In-Ground spa routinely has pH well above 8 upon weekly visits - what might be happening?

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QPSUtah

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In The Industry
Sep 12, 2024
110
Salt Lake City
Here are my measurements from my last visit...

8+
190 TA
350 CH
0 CYA
3200 Salt/TDS

Inground spa size is about 1400 gallons.

I adjusted pH down to 7.2 and the alkalinity came down to 170.

Customer has naturally high alkalinity in the water. Upon filling the spa a couple months ago, the alkalinity was well over 250 as I recall. He rarely uses it and keeps the pump running 24/7 right now in the winter with temperature set to 71 unless he's using it, then heats up to 100+. Pool also remains covered most of the time with vinyl cover (not insulated). I just started going there on a regular basis but everytime I show up I have had to add 2+ cups of acid to bring the pH down. The alkalinity has then been slowly dropping with each visit. Is the running pump just increasing the alkalinity through aeration? Even though there isn't really visible air/aeration happening.
 
The bicarbonate is in equilibrium with carbon dioxide.

High TA means high bicarbonate and high carbon dioxide.

As the carbon dioxide offgasses, the pH rises because the bicarbonate converts into carbon dioxide.

HCO3- + H+ --> H2O + CO2

So, you have to keep the TA low to prevent pH rise.

1739820895014.png
 
QPSUtah said:
Is the running pump just increasing the alkalinity through aeration?
Click to expand...
Alkalinity is increased by 1) adding baking soda, soda ash etc. or 2) fill water that has high TA.
Acid lowers pH and TA.
pH will rise naturally. Aeration will raise pH faster.
pH will rise faster when pH is low and when TA is high.

PH TA Relationship - Further Reading

www.troublefreepool.com www.troublefreepool.com
www.troublefreepool.com

Lower Total Alkalinity

A lower Total Alkalinity level can help a pool owner stabalize their pH increase. We'll explain how to lower TA with a little help from muriatic acid.
www.troublefreepool.com www.troublefreepool.com

Lowering Total Alkalinity (TA) Quickly with the Acid Aeration Method

Thought I'd make a post that might help others. We've had quite a few members this year with high TA, and I've been typing the same information over and over. I'm going to post this so I can link it to help others. High TA can cause rapid pH rise. High TA is typically caused by high TA fill...
www.troublefreepool.com www.troublefreepool.com
 
Carbon dioxide in a pool or spa is like carbon dioxide in a carbonated beverage like soda or beer.

The carbon dioxide comes out faster with heat or agitation.

Spas are hot and there is a lot of agitation, which causes the CO2 to be lost faster and the pH to rise faster.

As you can see from the graph, higher pH reduces the CO2 levels.

It is counterintuitive, but keeping the pH as close to 8.3 as possible while keeping the TA down to about 50 or 60 minimizes the CO2 effects.

You have to keep the CSI at about -0.3 to 0.0 to keep the plaster from etching or scaling.

This usually means keeping the calcium lower due to the high pH.
 
PoolStored said:
Alkalinity is increased by 1) adding baking soda, soda ash etc. or 2) fill water that has high TA.
Acid lowers pH and TA.
pH will rise naturally. Aeration will raise pH faster.
pH will rise faster when pH is low and when TA is high.

PH TA Relationship - Further Reading

www.troublefreepool.com www.troublefreepool.com
www.troublefreepool.com

Lower Total Alkalinity

A lower Total Alkalinity level can help a pool owner stabalize their pH increase. We'll explain how to lower TA with a little help from muriatic acid.
www.troublefreepool.com www.troublefreepool.com

Lowering Total Alkalinity (TA) Quickly with the Acid Aeration Method

Thought I'd make a post that might help others. We've had quite a few members this year with high TA, and I've been typing the same information over and over. I'm going to post this so I can link it to help others. High TA can cause rapid pH rise. High TA is typically caused by high TA fill...
www.troublefreepool.com www.troublefreepool.com
Click to expand...
I meant to say is the pump increasing *pH* through aeration? Even though there are no visible bubbles?
 
QPSUtah said:
I meant to say is the pump increasing *pH* through aeration? Even though there are no visible bubbles?
Click to expand...
Any heat or agitation causes the loss of CO2.

Higher temps = more CO2 loss and more pH rise.

More agitation like circulation, jets, air blowers etc. causes CO2 loss.

High TA creates high CO2, so that increases the rate of pH rise.

If the Pool or Spa are uncovered, the carbon dioxide will get out.

Covering reduces the loss of heat, water (evaporation) and carbon dioxide.

If you boil a pot of water on the stove, you will notice the formation of gas bubbles before boiling happens.

These bubbles are the dissolved gasses like nitrogen, oxygen and carbon dioxide coming out of solution due to the heat.

The partial pressure of oxygen in air at sea level is approximately 160 mmHg.

This is calculated by multiplying the atmospheric pressure at sea level (760 mmHg) by the percentage of oxygen in air (approximately 21%).

