Experiment 15 - Objectives To determine the available chlorine in commercial bleaching agents. To prepare a standardized sodium thiosulfate solution.
Experiment 15 – Introduction Natural fibers such as cotton, wool, and paper products tend to retain some of their natural color which is not white. Natural and synthetic dyes exist as a result of the presence of molecules with multiple bonds. The electrons of the multiple bonds absorb visible radiation, reflecting the colors that are not absorbed.
Experiment 15 – Introduction The electrons of the double/triple bonds are removed by oxidizing agents. Because the oxidized form of the molecules causing the colors/stains no longer absorb visible radiation, the fabric or paper appears whiter.
Experiment 15 – Introduction All the halogens and solutions of halogens are excellent oxidizing agents and are relatively safe to handle. Chlorine is an oxidant that is also an excellent bactericide used in swimming pools, drinking water, and sewage treatment plants.
Experiment 15 – Introduction Most cheap commercial bleaching agents contain the hypochlorite ion, ClO-. The ClO- ion removes light-absorbing electrons from the multiple bonds of molecules. Therefore, the absorption of visible radiation does not occur; all visible light is reflected/transmitted, giving the object a white appearance. The ClO- ion is generally in the form of a Na or Ca salt in the bleaching agent.
Experiment 15 – Introduction Industry expresses the strength of bleaching agents as the mass of molecular chlorine, Cl2, per unit mass of bleach (solution or powder). This mass ratio, expressed as percent, is called the percent available chlorine in the bleach.
Experiment 15 – Introduction Liquid laundry bleach is generally a 5.25% NaClO solution. Laundry bleach is manufactured by the electrolysis of a cold, concentrated NaCl solution. The chlorine gas and the OH- that are generated in the electrolysis cell combine to form the ClO- and, in the presence of Na, a NaClO solution. Cl2(aq) + 2OH-(aq) ClO-(aq) + Cl- + H2O(l)
Experiment 15 – Introduction The percent available chlorine in a 5.25% NaClO solution is calculated as: gCl2 x 100 = 5.25g NaClO x mol NaClO gsoln 100 g soln 74.44g x 1mol Cl2 x 70.90g Cl2 x 100 = 5.00% Cl2 1mol NaClO mol Cl2
Experiment 15 – Introduction Solid household bleaches often contain sodium perborate, NaBO2·H2O2·H2O, in which the bleaching agent is hydrogen peroxide, a compound that is also found in hair bleaches.
Experiment 15 – Procedure Starch acts as an indicator by forming a strong bond to molecular iodine; whereas molecular iodine has a yellow-brown color, the I2·starch complex is a deep blue. Just before the brown-yellow color disappears during the titration, you will add starch and continue titrating with sodium thiosulfate until the blue color disappears. Repeat twice and average the [Na2S2O3].
Experiment 15 - Procedure With the standardized Na2S2O3, you will titrate two different brands of bleach, two trials each. Calculate the percent available chlorine in each bleach.
Experiment 15 – Procedure Notes & Questions Procedure Notes Omit Parts A & D Questions 3, 5