Summer Research Program for Science Teachers

Michael J. O'Leary

 

Analysis of Bleach



In this experiment we will analyze and compare various laundry bleaches by testing their relative oxidizing powers. We will run two reactions. First we will use the bleach to oxidize iodide to iodine and then we will titrate the iodine to determine exactly how much was produced by the bleach. [9-12 Content Standard B- Chemical reactions]

Introduction


The active ingredient in most chlorine bleaches is sodium hypochlorite, NaOCl. The oxidizing action of hypochlorite ion, OCl-, kills germs and also decolorizes many stains and dyes. [9-12 Content Standard E- Understandings about science and technology] The quantity of hypochlorite ion in a sample of bleach can be determined by finding out how much iodine, I2, it can produce by oxidizing iodide ion, I. The quantity of iodine produced is measured by titrating it with sodium thiosulfate, Na2S2O3, which converts the colored iodine back to colorless iodide ion.

The equations are:

Oxidation of iodide ion to iodine with bleach:

2H+ + OCl- + 2I- I2 + Cl- + H20

Titrating iodine with thiosulfate:

I2 + 2S2032- 2 I + S4062-

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CAUTION! UNDILUTED BLEACH AND HYDROCHLORIC ACID CAN CAUSE CHEMICAL BURNS AND RUIN YOUR CLOTHES. BLEACH IS ALSO IRRITATING TO THE EYES. [Teaching Standard D- Ensure a safe working environment]

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Procedure

Obtain two burets and a 250 ml volumetric flask from the stockroom. One buret will be used for diluted bleach and the other for sodium thiosulfate.

(1) Measure out exactly about 25 ml of the liquid bleach assigned to you from the buret provided for unknowns. Collect this measured amount carefully from the buret in an Erlenmeyer flask or a beaker. Note the initial reading and the final reading on the unknown's buret to the hundredths of a ml (two decimal places).

(2) Pour this bleach into the clean (distilled water) but not necessarily dry 250 ml volumetric flask. Rinse your beaker or Erlenmeyer flask and anything else you used in the transfer (funnel, etc.) very thoroughly with distilled water and pour all the rinsings into the volumetric flask. Be sure that every last drop of the bleach you took gets into the volumetric.

(3) Fill the flask to the mark exactly with distilled water, using an eyedropper for the last bit if necessary. Stopper the flask and shake well to mix thoroughly.

(4) Prepare two burets by rinsing (three times) and filling (to near the zero mark), one buret with the diluted bleach solution, the other with 0.3000M sodium thiosulfate solution.

(5) Prepare a 100 or 150 ml Erlenmeyer flask with about 10 ml of a 10% KI solution in it; add about 10 ml of 2M H2504 to the flask and stir.

(6) Read the initial reading on the bleach buret and then run about 25 ml of the bleach solution into the Erlenmeyer with the acidified KI solution. Note the color change as iodine is formed Take the final reading on the bleach buret.

(7) Record the initial reading on the other buret and titrate the iodine in the Erlenmeyer flask with the sodium thiosulfate solution. The end point is the disappearance of the iodine color. To get a more sensitive endpoint, stop the titration when the red-brown iodine color is very faint but still visible. Add 2 ml of fresh 1% starch solution to the flask; a starch-iodine complex forms which has an intense blue color. Continue the titration until this blue color just disappears. Record the final buret reading. The endpoint is made more distinct by the addition of the starch solution, which combines with excess iodine to form the deeply colored complex. [Content Standard Unifying Concepts- Change, constancy, and measurement]


I2 + starch (starch-12 complex)

red-brown deep blue

(8) Repeat steps (5), (6) and (7) in a second trial. Do the same for a third trial. Be sure you refill the burets before you start a trial.


Calculations

If the exact Molarity of the sodium thiosulfate is known, you can use the balanced equations in the introduction to determine the Molarity of the bleach for OCl- ion. Note that half as many moles of the OCl- ions are required as S2O32- ions in the reactions. If you can do these calculations, find the Molarity of the OCl- in your bleach for each case and average the three values. Otherwise, for each titration calculate the ml of thiosulfate used per ml of bleach solution and average the values.

Obtain other students' results for other brands of bleach different from yours. Compare your results for different brands in the class. Take into account the original dilution, if there are significant differences between different students' work. See Report Sheet. [Teaching Standard B- Orchestrate scientific discourse]



PRELABORATORY ASSIGNMENT

NAME _________________________________________

LAB SECTION __________________ DATE _________________

(1) Define

(A) Oxidation

(B) Oxidizing agent [9-12 Content Standard B- Chemical reactions]

(2) Balance the following oxidation-reduction equation

SnCI2 + HCIO3 + HCI SnCI4 + H2O



REPORT SHEET

NAME __________________________________

LAB SECTION ____________ DATE _______________

Estimate all buret readings to two decimal places.


Type of bleach you used _________________________________



TITRATION I 1 I 2 I 3 I

ml amount bleach dilute bleach thio- sulfate dilute bleach thio- sulfate dilute bleach thio- sulfate

Final Reading

Initial Reading

Volume Used


(1) If the exact Molarity of the thiosulfate is not known, go to (2), next page.

From the Molarity of thiosulfate calculate the number of moles of thiosulfate used in each of the three titrations, the number of moles of hypochlorite in the diluted bleach, and then the Molarity of the hypochlorite in the diluted bleach for the three trials. Do this and find an average value. Show one set of calculations (attach sheet).

Molarity of hypochlorite in diluted bleach



Titration 1 ____________ Titration 2 ________________



Titration 3 ____________ Average ________________


(2) Do not fill in this section unless you did not complete section (1)

This page is to be used only if you did not know the exact concentration of the thiosulfate.



Calculate the ml thiosulfate used per ml of diluted bleach in each of the three trials. Average the three values. Show one set of calculations (attach sheet).

ml thiosulfate per ml diluted bleach

Titration 1 __________ Titration 2 ______________

Titration 3 __________ Average ______________

(3) Give any comparisons you made with other students' bleach of a different brand from yours.


(4) Questions


(a) In step 2 of the procedure, why is it not necessary to use a completely dry volumetric flask?

(b) In most titrations we add an indicator. Why is none used in this experiment?

[Teaching Standard B- Orchestrate scientific discourse]

 

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