Lab 9: Analysis Of Commercial Bleach

Lab 9: Analysis of Commercial Bleach
Introduction
Many common products are effective because they contain oxidizing agents.
Some products, which contain oxidizing agents, are bleaches, hair coloring agents, scouring powders, and toilet bowl cleaners. The most common oxidizing agent in bleaches is sodium hypochlorite, NaClO (or NaOCl). Commercial bleaches are made by bubbling chlorine gas into a sodium hydroxide solution. Some of the chlorine is oxidized from the molecular form (Cl2) to the hypochlorite ion, ClO-. Some of the molecular form is also reduced to the chloride ion, Cl-. This type of reaction, where the same type of element is both oxidized and reduced, is called a disproportionation reaction. The solution remains strongly basic. The net
ionic chemical equation for the process is:
Cl2(g) + 2OH-(aq) → ClO-(aq) + Cl-(aq) + H2O(l)
The amount of hypochlorite ion present in a solution of bleach is determined by an oxidation-reduction titration. One of the best methods is the iodine-thiosulfate titration procedure. Iodide ion, I-, is easily oxidized by almost any oxidizing agent. In acid solution, hypochlorite ions oxidize the iodide ions to form iodine, I2. The iodine that forms is then titrated with a standard solution of sodium thiosulfate.
The analysis takes place in a series of steps:
(1) Acidified iodide ion is added to hypochlorite ion solution, and the iodide is oxidized to iodine.
2H+(aq) + ClO-(aq) + 2I-(aq) → Cl-(aq) + I2(aq) + H2O(l) from: HCl bleach KI solution
(2) Iodine is only slightly soluble in water. It dissolves very well in an aqueous solution of iodide ion, in which it forms a complex ion called the triiodide ion. Triiodide is a combination of a neutral I2 molecule with an I- ion. The triiodide ion is yellow in dilute solution, and dark-brown when concentrated.
I2(aq) + I-(aq) → I3
-(aq)
iodine iodide triiodide
(3) The triiodide is titrated with a standard solution of thiosulfate ions, which reduces the iodine back to iodide ions:
I3
-(aq) + 2S2O3
2-(aq) → 3I-(aq) + S4O6
2-(aq)
from Na2S2O3 solution called diothionate ion
During this last reaction, the red-brown color of the triiodide ion fades to yellow and then to the clear color of the iodide ion. It is possible to use the disappearance of the color of the I3
- ion as the method of determining the end point, but this is not a very sensitive procedure. The addition of starch to a solution that contains iodine or triiodide ion forms reversible blue complex. The disappearance of this blue colored complex is a much more sensitive method of determining the end point. However, if the starch is added to a solution, which contains a great deal of iodine, the complex that forms may not be reversible. Therefore, the starch is not added until shortly before the end point is reached, when the solution has faded to a light yellow. The quantity of thiosulfate used in step (3) is directly related to the amount of hypochlorite initially present.
2
Materials
Bleach, containing NaClO
Hydrochloric acid, HCl, 3 M (in the hood)
Sodium thiosulfate solution, Na2S2O3, 0.100 M
Volumetric flask, 100-mL with stopper
Erlenmeyer flask, 125-mL or 250-mL
Potassium iodide, KI (s)
Starch solution, 2%
Graduated cylinder, 25-mL
Transfer pipet, 5-mL
Buret
Ring stand
Buret clamp
Pipet bulb
Safety Alert
 Concentrated bleach is damaging to the eyes, skin, and clothing. Hydrochloric acid is also hazardous. Both give off strong vapors. If you spill either solution on yourself, wash off with lots of water. Neutralize hydrochloric acid spills with baking soda
 Adding hydrochloric acid to bleach may cause chlorine gas to be given off.
Carry out step 3 in a fume hood.
 Always use a pipet bulb when pippetting. Never pipet by mouth.
Wear Chemical Splash Goggles and a Chemical-Resistant Apron.
Procedure
1. Dilute the concentrated bleach.
Use a pipet bulb and a 5-mL transfer pipet to measure 5.00 mL of a commercial bleach solution into a 100-mL volumetric flask. Dilute to the mark with distilled water, stopper, and mix well.
2.
Measure the potassium iodide.
Weigh out approximately 2 g of solid KI. This amount is a large excess over that which is needed.
3. Oxidize the iodide ion with hypochlorite ion.
Using a graduated cylinder, transfer 25 mL of the dilute bleach into an
Erlenmeyer flask. Add the KI and about 25 mL of distilled water. Swirl to dissolve the KI. Working in a fume hood and slowly with swirling, add approximately 2 mL of 3 M HCl. The solution should be a dark yellow to redbrown from the presence of I3
- complex.
4. Titrate the iodine.
Rinse the buret with distilled water and then three times with small portions of your 0.10 M sodium thiosulfate solution. Then, fill up the buret with the 0.10 M sodium thiosulfate solution. Titrate the bleach-iodine mixture in your
Erlenmeyer flask until the iodine color becomes yellow. Add one dropperful of starch solution. The blue/black color of the starch-iodine complex should appear. Continue the titration until one drop of Na2S2O3 solution causes the blue color to disappear. Record the final buret reading.
5. Repeat
Repeat the titration beginning with step 2 two more times.
Disposal
The solutions may safely be flushed down the drain with a large excess of
water.
After you have poured out a solution, leave the faucet running.