Saturation Index?

[LegalEagle45]

I have been reading about the saturation index and its importance to keeping my plaster and metals safe from corrosion. However, there seems to be some disagreement as to just how important this really is. Can anyone explain what the Saturation Index is and if it is something that I need to worry about?

[chem geek]

If you have a plaster pool or a pool with tile that has grout exposed to water, then saturation of the water with calcium carbonate is VERY important to prevent the plaster from dissolving or pitting. The plaster is composed of a combination of calcium silicates and rock materials that include calcium carbonate. The calcium carbonate will dissolve in water that is not saturated with calcium carbonate. This is why you must have a pH, Total Alkalinity (TA) and Calcium Hardness (CH) balance that is saturated. You can use The Pool Calculator to calculate the saturation index which should be near 0 (you have some leeway). I don't know anyone who disagrees with this, though there have been competing indices (such as Hamilton), but the index calculated in The Pool Calculator is consistent with true calcium carbonate saturation and is consistent with the Taylor watergram (except the watergram does not account for higher TDS as found in salt pools).

As for metal corrosion, that is another matter and is far more controversial. Low pH is the most harmful to metals as is very high oxidizer levels such as chlorine with no Cyanuric Acid (CYA) in the water. Higher salt levels are also more corrosive due to higher conductivity and in the case of stainless steel (and probably aluminum) the higher chloride levels are more corrosive.

So generally in vinyl pools one does not saturate the water with calcium carbonate, so some amount of calcium is usually in the water and if foaming is an issue, then around 120-150 ppm CH can be used to prevent such foaming.

[quantumchromodynamics]

The Saturation Index is critical for the proper longevity of your plaster. Anything below saturation significantly reduces the life of your plaster.

As far as metals go, there is far more controversy. The most important issues for protection of metals are pH, alkalinity and chlorine. All three of these have to be kept at ideal levels, or metals will be corroded.

Corrosion is also accelerated by salt in the water such as with a salt-water pool. In my experience, bromine seems to corrode copper at a faster rate than chlorine. This may be due to such factors as cyanuric acid mitigating the effects of chlorine where there is no similar mitigative chemical for bromine.

One benefit of having some calcium carbonate film on a metal is that it will act as a buffer in case the pH falls too low. Keeping your PH at the correct level will obviate this benefit. It may also form a partial barrier to chlorine. Calcium carbonate scale does clog heat exchangers and it forms an insulating layer that reduces the heater's efficiency. Therefore, it is not something you want to get too much of.

It is my belief that a negative saturation index may contribute to metal corrosion and that a neutral to positive Index may provide some moderate protection. I really don't know. I do not know of any credible sources that conclusively state that there is definitely a significant advantage or protection. I would only consider using an increased S.I as an adjunct risk management technique to protect metals if there were no significant adverse effects on anything else.

The most important metal to protect would be a copper heat exchanger. My primary considerations would be pH, alkalinity, chlorine or bromine levels, and mechanical erosion due to excessive water flow and/or turbulence.

The newer heat exchangers utilize more corrosion resistant metals, such as cupro-nickel and titanium. Copper is the softest and most vulnerable metal. Really good stainless steel is very resistant to most pool environments.

316 Stainless Steel contains 16% chromium, 10% nickel and 2% molybdenum. The molybdenum is added to help resist corrosion to chlorides. Stainless steel that contains nickel is not very magnetic at all.

It is also important not to over saturate the water with calcium. Scale can damage equipment like heaters and chloride to chlorine conversion cells and it can cause plaster to become rough.

[chem geek]

It is my belief that a negative saturation index may contribute to metal corrosion and that a neutral to positive Index may provide some moderate protection. I really don't know. I do not know of any credible sources that conclusively state that there is definitely a significant advantage or protection. I would only consider using an increased S.I as an adjunct risk management technique to protect metals if there were no significant adverse effects on anything else.

You might want to take a look at this discussion. This is part of the reason I think the saturation index by itself isn't as useful for predicting or preventing metal corrosion. I agree with you that pH is important and that the appropriate amount of TA for pH buffering is important (especially if Trichlor is used since that can have the pH drop quickly if the TA isn't kept high enough).

Keep in mind that most water in copper pipes in homes isn't saturated with calcium carbonate. In my own tap water, the pH is 7.7, TA is 80 ppm, and CH is 55 ppm. They do add 300-500 ppm phosphates to inhibit corrosion. They also use around 1 ppm monochloramine, CC, for sanitation, though they used to use 1 ppm FC instead (and didn't have any CYA to moderate its strength).

