Calcium Hypochlorite

[PaP dude]

I have a question.(Chem geek) A competitor down of the pool store that i work in sells a 73% calcium hypochlorite shock. Now, on their package (which is ONE POUND) it advertises to "treat" 16,500 gallons of water. I wouldn't really wouldn't be asking questions but the location that I work at sells the exact same thing (only in a different wrapper) but it is advertised to treat 10,000 gallons of water. So i really want to know how much does one pound of 73% calcium hypochlorite treat. Lets make up an ideal situation. The FC level is at a 0 and the CYA is at a 0 and the sun's UV rays don't play a role in this environment. If i had a pool that was 16,000 gallons and i threw in a pack of this calcium hypochlorite, where would my FC be at?

[chem geek]

You can use The Pool Calculator to figure this out. One pound of 73% Cal-Hypo in 10,000 gallons would raise the FC by 8.7 ppm (and the CH by 6.1 ppm). In 16,500 gallons, it would raise the FC by 5.3 ppm (and the CH by 3.7 ppm).

Remember the following simple rules with regard to chlorine chemicals (these rules are independent of the concentration):

For every 10 ppm FC added by Trichlor, it also adds 6 ppm to CYA.

For every 10 ppm FC added by Dichlor, it also adds 9 ppm to CYA.

For every 10 ppm FC added by Cal-Hypo, it also adds 7 ppm to CH (plus 2 ppm "extra" salt).

For every 10 ppm FC added by Sodium Hypochlorite (chlorinating liquid, unscented bleach), or Lithium Hypochlorite, it also adds 8 ppm "extra" salt (sodium chloride) beyond the 8 ppm salt that is added by ALL sources of chlorine when the chlorine gets used up.

While the Free Chlorine (FC) will get broken down (converted to chloride -- salt) from sunlight and oxidizing organics (mostly urea/ammonia) and killing algae, bacteria and viruses, the Cyanuric Acid (CYA) and Calcium Hardness (CH) do not go away and instead build up over time. The only way they get reduced is through dilution of the water, such as splash-out or backwashing followed by refill. Evaporation followed by refill (top-off) will NOT dilute the water. Instead, it will simply add to the water whatever is in the fill water.

From this map you can see the pan evaporation rate in inches per year for your area. A 4.5 foot average depth pool (6 feet deep end, 3 feet shallow end) is 54" which is on the low side of annual evaporation. This means that over a year, an uncovered pool will have over a full volume of its water evaporate and refilled, but this will NOT dilute its contents (evaporation concentrates the contents and refill dilutes them back to what they were before PLUS what is in the refill water). If the fill water has 150 ppm CH, then this will get added to the CH in the pool. TA will also get added, but TA will be more self-limiting as the outgassing of carbon dioxide will have the pH rise and later acid addition will lower both pH and TA resulting in a net lowering of TA.

By the way, I suspect that they meant to say that "Sodium Bisulfate" is used to lower pH, not "Sodium Bicarbonate" which raises Total Alkalinity (TA). Sodium Bisulfate, aka "dry acid", does lower the pH, but it also adds to the sulfate level in the water. Another alternative acid to lower pH is Muriatic Acid (which is usually 31.45% Hydrochloric Acid) as that only increases the chloride level in the water. Muriatic Acid is nasty stuff, however. A lower (half) concentration may fume less.

Hope this helps.

Richard

[PaP dude]

Thanks Richard,

This information is useful. I really like the pool calculator too!

It seems to me that cal-hypo and sodium hypochlorite both have one thing in common...that is they both end up making hypochlorous acid, which does all the work in making your water clean. Knowing this information, I did a mol to mol comparison of the grams of hypochlorite ion available in each product (one pound of 73% calcium hypochlorite vs. 2.5 gallons of 10.5% sodium hypochlorite). It makes sense to me that which ever one can supply the most hypochlorite, would be more effective. (Ignoring the respective cation associated with each along with their effects to the water, Ca2+ and Na+) Click here to see the stoichiometric calculations. **Please note, my work was strictly the calculations...were the additional text may be a bit misleading due to a bias opinion.

All in all, the pool calculator seems to be consistent with the results of these calculations comparatively speaking. I just didn't know the respective "strengths" of cal-hypo or sodium hypochlorite in a given volume of water.

Some one me that total alkalinity (sodium bicarbonate..baking soda) lowers pH! He had the idea somewhat confused i think (the idea that TA governs the pH by establishing a buffer.....with carbonic acid being a weak acid present in the water and sodium bicarbonate being the salt that contains the conjugate base of that weak acid)

[chem geek]

Your calculations are essentially correct except that chlorinating liquid goes by Trade % which is not the same as Weight % of sodium hypochlorite as found in bleach, for example. Trade % is the Volume % of Available Chlorine which is the Weight % of Available Chlorine times the density of the liquid. The Available Chlorine is the equivalent amount of chlorine gas that produces the same amount of hypochlorous acid in the water. Because the "ppm" of all chlorine measurements is referenced to chlorine gas (i.e. its molecular weight of 70.906 g/mole is used for the mole-to-ppm calculation), the net effect of Trade % is that it directly relates to the ppm effect in water. 1 gallon of 10.5% chlorinating liquid results in 10.5 ppm in 10,000 gallons because (10.5/100)*(1/10,000)*(1,000,000) = 10.5 ppm. With 6% bleach, which is a weight % of sodium hypochlorite, this has 5.71% weight % of available chlorine and a Trade % of 6.17%.