(1.3 x 10^-3)(0.21)31,999 = 8.735 ppm Oxygen

409.8 ppm = 4.098x10^-4 Atm

3.4x10^-2 x 4.098x10^-4 = 0.0000139332M Carbon Dioxide.

0.0000139332 x 44009 = 0.613 ppm Carbon Dioxide at equilibrium with the atmosphere.

(6.1 x 10^-4)(0.78)(28,013.4) = 13.329 ppm Nitrogen.

1740071736666.png

1740070122519.png

1740070204722.png
1740070594789.png
 
Last edited:
Your Carbon Dioxide is at about 3.74 ppm.

To minimize pH rise, try to keep the CO2 below 1 ppm.

The CO2 is about 3.8 times higher than it should be to prevent pH rise.

190/50 = 3.8.

To minimize the CO2, you need to keep the pH high and the TA low.
________________________________________________

X = Bicarbonate Molarity.

Y = Carbon Dioxide Molarity.

(Total Alkalinity/100086.9) = (0.5)X

X = (1/(10^(6.35- pH) + 1))(X + Y)

Y = (1 - (1/(10^(6.35-pH) + 1)))(X + Y)

Bicarbonate PPM = 61016.8X

Carbon Dioxide PPM = 44009Y
________________________________________________

TA = 190 and pH = 8.0

X = 0.0037967M, Y = 0.0000849975M

Bicarbonate PPM 0.0037967 x 61016.8 = 231.66248456

Carbon Dioxide PPM = 0.0000849975 x 44009 = 3.74 PPM.
________________________________________________

At pH = 8.0 and Total Alkalinity = 50

X≈0.000999132, Y≈0.0000223678

Bicarbonate PPM = 61016.8 X 0.000999132 = 60.9638374176

Carbon Dioxide PPM = 44009 x 0.0000223678 = 0.98 PPM.
________________________________________________

At pH = 7.6 and Total Alkalinity = 50

X = 0.000999132, Y = 0.0000561853

Carbon Dioxide PPM = 0.0000561853 x 44009 = 2.47 PPM.
________________________________________________

(100/100086.9) = (0.5)X, X = (1/(10^(6.35- 8) + 1))(X + Y), Y = (1 - (1/(10^(6.35-8) + 1)))(X + Y) - Wolfram|Alpha

Wolfram|Alpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.
www.wolframalpha.com www.wolframalpha.com

www.omnicalculator.com

PPM to Molarity Calculator

If you need to convert ppm (parts per million) to molarity for solutions, this calculator is for you!
www.omnicalculator.com www.omnicalculator.com
 
Last edited:
The Henry's law constant for carbon dioxide in water at 25°C is approximately 3.1 x 10^-2 M/atm.

This means that for every 1 atmosphere of partial pressure of carbon dioxide, the concentration of dissolved carbon dioxide in water will be 3.1 x 10^-2 moles per liter.


The partial pressure of co2 in atmosphere is about 0.3 millimeters of mercury (mmHg) (3.95×10^-4 atm (atmospheres)).

Calculation: Since CO2 makes up a very small fraction of the atmosphere (around 0.04%), when you multiply this percentage by the total atmospheric pressure (760 mmHg at sea level), you get a partial pressure of CO2 around 0.3 mmHg.

1739988415918.png

S(CO2) = [3.1 x 10^-2 mol/L atm][4.0X10^-4 atm] = 1.2x10^-5 mol/L.

1.2 x 10^-5 mol/L = 0.528 ppm.

So, the Henry's Law Solubility for carbon dioxide is only about 0.528 ppm.

Any amount over this concentration will offgass until the level reaches 0.528 ppm.

As the CO2 is lost, the pH rises and the amount of CO2 drops due to lost CO2 and conversion of CO2 to HCO3.

1739990049235.png
 
Last edited:
The Henry's law constant for carbon dioxide (CO2) at 25°C is approximately 3.3 x 10^-2 mol/(Latm).

When the temperature of a system changes, the Henry constant also changes.

At 77 F.
1739988937242.png
pCO2, pCO2, or PCO2 is the partial pressure of carbon dioxide (CO2).

The pCO2 of Earth's atmosphere has risen from approximately 280 ppm (parts-per-million) to a mean 2019 value of 409.8 ppm.

409.8 ppm = 4.098x10^-4 Atm

3.4x10^-2 x 4.098x10^-4 = 0.0000139332M Carbon Dioxide.

0.0000139332 x 44009 = 0.6131861988 ppm Carbon Dioxide at equilibrium with the atmosphere.

3.74/0.6132 = 6.1 times too much carbon dioxide over the equilibrium amount.
 
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At a pH of 8.3 and a Carbonate Alkalinity of 50 ppm.

X = 0.000999132, Y = 0.0000112104

Bicarbonate PPM = 61016.8X0.000999132 = 60.96 PPM.

Carbon Dioxide PPM = 44009 x 0.0000112104 = 0.4933584936 PPM.

This would result in zero pH rise.

At a TDS of 1,000 ppm and a temp of 104, the Calcium Hardness should be 80 to prevent scaling.