Richard

[quantumchromodynamics]

Here is a quote from the reference you cite:

________________________________________________________________________________

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"Dr. Dillon just gave me a lesson on "never say never" last week. I am now more inclined to think "sometimes" or "it depends" are very important terms that must be used regularly in this craft. Quoting page 70 of "The Corrosion Engineer's Reference Book"..."If the index is a minus quantity, calcium carbonate does not precipitate, and the probability of corrosion (if dissolved oxygen is present) will increase with an increase in the negative value of the index." -Allen.D.Book"

________________________________________________________________________________

_____

I think that the primary issue is why the index is negative. If it is negative because pH and/or alkalinity are too low, then it will be significant. If it is low due to calcium, then there may be little or no effect. I would not consider the index a reliable predictor of corrosivity.

I think that people need to be aware of the early warning signs of metal corrosion and take remediative action quickly to limit the damage, and to determine and rectify the cause.

People should always choose the best available metals, such as high quality stainless steel.

Richard, do you think that adding calcium chloride can actually increase the risk of corrosion?

First, it increases the chloride concentration and second, calcium chloride is a salt, which provides an electrolyte and increases the conductivity of the water.

[chem geek]

Richard, do you think that adding calcium chloride can actually increase the risk of corrosion?

First, it increases the chloride concentration and second, calcium chloride is a salt, which provides an electrolyte and increases the conductivity of the water.

Yes, there is likely some small increase in corrosion rates at higher TDS, specifically dissolved charged ions that increase conductivity. To the extent that intentionally adding calcium chloride increases this, then there is some increase in conductivity and therefore corrosion. However, it's a small increase and furthermore the conductivity will increase over time from continued use of chlorine since chlorine converts to chloride when it gets used up (and typically sodium is added either in chlorine products or in pH Up products). Also, the chloride levels do affect stainless steel corrosion (see this EPA PDF file). However, both conductivity and chloride levels need to get fairly high before they become issues. Saltwater chlorine generator (SWG) pools would be closer to that "edge", but it really depends on the strength of the materials. Even a plain non-SWG pool will corrode zinc, for example, which is why it isn't used as a material in pools. Aluminum and lower quality stainless steel are more susceptible.

In practice I think the pH is the most important factor, the second would be using chlorine with no CYA in the water, especially if combined with saltwater (3000 ppm TDS) levels, third would be higher TDS levels. Very low TA would be a problem, but unless acidic sources of chlorine were in use, such as Trichlor, low TA of even 50 ppm is not a problem with hypochlorite sources of chlorine.

Richard

[RMS]

I have been reading about the saturation index and its importance to keeping my plaster and metals safe from corrosion. However, there seems to be some disagreement as to just how important this really is. Can anyone explain what the Saturation Index is and if it is something that I need to worry about?

An honest answer is that the Saturation Index comes from the Langelier Index which was first used for public water systems. In the 1950's it was introduced to residential pools simply because there wasn't any other index around. Great for your local water company, not so much for pools. Here's where you want to keep your levels:

Chlorine 1.5-3.5ppm

pH 7.6-7.8, not 7.2. You'll use too much acid. pH naturally rises in water through swimmers.

Alkalinity 80-100ppm

Calcium 150-250ppm

I know this may upset some people because they live and die by the Langelier Index and the Saturation Index. Sorry to upset you.

http://www.clean-pool-and-spa.com/swimming...nance-tips.html

[chem geek]

If you look at the derivation of the saturation index at the bottom of my spreadsheet here you will see that there is NOTHING in the derivation specific to public water systems, to pools, or to any other aspect of the environment. It is simply and ONLY a formula based on chemical equilibrium saturation with calcium carbonate. It is not a rate equation, so will not tell you how quickly any dissolving of plaster or formation of scale will take, but it does tell you if it is possible in either direction (because it is based on thermodynamics) -- when the index is near zero there can be no dissolving nor scaling. When the index is negative, then dissolving of calcium carbonate is possible, but may take a long time; when the index is positive, then scaling can occur, but it can take a long time.

So use of the index is very reasonable, but the Langelier index used by the pool industry isn't up to date with the latest equilibrium constants and temperature dependency. My spreadsheet is closer to current info and pretty much matches the Taylor watergram (though the watergram doesn't have TDS as a variable which affects ionic strength).

The only real controversy for the index is with regard to metal corrosion (as opposed to dissolving calcium carbonate in plaster). Some believe that saturating the water with calcium carbonate forms a thin film on metals to inhibit corrosion while others think that isn't particularly important and other factors (especially pH) are more important. See this link, for example.

