Volumetric analysis

            The molarity of the nitrite solution was calculated.

Objective2(B): To determine the average molarity of the thiosulfate solution from sodium thiosulfate pentahydrate against iodine titration.

        To determine the average concentration of hypochlorite in the chlorox .

 (B) METHODS

23. A burette was filled up with sodium thiosulfate solution .
24. About 0.2 g potassium iodate was weight and it was dissolved in distilled water in a 100-mL volumetric flask.
25. 25-mL of the potassium iodate solutionwas pipette into a 250-mL conical flask.1 g of potassim iodide and followed by 5-mL of sulfuric acid (1 M) was added.
26. The liberated iodine was titrated with the thiosulfate solution immediately.About 100-mL distilled water followed by 1 to 2 –mL of freshly prepared starch solution was added   when the colour of the mixture is pale yellow.
27. The titration was continue until the titration is colourless.
28. The titration was repeated one more time and the average molarity of the thiosulfate solution was calculated.
29. 10-mL of the chlorox solution was pipette into a conical flask.40-mL of distilled water,1 g of potassium and 10-mL of sulfuric acid (1M)  was added.
30. The liberated iodine was titrated with the standart sodium thiosulfate solution.
31. The titration was repeat and the average of concentration of hypochlorite in the chlorox (expressed in % Cl2) was calculated.

 RESULT AND CALCULASIONS:

(A) STANDARDISATION OF KMnO4

Mass of sodium oxalate = 0.2998 g

Mass of potassium permanganate = 0.8190 g

Titration:

Average volume =(9.25+9.20+9.40) / 3 = 9.28 ml 

Observation :

Purple colour     change          Pink pale colour

The molarity of the potassium permanganate solution :

5 C2O42- + 2MnO4- + 16 H+       10 CO2 + 2 Mn2+ + 8 H20

aA(aq) + bB(aq)      Products

 MAVA   =   a

 MBVB        b

n(sodium oxalate) = 0.2998 g / 123.99 gmol-1

                             = 2.4179x10-3   mol

 Molarity of sodium oxalate =   2.4179x10-3 mol

                                                   0.1L

                                                                    

                                           =0.024179 M

MAVA   =   a

 MBVB        b

0.024179 M x 25  =  5

MB X  9.28                 2

MB = M potassium permanganate = 0.02605 M

The potassium permanganate solution now becomes the ‘’ standard solution’’ after it has been standardized against sodium oxalate.

This solution was used to determine the quantity of nitrite ions in a Back titration in the first part.

Volume of potassium permanganate = 10mL(excess) added to the nitrite solution under acid conditions.

Because there is an excess ,the excess was titrated against iron(ll) ammonium sulfate solution.

                       5 NO2- + 2MnO4 + 6 H+                              5 NO3- + 2Mn2+ + 3 H2O

             

                2 MnO4- + 10 Fe 2+ + 16 H+                     10 Fe3+ + 2Mn2+ + 8H20

Molarity of standard iron (ll) ammonium = 0.011 M

Average volume = (29.00+29.30)/2 = 29.15 ml

Observation :

Purple colour              change                      pale pink

     

(2 MnO4- + 10 Fe 2+ + 16 H+      10 Fe3+ + 2Mn2+ + 8H20 ) ÷ 2

 MnO4- + 5 Fe 2+ + 8 H+      5 Fe3+ + Mn2+ + 4H20 )

MVFe2+        = 5

MV MnO4-      1

0.011 M(29.15 ml) = 5

M ( 8ml)                    1

M MnO4-     = 8.01625 X 10-3M

n(MnO4-) initial = 0.8190 g     = 5.1835x10-3mol

                            158gmol-1        

n(MnO4-) excess =  0.02605 M x 10

                                                         1000                    = 2.605x10-4 mol

n(MnO4-) reacted =  (5.1835x10-3mol-2.605x10-4 mol)=4.923X10-3mol

 From equation:

   5 NO2- + 2MnO4 + 6 H+                              5 NO3- + 2Mn2+ + 3 H2O

2 mol Mn04-  ==  5 mol NO2-

4.923X10-3mol == 5 x 4.923X10-3mol of NO2-    = O.O1231 mol of NO2-

                                                                2

[NO2-] = O.O1231 mol X 1000

                      8 mL

                = 1.53875 M

RESULT FOR (B)

Weight of potassium iodate = 0.2096 g

Weight of potassium iodite = 1.02 g

Table of titration of sodium thiosulfate solution with potassium iodate.

