This page was last updated on May 16, 2005
Questions? Comments? Drop me a note...
This page is maintained by Doug Chapman
There is some really neat chemistry going on here, and the procedure isn't as involved as it may seem. Your goal here is to determine the weight percent of sodium hypochlorite (NaClO) in commercial bleach. You will do this by titrating with a standardized thiosulfate (S2O32-(aq)) solution.
Part I. Standardization of sodium thiosulfate
The net ionic equation for the titration reaction is
IO3-(aq) + 6H+(aq) + 6S2O32-(aq) ----> I-(aq) + 3S4O62-(aq) + 3H2O(aq)
In this reaction, iodate ion (IO3-(aq)) is the limiting reactant, and we know (at least we should know!) the concentration of the standard iodate solution. In this titration, we use starch as an indicator - we titrate the standard KIO3 solution with sodium thiosulfate until the solution is a pale yellow; at this point, we add some starch (which complexes with iodine to produce a blue color) and then we continue titrating with sodium thiosulfate until the solution is colorless.
We know how many mol of iodate ion we started with, because we pipetted 10.00 mL of ~0.02 M KIO3:
mol IO3- = (~0.02 mol/liter) * (.01000 liter) (be sure to use the real concentration of the potassium iodate!!)
From the above net ionic equation, we see that iodate and thiosulfate react in a 1:6 ratio, so to find mol thiosulfate ion in the solution, we convert mol iodate to mol thiosulfate:
mol IO3- * (6 mol S2O32- / 1 mol IO3-)
Then, to find the molarity of the thiosulfate, we calculate
M = (mol thiosulfate / volume thiosulfate delivered, L)
You will perform at least three determinations - present your data in a table (with a title!) Calculate and report the average molarity of the thiosulfate solution from your trials.
Part II, Titration of sodium hypochlorite in bleach
Here, the pertinent net ionic equation is
ClO-(aq) + 2S2O32-(aq) + H2O(l) ----> S4O62-(aq) + Cl-(aq) + 2OH-(aq)
Notice that you added 10.00 mL of bleach to a 100 mL volumetric flask and diluted to the mark with water - this will be very important later!!
The titration is done in exactly the same fashion as was done earlier (i.e., using starch as an indicator.) You should have made two determinations.
For the calculations, notice that we know the concentration of the thiosulfate - we standardized it in part I (and we will use the average concentration.) When we titrate a bleach sample until it is colorless, we know how many mol thiosulfate were added, since
mol S2O32- = (ave molarity S2O32- from part I) * volume S2O32- delivered, L
From the above net ionic equation, we see that hypochlorite ion and thiosulfate ion react in a 1:2 ratio, so to calculate mol hypochlorite in the bleach sample, we convert mol thiosulfate to mol hypochlorite:
mol S2O32- * (1 mol ClO- / 2 mol S2O32-)
We are interested in the concentration and weight per cent of sodium hypochlorite, NaClO, in the bleach. Since sodium and hypochlorite ions are in a 1:1 ratio, mol ClO- = mol NaClO.
The concentration of the sodium hypochlorite in the bleach (in mol/liter) is given by
[NaClO] = mol sodium hypochlorite / volume of bleach, L
Notice that we diluted the bleach by a factor of 10!! You must multiply the calculated molarity by 10 to get the correct answer!!
Report your trials for the NaClO titration and the average [NaClO] (you will do at least three trials) in Table form.
To calculate the weight percent sodium hypochlorite in the bleach, we do the following:
we know the concentration of sodium hypochlorite (in mol/liter) in the bleach (we calculated it above.) We can convert this quantity to units of g/L using the molar mass of sodium hypochlorite:
(mol NaClO / L) * (74.44 g NaClO/ mol NaClO) = g NaClO / liter
This gives us the number of grams NaClO per liter of bleach. To get a weight percent, we need to know how many g of NaClO we have per gram of bleach. To do this, we need to know the density of bleach. Assume that the density of the bleach is 1.10 g/mL, and therefore, 1 liter of bleach has a mass of 1100 g. To get the weight percent of sodium hypochlorite, we divide g NaClO/liter (from above) by 1100 g (and multiply by 100 to get a percent:)
weight % = (g NaClO / liter )*(1 liter / 1100 g) * 100
Present these results in a table also. Since you did at least three determinations of the bleach, calculate the weight percent of sodium hypochlorite for each determination separately and report the average of your results, along with the standard deviation.