Muratic Acid problem with the PoolEquations.xls spreadsheet

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I

itabb

0
Jun 22, 2007
71
Atlanta, GA
Measured pH 8.3
Total Alkalinity (ppm CaCO3) 159
Free Chlorine (ppm Cl2) 12.0
Cyanuric Acid (ppm CYA) 60
Calcium Hardness (ppm CaCO3) 380
Total Dissolved Solids (ppm) 3,439
Total Sulfate (ppm SO42-) 0
Total Borate (ppm Boron) 50.3
Total Ammonia (ppm Nitrogen) 0.0
U.S. Gallons 15,000
Temperature (oF) 89


From here, I want to adjust my PH to 6.8. So I set my goal to 6.8. It says I need 304 oz of Muratic Acid. So I put 304 oz of acid in the apply next apply column and hit calculate. PH is 6.5!!! What is going on here? Also, TA drops to 81, which is perfect, by the way.

I also tried just changing my PH to 6.8, then hit calc Acid/Base/TA and it said 247 oz Acid, and it entered it and calcuated for me and seems right, I am going with 2 gallons of acid for now.
 
I have found that you need to press "Clear All Inputs" sometimes to get things to work correctly. I get 309 oz when I try what you said. Hopefully Richard can explain what is happening.

Keep in mind that you don't want to do a very large PH shift like that all at once, even when using Richard's spreadsheet. I would add a gallon of acid, wait, retest, recalculate, repeat. There are too many potential inacuracies in the calculation. Even if Richards spreadsheet is perfect, you can never know your current numbers very accurately. A couple of measurement errors that all happen to be off in the same direction could throw the calculation way off.
 
In this case, it's not a situation of not clearing all inputs. After entering in the number you listed into the "Initial" column and then pressing "Reset Goal = Initial" to copy them into the "Goal" column (though they can be entered manually if desired), you will notice that there are numbers in the blue area for "Acid to add to decrease pH", BUT there are also numbers in the blue area for "Buffer to add to increase alkalinity". If you had entered BOTH the acid and buffer (baking soda) numbers into the orange column and then clicked on "Calculate pH/TA" you would get no change in the Goal numbers except for an increase in TDS (which is expected).

The numbers in the blue column assume you want ALL the values in the Goal column -- BOTH pH AND TA. If instead of manually entering the numbers into the orange area you clicked on the "Calculate Acid/Base/Buffer" then these would essentially be copied over for you, though there are some subtle improvements in the numbers based on the change in ionic strength due to the change in TDS (i.e. I iterate to lock on on the accurate number).

What you were doing, and wanting as a result, was wanting to know how much acid to add to achieve a pH REGARDLESS of the TA. You do that by clicking on the "Calculate Acid/Base/TA" button. However, the numbers in the blue area do not represent the answer for that question -- they are for the "Calculate Acid/Base/Buffer" question. So normally, you ignore what is in the blue column, at least for acid/base calculations where you don't care about TA, and just click on the "Calculate Acid/Base/TA" button to get an answer. Technically, the calculation for finding the amount of acid or base independent of TA is much more complicated and requires iterations so that's is one reason why I don't show that automatically (the other is that the blue column is a WYSIWYG for going from Initial to Goal).

Also, be aware that the blue column may not be quite right for Borates in all cases since they are used for multiple purposes (to increase borates and to increase pH) and I have no way of knowing the purpose so I put any Initial/Goal Borates change into the lower Borates entry section and put any pH base effect into the Base section, but doing both at the same time may not be correct. If you press one of the Calculate buttons, you'll always get a correct result since I'm able to sort things out through those more complex iterative formulas.

I hope that made sense. Also, JasonLion is absolutely correct that though the spreadsheet may be accurate, the inputs most certainly are not. The error in the pH, in particular, can change numbers dramatically. This is especially true for acid/base amounts when the Initial or Goal pH is low -- below 7.2 -- since the carbonate buffer really resists changes at that point so an error of even 0.1 in pH ends up being a lot of acid or base. Of course, that cuts both ways. If you add too much, you won't be moving the pH that much more.

Richard
 
FC 34 (68 drops, 10ml)
PH 7.4

Yesterday, I did as I described above. I added enough 20 mule team to go from 30 to 50ppm Borates (4 boxes and 20 oz). I also lowered PH to 6.8 using 2 gallons of Muratic acid. Not all at once, of course, but over a couple of hours and without testing in between. I also added 250 oz of bleach to raise the chlorine from 12 to 20ppm, which seems like the right amount of bleach. Then I ran the waterfalls all night. As you can see, PH is perfect. Right were I'd expect it. However, chlorine is not. I ran the pump all night, and turned the aquarite off. I had some trouble testing the chlorine. The DPD test says solid 25. The first titrate test bleached out. I only used one scoop. So I retested, using 2 scoops, and it worked, if you could say that FC 34 worked.

Any idea what is going on here? Am I testing totally wrong or something?
 
There are too many things happening at once to be sure of what is going on. Most importantly, the PH test is not reliable with FC above 15, and can be a little iffy at FC 12. Also, there is no way of knowing how far the waterfall moved the PH, so no telling what the PH was after the acid was all added.

I agree that the chlorine rise is surprising. Unless the SWG was actually running the entire time on boost mode I don't see how you could have been that far off on chlorine, actually even with the SWG, FC 34 is still too much. I wonder if the chlorine test is unreliable at extreme PH levels.

