I need help with writing an introduction for lab report and also help with organizing experimental details I already wrote the procedures and just need help with adding more details on them. The experiment is about (Determination of ascorbic acid in vitamin C tablets). the procedures file is attachedTitrations with Potassium Bromate:
– Determination of ascorbic acid in vitamin C tablets
1) 38I- 1: Preparation of potassium bromate
a) Weigh approximately 1.3 g of potassium bromate (to the nearest 0.1 mg) in a 500 mL
volumetric flask
b) Dissolve with 200 mL of distilled water and then dilute to the mark
2) 38H- 1: Preparation of sodium thiosulfate (making 0.05 M not 0.1 M)
a) Boil about 1 L of distilled water for 10 to 15 minutes. Allow the water to cool to room
temperature and then add about 12.5 g Na2S2O3*5H2O and 0.1 g of Na2CO3
b) Stir until solid has dissolved
c) Transfer the solution to clean glass or plastic bottle and store in a dark place
3) 38G-1b: Preparation of starch indicator
a) Rub 1 g of soluble starch and 15 mL of water into a paste
b) Dilute to about 500 mL with boiling water and heat until the mixture is clear
c) Cool and store in a tightly stoppered bottle
d) For most titrations, 3 to 5 mL of indicator are used
4) 38I-2: Standardization of sodium thiosulfate against potassium bromate
a) Pipet 25.00 mL aliquots of the KBrO3 (potassium bromate) solution into 250 mL conical
flasks and rinse the interior walls with distilled water
b) Treat each sample individually beyond this point
c) Introduce 2 to 3 g of KI and about 5 mL of 3 M H2SO4
d) Immediately titrate with Na2S2O3 (sodium thiosulfate) until the solution is pale yellow
e) Add 5 mL of starch indicator nd titrate to the disappearance of the blue color
f) Calculate the concentration of the thiosulfate solution
Need volume thiosulfate titrated
Known concentration of potassium bromate
Bromate reacts with KI and H+ which forms I2 – known value
Titrate KBrO3 with sodium thiosulfate, use known concentration of KBrO3
Take bromate to iodine to find how much iodine is formed. Take iodine to
thiosulfate to find out how much thiosulfate used. Divide the amount of
thiosulfate used by volume of thiosulfate delivered.
BrO3- + 6I- + 6H+ → Br- + 3H2O + 3I2
5) 38I-3: Determination of ascorbic acid in vitamin c tablets
a) Crush 3-5 tablets, weigh them, find total mass and divide by number of tablets.
b) Weigh out individual 0.40 to 0.50 g samples (to the nearest 0.1 mg) into the dry 250 mL
conical flasks
c) Treat each sample individually beyond this point
d) Dissolve the sample in 50 mL of 1.5 M H2SO4 then add about 5 g of KBr (bromide)
KBr added to push forward reaction because bromide + bromate = bromine
e) Titrate immediately with potassium bromate (KBrO3) prepared in Step 1 until it turns
faint yellow due to excess Br2. Do three replicates.
f) Record the volume of KBrO3 used
g) Add 3 g of KI and then back-titrate with thiosulfate until the yellow turns clear
h) Once it turns clear, add 5 mL of starch indicator (will turn solution blue) and titrate with
standard 0.05 M Na2S2O3 until clear again
i) Calculate the average mass (in milligrams) of ascorbic acid (176.12 g/mol) in each tablet
Moles of ascorbic acid is equal to amount of bromine used
1) Forward titration:
a) x amount of excess bromine
b) comes from bromate (KBrO3) + bromide (KBr)
2) Back titration: x amount of bromine + iodide (KI) = bromine + iodine
a) Excess iodide from KI leads to reaction with iodine with causes triiodide (iodine +
b) Triiodide is what the starch indicator reactions with and you need excess KI to form the
c) Amount of excess bromine determines how much iodine is made because iodine is in
d) Once iodine has reacted, there is no more iodine to combine with excess iodide to create
triiodide to react with starch indicator, so the solution goes clear
e) So, amount of iodine corresponds to excess Bromine from forward titration
3) Forward titration: Flask contains ascorbic acid and KBr (bromide), add bromate to that by
titrating and that bromate being added causes formation of bromine which reacts with ascorbic
acid. The bromine created reacts with ascorbic acid. Once all the ascorbic acid has reacted, there
will be excess bromine and that’s what causes the color change to pale yellow.
4) Back titration tells you how much you overshot with forward reaction – find excess of bromine
I2 + 2S2O32- → 2I + S4O62- we get the concentration of I2 because we know the concentration of 2S2O32Br2 + 2I- → 2Br- + I2 we use the concentration of I2 to get the concentration of Br2
BrO3- + 5Br- + 6H+ → 3Br2 +3H2O we use the concentration of Br2 to get the concentration of BrAscorbic acid + Br2 → dehydrated ascorbic acid + 2Br- + 2H+ we use the concentration of Brto get the concentration of Ascorbic acid

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