Swimming Pool Alkalinity Question

I have an in ground pool with a Pebble Tech finish. It holds 10,500 gallons. I am using chlorine tablets in a floater, about 2 a week.

The Chlorine / Bromine test are dead on (Cl 2 ppm / Br 4 ppm) and the pH is 7.5, so far so good.

The problem is the alkalinity. It is constantly coming in at 140 ppm and the recommended range is 80 to 120 ppm. Everything I read says to add muriatic acid to lower the alkalinity. When I do the PH drops and I see no significant change in alkalinity.

I live in the greater Phoenix, AZ area. The local pool supply company says I should live with it because the water here is hard. The anal part of me just won’t let this go. Is there anything I can do short of installing a soft water system? I know it is contributing to the ugly scaling around the pool.

P.S. Any tips on how to remove scaling from Pebble Tech would be appreciated as well.

What is the total and calcium hardness of your water and what is the phenolphthalein (pH titrated to ~8.2) alkalinity of your water since I am assuming that the value reported for alkalinity is for methyl orange titrated to ~ pH=8.2-8.4 (this not as important as the hardness values)? Also need total dissolved solids and with these values a scaling index (Ryznar or Langlier) can be calculated for the water.


I am using a DPD test kit. So the values you need are not readily available. Do you think if I take a water sample to the pool company they could give me these values? Or better yet is there a test kit you can suggest I should buy?


The pool supply place can most likely give you a hardness value. I doubt that they will be able to do the rest.

Who is doing the alkalinity, pH, Cl and Br testing for you? I assume the Cl and Br values are residual values? Do the tablets you use also contain bromine?

Do you have trouble with any scale buildup in any plumbing units in your house?

Sorry for all the questions but this will help sort out a couple of issues and non-issues.

I am doing the water test myself with a kit I bought from the pool supply store. I have taken water samples in to the store to confirm my results.

The following is is what I got off the chlorine tablet container. I don’t believe it contains any bromine.

Active Ingredients:
Trichloro-s-triazinetrione …99.4%
Other Ingredients: …0.6%
Total …100%
…(Available Chlorine - 90%)

The house is almost two years old and as of yet I haven’t seen any scale. However the pool edges and the rock water fall have scale. At least I believe it is scale. It is white and very hard.

I don’t mind questions if you don’t mind asking :wink:


I just received a water quality report from Arizona American Water.

Hardness (grains/gallon) is 7 and the source is Natural calcium/magnesion content.

Does that help?

It helps but here’s what we have at this time:
[li]Alkalinity = 140 ppm as CaCO3[/li][li]pH = 7.5[/li][li]Total Hardness = 7 grains/gallon = 120 ppm as CaCO3[/li][/ul]
Here unfortunately are more questions whose answers will help me to resolve my initial calculations summarized below:
[li]What is the temperature of the water for which all or most of these measurements were made? [/li][li]Can the pool supply place provide a rough estimate of the total dissolved solids (TDS) in ppm? [/li][li]What are you using to get the pH values, test strips that change color or are you or someone else using a pH meter? [/li][li]Is the pH value of 7.5 you report above taken after adding muriatic acid and if so, what was the pH before adding the acid (very important regarding discussion that follows)?[/li][/ol]
By making assumptions regarding the calcium hardness (as opposed to total hardness) and TDS values and using your values for pH and alkalinity I am getting Saturation Index (Langlier Index) numbers that are actually negative which indicates the water should be naturally corrosive. This would be expected if the pH value you report is after adding acid and would indicate that you should be eliminating soft scale. The other unknown, for which acid addition will not help, is if you have high sulfate in your water, which when combined with calcium will create hard scale. Soft scale can be easily removed using vinegar, hard scale cannot be removed with vinegar. You might try a simple test on a spot of scale and see what happens. If soft scale the surface should start bubbling indicating the release of CO2 gas.

You may still want to respond to my questions first before resorting to the vinegar test but that is up to you.

Now that there’s a pool person in here, I have a slight hijack (well maybe not so slight). Why am I having trouble changing numbers in my pool? When I do the tests, my pH is low, and stabilizer is at 0. I’ve added pH plus, but the pH seems to stay the same. Also I’m using stabilzer and stabilized chlorine and yet it always stays at zero. BTW do you have any tips for dissolving stabilizer. What I’ve been doing is using one of those BIG paint buckets, filling it with very hot water (almost boiling) thowing in a couple of capfulls of stabilizer (maybe a quarter cup at a time) milxing it with a paint mixer on a drill, and even that takes a very very very long time. One more thing, the last time I shocked it, the chlorine shot up to 10 and took about 3 days to come back down to three, does that seem about right?