By the way, Trichlor and Dichlor also both provide hypochlorite -- actually hypochlorous acid -- via the following net reactions:

Trichlor + 3H₂O --> CYA + 3HOCl

Dichlor + 3H₂O --> CYA + 2HOCl + Na⁺ + OH^-

You can see a cost comparison of various chlorine sources where you can see their relative chlorine content by weight here and also accounts for the net acidity of many of the chlorine sources. You can do more complete calculations beyond The Pool Calculator by using my spreadsheet here.

Also, you should understand that there is a misconception in the industry that the pH of the chlorine source determines the net effect on pH in water. The hypochlorite sources of chlorine have a high pH and they do raise the pH when initially added to the water, but regardless of the source of chlorine when the chlorine gets used up via breakdown from sunlight or oxidation of an organic (including urea/ammonia and algae) this is an acidic process with the net effect being virtually no pH change from hypochlorite sources of chlorine. This is described chemically here. However, in bodies of water with aeration, including most spas and especially those with ozonators and in pools with SWG systems (since they produce hydrogen gas bubbles that aerate the water), the pH will rise when using a hypochlorite source of chlorine. The aeration causes carbon dioxide to outgas which makes the pH rise and in spas where the water is hotter it also causes outgassing of hypochlorous acid itself (so some of it doesn't breakdown to lower the pH).

Finally, the graphs that are typically shown in CPO and similar courses are incomplete. The graph on the left is what is normally shown, but does not apply to pools with CYA. The graph on the right is the correct graph when CYA is present, in this case with 3 ppm FC and 30 ppm CYA.

There are several things to note about the true graph when CYA is present. The first most obvious fact is that hypochlorous acid concentration is significantly reduced, and I’m not talking about 50% reductions or factors of 3 or 5, but rather multiple orders of magnitude – a factor of around 30 in this case. In fact, there are two useful rules of thumb that are reasonably accurate when the CYA level is 5 times or more higher than the FC level (both in their respective ppm) and the pH is near 7.5:

RULE #1: The effective chlorine (hypochlorous acid) concentration is reduced by a factor roughly equal to the CYA concentration in ppm. 30 ppm CYA reduces the amount of disinfecting and oxidizing chlorine by roughly a factor of 30.

RULE #2: The effective chlorine (hypochlorous acid) concentration is roughly proportional to the FC/CYA ratio. So to maintain the same concentration of disinfecting and oxidizing chlorine when doubling the CYA level, one must double the FC level to keep the FC/CYA ratio roughly constant.

Another item to note is how flat the hypochlorous acid curve becomes. CYA is essentially a hypochlorous acid buffer and resists changes in the hypochlorous acid concentration with changes in pH. This is more readily seen looking at a log graph as shown below.

Now you can see more clearly how the hypochlorous acid concentration is more stable with changes in pH when CYA is present, especially to a rise in pH. For comparison, going from a pH of 7.5 to 8.0 when no CYA is present causes the hypochlorous acid concentration to drop by 53% (roughly cut in half) while at 3 ppm FC and 30 ppm CYA going from a pH of 7.5 to 8.0 causes the hypochlorous acid concentration to drop by only 14%. Now, realistically, the drop of 53% isn’t as big a deal as it seems since the absolute hypochlorous acid concentration is much higher. Even if the FC without CYA were only 1 ppm, a change in pH from 7.5 to 8.0 has the hypochlorous acid concentration drop from 0.48 ppm to 0.23 ppm. With 3 ppm FC and 30 ppm CYA, it drops from 0.042 ppm to 0.036 ppm. So even though the drop is much larger in percentage terms without CYA, the absolute concentration is still vastly higher. The large drop in FC with no CYA and changes in pH is more important in water treatment where higher concentrations of chlorine are used and CT values are critical, but in pools and spas this worry about pH is vastly overstated, with or without CYA.

As an interesting side note, the effect of CYA on keeping hypochlorous acid more constant with changes in pH works the other way around as well. When you add chlorine to a pool with CYA, the pH goes up more (or with acidic chlorine, the pH drops less) than it would if you added the chlorine in a pool with no CYA. And when the chlorine gets used up, the pH drops more in a pool with CYA than in a pool without CYA.

