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

[JamesW]

If you want to avoid pH rise in a hot tub, you probably need to be in the range of pH = 7.8 to 8.0 with a carbonate alkalinity of 30 to 50 ppm.

30 ppm Carbonate Alkalinity gives you very little room for error because any acid added can crash the pH fast.

For example, in a 700 gallon Spa/Hot Tub, 1 oz acid will lower the TA by 5.6 ppm.

5.35 oz acid will lower the TA by 30 ppm in a 700 gallon Spa/Hot Tub.

At 30 ppm carbonate alkalinity, you should not use trichlor tabs as they can and will crash the pH and alkalinity quickly and destroy the heater.

 

[JamesW]

For a Hot Tub at 104 degrees F, the Henry's Law Constant will be about 0.0232 Mol/L*Atm, which is lower than the reference (0.034 Mol/L*Atm) at 77 degree F.

So, the water can only hold about 68% of the CO2 at 104 F compared to at 77 degrees F.

2.32x10^-2 x 4.098x10^-4 x 44009 = 0.4184 ppm Carbon Dioxide at equilibrium with the atmosphere at 104 F.

Your Carbon Dioxide is at about 3.74 ppm.

3.74/0.4184 = 8.94 times more CO2 than equilibrium at 104 F.

Getting down to 1.00 ppm CO2 is probably the lowest realistic level of CO2 that you could do unless you were super diligent about testing and balancing daily.





Here is the table showing Henry’s Law constant for CO₂ in water at temperatures from 50°F to 110°F:

Temperature (°F)	Temperature (°C)	Henry's Constant (mol/L·atm)
50.0	10.0	0.0521
53.2	11.8	0.0494
56.3	13.5	0.0469
59.5	15.3	0.0446
62.6	17.0	0.0424
65.8	18.8	0.0404
68.9	20.5	0.0384
72.1	22.3	0.0366
75.3	24.0	0.0349
78.4	25.8	0.0333
81.6	27.5	0.0318
84.7	29.3	0.0303
87.9	31.1	0.0290
91.1	32.8	0.0277
94.2	34.6	0.0265
97.4	36.3	0.0253
100.5	38.1	0.0242
103.7	39.8	0.0232
106.8	41.6	0.0223
110.0	43.3	0.0213

Here is the updated table showing Henry’s Law constant for CO₂ in water at 5°F increments from 50°F to 110°F:

Temperature (°F)	Temperature (°C)	Henry's Constant (mol/L·atm)
50	10.0	0.0521
55	12.8	0.0480
60	15.6	0.0442
65	18.3	0.0409
70	21.1	0.0378
75	23.9	0.0350
80	26.7	0.0325
85	29.4	0.0302
90	32.2	0.0281
95	35.0	0.0262
100	37.8	0.0244
105	40.6	0.0228
110	43.3	0.0213

 

[JamesW]

As the CO2 leaves the water, some carbonate combines with a hydrogen ion to become bicarbonate, bicarbonate combines with hydrogen to become carbonic acid and carbonic acid becomes CO2 and water.

This continues as long as the CO2 concentration is above the equilibrium amount, which is about 0.42 ppm for Spas (104 F) and 0.6 ppm for unheated pools (77 F).

As the hydrogen ions are removed from the water as free ions, the pH rises.

pH is the negative log of the hydrogen ion molarity.

The formula to calculate pH is: pH = -log[H+] where [H+] represents the molar concentration of hydrogen ions in a solution.

[H+] = 10^-pH.

pH = 7.0 means that the hydrogen ion concentration is 10^-7M.

pH = 8.0 means that the hydrogen ion concentration is 10^-8M.

So, higher pH means fewer hydrogen ions.

As the pH rises, the CO2 level falls and the pH rise slows down.

CO2 + H2O <--> H2CO3 <--> H + HCO3 <--> 2H + CO3

 

[QPSUtah]

As the CO2 leaves the water, some carbonate combines with a hydrogen ion to become bicarbonate, bicarbonate combines with hydrogen to become carbonic acid and carbonic acid becomes CO2 and water.

This continues as long as the CO2 concentration is above the equilibrium amount, which is about 0.42 ppm for Spas (104 F) and 0.6 ppm for unheated pools (77 F).

As the hydrogen ions are removed from the water as free ions, the pH rises.

pH is the negative log of the hydrogen ion molarity.

The formula to calculate pH is: pH = -log[H+] where [H+] represents the molar concentration of hydrogen ions in a solution.