Richard

[RMS]

If you look at the derivation of the saturation index at the bottom of my spreadsheet here you will see that there is NOTHING in the derivation specific to public water systems, to pools, or to any other aspect of the environment. It is simply and ONLY a formula based on chemical equilibrium saturation with calcium carbonate. It is not a rate equation, so will not tell you how quickly any dissolving of plaster or formation of scale will take, but it does tell you if it is possible in either direction (because it is based on thermodynamics) -- when the index is near zero there can be no dissolving nor scaling. When the index is negative, then dissolving of calcium carbonate is possible, but may take a long time; when the index is positive, then scaling can occur, but it can take a long time.

So use of the index is very reasonable, but the Langelier index used by the pool industry isn't up to date with the latest equilibrium constants and temperature dependency. My spreadsheet is closer to current info and pretty much matches the Taylor watergram (though the watergram doesn't have TDS as a variable which affects ionic strength).

The only real controversy for the index is with regard to metal corrosion (as opposed to dissolving calcium carbonate in plaster). Some believe that saturating the water with calcium carbonate forms a thin film on metals to inhibit corrosion while others think that isn't particularly important and other factors (especially pH) are more important. See this link, for example.

Richard

Thanks for the info chem geek. You have really researched this topic and for that I thank you. This is what I know:

I have an 80,000 gallon pool with the alkalinity of 70 and the pH of 7.6. I want to get the alkalinity to 100. I add about 570oz. of Sodium Bicarb. with the method above and allow it to sit for 2-3 hours. Come back, fire up the pump for 5 hours, (full turn-over) and re-take the alkalinity test. The alkalinity is 100 and the pH is still 7.6 and it will stay there for 3 months. All this with 2000-3000 people per week swimming in the pool. Maybe add 1 gallon of acid every 25 days or so to keep the pH in check.

Doing this, among other things, has saved the YMCA nearly $7000 in chemicals since July 08. They couldn't be happier and the members couldn't be happier because now they now have a well balanced pool that uses 80% less chemicals than in previous years. And it's all documented.

Best regards

http://www.clean-pool-and-spa.com

[chem geek]

I'm not clear on what exactly it is that you are doing differently that is saving 80% on chemicals. A higher TA level will result in a faster pH rise so if one has the TA too high then they will end up using more chemicals to keep the pH down -- i.e. more acid to lower pH and then more baking soda to raise the TA in a never-ending cycle. If the TA is lower, then the rate of pH rise is slower, all assuming a hypochlorite source of chlorine is used. Did you have your TA much higher in the past or tried to have the pH lower whereas now you don't have the TA as high and also have a higher pH target?

Since you hardly add acid that often, you probably hardly add baking soda very often either so it sounds like what is working for you isn't the way you are adding the chemical but rather not having too high a TA level in the first place. Is that right?

Though I've rarely had to raise the TA in my own pool except when I used Trichlor pucks years ago, at that time adding baking soda (Alkalinity Up) didn't result in a pH rise and only raised the TA even though my pump was running. I did, however, pour very slowly over a return flow for thorough mixing. Normally, however, I used pH Up (sodium carbonate) since with Trichlor I needed to raise both pH and TA, but sometimes I just used baking soda to mostly raise just the TA. These days in my 16,000 gallon pool, I only use chlorinating liquid that costs around $15 per month and I add a very small amount of acid every month or two similar to your situation. It's easy to get a stable pH if one doesn't let the TA get too high. TA is mostly bicarbonate so represents how much pools are over-carbonated and just like a beverage, they outgas carbon dioxide and this causes the pH to rise (with no change in TA for technical reasons I won't get into here).

[billp]

I have an 80,000 gallon pool with the alkalinity of 70 and the pH of 7.6. I want to get the alkalinity to 100.

If you want the alkalinity at 100 WHY did you wait until it was 70 to start making adjustments??

I add about 570oz. of Sodium Bicarb. with the method above and allow it to sit for 2-3 hours.

Letting it sit on the bottom makes no sense chemistry-wise.

Come back, fire up the pump for 5 hours, (full turn-over) and re-take the alkalinity test. The alkalinity is 100 and the pH is still 7.6 and it will stay there for 3 months.

What happens at the end of 3 months?

...Doing this, among other things, has saved the YMCA nearly $7000 in chemicals since July 08. They couldn't be happier and the members couldn't be happier because now they now have a well balanced pool that uses 80% less chemicals than in previous years. And it's all documented.

