Experiment 2: Analysis of Commercially Available Cleansers

 

Introduction

 

The ability of a household cleanser (like Ajax, Comet, etc.) to remove stains is related to the amount of oxidizing agent in it.  Until recently, the oxidizing agent in most cleansers was sodium hypochlorite, NaOCl, which dissolves in water as follows:

 

                            H2O

         NaOCl(s)     ®      Na+(aq)         +       OCl- (aq)                                                                                (1)

 

In this experiment we will assume that the oxidizing agent in cleansers is NaOCl, and we will determine, by titration, the % NaOCl in a commercial cleanser.

 

The method by which this will be accomplished is to first add an excess of potassium iodide, KI, to a dissolved, acidified sample of cleanser.  The following reaction will occur:

 

         OCl-(aq)        + 2H+(aq)   + 2I-(aq) ®    I2(aq)   +   Cl- (aq)+ H2O(l                                                           (2)

 

I2(aq) is yellow when dilute to red-brown when concentrated.  The amount of I2(aq) formed by this reaction will then be measured by titrating it with a standardized solution of sodium thiosulfate, Na2S2O3, which reacts with I2 (Aq) as follows:

 

         2S2O32-(aq)    +       I2(aq)   ®     S4O62-(aq)      +       2I-(aq)                                                                  (3)

 

From the volume of standardized Na2S2O3 solution used, the % NaOCl in the sample can be calculated.

 

The instructions for this experiment are written in the traditional, compact form.  This form does not include reminders to clean/zero the balance before use, or reminders to rinse pipet/buret first with deionized water, then with the solution to be measured.  These are considered to be basic laboratory techniques, which you would know to do without being reminded. All Chem 401 experiments will be written this way.

 

Primary Standards

 

A primary standard is a material that can be quantitatively used to determine the concentration of other substances.

 

In order to be a good primary standard, a substance must be
         a) of high purity
         b) stable
         c) no waters of hydration (not hydroscopic or hygroscopic)
         d) have a high molar mass
         e) be soluble in the solvent of interest.

 

In this experiment, we need to know the concentration of the sodium thiosulfate very accurately, because it will be used to stoichiometrically determine the concentration of hypochlorite ion in our cleanser. However, it forms a hydrate, with 5 waters of hydration. This means that it cannot be weighed out accurately as a primary standard. We must prepare it at an approximate concentration and then standardize it against a primary standard.


The primary standard that we will be using is potassium iodate, KIO3. Potassium iodate can be used to standardize the sodium thiosulfate by the following reactions.

 

First, a know amount of KIO3 (limited reactant) is reacted with an excess of KI in an acidified solution:

 

                      IO3- + 5I- + 6H+ ® 3I2 + 3H2O                                                                              (4)

 

This produces a known amount of I2 in solution. (You may be asking yourself, why didn’t we just weigh out the right amount of iodine and dissolve it? The reason is that it cannot act as a primary standard either. Iodine is not stable enough. It sublimes at a fairly low temperature!)

 

This known iodine concentration can then be titrated against the unknown (or only approximately known) thiosulfate concentration, based on the reaction:

 

                     2S2O32-     +       I2(aq)   ®      S4O62-         +       2I-                                                            (5)

 

Note this is the same reaction as shown for the analysis of the cleanser above (3). During the first half of the experiment, it is the unknown that we are standardizing against a primary standard. Once its concentration is known, it becomes a secondary standard, which can be used to determine the concentration of an unknown amount of iodine. This time the iodine will come from the reaction with the hypochlorite we are trying to determine.

II. Experimental Procedures

A. Equipment needed

Set-up in lab: 25mL burette, funnel

Chemicals in lab: Na2S2O3•5H2O, KIO3, KI, 1M HCl, 1% starch solution, cleanser

B. Disposal

Pour this titrated solution into the large bottles labeled "Used I-, S4O62-, Starch, Cleanser Waste".

C.  Experimental considerations:

1.      Cleansers are designed to form suds. Be careful not to allow the Erlenmeyer flask to overflow. Tapping the flask flat on the table to break air bubbles can sometimes prevent a flask from overflowing.

