Conductivity when mixing salt mix. C1*v1=c2*v2 doesn’t work

[hart24601]

Hi Randy and chemistry gurus. A friend sent me a question and I am not sure the answer. I wonder if it’s because various elements in salt mix have different conductivity? We were coworkers in molecular biology so have a decent understanding of chemistry but far from a chemist, so might as well ask the reef chemist!

“I have a reef salinity problem and am looking for a math formula to solve it. The other day my reef was at 60 mS/cm conductivity and I did a water change with 48 mS salinity and about 40 gallons water change, should have calculated with the dilution formula but I just went for it, and dropped the tank salinity to 50 mS. A bit too low for long term reef care. The optimum target salinity is 53 mS. I know the dilution formula C1 x V1 = C2 x V2 and it does not seem to work in this application. I’m looking for a math formula to increase the tank salinity from 50 mS to 53 mS. Not sure how to do it mathematically. I can certainly add more salt a bit at a time until 53 mS is reached. For me math can solve the problem ‘how much water to pull from the tank and how salty the new solution should be to obtain the 53 mS target’, that would be fun. Any math ideas?



Tank 110 gal at 50 mS

Target tank 110 gal at 53 mS

10 gal water pull at salinity 50 mS from the tank at what 10 gal water replacement salinity ?mS is needed for the 110 gal tank to reach 53mS?”

Any thoughts Randy?

 

[KrisReef]

 

[Dan_P]

Hi Randy and chemistry gurus. A friend sent me a question and I am not sure the answer. I wonder if it’s because various elements in salt mix have different conductivity? We were coworkers in molecular biology so have a decent understanding of chemistry but far from a chemist, so might as well ask the reef chemist!

“I have a reef salinity problem and am looking for a math formula to solve it. The other day my reef was at 60 mS/cm conductivity and I did a water change with 48 mS salinity and about 40 gallons water change, should have calculated with the dilution formula but I just went for it, and dropped the tank salinity to 50 mS. A bit too low for long term reef care. The optimum target salinity is 53 mS. I know the dilution formula C1 x V1 = C2 x V2 and it does not seem to work in this application. I’m looking for a math formula to increase the tank salinity from 50 mS to 53 mS. Not sure how to do it mathematically. I can certainly add more salt a bit at a time until 53 mS is reached. For me math can solve the problem ‘how much water to pull from the tank and how salty the new solution should be to obtain the 53 mS target’, that would be fun. Any math ideas?



Tank 110 gal at 50 mS

Target tank 110 gal at 53 mS

10 gal water pull at salinity 50 mS from the tank at what 10 gal water replacement salinity ?mS is needed for the 110 gal tank to reach 53mS?”

Any thoughts Randy?

Is the volume of water actually 110 gallons or is that the size of the sump plus aquarium? I ask because the salinity drop seems off.

 

[hart24601]

From my friend:

Yes the 110 is for the whole system, display tank and sump. Not sure how much water displacement from rock, corals, and equipment. Hope my EC meter is working ok. I might have a standard solution for checking the EC meter.

 

[Dan_P]

From my friend:

Yes the 110 is for the whole system, display tank and sump. Not sure how much water displacement from rock, corals, and equipment. Hope my EC meter is working ok. I might have a standard solution for checking the EC meter.

When you did a 40 gallon water change with 48 mS on a 60 mS system and ended with 50 mS, the amount of water in your system must have been around 100 gallons not 110. That is why the math did not work.

 

[hart24601]

Talking with him that seems like a reasonable conclusion. Thanks for your help! Trying to get him to join here!

 

[Randy Holmes-Farley]

I agree that volume is the biggest error, but not the only one.

The mixing equation is not exactly accurate for conductivity (it would be for actual salinity). Conductivity of more dilute ions is higher than more concentrated ones.

One can see this as follows for an extreme case.

Take
one part 35 ppt seawater (conductivity 53.07 mS/cm)
and
one part pure fresh water (conductivity 0 mS/cm)

to give a salinity of 17.5 ppt

What is the conductivity?
According to this calculator it is 28.42 mS/cm

The exact mixing equation would give

53.07/2 = 26.54, which is lower than the actual result

Oceanlife - Water Salinity Converter en

Oceanlife Water Salinity Converter - Software per la conversione dei parametri fondamentali dell'acqua marina

www.oceanlife.it

This article goes into substantial detail and has a nice table (Table 1b):

https://www.onsetcomp.com/files/manual_pdfs/15260-B-Conductivity-Assistant-Calculations-Sea-Water.pdf

Seawater at 40 pSU and 25 deg C has conductivity of 59.74 mS/cm,

but seawater at half that salinity (20 PSU) at the same temp has more than half the conductivity: 32.09

 

[hart24601]

Awesome. This is exactly what he was after, thank you so much Randy!

 

[Vance825]

The mixing equation is not exactly accurate for conductivity (it would be for actual salinity). Conductivity of more dilute ions is higher than more concentrated ones.

I did a dilution with some stock saltwater and RO water. The resulting conductivity of the diluted sample was higher than the expected x/2.

My conductivity meter readings were:
53.1 mS with 40 ml saltwater and added 40 ml 0.0 mS RO water and the diluted conductivity was 29.6 mS. Expected 53.1/2 = 26.6 mS.

I am planning to upgrade to a salinity meter soon. Thank you for everyone’s comments they are very helpful.

 

[hart24601]

I did a dilution with some stock saltwater and RO water. The resulting conductivity of the diluted sample was higher than the expected x/2.

My conductivity meter readings were:
53.1 mS with 40 ml saltwater and added 40 ml 0.0 mS RO water and the diluted conductivity was 29.6 mS. Expected 53.1/2 = 26.6 mS.

I am planning to upgrade to a salinity meter soon. Thank you for everyone’s comments they are very helpful.

Welcome to r2r!!!!