I just loath being forced into this response because it involves chemical reaction dynamics, and that is I am sure is guaranteed to bore everyone here to tears; and it does not advance anyone's interest in sailing, and makes me feel like somebody I do not wish to be.
The key word in my quote is available chlorine
There are many different rates of reaction in chemistry, as there are mechanisms
In this case as concentration goes down, reaction rate goes down logarithmically
In plain English there is less chlorine available, so you need to add more.
Now this analogy is a stretch, but it might help anyone who wishes to understand: just dilute your battery acid 10% and expect to only get a drop of 10% output.
This is my last post.
Cheers
Dick
Water tanks
Moderator: Jim Walsh
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tartansailor
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Tom Keevil
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- Location: Cape Dory 33 "Rover" Hull #66
Perils of Internet Info
The posted article about chlorination is a good example of why you shouldn't believe everything you find on the web. It is a verbatim copy of a web page from the U of Hawaii that must have a typographical error in it.
As John Vigor has astutely pointed out, 1/4 cup of bleach is not one-tenth of 1/2 cup of bleach. Commercial bleach is 5% sodium hypochlorite by weight. (The article then uses % by volume, but let's ignore that small problem.) A solution that is 5% is 50,000 ppm. Diluting this 1:10 will produce a 5,000 ppm solution. A gallon is 128 fluid oz, so 1/10 of that is 13 oz, as stated in the article. Since a cup is 8 oz, it would take 1.6 cups of bleach per gallon, not the stated 1/2 cup. I expect that if you look in the original book you would find 1 1/2 cups as the suggested quantity, and that it was miscopied onto the web page. To get a 500 ppm solution would require 100:1 dilution or 1.3 oz (0.16 cup). A Tablespoon is 1/256 of a gallon, so that dilution would produce 195 ppm. Clearly they are suggesting measuring devices that most people have on hand to produce approximately correct concentrations.
I respectfully disagree that this has anything to do with rates of chemical reactions. The chlorine is allowed to sit for 3 - 24 hrs, which is more than enough time for the lethal reactions to proceed to completion at any reasonable concentration of chlorine. The exponential nature of the reaction is irrelevant.
Available chlorine is that in the solution which has not yet reacted with anything. In pure bleach that is all of it. After it has been added to water, it is whatever is left over after the reactions are complete. You need to add more chlorine to filthy water to get the same final available chlorine level. These recipes assume that the water is fairly decent to begin with (not hazy or opaque).
I would also point out that the correctly calculated 0.025% chlorine corresponds to 250 ppm. Most domestic water is less than 2ppm, swimming pools are less than 5 ppm, and it would be the rare person who could not taste the 250 ppm recommended by Peggie Hall. Also note that Peggie's instructions produce a solution acceptable to the quoted text.
As a minor point, the calculation of Liters x ppm = mL works only if you use mL as an abbreviation for microliters rather than the more widely accepted milliliters. The final answer in cc's is fine.
My wife Jean just read this over my shoulder, gave me smile, and noted that I have now joined the ranks of retired people with nothing better to do than write tedious posts like this one.
On the other side of the recently retired coin, we spent five months living on Rover this spring/summer/fall while we slowly explored bits of the Pacific Northwest. We never made it back home to the Oregon coast, and the boat is now moored in Olympia while we're back home in Ashland.
As John Vigor has astutely pointed out, 1/4 cup of bleach is not one-tenth of 1/2 cup of bleach. Commercial bleach is 5% sodium hypochlorite by weight. (The article then uses % by volume, but let's ignore that small problem.) A solution that is 5% is 50,000 ppm. Diluting this 1:10 will produce a 5,000 ppm solution. A gallon is 128 fluid oz, so 1/10 of that is 13 oz, as stated in the article. Since a cup is 8 oz, it would take 1.6 cups of bleach per gallon, not the stated 1/2 cup. I expect that if you look in the original book you would find 1 1/2 cups as the suggested quantity, and that it was miscopied onto the web page. To get a 500 ppm solution would require 100:1 dilution or 1.3 oz (0.16 cup). A Tablespoon is 1/256 of a gallon, so that dilution would produce 195 ppm. Clearly they are suggesting measuring devices that most people have on hand to produce approximately correct concentrations.
I respectfully disagree that this has anything to do with rates of chemical reactions. The chlorine is allowed to sit for 3 - 24 hrs, which is more than enough time for the lethal reactions to proceed to completion at any reasonable concentration of chlorine. The exponential nature of the reaction is irrelevant.
Available chlorine is that in the solution which has not yet reacted with anything. In pure bleach that is all of it. After it has been added to water, it is whatever is left over after the reactions are complete. You need to add more chlorine to filthy water to get the same final available chlorine level. These recipes assume that the water is fairly decent to begin with (not hazy or opaque).
I would also point out that the correctly calculated 0.025% chlorine corresponds to 250 ppm. Most domestic water is less than 2ppm, swimming pools are less than 5 ppm, and it would be the rare person who could not taste the 250 ppm recommended by Peggie Hall. Also note that Peggie's instructions produce a solution acceptable to the quoted text.
As a minor point, the calculation of Liters x ppm = mL works only if you use mL as an abbreviation for microliters rather than the more widely accepted milliliters. The final answer in cc's is fine.
My wife Jean just read this over my shoulder, gave me smile, and noted that I have now joined the ranks of retired people with nothing better to do than write tedious posts like this one.
On the other side of the recently retired coin, we spent five months living on Rover this spring/summer/fall while we slowly explored bits of the Pacific Northwest. We never made it back home to the Oregon coast, and the boat is now moored in Olympia while we're back home in Ashland.
Tom and Jean Keevil
CD33 Rover
Ashland OR and Ladysmith, BC
CD33 Rover
Ashland OR and Ladysmith, BC
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Phil Shedd
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Rothesay NB Canada
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