Ask A Scientist Chemistry Archive

name         Claudia B.
status       other

Question -   I am checking a magazine article about maintaining color in clothes when 
doing laundry. Our writer says that if your water comes from a municipal water system, 80-90% in 
US do, it probably contains trace amounts of chlorine and that this disinfectant additive acts as
a subtle bleach on clothes/fabrics. Do you agree? Is "trace" the correct word for the amount?
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Claudia,

All municipal water supply chlorinators care about is that there is at least a "trace" of chlorine 
in the water. If a trace is present, it means that there are no oxidizable pathogens present. Any 
amount of chlorine more than a "trace" is money wasted and will likely affect the flavor of the 
water and make the consumers unhappy.

So, I cannot tell you how much a "trace" might be -- it all depends on what the plant operator 
considers sufficient. Even so, the amount present is not likely to have much if any effect on 
fabric color stability unless the color is some sort of home-brew, fragile dye. Your writer 
said, "... acts as a subtle bleach on clothes/fabrics." I suppose there could be a grain of 
truth in this assertion. However, it is also likely that most modern dyes would not be 
significantly affected.

Regards,
ProfHoff 695
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It is true that most (I am not sure of the exact figure) municipal water supplies may contain 
trace amounts of residual chlorine as the hypochlorite ion. The amount is far too small to cause 
bleaching of dyed fabrics. First, because the amount is so small (The taste / odor would be so 
terrible you would never use it.) Second, heat (like in hot or warm water), metal ions, and 
"dirt" in the
clothes all "conspire" to reduce the hypochlorite to inactive chloride ions. So the chlorine 
"available" to bleach fabrics is even much smaller than residual concentration. The amount of 
chlorine residual is much less than in a swimming pool, and there the concentration of chlorine 
is of the order of parts per million, so the amount present in tap water is very small.
    Chlorination of water has become the target of various groups as causing all sorts of 
	diseases, toxic problems and so on. Some of the concerns are credible and should be (and are) 
	being investigated. On the other hand, the chlorination of water is perhaps the single most 
	important advance in public health of the 20th century. One need only read history about the 
	ubiquitous outbreaks of water borne diseases in the past  to confirm the extraordinary value 
	of chlorination of municipal water supplies. Are there safer, better ways? Probably, and those
	possibilities should be explored and tested, but at present nothing has proven better and safer 
	on a large scale as chlorination.
	
Vince Calder
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Claudia,

"Trace" is okay to use.  But, in water chemistry the term chlorine "residual" is used.  One of the 
advantages of chlorine as a disinfectant is that a leftover or residual concentration can be 
maintained in the water distribution system, ensuring good sanitary water quality.  In drinking 
water, a residual of 0.2 mg/l of chlorine is optimal.  I hope that this answers your question.

Sincerely,
Bob Trach
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