Use MA to neutralize pH effects of bleach?

[rimshaker]

When you guys add bleach, do you also add some muriatic acid to neutralize the pH effects?

From other sites i've read, 1 gal of sodium hypo is pH neutralized with 5 fl.oz. of MA.

If i'm gonna shock and initially bring the pH down to about 7.2 for higher sanitation strength, adding several gallons of bleach will raise pH back up. Wouldn't that cancel out my initial efforts of lowering to 7.2 in the first place?

Or should I lower pH to around 7.0 to compensate for the bleach before shocking?

 

[JasonLion]

When you first add bleach the PH will go up, then as the FC gets used up the PH will go back down to almost exactly the point where it started. If you were to neutralize the initial PH change with acid the final PH would end up very low.

The normal advice is to have PH between 7.2 and 7.4 before shocking for maximum effectiveness. This takes into account the temporary PH change that will be caused by adding a large amount of bleach. The only time you might need to do something differently was if you have a very high CYA level, over 100, and are therefor adding really large amounts of bleach.

 

[chem geek]

As an example, if you were to add 3 gallons of 6% bleach to a 15,000 gallon pool that has 30 ppm CYA, then the following shows the change in various water chemistry parameters after adding the bleach and then after it is used up:

FC ..... 0 ... 12.3 ..... 0
pH ... 7.5 .... 8.1 ... 7.5
TA ... 100 ... 109 ... 100
DC ..... 0 .... 0.24 ..... 0

If you first lower the pH with 24 fluid ounces of Muriatic Acid, you get the following sequence

FC ..... 0 ..... 0 ... 12.3 ..... 0
pH ... 7.5 ... 7.2 ... 7.6 ... 7.2
TA ... 100 ... 94 ... 103 ... 94
DC ..... 0 ..... 0 ... 0.30 ..... 0

The "DC" is disinfecting chlorine (i.e. hypochlorous acid) so you can see that first lowering the pH does result in more disinfecting chlorine, but not by as much as one might think with the traditional chlorine graphs. The reason is that CYA "buffers" chlorine (specifically, hypochlorous acid) so resists changes in its concentration with changes in pH. So normally when shocking a pool at reasonable CYA levels one doesn't worry about the pH jump, but if shocking at high levels for mustard/yellow algae or at high CYA levels, then lowering the pH first makes more sense. Normal shock level is a disinfecting chlorine (DC) of around 0.30

Now if you needed to lower your TA level anyway, then adding acid before shocking does two things at once as you'll end up with lower TA as you could see above (and, if desired, you could then let aeration make the pH rise without any change in TA).

Repeating the same analysis at a higher CYA level of 80 ppm and using 8 gallons of 6% bleach I get the following:

FC ..... 0 ... 32.9 ..... 0
pH ... 7.5 .... 8.8 ... 7.5
TA ... 100 ... 123 ... 100
DC ..... 0 .... 0.21 ..... 0

If you first lower the pH with 30 fluid ounces of Muriatic Acid (it takes more since the CYA buffer is slightly stronger and I still assumed the same 100 for TA), you get the following sequence

FC ..... 0 ..... 0 ... 32.9 ..... 0
pH ... 7.5 ... 7.2 ... 8.5 ... 7.2
TA ... 100 ... 92 ... 115 ... 92
DC ..... 0 ..... 0 ... 0.25 ..... 0

If you add a larger amount of acid, 60 ounces, to get the pH down to 7.0, then you have the following

FC ..... 0 ..... 0 ... 32.9 ..... 0
pH ... 7.5 ... 7.0 ... 7.8 ... 7.0
TA ... 100 ... 84 ... 108 ... 84
DC ..... 0 ..... 0 ... 0.30 ..... 0

So significantly lowering the pH before shocking makes sense when you have to add a lot of chlorine to get to a high FC, but it's not as necessary at lower normal shock levels.

Richard

 

[rimshaker]

Richard, much thanks! That's exactly what I needed to know.

I do have a high CYA somewhere in the 90-100 range so I lowered pH to about 7.1 over several days, and then dumped 7.5 gal of 10% bleach to get to shock levels.

Wish I could get down to 50 CYA, but don't have any easy options.