Calculate available chlorine?

[Sully207]

New to TFP, and thankful to all the advice. I've been buying up bleach and liquid chlorine for the pool (to the point where people must think I'm trying to dispose of a dead body! :> )
I bought some Liquid Chlorine, and the bottle only shows: 10% Sodium-Hypochlorite and 90% Other Ingredients. Nowhere does it indicate a % available chlorine.
I assume available chlorine is NOT 10%...as I have bottles of bleach that show 7.5% Sodium-Hypochlorite and 92.5% Other...yields 7.17% available chlorine.
When I log the additions of the 10%/90% liquid chlorine into my PoolMath, I put it in as 9%. Is that good enough?
Some of the math I've Googled, gets into specific gravity measurements and a bunch of other over my head stuff, that I'm hoping just guessing 9% will suffice

 

[HermanTX]

Welcome to TFP and don't be timid on asking questions. Everyone is learning.
If your Liq. Chlorine states 10% Sodium Hypochlorite, then that is the number you put in Pool Math for calculations of how much liq. chlorine to add. The app does the internal calculations. You can play around and put in 6% and you will see you will need to add more volume for the same affect in PPM you are targeting.
Hope this helps.

 

[mgtfp]

The number you need to calculate the FC-increase by adding bleach, is the Trade-% value, which is volume-% available chlorine (Cl₂). What you have, should be weight-% sodium hypochlorite (NaOCl).

Procedure has been described by Chem Geek in this thread:

Re: Breakdown of bleach over time by storage temperature

Yes, Trade % is % Available Chlorine by volume. The 7.86% Available Chlorine you see on the bleach bottle is % Available Chlorine by weight. The 8.25% is the weight % of sodium hypochlorite. Available Chlorine is in molecular chlorine (Cl₂) units. 8.25 * (70.906 g/mole Cl₂ / 74.442 g/mole NaOCl) = 7.86. The Trade % for this bleach depends on its density (specific gravity) which is around 1.10 g/ml so Trade % = 7.86 * 1.10 = 8.65%. If you were to add one gallon of this bleach to 10,000 gallons of water, it would increase FC by 8.65 ppm.

As for what to use for price per ounce, it doesn't matter so long as you are consistent for all products. I think that the Trade % is the most useful since it relates directly to the volume of product you would add and usually the price of the product is per volume (i.e. gallon) and not per weight. So Trade % is better to use when figuring the price of chlorine per fluid ounce.

Just replacing numbers from Chem Geek's post with your numbers:

The 10% should be the weight-% of sodium hypochlorite. Available Chlorine (in weight-%) is in molecular chlorine (Cl₂) units. 10% * (70.906 g/mole Cl₂ / 74.442 g/mole NaOCl) = 9.52%. The Trade % for this bleach depends on its density (specific gravity) which is around 1.1 g/ml so Trade % = 9.52% * 1.1 = 10.48%. If you were to add one gallon of this bleach to 10,000 gallons of water, it would increase FC by 10.48 ppm.

Now, PoolMath is another story again. As far as I know, it assumes that percentages above aboout 10% are usually liquid chlorine versions, that use trade-%, and the "Bleach Chlorine %" value is taken 1:1 by PoolMath. If it is smaller than ten (maybe 9, not sure where the exact cut-off is), it assumes labelling as %-NaOCl, and adjusts the value. When you enter e.g. 8.25%, PoolMath will use 8.65% for the calculation.

In your case, PoolMath will interpret a value around 10 as Trade-%. If your chlorine really specifies %-NaOCl, then you have to calculate the Trade-% first, and put that value int PoolMath.

But: Since the chlorine decays pretty quickly anyway, your calculations will never be that accurate anyway, because you never know the exact strength of the chlorine in your bottle. So I'd say, that it doesn't really matter in the end, which value you put in...