2.      Your samples of cleanser also contain grit that is helpful in scrubbing. This material will not dissolve.

D. Before Starting Experimental Work (Before Class)

1.      In your notebook, enter the experiment title, date, your name and name of partner. Number each page.

2.      Write the purposes of the lab. 

3.      Write out all reactions that are involved in the analysis.

4.      Write an executive summary of the procedures described in this lab. Include all major procedural steps that you will need to follow.

E. Procedure 1. Preparation and standardization of .05 M Na2S2O3.

         a.       To prepare the ~0.05M sodium thiosulfate solution: Add approximately 2.5 g of Na2S2O3•5H2O into a 400 or 500 mL beaker. Record the weight in your lab notebook. Add 200 mL of deionized water (with your large graduated cylinder) and stir the mixture until the Na2S2O3•5H2O has completely dissolved.  Condition and fill a 25.00 mL buret with the sodium thiosulfate solution.

b.      To prepare the potassium iodate (primary standard) solution: Accurately weigh out 0.16 g of KIO3 into a weighing boat and transfer to a 100mL volumetric flask.  Record the weight in your lab notebook. Add enough deionized water to half fill it.  Swirl the mixture until the KIO3 has completely dissolved.  Carefully add more DI water until the bottom of the meniscus reaches the mark on the neck of the flask.  Cap the flask and invert it with shaking 15-20 times to insure complete mixing. 

c.       Preparation of a solution of known iodine concentration: Pipet 15.0 mL of the KIO3 solution (using your graduated pipette at two volumes that add up to 15.0 mL) into a clean 250 mL Erlenmeyer flask.  Add 30 mL deionized water and 0.4 g KI.  Swirl the mixture until the KI completely dissolves.  Add 10 mL of 1 M HCl (using your small graduated cylinder) and swirl for 1 minute.  A brownish red color will appear due to the presence of aqueous iodine.

         d.      Standardization of the sodium thiosulfate (secondary standard) solution: While swirling the solution from step c, add the Na2S2O3 solution from the buret until the brown fades to a pale yellow.  Add ~4mL of starch suspension (using your small graduated cylinder).  The solution in the flask should turn dark blue because of a complex that starch forms with iodine. (You do not need to record the volume at which you added the starch.) Continue adding the Na2S2O3 solution drop-wise with swirling until the dark blue color disappears.  Record the total volume of Na2S2O3 solution delivered (from the start of the titration to where the indicator color has disappeared) to 2 decimal places in Table 1. (It is not necessary to record the volume at which the starch was added!)

         Pour this titrated solution into the large bottles labeled "I-, S4O62-, Starch, Cleanser used”.  Rinse the 250 mL Erlenmeyer flask with tap water 3 times and with deionized water 2 times.

e.       Repeat Steps c and d two more times, for a total of 3 titrations. If all volumes agree within 0.06mL, continue to Procedure 2. If not, perform a 4th titration. 

E. Procedure 2. Titration of the Cleanser

To a clean 250mL Erlenmeyer flask add 50mL deionized water and 0.5 g KI.  Swirl until the KI has dissolved.  Add an accurately weighed sample of cleanser (about 10 g) and swirl for 4 minutes.  The insoluble polishing powder will remain, but will not interfere with the titration.  Add 25 mL 1M HCl using your large graduated cylinder. (Add the acid slowly, or else the sample may foam out of the container!) Swirl for 30 seconds, and then titrate with the Na2S2O3 solution until only a small amount of yellow remains (in the presence of the original blue-green color).  Add 4mL starch suspension and titrate until the dark blue color disappears.[1] 

Repeat this procedure with 3 more samples of the cleanser, for a total of 4 titrations.  Review these results with your instructor.  After your instructor has approved the values, pour the unused KIO3, 1 M HCl, and Na2S2O3 solutions into the large plastic bottles labeled "Used I-, S4O62-, Starch, Cleanser".

