Re: Why litmus paper doesn't change color in - household bleach?

To answer your question(s) we have to consider several separate factors 
that will come together to explain the chemical behaviour of Sodium 
Hypochlorite. One important conclusion implied in your question, however, 
must be addressedt, for it is misleading.  You have assumed that the extent 
of the reaction with litmus by Sodium Hypochlorite is directly responsible 
for its desinfecting effect.  Not quite!. 

Concentrated bleach is a 5.25% solution of Sodium Hypochlorite, NaOCl, at 
pH 11 or higher. At this very basic conditions, bleach is relatively 
stable. Nevertheless, a very slow decomposition occurs at room temperature 
generating Chloride ions (Cl-) and Chlorate ions (ClO3 -).  This 
decomposition is accelerated by heat.  

3 NaOCl   -->   2 NaCl   + Na ClO3

A second reaction also occurs when the bleach solution is brought in 
contact with compounds that are reach in electrons, or that have a weak 
hold on their electrons. Sodium Hypochlorite decomposes into Sodium 
Chloride (NaCl) and atomic Oxygen (O).  Atomic Oxygen is the bleaching 
agent.  Its most dramatic effect is with coloured substances, which it 
readily attacks forming oxidation products that are colourless.  

NaOCl    -->    NaCl + O

As the composition of the bleach solution  is altered, the presence of 
Chloride ions together with Hypochlorite ions starts forming Chlorine gas 
(Cl2).  This reaction becomes more noticeable as the solution becomes less 
alkaline (basic).

NaOCl + H2O   -->    NaOH + HOCl

HOCl + Cl-    -->   Cl2 + OH-

Lets next look at litmus ( a blue pigment isolated from lichens) that turns 
red under acidic conditions (pH less then 7), and turns blue at neutral to 
basic conditions (pH larger or equal to 7). Paper impregnated in litmus is 
available as both red litmus (to test for bases) and blue litmus (to test 
for acids). I don’t know what type of litmus paper you used for your 
experiment. I have repeated your experiment using both red and blue litmus 
paper with concentrated bleach and diluted bleach.  I also used a pool 
strip to determine the presence of free chlorine. The following table shows 
my results. 
    

red litmus		immediately			after 30 seconds
conc bleach  	     blue spot with white edges         white spot
	
dil. Bleach (1:20)   blue spot with light blue edges 	slowly turning 
                                                        white



blue litmus		immediately			after 30 seconds	
conc bleach          blue centre with white edges 	white spot
	
dil. Bleach (1:20)   light blue spot edges 		light blue spot
		     slowly turning white


chlorine indicator 	immediately	                after 30 seconds	
	
conc bleach		purple	                        purple
	
dil. Bleach (1:20)	deep purple			deep purple
		

The initial blue colour of the red litmus confirms the strong basic 
conditions of the original hypochlorite solution. A 1:20 dilution would not 
affect the pH much the one order of magnitude , lowering the pH from 11 
down to 10. Still a very basic solution. For this reason the results of the 
diluted bleach solution are not much different then the original 
concentration.  What is different is the speed with which the pigments are 
oxidized by the hypochlorite. The more diluted the solution the lesser the 
oxidative power of the bleach solution.  What increases with dilution, 
however, is the release of chlorine gas.  Together, the chlorine and the 
free Oxygen, will give bleach its disinfecting and bleaching powers.  

Concentrated Sodium Hypochlorite will function as a very strong oxidant, 
literary burning the compounds it comes in contact with.  At more diluted 
concentrations (in the range of those used in washing machines or house 
cleaning conditions,) bleach will act less as an oxidant and more as 
a source of Chlorine gas, which in turn will react with micro organisms and 
function as a disinfectant, react with metals to form metallic chlorides 
(salts) (an effect that will show as corrosion of metalic surfaces) or with 
organic compounds to form chlorinated organic compounds (which are 
environmentally toxic).

I would encourage you to repeat your experiment and note all details as 
carefully as possible.  Let us know.


Review Web Sites and Books:

 1. Acids and Bases 

      Pardue Chemistry Department. General Chemistry Help Homepage: General 
      Chemsitry Topic Review Acids and Bases.       
      http:/
/chemed.chem.purdue.edu/~genchem/topicreview/index.html  


 2. Chemistry of Sodium Hypochlorite:

     Brady, J.E. and Hiolum, J. R. (1993). Chemistry: The Study of Matter 
     and Its Changes.  John Wiley & Sons, Inc.  Pp: 891-892.