You can manage the pH at 7.9 to 8.3 if you have a pH meter.

Keep the CSI between about -0.3 and 0.0.

1739991041131.png1739991207618.png
 
TA = 100 ppm.

1740000725226.png

pH......PPM CO2

7.0........19.69
7.2........12.42
7.4..........7.84
7.6.........4.945
7.8.........3.12
8.0.........1.9687646195
8.2.........1.2422
8.4.........0.7837782855

You can get the PPM CO2 by finding the PPM CO2 at the pH and multiplying by the TA/100.

Example: 190 TA and 8.0 pH.

PPM CO2 = 1.9687646195 x 190/100 = 3.74 ppm.

Example: 60 TA and 8.0 pH.

PPM CO2 = 1.9687646195 x 60/100 = 1.1812587717 ppm.

(60/100086.9) = (0.5)X, X = (1/(10^(6.35- 7.6) + 1))(X + Y), Y = (1 - (1/(10^(6.35-7.6) + 1)))(X + Y) - Wolfram|Alpha

Wolfram|Alpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.
www.wolframalpha.com www.wolframalpha.com

PPM CO2 = 44009Y
 
Last edited:
TA = 60.

1740001883009.png


pH......PPM CO2

7.5........3.735
7.6........2.967
7.7........2.357
7.8........1.87
7.9........1.487
8.0........1.18
8.1........0.9383
8.2........0.745
8.3........0.592
 
At a TA of 190, the pH will stop rising somewhere between about 8.3 and 8.8.

TA = 190. pH = 8.7.

ppm CO2 = 0.7463574328
__________________________________

TA = 190. pH = 8.8.

ppm CO2 = 0.59285 ppm.
_______________________________________________________

(190/100086.9) = (0.5)X, X = (1/(10^(6.35- 8.7) + 1))(X + Y), Y = (1 - (1/(10^(6.35-8.7) + 1)))(X + Y) - Wolfram|Alpha

Wolfram|Alpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.
www.wolframalpha.com www.wolframalpha.com
 
Note: You need to use Carbonate alkalinity.


Results:
pH: 8.000000000000
Carbonate Alkalinity: 50.000000000000
Bicarbonate PPM: 60.963822438301
Carbon Dioxide PPM: 0.984383356736
Bicarbonate Molarity: 0.000999131755
Carbon Dioxide Molarity: 0.000022367774

X = 0.000999132, Y = 0.0000223678

(50/100086.9) = (0.5)X, X = (1/(10^(6.35- 8) + 1))(X + Y), Y = (1 - (1/(10^(6.35-8) + 1)))(X + Y) - Wolfram|Alpha

Wolfram|Alpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.
www.wolframalpha.com www.wolframalpha.com

Carbonate Alkalinity (Adjusted TA) = TA – (CYA X CYA C.F) – (Borate x Borate CF)

Borate C.F (correction factor) based on pH.

pH.......CF

7.2.....0.051

7.4.......0.0786

7.6......0.1248

7.8......0.1989

Cyanuric Acid correction factor based on pH.

pH........CF

6.0..........0.047

6.1..........0.05

6.2..........0.065

6.3..........0.08

6.4..........0.10

6.5..........0.11

6.6..........0.13

6.7..........0.15

6.8..........0.17

6.9..........0.20

7.0.......0.22

7.1.......0.24

7.2.......0.26

7.3.......0.28

7.4.......0.30

7.5.......0.32

7.6.......0.33

7.7.......0.34

7.8.......0.35

7.9.......0.36

For example, if the pH = 7.6, TA = 90, Borate = 50 and CYA = 70, the adjusted (Carbonate) alkalinity is 90 - (70 x 0.33) – (50 x 0.1248) = 60.66.

X = Bicarbonate Molarity.

Y = Carbonate Molarity.

Bicarbonate PPM = 61016.8X

Carbon Dioxide PPM = 44009Y

(Carbonate Alkalinity/100086.9) = (0.5)X

X = (1/(10^(6.35- pH) + 1))(X + Y)

Y = (1 - (1/(10^(6.35-pH) + 1)))(X + Y)
 
Last edited:
Here is a reference for a Carbon Dioxide Level of 1.00 ppm.

pHCarbonate Alkalinity (ppm)
7.05
7.16
7.28
7.310
7.413
7.516
7.620
7.725
7.832
7.940
8.051
8.164
8.281
8.3101
8.4128
1740065475511.png


1740064918310.png

1740065044561.png

pHCarbonate Alkalinity (ppm)
7.28
7.310
7.413
7.516
7.620
7.725
7.832
7.940
8.051
8.164
8.281
8.3101
8.4128
8.5161
8.6202
8.7255
8.8320

pHCarbonate Alkalinity
7.05.08
7.16.39
7.28.05
7.310.13
7.412.76
7.516.06
7.620.22
7.725.46
7.832.05
7.940.35
8.050.79
8.163.94
8.280.50
8.3101.35
8.4127.59
 

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