[li]The water temperature is in the 86 - 90 deg range. The pool is not heated and it is not covered.[/li]
[li]TDS is 125 ppm. The pool supply company has a meter.[/li]
[li]I am using a test kit made by a company called Taylor. It looks similar to this. A cell is filled with water then a solution is added. In the case of pH Phenyl Red is used. I then match it with a color standard. The color standards I can read against are 6.8, 7.2, 7.5, 7.8, and 8.2.[/li]
[li]I always take the pH 24 hours after adding any chemicals. Generally I only need to add about 16 ounces of acid once a month. And I do that when the pH shows a solid 7.8. The last time I added acid was three days ago.[/li][/ol]

I’l hold off on the vinegar. When and if I do use vineger will it have a significant effect on my chemical levels in the water?


[nitpick]There is no such thing as a TDS meter. TDS is an empirical test in which a filtered water sample is evaporated and all remaining solids are weighed. In most situations, a good correlation can be made between a water sample’s conductivity and it’s TDS, such that a meter reading conductivity and temperature may be able to give an approximate value for TDS, but that is not the same as a TDS test.[/nitpick]

Stan Doubt was correct in his statement regarding TDS. To be more specific:
[li]Alkalinity expressed as 140 ppm as CaCO3 which equals ~ 85 ppm as HCO3- or CO3-2[/li][li]Hardness as 120 ppm as CaCO3 which equals ~ as Ca+2 as 48 ppm.[/li][/ul]
The alkalinity and hardness alone account for ~135 ppm as the ions with the understanding that most water contains sodium, chloride and other constituents. Therefore, the statement that TDS is 125 ppm is suspect. Who supplies your water? Is it from wells (in which case you should be able to get water quality data from the well driller or is from municipal water (in which case call up the water utility and ask for their water quality report for the last reporting year, either 2003 or 2004).

Just as a side note regarding the vinegar, I’m talking about about a small area of several square inches and maybe one or two ounces of vinegar and no it will not effect your overall pool pH.

Regarding pool pH, your acid additions seem to be doing a very adequate job regarding potential soft scale (Rynar or Langlier indices) but your experience suggests hard scale. Make sure you ask or try to find out from your water supplier what the sulfate concentration is. Also ask about phosphate levels.

I will be on vacation for a couple of days and not have computer access but I will try to respond to you on late Saturady or Sunday.

You are 100% correct and it is not, in my opinion, a nitpick.

I have Arizona American Waters 2004 annual report. I scanned it and put it here . The file name is 2004-annual.jpg and it is roughly 250kb. If you don’t mind D/Ling it I hope it has the information you need.

By the way I did my weekly test this morning. Chlorine and pH are perfect. Still the alkalinity is at 140ppm. Do you think shooting for 100ppm is asking to much?


After looking at your analysis, I would say that you are asking for too much. Given the original pH of 8.7 and an adjusted pH of 7.5 I would say you are definitely doing the best you can do.

As a follow up to my earlier response:

As I mentioned earlier the scaling index is an empirical determination based on the pH, temperature, total dissolved solids (TDS), alkalinity and calcium hardness of the water.

Alkalinity (in a somewhat simplified definition) is the equilibria that exists in natural waters between bicarbonate-carbonate-hydroxide-Free CO2. It is difficult to radically reduce alkalinity short of significantly lowering the pH (below 6.0) and stripping out the CO2 in a decarbonator. Therefore, changing alkalinity significantly is not going to happen.

Hardness, short of running the water through a softener, is also not going to be easily be reduced.

Temperature and TDS are also not going to be changed very easily, therefore the normal way of dealing with scaling index problems is to change the pH which can be easily be accomplished by the addition of muriatic acid (HCl). The problem you may be facing is that the soft scale may be forming at the elevated pH’s before you add acid and the pH is not lowered enough to redissolve the soft scale. The only way to find out for sure is to try the vinegar test that I discussed earlier.

There are several other possibilities regarding the scale you mention but without any idea of what the phosphate and sulfate levels are it is hard to say at this time.

I hope this helps.


Thank you for all the input. I’ll give the vinegar a try. At least now I kow I’ve done what I can.

Again Thanks

You’re welcome.

I would also suggest calling up the water department directly and speaking with one of the staff (i.e., get past the receptionist) since they have much more water quality data than what is reported on their website and the annual report (the annual report that you posted is a regulatory requirement rather than a summary of data) that they post. If you can get to the right person you may be able to get the following data:
[li]Phenophthalein (or “P”) Alkalinity as CaCO3[/li][li]Methyl Orange (or “M”) Alkalinity as CaCO3[/li][li]Sulfate as ion[/li][li]Total Phosphate[/li][/ul]
Let me know how your test goes and if you get any more data!