I suggest you get a glass cup and leave a cup of pool water in direct sunlight all day. That should burn off quite a bit of the chlorine so there will be some hope of getting below 15 by this evening and doing the PH test on that. (Chlorine levels appear to fall much more rapidly in shallow containers.)
 
I don't know why 250 ounces of bleach would have the FC go from 12 to 34 (instead of to 20) except that any of the "drop" counting tests are affected by static electricity which makes the subsequent drops smaller (hence you need more drops and think the level is higher than it really is). See this link for more information about this and how to overcome it (by wiping the dropper periodically).

As JasonLion says, the pH test is not reliable at high chlorine levels. According to the Taylor "Pool & Spa Water Chemistry" manual:

----------------------

FALSE READINGS: high levels of chlorine (usually > 10 ppm) will quickly and completely convert phenol red into another pH indicator (chlorphenol red). This new indicator is a dark purple when the water's pH is above 6.6. Unfortunately, some pool operators mistake the purple color for dark red and think the pool water is very alkaline and wrongly add acid to the pool.

When a sanitizer level is not extreme, only some of the phenol red may convert to chlorphenol red. However, purple + orange (for example, pH 7.4) = red. This error is more subtle as no purple color is observed and the operator does not suspect that a false high pH reading has been produced. Some operators neutralize the sanitizer first by adding a drop of chlorine neutralizer (i.e. sodium thiosulfate). However, thiosulfate solutions have a high pH and, if heavily used, may cause a false higher sample pH.

-----------------

I've noticed that roughly speaking each drop of thiosulfate seems to raise the pH in the test by about 0.05-0.1 so the effect is pretty strong (unfortunately); otherwise, one could just simply neutralize the chlorine. The Taylor Phenol Red solutions apparently have some amount of thiosulfate in them already and that is accounted for in their color comparators.

Richard
 
Last night, I tested the pool, twice. I'm almost out of titrate.
FC 15.5
PH 7.2

I added 140 oz of bleach for a target goal of 20ppm FC. I had to close the pool cover immediately afterwards due to a sever thunderstorm approaching, and the pool stayed under cover all night. When I get around to testing this morning (hopefully have enough titrate for one more go), we'll see if the same problem is not reproduced.

I had a full day of sun exposure yesterday, so going from 34 to 15 in 8 hours of sun seems correct for 60ppm CYA. It just does not make sense how I got 34ppm FC.

On a side note, my chlorine spa is just a gem. I was checking FC last night, because of the sun exposure.
Titrate test 1: FC 3
Titrate test 2: FC 2.5
DPD Test: FC 2
All tests 0CC
PH 7.6

I had my wife verify the 2 reading on the DPD test. I'm confident that the spa is above 2ppm FC. Why shouldn't the DPD test look more like 3 instead of 2. I mean, it exactly matched 2. No question. Is the DPD test just always short? Is there some way I could get a baseline where I know the FC is exactly precisely 3 and check the reagents? Even diluting bleach will have some error for the age of the bleach, right? Or am I just completely missing something here?

ps: thanks for the link. I knew about holding it directly upright, but I did not know about wiping off the static eletricity. Thanks.
 
This am:
Pool
FC 21
CC 0
PH 7.2
Borates 50
Temp 89
crisp and clear

Spa:
FC 5.5 (titrate)
FC 3 (DPD)
FC 4 (DPD 50% diluted filtered water)
CC 0
PH 7.4
Borates 30
Temp 85
crisp and clear
I added a bit o' Muric and Clorox last night.
It's annoying how off the DPD color test seems to be.

What I find interesting is the resulting increase in chlorine, assuming no burn off last night because the cover was on, indicates a 12,500 gallon pool instead of a 15000 gallon pool.

Re-measure: The pool is 32 long by 18 wide. It is 3ft in the shallow end and 6 feet at the drains. There is an 8' by 8' cut out of the pool in the shallow end for the spa. I calculated by 2 pools. 1 8x8, 3ft, and 1 24x16 3-6ft deep. Using Bleachclac, 8x8@3f is 1440. The rest is [email protected] is 12,960. That's 14400, so I have been using 15000 to make it easier, though with features, such as steps and a deep end bench, maybe it is closer to 14000. Let me know what you think.

So if I was off on the titrate test and the pool was really 16FC, and really 14000 gallons, then that is 20.82FC I raised to, which would show up as 21. Although the pump has run continuously, I suppose there could still be regional variances. I sample from about 6 inches below the surface in front of the skimmer.

One other question. How long do I keep my pool at 20ppm FC to ensure whatever bad is in it is burned out?
 
Depending on how rounded the corners are it could easily be under 14,000 gallons. Even if it was somehow 12,000 gallons it is still difficult to explain the FC reading of 34. Everything else has been reasonably consistant, so I wouldn't worry about it.
 

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Generally speaking, you are done with shocking when all of the following is true: 1) the water is crystal clear (not cloudy or dull and no visible signs of algae), 2) the Free Chlorine (FC) level holds overnight (< 0.5 ppm FC drop, usually), 3) there is no significant measurable Combined Chlorine (CC) (< 0.5 ppm usually -- often < 0.2 ppm).

When fighting yellow/mustard algae, one may shock for an extra day or two even with the above conditions just to make sure. For green algae, that's not necessary.
 
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