Since 75% of the chemical cost is chlorine and since this doesn't really affect chlorine consumption I think it is the "other things" you don't mention that are saving the $$$.

Alkalinity does not change rapidly under normal conditions so if you have a target of 100 there is NO reason to wait until it is 70 (or 130) to start making adjustments. It is always easier to make small adjustments rather than big ones. And there is no reason to leave chemicals sitting on the bottom of the pool.

It would be interesting if you would include chlorination method, pH control, other chems used, instrumentation if any, and just what chemicals you saved $7,000 on and what "other things" you did.

[billp]

A higher TA level will result in a faster pH rise so if one has the TA too high then they will end up using more chemicals to keep the pH down -- i.e. more acid to lower pH and then more baking soda to raise the TA in a never-ending cycle. If the TA is lower, then the rate of pH rise is slower,

Total Alkalinity (TA) is a measure of the buffering capacity of the water. The higher the buffering capacity (TA), the more resistant it is to pH change. So high TA results in SLOWER pH (rate of) change, lower TA results in FASTER pH change. The amount of chemical needed to change the TA increases as the TA is higher or lower than your target TA, as you said.

[PaulR]

A higher TA level will result in a faster pH rise so if one has the TA too high then they will end up using more chemicals to keep the pH down -- i.e. more acid to lower pH and then more baking soda to raise the TA in a never-ending cycle. If the TA is lower, then the rate of pH rise is slower,

Total Alkalinity (TA) is a measure of the buffering capacity of the water. The higher the buffering capacity (TA), the more resistant it is to pH change. So high TA results in SLOWER pH (rate of) change, lower TA results in FASTER pH change. The amount of chemical needed to change the TA increases as the TA is higher or lower than your target TA, as you said.

High TA will buffer the pH against changes induced by adding acid (for example); with high TA it requires more acid to drop pH than it does when TA is low.

In water left to itself, however, extensive experience shows that with high TA, pH will climb faster than when TA is low. In my own pool with fill water at TA 150, I start out needing to add acid every week or so to keep the pH in the 7.5-7.8 range; with TA under 100, the pH stays in range much longer.

--paulr

[chem geek]

A higher TA level will result in a faster pH rise so if one has the TA too high then they will end up using more chemicals to keep the pH down -- i.e. more acid to lower pH and then more baking soda to raise the TA in a never-ending cycle. If the TA is lower, then the rate of pH rise is slower,

Total Alkalinity (TA) is a measure of the buffering capacity of the water. The higher the buffering capacity (TA), the more resistant it is to pH change. So high TA results in SLOWER pH (rate of) change, lower TA results in FASTER pH change. The amount of chemical needed to change the TA increases as the TA is higher or lower than your target TA, as you said.

You are forgetting that TA mostly measures bicarbonate in the water and that carbonates in the water have TWO effects. One of them is what you describe which is as a pH buffer, but the other one is that they are a SOURCE OF RISING PH themselves. This is because water is intentionally over-carbonated and just like a lovely beverage, the excess carbonation will outgas carbon dioxide. When this happens, the pH rises since the carbon dioxide comes from carbonic acid so essentially an acid is getting removed from the water (technically, the carbonic acid turns into carbon dioxide and water and it is the carbon dioxide that outgasses leaving the water behind).

A higher TA will result in a faster rate of pH rise. If you don't believe me, then go ahead and increase the TA and see what happens to the rate of pH rise (do this in bucket tests if you don't want to affect your pool/spa). Aeration also speeds up the rate of pH rise. A lower pH also speeds it up. The only time you want the TA to be higher is when using acidic chemicals such as Trichlor pucks/tabs in a pool since they are acidic. Even Dichlor is net acidic since though its initial addition is near pH neutral, the consumption/usage of chlorine is acidic so the net effect from Dichlor would be a drop in pH were it not for outgassing of carbon dioxide. Hypochlorite sources of chlorine (chlorinating liquid, bleach, Cal-Hypo, lithium hypochlorite) are pH neutral except for the small amount of "excess lye" in some of these products. Rising pH mostly comes from carbon dioxide outgassing. This chart shows how far out-of-equilibrium pool/spa water is in terms of the amount of carbon dioxide in water vs. the much smaller amount in air (at various pH and TA of the water).

