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Monday, August 26, 2013

The Determination of Ascorbic Acid in Vitamin C Talbets by Titration with Potassium Bromate - Lu Le Laboratory

Standardization of Sodium Thiosulfate against Potassium Bromate

Iodine is generated by the reaction between a known volume of standard potassium bromate and an unmeasured excess of potassium iodide:

BrO3- + 6 I- + 6 H+ Br- + 3 I2 + 3 H2O

The iodine produced is titrated with the sodium thiosulfate solution.

1.     Pipet 25.00-mL aliquots of the KBrO3 solution into 250-mL a conical flask and rinse the interior walls with distilled water.
2.     Treat each sample individually beyond this point. Introduce 2 to 3 g of KI and about 5-mL of 3 M H2SO4
3.     Immediately titrate with Na2S2O3 until the solution is pale yellow.

4.     Add 5-mL of starch indicator, and titrate with Na2S2O3 to the disappearance of the blue color.

5.     Calculate the concentration of the thiosulfate solution.

The Determination of Ascorbic Acid in Vitamin C Tablets by Titration with Potassium Bromate

Ascorbic Acid, is cleanly oxidized to dehydroascorbic acid by bromine:

An unmeasured excess of potassium bromide is added to an acidified solution of the sample. The solution is titrated with standard potassium bromate to the first permanent appearance of excess bromine; this excess is then determined iodometrically with standard sodium thiosulfate. The entire titration must be performed without delay to prevent air oxidation of the ascorbic acid.

1.     Weight (to nearest milligram) 3 to 5 vitamin C tablets [1]. Pulveize them thoroughly in a mortar, and transfer the powder to a dry weighting bottle.
2.     Weight individual 0.40-g to 0.50-g samples (to the nearest 0.1mg) into dry 250-mL conical flasks.
3.     Treat each sample individually beyond this point. Dissolve the sample [2] in 50-mL of 1.5 M H2SO4; then add 5g of KBr.

4.     Titrate immediately with standard KBrO3 to the first faint yellow due to excess Br2. Record the volume of KBrO3 used.

5.      Add 3 g of KI and 5-mL of starch indicator; back-titrate [3] with standard 0.05 M Na2S2O3.

6.     Calculate the percentage of ascorbic acid in the sample.

[1] This method is not applicable to chewable vitamin C tablet.
[2] The binder in may vitamin C tablets remains in suspension throughout the analysis. If the binder is starch, the characteristic color of the complex with iodine appears on addition of KI.
[3] The volume of the thiosulfate needed for the back-titration seldom exceeds a few milliters.

Report Sheet
Concentration of Standard KBrO3 (aq)
0.0150 M
Concentration of Standard Na2S2O3 (aq)
0.0500 M
Weight of Vitamin C Samples

Sample 1
0.4000 g
Sample 2
0.4000 g

Test 1

Volume of KBrO3 (aq)
51.25 mL
Volume of Na2S2O3 (aq)
3.50 mL
Test 2

Volume of KBrO3 (aq)
51.20 mL
Volume of Na2S2O3 (aq)
3.10 mL
Weight of Vitamin C
(3x0.0150 x V(KBrO3) – (0.0500 x V(Na2S2O3) /2 ) x 176.12 x 10-3
Test 1
0.3910 g
Test 2
0.3920 g
Average Weight of Vitamin C
0.3920 g
Percentage Error
2.00 %
Assay of Vitamin C Sample

Saturday, August 24, 2013

Synthesis of Chloroform from Acetone and Bleach - Haloform Reaction - Lu Le Laboratory

Chloroform is an organic compound with formula CHCl3. It is one of the four chloromethanes. The colorless, sweet-smelling, dense liquid is a trihalomethane, and is considered somewhat hazardous. Several million tons are produced annually as a precursor to PTFE and refrigerants, but its use for refrigerants is being phased out.

During prolonged storage in the presence of oxygen, chloroform converts slowly to phosgene, releasing HCl in the process. To prevent accidents, commercial chloroform is stabilized with ethanol or amylene, but samples that have been recovered or dried no longer contain any stabilizer. Amylene has been found ineffective, and the phosgene can affect analytes in samples, lipids, and nucleic acids dissolved in or extracted with chloroform. Dissolved phosgene cannot be removed by distillation or carbon filters, but it is removed by calcium hydroxide or activated alumina. Phosgene and HCl can be removed from chloroform by washing with saturated aqueous carbonate solutions, such as sodium bicarbonate. This procedure is simple and results in harmless products. Phosgene reacts with water to form carbon dioxide and HCl, and the carbonate salt neutralizes the resulting acid.  (Wikipedia)

To prepare chloroform easily, using acetone and bleach to generate a haloform reaction may be a good method. The equation and mechanism is shown below.

Reaction Equation



1.     Acetone: 36.1536 g
2.     12% sodium hypochlorite (or industrial bleach): 1000.0 mL 

3.     Magnesium sulfite anhydrous: Amount


1.     Add 1000.0 mL filtered 12% sodium hypochlorite in a serum bottle.[1]

2.     Chill the bottle containing with 12% NaOCl(aq) in an ice bath.[2]

3.     Add 36.1536 g of acetone into the bottle slowly.

4.     Stir the mixture thoroughly and incubate on ice for 1~2 hours.

5.     Collect the lower layer.

6.     Dry the crude product with magnesium sulfate anhydrous.

7.     Distill the dried product and collect the distillate at 61~62.

 My fluid cooling system

8.     Dry the product again with magnesium sulfate anhydrous.

9.     Figure out the yield of chloroform.
10.  Preserve the product in a brown glass bottle.


[1] There may have some precipitate in the bottom of the bottle of industrial bleach. That is because sodium hypochlorite may disproportionate at room temperature and produce sodium chloride and sodium chlorate, and the solubility of sodium chloride is the worst in these three salts. So the NaCl will salt out and form precipitate.

[2] Haloform reaction is an exothermic reaction so the temperature of the mixture would raise high (about 70) if cooling is not sufficient. High temperature may lead side reaction occur:

3 ClO-(aq) ClO3-(aq) + 2 Cl-(aq)

This side reaction may reduce the concentration of hypochlorite ion and cause decrease in the yield. So it is important in controlling the temperature of the reactant.

Report Sheet

Volume of 12% sodium hypochlorite
1000.0 mL
Density of 12% sodium hypochlorite (24.0)

Test 1
1.1557 g/mL
Test 2
1.1618 g/mL
1.1586 g/mL
Mole of sodium hypochlorite
1.8675 mol
Weight of acetone
36.1536 g

= 0.6225 mol
Theoretical weight of chloroform
74.2980 g
Weight of Chloroform
34.6337 g