When finished, rinse your buret several times with DI water, fill it with DI water, stopper it, and return it to its original location. Also refill your DI wash bottle for use in the next experiment.

III. Further Instructions

A. Required Calculations

1.      Q-test: Apply the Q-test to see if any of the 3 or 4 volumes from your standardization of sodium thiosulfate (Procedure 1d/e) should be eliminated from the average volume calculation. Be sure to list the calculated value of Q, the critical value of Q from the table, and your decision to accept or reject the suspect data point.

2.      Determination of average volume of sodium thiosulfate solution used: Calculate the average volume of sodium thiosulfate used to titrate the 15mL portion of the potassium iodate primary standard. In determining the average, only use those values that the Q-test told you to retain.

3.      Determination of the concentration of the sodium thiosulfate solution. Use the average volume of sodium thiosulfate to calculate the molarity of the Na2S2O3 solution. Use the stoichiometry from the reactions shown in the introduction section. Be sure to use good dimensional analysis and show all of your units!
3a) First calculate the molar mass of KIO3.
3b) Next calculate the molarity of the KIO3 solution.
3c) Calculate the moles of KIO3 (or IO3-) in your 15mL aliquot.
3d) Use stoichiometric factors (from equations 4 & 5) to convert mole IO3- to mole S2O32-.
3f) Calculate the molarity of the S2O32- solution from the average titration volume
      determined in calculation 2.

4.      Determination of the % NaOCl in the cleanser: From the volume of Na2S2O3 used in Procedure 2, calculate the % NaOCl in the cleanser. Note, since we had a different mass of cleanser in each trial, we must calculate the % NaOCl for each trial separately.
4a) Calculate the mol S2O3- used in trial #1 from the titration volume from procedure 2,
      trial #1 and the molarity from calculation 3.
4b) Calculate the mol of OCl- in trial #1 using stoichiometric factors from equations 3 & 2.
4c) Convert mol OCl- to g NaOCl using the molar mass of NaOCl.
4d) Determine % bleach (NaOCl) in trial #1
4e) Repeat calculations 4a to 4d for the trials #2, #3, &#4.

5.      Q-test: Apply the Q-test to see if any of the four % NaOCl values from your standardization of sodium thiosulfate should be discarded. Be sure to list the calculated value of Q, the critical value of Q from the table, and your decision to accept or reject the critical data point.

6.      Determination of the Average, Standard Deviation, and %RSD for the Cleanser Analysis:
Calculate the Avg., s.d., and % rsd for the %NaOCl in the cleanser, using Q-test accepted values.

B. In-Class Work

1.   All data collection must be completed during class time.

2.   Perform calculations A1, A2 & A3.

C. Results: at home

1.      Complete all remaining calculations in Section A

2.      Place a copy of the Q-table critical values (from the Lab Manual Introduction) inside the back cover of your notebook.

D. Discussion and Experimental Summary:

After all calculations are complete:

1.      Name 5 qualities of a good primary standard.

2.      Why can’t we use I2 as a primary standard?

3.      Was the analysis of the cleanser precise? Give some limitations on the reproducibility of the analysis.

4.      Place a copy of the Q-Table Critical values in your notebook for future reference.

5.      Most reactions that occurred during this experiment are oxidation/reduction reactions (Redox). For reactions 2, 3, and 4, fill in the following table. (Hint: decide for the iodine/iodide pair first and then fill in the oxyanions.)

Reaction

Substance Oxidized

Substance Reduced

Oxidizing
Agent

Reducing
Agent

2

 

 

 

 

3

 

 

 

 

4

 

 

 

 

 

Table 1: Standardization of Sodium Thiosulfate Solution.

Mass of KIO3 _______________________                  Mass of Na2S2O3·5H2O_________________

KIO3 dissolved in _____________ mL H2O            Na2S2O3·5H2O dissolved in ____________mL H2O

Run #

1

2

3

(4)

Aliquot Size
(mL)

 

 

 

 

Burette Ending Volume (mL)

 

 

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محمد رجب احمد

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