If you read the numerous posts on this forum, you will see that this is very, very true. Lowering the TA substantially reduces the rate of pH rise for those using the Dichlor-then-bleach method (it does so for everyone, but Dichlor-only users actually need something to raise the pH a bit so they don't want the TA to be too low). The use of 50 ppm Borates as an additional pH buffer helps because it is not a source of rising pH itself (i.e. it doesn't outgas anything).

I know that this is one of the most counter-intuitive things in all of pool/spa water chemistry, but it's true and is one reason the recommendations for TA in pools is 80-100 ppm for hypochlorite sources of chlorine while for Trichlor its 100-120 ppm. It's not just for more pH buffering, but to help compensate for the acidity of Trichlor with the net result that less base (pH Up) needs to be used. In theory, one could have a high enough TA and aeration when using Trichlor such that the pH is roughly stable and the net effect would be a lowering of TA over time.

[billp]

I was addressing the strictly chemistry side of the statement. You threw in a lot of other variables and were looking at it from another side..

The important thing is we never heard back from the guy, he never answered the questions, esp how can you save $7,000 by throwing in bicarb?

[chem geek]

I was addressing the strictly chemistry side of the statement. You threw in a lot of other variables and were looking at it from another side..

The important thing is we never heard back from the guy, he never answered the questions, esp how can you save $7,000 by throwing in bicarb?

OK, sorry I misunderstood. I now understand that when you said higher TA results in a lower rate of pH change you were referring to chemical additions, not what happens to the water exposed to air (i.e. when outgassing can occur).

[RMS]

I was addressing the strictly chemistry side of the statement. You threw in a lot of other variables and were looking at it from another side..

The important thing is we never heard back from the guy, he never answered the questions, esp how can you save $7,000 by throwing in bicarb?

Well, here I am and I will answer your question on how I helped the Y save so much money.

Before me:

The guards (mostly 16-17 yr old kids) were using 35 gallons of acid per month at $35.99 per case of 4 gallons. $323 per month times 12 months is $3886 per year. Calcium per year $479, Bicarb $180, Salt $957 and Phosfree $1583 (at $32.99 per bottle/4 per month)

Total=$7085 per year

Acid to drop the pH to 7.0, then 25lbs to bring it up to 7.4. After 2 days the pH is 7.6, then 2 more gallons of acid, now it's below 7.0, so 50lbs. of bicarb, etc...throughout the entire month. There were days when they would put in 2 gallons of acid at 8:30pm, then add 50lbs. of bicarb at 9pm.

Salt (50lb. bags) and sometimes the salinity in the water was 3700 - 3900. Burned out a Chloromatic cell ($4500) and went through 3 of them in 2 years.

After me:

Acid $73 (this includes both the pool and hot tub)

Calcium $25.99 (1 bag per year)

Bicarb $60 (pool and hot tub)

Salt $545

Phosfree $0 (don't need it)

Total=$703 per year

Difference=$6342

This past June our chemical order was $65 for the entire month.

Not to mention we haven't had to purchase another Cloromatic cell because it is properly cleaned each month.

So I didn't save the Y money by "throwing in bicarb." I did it using common sense, proper training, and an understanding of what the pool needed, how it needed it, and when it needed it. I kept it simple.

The members are happy, the pool is happy, and so is the management. And it's all documented. Nuff said.

http://www.clean-pool-and-spa.com

[chem geek]

Before you, they were trying to keep the TA too high and adjusting too wildly so were in a classic swing between using acid which lowered pH and TA and then used bicarb to raise the TA, but because the TA level was too high to begin with they ended up raising the pH as well since the outgassing of carbon dioxide is faster at higher TA and lower pH (see this chart as a rough guide) and with more aeration.

Now you don't try and lower your pH so much and you don't use as much bicarb and you have a lower TA level. If you targeted an even lower TA level, you'd probably be able to eliminate the bicarb completely. That's the case in my own pool where I only add 12.5% chlorinating liquid and a small amount of acid. Of course, at your Y you've a lot more splashing and aeration so the pH rise will be faster, but the lower the TA level and the higher your target pH then this effect is minimized. If you lower your target TA level, then you'd need to raise the CH or pH target accordingly to keep the saturation index near zero. Basically, the rule is that if you are using a hypochlorite source of chlorine and the pH is tending to rise (and it's not due to plaster curing or other source of pH rise), then lowering the TA level reduces this rate of rise even though this is counter-intuitive.

What I don't understand is what you did to reduce the amount of salt needed and especially the amount of calcium. These normally only get lowered via dilution of water so how did you reduce that? For the calcium, perhaps you are using at least some Cal-Hypo whereas before they used sodium hypochlorite, is that right?

Richard