Richard

[PaP dude]

"There are several things to note about the true graph when CYA is present. The first most obvious fact is that hypochlorous acid concentration is significantly reduced, and I’m not talking about 50% reductions or factors of 3 or 5, but rather multiple orders of magnitude – a factor of around 30 in this case. In fact, there are two useful rules of thumb that are reasonably accurate when the CYA level is 5 times or more higher than the FC level (both in their respective ppm) and the pH is near 7.5:

RULE #1: The effective chlorine (hypochlorous acid) concentration is reduced by a factor roughly equal to the CYA concentration in ppm. 30 ppm CYA reduces the amount of disinfecting and oxidizing chlorine by roughly a factor of 30.

RULE #2: The effective chlorine (hypochlorous acid) concentration is roughly proportional to the FC/CYA ratio. So to maintain the same concentration of disinfecting and oxidizing chlorine when doubling the CYA level, one must double the FC level to keep the FC/CYA ratio roughly constant."

Chem Geek,

You were helping me out with this information a while back. I am trying to understand basically mathematically how to apply rule 1.

First question, when you say the effective chlorine concentration is reduced by a factor roughly equal to the CYA concentration in ppm, are you referring to the ppm of FC or the hypochlorous acid concentration as a percentage? (If that makes any sense ) For example, if my FC is a 5 and my CYA is a 30, the hypochlorous acid concentration would be reduced by a factor of 5/30? This is how I interpret it. Is this wrong? (I have a feeling it might be)

Secondly, what is a good FC/CYA ratio? I think I read that keeping your FC 10% of your CYA is good.

Lastly, I read that you yourself using 12.5% chlorinating liquid have not had to shock your pool this season. Please tell me how you do this because I am shocking about every 8 days. ( I do live in Florida, and we get tons of rain and my pool is in direct sunlight pretty much all day. Most recent test: CYA = 60 TA = 80, pH = 7.3, CH = 210, FC = 3.5, CC = 0 )

[chem geek]

The rough rule of the FC/CYA ratio is that this represents the EQUIVALENT FC if there were no CYA present. So, 3 ppm with 30 ppm CYA is roughly equivalent to 0.1 ppm FC with no CYA (at a pH near 7.5). The hypochlorous acid concentration is roughly half of this amount, with the other half being hypochlorite ion.

A decent FC/CYA ratio is 10%. A minimum of 7.5% is usually needed in manually dosed pools to keep away algae; SWG pools can usually go as low as 5% before running into trouble with algae.

With your CYA of 60, you shouldn't let the FC get below 4.5 ppm so targeting 6 ppm is safer. Why are you shocking every 8 days? What is it about your pool that makes you think you need to shock it? Do you measure CC after 8 days?

Richard

[PaP dude]

Well, I usually test my water twice a week. For the most part, everything stays pretty much were I like to keep it. After a big rainstorm I usually have to add a little baking soda to adjust TA, then adjust pH accordingly. I then adjust FC if needed. (Maybe I am just misusing my terminology. Any time I add chlorinating bleach (10.5%) to my pool, I consider this shocking, even if it only small amounts.) Usually, I notice that the FC just slowly drops as the week progresses. It is in direct sunlight all day long and gets minimal use. I usually go in after I do my long distance running. (I do get pretty sweaty and I don't usually rinse off before hand. But this is once or twice a week. ) Based on what I have told you about my pool situation, do you think my FC should drop like it is? I will try to keep my FC about 10% of my CYA like you suggested. I also bought k-2006 test kit that you had recommended so I usually test the FC and CC (as well as everything else) every time I test.

[chem geek]

Shocking usually refers to an elevated FC level done infrequently as needed -- it does not refer to normal addition of chlorine to maintain an FC level. What pool stores call "pool shock" vs. regular chlorine is identical chlorine, sometimes even in concentration. They sometimes charge more for it when they call it "liquid shock", but it's the same as unscented bleach except for strength.

In most pools with 30-50 ppm CYA that get direct sunlight and have light bather loads, they will typically lose around one-third to one-half of the FC per day so chlorine pretty much needs to be added every day or certainly every other day. Pools with 60-80 ppm CYA typically lose a bit less, especially if they have deeper water (say, 8 foot deep ends instead of a 6 foot deep end or 4 foot above-ground pools), losing around one-quarter to one-third of the FC per day. Pools with pool covers lose a lot less, sometimes less than 1 ppm FC per day.

It is unusual to have your FC dropping so slowly given the CYA of 60 ppm and being in full sun. Consider yourself very lucky. At 3.5 ppm, it would not be unusual to lose 1 to 1.5 ppm FC in one day even with no use.

By the way, if you don't want to worry about the FC level relative to CYA, you can use a weekly PolyQuat 60 algaecide or you can use a phosphate remover -- either will prevent algae even if the FC gets too low relative to CYA. For killing pathogens (i.e. bacteria, viruses) it takes a very small amount of chlorine so even 1 ppm FC would be sufficient and the only reason not to go below that is to have enough reserve to not run out locally -- but this 1 ppm FC would be the minimum that would occur when you next add chlorine (so with the daily loss you'd need to start out higher).

Richard