Sodium Hypochlorite

[easttn]

What does the percentage of sodium hypochlorite on a bottle of bleach or chlorinating liquid mean, by weight or volume?
Ive looked on a bottle of bleach but it doesnt specify.

Sodium Hypochlorite .... 5.25% - (5% available chlorine)
Sodium Hypochlorite .... 6% ---- (5.7% available chlorine)
Sodium Hypochlorite .... 8.25% - (7.86% available chlorine)
Sodium Hypochlorite .... 10% --- (8.8% available chlorine)
Sodium Hypochlorite .... 12.5% - (10.8% available chlorine)

 

[JasonLion]

There are three percentages that tend to come up, by weight, available chlorine, and trade. Household bleach is labeled by weight percentage, and occasionally also by available chlorine. Liquid chlorine, 10% and higher, is usually labeled by trade percentage.

6% by weight is 6.17% trade, which is also 5.71% available chlorine.

 

[easttn]

Well that explains the bigger gap between the available chlorine and percentage for 10% and 12.5%.

The "bleaches" available chlorine is 95% of the weight percentage. Is 10% trade is really 9.24% and 12.5% is really 11.34% weight percentage? How did you calculate that?

 

[JasonLion]

Weight percent of sodium hypochlorite = (GPL available chlorine x 1.05) / (10 x (specific gravity of solution))

Weight percent available chlorine = GPL available chlorine / (10 x (specific gravity of solution))

Trade percent available chlorine = GPL available chlorine / 10

Available chlorine is in chlorine gas equivalents, which is Cl2.

GPL is grams per liter.

The specific gravity of the solution varies with the concentration of sodium hypochlorite and also with the amount of excess lye. These days excess lye is typically quite low, but that does occasionally vary.

 

[easttn]

So trade percent doesnt take into factor the specific gravity of the solution (the actual weight per gallon).

From the many msds sheets Ive been through this is an "average" specific gravity that I come up with:
12.5% = 1.2
10% = 1.16
8.25% = 1.13
6% = 1.09
5.25% = 1.08

Available chlorine is in chlorine gas equivalents, which is Cl2.

That is something else Ive been wondering about. Sodium Hypochlorite only has 1 Cl.
Molar mass of Chlorine (Cl) = 35.453 g/mol
Molar mass of Chlorine Gas (Cl2) = 70.906 g/mol
Molar mass of Sodium Hypochlorite (NaClO) = 74.442 g/mol

70.906 / 74.442 = 0.95
Chlorine Gas / SodHypo equals 95% which is the difference between the weight percentage and available chlorine percentage for bleach. How did that come to be since sodium hypochlorite only has 1 Cl and chlorine gas is Cl2? Seems as though you would use Chlorine (since it only has 1 Cl). My math or equation fell apart there.

 

[JasonLion]

There are mildly entertaining, but irrelevant, historical stories about how the choice of units came about. It doesn't make any logical sense, just go with it. Loosely speaking, when the measurement units were decided they didn't have the chemistry knowledge we have today. And since then everyone has just gone with the existing standard rather than inventing a new more logical unit.

 

[easttn]

Ok. Well thanks for the replies.