be washed into flask with distilled water so that no acid loss occurs .