[H+] = 10^-pH.

pH = 7.0 means that the hydrogen ion concentration is 10^-7.

pH = 8.0 means that the hydrogen ion concentration is 10^-8.

So, higher pH means fewer hydrogen ions.

As the pH rises, the CO2 level falls and the pH rise slows down.

CO2 + H2O <--> H2CO3 <--> H + HCO3 <--> 2H + CO3


View attachment 629024

Thank you for the detailed response! So the covering of the spa and the heat (though he keeps it at 70 degrees when not being used, and he doesn't use it often), along with the high alkalinity is driving the pH rise between visits even without much visible aeration. Sounds like the first order of business will be to reduce TA either through repeat daily visits and dosing with acid, or take up an "aerator" of sorts to be able repeat dose with acid to bring the TA down to below 100. Then perhaps see if the pH is a little more stable. But yeah, with the naturally high calcium and TDS level, even with an alkalinity of 65, I'm seeing a pH ceiling of 8.22 and a need to keep the pH between 7.7 and 7.9 for proper LSI (for water temperature between 70 and 100).

 

[JamesW]

You can use CO2 as a pH reducing strategy by getting an automated dosing system that monitors the pH and injects the CO2 automatically.

There are many different CO2 systems available.

For a Spa, the amount of CO2 should be reasonable.

You still need to reduce the TA, but a CO2 system might be worth investigating.

 

[JamesW]

I'm seeing a pH ceiling of 8.22 and a need to keep the pH between 7.7 and 7.9 for proper LSI (for water temperature between 70 and 100).

If you reduce the calcium hardness and TA, you can keep the pH in the 8.0 to 8.3 range.

In my opinion, a pH at up to 8.3 is fine as long as you can accurately measure that level, like with a calibrated meter.

For a Public/Commercial Pool, that would not be allowed by code.

As a business, you could potentially have issues keeping the pH higher than public pool code for residential pools.

 

[JamesW]

I'm seeing a pH ceiling of 8.22

pH: 8.22
Carbonate Alkalinity: 60
Carbon Dioxide PPM: 0.712

7.7 and 7.9 for proper LSI (for water temperature between 70 and 100).

pH: 7.7
Carbonate Alkalinity: 60
Carbon Dioxide PPM: 2.357
_________________________________________
pH: 7.9
Carbonate Alkalinity: 60
Carbon Dioxide PPM: 1.487
__________________________________________
pH: 7.9
Carbonate Alkalinity: 50
Carbon Dioxide PPM: 1.24

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[JamesW]

TABLE 3 DISINFECTANT LEVELS AND CHEMICAL PARAMETERS UNDER NORMAL OPERATING CONDITIONS
Stabilized Chlorine measured in milligrams per liter see note 2	POOLS	SPA POOLS	SPECIAL PURPOSE POOLS
pH 7.2 to 7.6	2.0, see note 1	3.0, see note 1	2.0, see note 1
pH 7.7 to 8.0	3.0, see note 1	5.0, see note 1	3.0, see note 1
Non-Stabilized Chlorine measured in milligrams per liter see note 2
pH 7.2 to 7.6	1.0, see note 1	2.0, see note 1	2.0, see note 1
pH 7.7 to 8.0	2.0, see note 1	3.0, see note 1	3.0, see note 1
Bromine measured in milligrams per liter	4.0, see note 1	4.0, see note 1	4.0, see note 1
Iodine measured in milligrams per liter	1.0, see note 1	1.0, see note 1	1.0, see note 1
pH	7.2 to 7.8	7.2 to 7.8	7.2 to 7.8
Cyanuric acid measured in milligrams per liter	10 to 100	10 to 100	10 to 100
Minimum water temperature measured in degrees Fahrenheit see Subsections R392-302-25(4)(b) and R392-302-25(5)	78	78	78
Maximum water temperature measured in degrees Fahrenheit see Subsections R392-302-25(4)(c) and R392-302-25(5)	104	104	104
Calcium Hardness measured in milligrams per liter as calcium carbonate	200, see note 1	200, see note 1	200, see note 1
Total Alkalinity Measured in milligrams per liter
Plaster pools	100 to 125	80 to 150	100 to 125
Painted or Fiberglass pools	125 to 150	80 to 150	125 to 150
Saturation Index see Table 2	-0.3 to +0.3	-0.3 to +0.3	-0.3 to +0.3
Chloramines or combined chlorine residual measured in milligrams per liter	0.5, see note 3	0.5, see note 3	0.5, see note 3

Note 1: Minimum value

Note 2: Maximum value of free chlorine is ten milligrams per liter as described in Subsection R392-302-25(1)(c)

