Alk increase affect on ph?

Reef. · Mar 22, 2023

Changing salt, the new salt is 1.5dkh higher than my old salt, if I were to start running my tank at the higher alk would the ph boost from 7dkh to 8.5 be significant or marginal at best? Thanks.

Randy Holmes-Farley · Mar 22, 2023

Reef. said:
Changing salt, the new salt is 1.5dkh higher than my old salt, if I were to start running my tank at the higher alk would the ph boost from 7dkh to 8.5 be significant or marginal at best? Thanks.

About 0.08 pH units if everything else is the same.

arking_mark · Mar 22, 2023

Randy Holmes-Farley said:
About 0.08 pH units if everything else is the same.

@Randy Holmes-Farley How do you make that calc? Based on my equations with indoor CO2 at 600, I get 0.06 change.

Randy Holmes-Farley · Mar 22, 2023

arking_mark said:
@Randy Holmes-Farley How do you make that calc? Based on my equations with indoor CO2 at 600, I get 0.06 change.

You calculation is likely more accurate if you use something like equation 2 from this article:

https://reefs.com/magazine/chemistry-and-the-aquarium-the-relationship-between-alkalinity-and-ph/
I just used the first term from eq 2 and ignored the second one for this estimate.

arking_mark · Mar 23, 2023

Randy Holmes-Farley said:
You calculation is likely more accurate if you use something like equation 2 from this article:

https://reefs.com/magazine/chemistry-and-the-aquarium-the-relationship-between-alkalinity-and-ph/
I just used the first term from eq 2 and ignored the second one for this estimate. The bigger the pH change, the more error in my estimate. Thought of differently, term 1 makes the pH and alk linearly related, but as can be seen from the graph, the line actually curves due to the second term, making my estimate an overestimate.

I'm using the CO2Sys model written in Python.

Randy Holmes-Farley · Mar 23, 2023

arking_mark said:
I'm using the CO2Sys model written in Python.

Then I expect yours is the more accurate number. :)

arking_mark · Mar 23, 2023

Randy Holmes-Farley said:
Then I expect yours is the more accurate number. :)

Ok. I was concerned that my calculations has been off...now I feel better. ;)

Randy Holmes-Farley · Mar 23, 2023

OK, you inspired me to download an xl version of Co2sys.

Using a pCO2 of 400 and 25 deg C, I got a pH difference of 0.0704
Using a pCO2 of 600 and 25 deg C I got a pH difference of 0.0731

So I'm now not sure why you got a slightly different value. Maybe your program uses different constants or makes different assumptions.

arking_mark · Mar 23, 2023

Randy Holmes-Farley said:
OK, you inspired me to download an xl version of Co2sys.

Using a pCO2 of 400 and 25 deg C, I got a pH difference of 0.0704
Using a pCO2 of 600 and 25 deg C I got a pH difference of 0.0731

So I'm now not sure why you got a slightly different value. Maybe your program uses different constants or makes different assumptions.

par1type = 1 # The first parameter supplied is of type "1", which means "alkalinity"; type "4" which means Seawater partial pressure of CO2; 8" which means "Aqueous CO2""; "9", which is "Dry mole fraction of CO2""

alk = 8.5

par1 = alk*1000*0.36 # value of the first parameter

print("Tank measured Alk =", alk, "dKH (", par1, ")")

#par1 = 1500

#print("Tank target CO2 =", par1, "matm ")

par2type = 9 # The second parameter supplied is of type "3", which means "pH"; type "4" which means Seawater partial pressure of CO2; 8" which means "Aqueous CO2""; "9", which is "Dry mole fraction of CO2""

# par2 = 7.73 # value of the second parameter

# print("Tank measured pH =", par2, "NBS")

par2 = 600

print("Air CO2 =", par2, "ppm")

sal = 35 # Salinity of the sample

tempin = 25 # Temperature at input conditions

tempout = 25 # Temperature at output conditions

presin = 0 # Pressure at input conditions

presout = 0 # Pressure at output conditions

sil = 50 # Concentration of silicate in the sample (in umol/kg)

po4 = 2 # Concentration of phosphate in the sample (in umol/kg)

pHscale = 4 # pH scale at which the input pH is reported ("1" means "Total Scale") ("3" means Free Scale)("4" means NBS)

k1k2c = 4 # Choice of H2CO3 and HCO3- dissociation constants K1 and K2 ("4" means "Mehrbach refit")

kso4c = 1 # Choice of HSO4- dissociation constants KSO4 ("1" means "Dickson")

Reef. · Mar 23, 2023

Keep going guys, it’s getting closer to 0.1 with every post, might be worthwhile moving to 8.5 after all :)

arking_mark · Mar 23, 2023

I'm curious if @Randy Holmes-Farley can see why we get slightly different answers...

Randy Holmes-Farley · Mar 23, 2023

I don’t see an obvious difference, but I didn’t look into the actual values of the various constants used.
I’ll post the link I used if you want to compare it.

arking_mark · Mar 23, 2023

Randy Holmes-Farley said:
I don’t see an obvious difference, but I didn’t look into the actual values of the various constants used.
I’ll post the link I used if you want to compare it.

Sure...that would be great.