The Big Picture In Pool Chemistry
Understanding the relationships to avoid wasted time and money
By Greg Schmidt
Have you ever added a particular chemical to a pool and wondered why it didn’t have the effect you expected? You calculated carefully, weighed the amount with the pump room scale, and spread it carefully around the perimeter, yet the result was not what it should have been. What gives? This article is intended to help increase your understanding of how the various chemicals work together (and against each other), so you don’t waste time and money.
You did a CSI test (Calcium Saturation Index) and found the Total Alkalinity (TA) was too low, only about 70 PPM. Your target is 100 PPM because you also try to keep the Calcium Hardness (CH) at 300 PPM to establish the magical triple that seems to stabilize both values. The pool is 300,000 gallons, so you carefully calculated the baking soda dosage as follows:
18 lbs = x lbs
120,000 gals 300,000 gals
Proportion #1 compares your pool to the million-pound pool.
Solving for x = 45 lbs. It takes 45 lbs to get 10 PPM increase in the pool, and you need to increase 30 PPM to hit the target. Now proportion #2 is then:
45 lbs = x lbs
10 PPM 30 PPM
Again solving for x = 135 lbs. You carefully added 135 lbs (2.7 bags).
Afterward, you retested the CSI and had the same result. TA is 70 PPM! What happened? Well, you just wasted about an hour of time and three bags of sodium bicarbonate (about $70). That’s what happened, but why? If you are using a chemical controller, which, of course, you should, you may have just told the controller to neutralize the bicarb addition. Remember that baking soda is a basic chemical, and so in addition to raising TA, it also raises pH. Raising pH will activate the acid feeder. If you’re on gas chlorine, you will get a double whammy because the raised pH will also lower the ORP (Oxidation Reduction Potential) and activate the chlorine feeder. Gas chlorine creates two acids when it reacts with water: hypochlorous acid (free chlorine) and hydrochloric acid. Now do you see what happened? The controller is just doing its job by trying to meet the set points that you’ve just messed up with the baking soda. The acid and chlorine feed activated by the controller neutralized the baking soda’s pH increase, giving a TA raise of zero. That’s because hydrochloric acid is the most commonly used chemical to lower TA. How do you avoid this? More on that later. First, let’s learn about the relationships that created this phenomenon.
The Relationships At Play
There are several key relationships in pool chemistry that operators and pool managers need to understand in order to make it all work.
First, the relationship between the make-up water and overall chemistry is a critical piece of the puzzle and cannot be ignored. In fact, make-up/fill water should determine what chemicals are logical for use at the pool. For example, my make-up water (fill water) has extremely high TA and pH. I use gas chlorine AND carbonic acid to maintain the target pH of 7.3. On the contrary, pools with acidic make-up water want to avoid gas chlorine because gas makes those two acids. In a pool with acidic make-up water, it makes sense to use bleach or cal-hypo—both high pH chemicals that will help neutralize the acidic fill water and again hit the target of 7.3. A powerful base like sodium hydroxide can counter the acidic make-up water and gas chlorine. Here, I use three acids and no base—and my pH is right at the target of 7.3.
Second, the relationship between pH and chlorine effectiveness (ORP) is essential to understand; the higher the pH, the lower the ORP (oxidizing power). As AFO Instructors, we teach our operators to maintain as low of pH as the code will allow, which is typically 7.2. Why is my target 7.3 then? Just so I have some wiggle room, in case my test kit doesn’t agree with the health inspector’s. The 7.3 set-point is used to provide maximum ORP—terrific oxidation to eliminate the number-one enemy—combined chlorine!
Third, let’s look at the relationship between water clarity and ORP vs. filtration. Pristine water is usually thought of as a function of great filtration. It certainly has a lot to do with it, but truly sparkling water is only obtained through outstanding ORP. I recommend an ORP set-point of 850 mV or higher. At the target pH of 7.3, the free chlorine should test around 3.5-4.5 PPM. Killing the pathogens is generally accomplished at much lower ORP (650 mV for most bugs). In the AFO course, we teach a two-third/one-third ratio of oxidation vs. filtration to attain sparkling, pristine water.
Lastly, the relationship between pH, ORP, and combined chlorine must be understood. I noted previously that the number-one enemy as an operator is combined chlorine (AKA chloramines). The best weapon against the nasty odor and eye-burn associated with this stabilized form of chlorine is high ORP. Low pH, combined with a high ORP set-point, will both prevent the formation of chloramines and promote the elimination of those that have formed. The goal of high ORP is the maintenance of continuous breakpoint chlorination. This can be achieved through the careful management of low pH and effective oxidation. Ideally, there would be a secondary oxidizer in addition to chlorine, to provide most of the ORP, and thus do most of the chloramine eradication. Probably the best choice for secondary oxidation is Medium Pressure UV; since it’s not a chemical at all, it has no effect on the pool’s chemistry. It not only kills the worst bugs (like cryptosporidium) but also nukes chloramines and prevents them from forming at all.
OK, now that we’ve covered some of the relationships, what about that bicarb scenario? Since you know what the big picture is when adding baking soda to the pool, remember to first test the water for chlorine residual, making sure there is an adequate residual to provide good oxidation without need for chlorine feed for one turnover. Now, close the influent and effluent valves to the controller flow cell. The no-flow alarm will come on and prevent any chemical feed. Put 135 pounds of baking soda into the pool (heck, just put three bags in, that’s close enough). Wait one turnover, testing the free chlorine at the halfway point to make sure you still have the required residual. Now reopen the flow cell valves and allow the feed cycles to begin again. The mixing after one turnover will minimize the pH increase and lowered ORP caused by the baking soda, and you’ll get the 30 PPM TA rise. Woohoo!
Greg Schmidt, AFO Instructor/LGIT/WSIT, is the Aquatic Center Manager for Eastern Washington University in Cheney, Wash. Reach him at email@example.com.