Chemistry Lab Report for Vitamin C Estimation by Back(Idiometric) Titration

Topics: Titration, Vitamin C, Iodine Pages: 12 (3317 words) Published: February 28, 2013
Vitamin c Estimation by back(idiometric) titration Mahindra (UWC of India) Chemistry lab report for back titration Name: Ashenafi Asfaw Beyene

Back (Indirect) Titration
Vitamin C estimation by Back Titration

Table1: The amount of volume (in cm3) and concentration (in M) of KI, KIO3, and H2SO4 which were used in the experiment of Vit C estimation by back titration. VKI added to the solution/cm3Concentration of KI added to the solution/MVolume of KIO3 added to the flasks ±0.08/cm3Volume of H2SO4 added to the solution±0.08/ cm3Concentration of H2SO4 added to the solution/M

Qualitative data
Colour of iodine is yellow brown
Colour of KIO3=colourless liquid
Colour remain the same after adding H2SO4 to the solution (colourless) Colour of KIO3 and H2SO=colourless
Colour of the solution after adding 0.2MKI on KIO3 and H2SO is dark yellow brown (dark red colour appeared) After titrating standardized 0.1M sodium thiosulphate colour of iodine changed from yellow brown to pale yellow Colour change after adding indicator (starch) to the solution is blue (dark brown colour appears On adding Na2S@O3 colour changes to pale yellow

Reactions: KIO3 (aq) + 5KI (aq) + 3H2SO4(aq) → 3H2O(l) + 3I2(aq) + 3K2SO4(aq) I2 (aq) + I-(aq) = I3-(aq) 2Na2S2O3 (aq) + I2(aq) → Na2S4O6(aq) + 2NaI(aq)

Table2: Burette reading for volume of Na2S2O3 (for standardization of Na2S2O3) used for the reaction of 10cm3 of 0.01 KIO3 and 5cm3 of 1,0M H2SO4 with 15cm3 of 0.2M excess KI for the formation of iodine Initial readingcm3±0.1/cm3Final readingcm3±0.1/cm3Difference in readingcm3±0.2/cm3 5.911.45.5


Average volume of burette reading=(5.5+5.3+5.3) cm3/3 = 5.37cm3 Average volume of Na2S2O3 (for standardization of Na2S2O3) =5.37cm3

Colour of tablet is orange
Mass of tablet=1.02g±0.02g which was calculated by subtracting mass of beaker from mass of tablets and beaker Colour of vitamin c solution made by -----is light orange
Colour after adding KIO3 to vitamin c solution= no colour change (remain the same) Colour change after adding H2SO4 =no colour same as vit C orange colour Colour change after adding KI to above solution darker colour appeared (brown) reddish in colour On adding sodium thiosulphate become light yellow

Adding 2cm3 of 1% starch indicator and adding sodium thiosulphate solution drop wisely, the dark brown colour of starch iodine complex was disappeared Reactions: KIO3 (aq) + 5KI(s) + 6H+ (g) → 3I2 (aq) + 6K+(s) + 3H2O (aq) C6H8O6 (ascorbic acid) (s) + I2 (aq) → C6H6O6 (dehydroascorbic acid)(S) + 2I- + 2H+(g) + excess I2(aq)

2Na2S2O3 (aq) + I2 (aq) → Na2S4O6 (aq) + 2NaI (aq)

Table3:Burette reading for volume by using 1.0Mof Na2S2O3 for standardization for back titration with I2 ,10cm3 of diluted vit c ,10cm3 of 0.01M KIO3, 5cm31.0Msulphuric acid and15cm3 of 0.2M KI for the formation of iodine(I-) Initial readingcm3±0.1cm3Final reading cm3±0.1 cm3Difference in readingcm3±0.2 cm3


Qualitative data for table:
Average volume of Na2S2O3 for back titration with I2= (2.5+2.5+2.6) cm3/3=2.5cm3

Iodometric Titration of Vitamin C Calculations

The Mass of KIO3 =2.14*10-2g
Mass of KIO3 calculated as
mKIO3=nKIO3 X MKIO3 =CKIO3 x VKIO3 x MKIO3 0.01M *10cm3*214/1000 =2.14*10-2g Average volume of Na2S2O3 (for standardization of Na2S2O3) =5.37cm3 Mass of tablet=1.02g±0.01g
Average volume of Na2S2O3 for back titration with I2=2.53 cm3

The balanced chemical reaction of KIO3 and KI is as follows: KIO3 (aq) + 5KI (aq) + 3H2SO4 (aq) → 3H2O (l) + 3I2 (aq) + 3K2SO4 (aq) 2Na2S2O3 (aq) + I2 (aq) →...
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