Table of titration of sodium thiosulfate solution with chlorox solution

Average volume of sodium thiosulfate solution (ml) = 14.00+14.10+14.10

                                                                              3

                                                                          = 14.07 ml

Observation :  

Blue black              change to        colourless  

CALCULATION:

Molarity of the sodium thiosulfate solution

Mpotassuim iodate=   n potassuim iodate                                n= 0.2096

                                                      Vpotassuim iodate                                          214.00

                                                                                  = 9.794 x 10-4 mol

                                = 9.794 x 10-4 mol

                                             0.1 L

                                = 9.794x10-3 M

2S2032- + l2                       S4062- + 2l-

M sodium thiosulfate V sodium thiosulfate    =   2

M potassuim iodate Vpotassuim iodate                   1        

                                           Msodium thiosulfate           =   2  x Mpotassuim iodate Vpotassuim iodate

                                                (average)                          1                Vsodium thiosulfate

                                                                  =   2  x (9.794 x 10-4x 25.00 )

                                                                       1                14.07

                                                                  =  3.480x10-3 M

Molarity of hypochlorite:

Chlorox solution = 10 mL

OCl-  + 2I-  + 2H+              I2  + Cl-  + H2O

2S2032- + l2                       S4062- + 2l-

2 MnO4-  + 10 Fe 2+  + 16 H+            10 Fe 3+  + 2Mn2+  + 8H2O

From the equations above,

 MaVa  =  a

 MbVb       b

DISCUSSIONS:

1. The potassium permanganate solution now becomes the ‘’ standard solution’’ after it has been standardized against sodium oxalate. At first,from this experiment I had calculated the molarity of potassium permanganate solution by using this formula :

aA(aq) + bB(aq)      Products

 MAVA   =   a

 MBVB        b

2. The molarity is known exactly.This solution was used to determine the quantity of nitrite ions in a Back titration.

3.The acid which use to acidic the potassium permanganate solution is sulfuric acid. Hydrochloric acid cannot be use because it can react with potassium permanganate and nitric acid is oxidizing agent, so it not suitable to use.

4. Titration which use potassium permanganate does not need any indicator because it indicate itself.In a direct titration,the titrant is added to analyte until end point is observed whereas in a back titration, a known excess of a standard reagent is added to the analyte. The end point is when the solution change color to pale pink.

5.From the first titration, the molarity of potassium permanganate is determined and in the second titration it react as standard solution then can determined the molarity of nitrite solution.

6.Note that for the HCl, use must be made of ultrapure HCl. Normal (reagent grade) 0.1 M HCl can have a significant amount of iodide (~100-300 mM) in it, which can easily be avoided by using triple distilled ultrapure HCl.

7.It is of great importance to calibrate the 5 cm3 and 3 cm3 pipettes with distilled water and to note their precise volumes. The volumes should remain fixed for the duration of the cruise. They are important for establishing the dilution factor to be used for further work on the samples. The standardization of the acid, of course, takes account of the volumes.

8.There should no excess of chlorine in the hypochlorite solution.

CONCLUSION:

FOR (A)

Molarity of potassium permanganate =  0.02605 M

Molarity of nitrite solution                         = 1.53875 M

FOR (B)

The average molarity of the thiosulfate solution =3.480x10-3 M

The average concentration of hypochlorite in the chlorox =

References :

1.Inorganic laboratory manual (1st year)-Department of Chemistry ,UM

2. 

3. John Lambert, the essentials of volumetric analysis. 1966. Heinemann Educational Books Ltd. London