Note 3: Maximum value of chloramines or combined chlorine residual as described in Subsection R392-302-25(1)(g)

Section R392-302-25 - Disinfection and Quality of Water, Utah Admin. Code 392-302-25 | Casetext Search + Citator

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[JamesW]

Henry's Law Constants for CO2 in Water​

Temperature (°F)	Temperature (°C)	kH (mol/L⋅atm)
32	0.0	7.037e-2
37	2.8	6.448e-2
42	5.6	5.919e-2
47	8.3	5.443e-2
52	11.1	5.013e-2
57	13.9	4.625e-2
62	16.7	4.273e-2
67	19.4	3.954e-2
72	22.2	3.664e-2
77	25.0	3.400e-2
82	27.8	3.160e-2
87	30.6	2.940e-2
92	33.3	2.739e-2
97	36.1	2.555e-2
102	38.9	2.387e-2
107	41.7	2.232e-2
112	44.4	2.090e-2
117	47.2	1.959e-2
122	50.0	1.838e-2
127	52.8	1.727e-2
132	55.6	1.624e-2
137	58.3	1.529e-2
142	61.1	1.441e-2
147	63.9	1.359e-2
152	66.7	1.283e-2
157	69.4	1.213e-2
162	72.2	1.147e-2
167	75.0	1.086e-2
172	77.8	1.029e-2
177	80.6	9.758e-3
182	83.3	9.262e-3
187	86.1	8.798e-3
192	88.9	8.364e-3
197	91.7	7.957e-3
202	94.4	7.576e-3
207	97.2	7.218e-3
212	100.0	6.883e-3

Henry's Law Constants for CO2 in Water​

Temperature (°F)	Temperature (°C)	kH (mol/L⋅atm)
30	-1.1	7.579e-2
35	1.7	6.932e-2
40	4.4	6.352e-2
45	7.2	5.831e-2
50	10.0	5.361e-2
55	12.8	4.937e-2
60	15.6	4.554e-2
65	18.3	4.208e-2
70	21.1	3.893e-2
75	23.9	3.607e-2
80	26.7	3.347e-2
85	29.4	3.110e-2
90	32.2	2.893e-2
95	35.0	2.695e-2
100	37.8	2.514e-2
105	40.6	2.348e-2
110	43.3	2.196e-2
115	46.1	2.055e-2
120	48.9	1.926e-2
125	51.7	1.807e-2
130	54.4	1.698e-2
135	57.2	1.596e-2
140	60.0	1.502e-2
145	62.8	1.416e-2
150	65.6	1.335e-2
155	68.3	1.260e-2
160	71.1	1.191e-2
165	73.9	1.126e-2
170	76.7	1.066e-2
175	79.4	1.010e-2
180	82.2	9.575e-3
185	85.0	9.087e-3
190	87.8	8.630e-3
195	90.6	8.203e-3
200	93.3	7.802e-3
205	96.1	7.427e-3
210	98.9	7.076e-3
215	101.7	6.745e-3

 

[JamesW]

CO2 PPM Calculator for equilibrium ppm at a given temperature.

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Temperature vs CO2 PPM​

Temperature (°F)	Temperature (°C)	CO2 PPM
30	-1.1111	1.3669
35	1.6667	1.2503
40	4.4444	1.1456
45	7.2222	1.0516
50	10.0000	0.9669
55	12.7778	0.8905
60	15.5556	0.8214
65	18.3333	0.7588
70	21.1111	0.7021
75	23.8889	0.6505
80	26.6667	0.6036
85	29.4444	0.5608
90	32.2222	0.5218
95	35.0000	0.4861
100	37.7778	0.4534
105	40.5556	0.4235
110	43.3333	0.3960
115	46.1111	0.3707
120	48.8889	0.3474
125	51.6667	0.3260
130	54.4444	0.3062
135	57.2222	0.2879
140	60.0000	0.2710
145	62.7778	0.2553
150	65.5556	0.2408
155	68.3333	0.2273
160	71.1111	0.2147
165	73.8889	0.2031
170	76.6667	0.1922
175	79.4444	0.1821
180	82.2222	0.1727
185	85.0000	0.1639
190	87.7778	0.1556
195	90.5556	0.1479
200	93.3333	0.1407
205	96.1111	0.1340
210	98.8889	0.1276
215	101.6667	0.1216

 

[JamesW]

CO2 PPM Calculator​

Temperature (°F): 93
Temperature (°C): 33.89
Results
CO2 PPM: 0.5000
Henry's constant: 0.0277 mol/L⋅atm