RO/DI water plus Red Sea Salt = 7.1 pH?

[whybenormal]

I have another thread where everyone has been trying to help solve my pH problem.
I think we might have just hit upon something strange contributing to it.​

I bought Red Sea Salt (switching over from Reef Crystals slowly, as I was out of salt anyway) and mixed 9 gallons with water from our RODI unit. Since Red Sea is supposed to mix up at 8.1-8.3 I thought this would be a good move.
After mixing and bring up to 77degress, I tested the ph and it still read 7.1 - as it did before adding the salt.
Salinity was perfect 1.026.
I tested the pH meter in my high alkaline bottled water (9.5+) it read 9.7
My husband did a few tests, he mixed some with tap water, same results. 7.1
Bottled water, started 6.9, after mixing with salt 8.1
Distilled water, started at 8.1, after salt 8.4.

It seems to me the problem is with our water supply.
We are on a community well.
What about our water would keep the pH from going up?

Before we had the RODI I bought water at the LFS, but that was a 36 gal tank, not 75.

 

[MarineandReef Jaron]

It is completely possible to have CO2 in the source water. I would aerate the water for a few hours or overnight and see if the pH stays depressed.

 

[MamaMolo]

I have another thread where everyone has been trying to help solve my pH problem.​

I think we might have just hit upon something strange contributing to it.​

I bought Red Sea Salt (switching over from Reef Crystals slowly, as I was out of salt anyway) and mixed 9 gallons with water from our RODI unit. Since Red Sea is supposed to mix up at 8.1-8.3 I thought this would be a good move.
After mixing and bring up to 77degress, I tested the ph and it still read 7.1 - as it did before adding the salt.
Salinity was perfect 1.026.
I tested the pH meter in my high alkaline bottled water (9.5+) it read 9.7
My husband did a few tests, he mixed some with tap water, same results. 7.1
Bottled water, started 6.9, after mixing with salt 8.1
Distilled water, started at 8.1, after salt 8.4.

It seems to me the problem is with our water supply.
We are on a community well.
What about our water would keep the pH from going up?

Before we had the RODI I bought water at the LFS, but that was a 36 gal tank, not 75.

What are you using to check salinity?
Did you calibrate the ph meter?
Is the TDS zero out of rodi?

 

[whybenormal]

I have

What are you using to check salinity?
Did you calibrate the ph meter?
Is the TDS zero out of rodi?

To check salinity I use my refractometer.
I have 3 different pH meters, 2 can be calibrated and I have the solution and do. The other is my Seneye. I also have the API test kit.
The TDS out of the RODI was 6 vs out of the tap at 109.

 

[whybenormal]

It is completely possible to have CO2 in the source water. I would aerate the water for a few hours or overnight and see if the pH stays depressed.

I will try that.
Given the ongoing pH issues with my tank though, I'm thinking this may be the root of the problem.

 

[Randy Holmes-Farley]

If the pH us actually 7.1 when first mixed then simple aeration will raise it.

Using tap water carries risks.

 

[whybenormal]

If the pH us actually 7.1 when first mixed then simple aeration will raise it.

Using tap water carries risks.

I used RODI water. I know not to use tap, (and have finally convinced my husband of that.)

My husband mixed up some salt with tap water just to see if it would mix up to 8.1-8.3 pH, as it states it should.
All water NOT connected to our house mixed up 8.1-8.3.
Only RODI and tap water didn't change.
I hoped you might have an idea why.

 

[arking_mark]

There is a mathematical model for seawater pH, Alk, and CO2. Given any two of these measurements, you can calculate the 3rd.

Outdoor air is ~400ppm CO2. So aerating your water outside for about an hour fixes one of the variables.

Typical accuracy of calibrated alk measurements are +/- 0.5 and typical accuracy of calibrated pH meters are +/- 0.1.

Take a cup of your salt mix and aerate it for an hour with an aitstone. Measure pH and Alk.

Properly measured and mixed Red Sea Salt should have an Alk of 8dKH.

Your expected pH should be 8.27 NBS.

 

[Randy Holmes-Farley]

I used RODI water. I know not to use tap, (and have finally convinced my husband of that.)

My husband mixed up some salt with tap water just to see if it would mix up to 8.1-8.3 pH, as it states it should.
All water NOT connected to our house mixed up 8.1-8.3.
Only RODI and tap water didn't change.
I hoped you might have an idea why.

If you used 0 ppm tds ro/di, then the pH measurement is in error.

 

[Randy Holmes-Farley]

I will try that.
Given the ongoing pH issues with my tank though, I'm thinking this may be the root of the problem.

I don’t believe it is anything except test error. There is no type of 0 ppm tds ro/di that will make seawater ten times more acidic than normal.

 

[Randy Holmes-Farley]

If the di depleted, or perhaps you used the wrong type of di resin (cation binding only) then the effluent might be acid and deplete the alk. Check the alkalinity in the pH 7.1 water.

 

[whybenormal]

I don’t believe it is anything except test error. There is no type of 0 ppm tds ro/di that will make seawater ten times more acidic than normal.

Randy, What if the TDS of the ro/di water was 6?
The pH meter tested everything else correctly, including the calibration solutions, so we were inclined to trust it over the other two (the Seneye and the 2nd in tank monitor)
Today the pH in the tank measures 8.0 btw.

 

[Randy Holmes-Farley]

Randy, What if the TDS of the ro/di water was 6?
The pH meter tested everything else correctly, including the calibration solutions, so we were inclined to trust it over the other two (the Seneye and the 2nd in tank monitor)
Today the pH in the tank measures 8.0 btw.

If it was 6 ppm tds of H+ because the cation resin depleted, then that could lower the alk and the pH. Check the pH of the di effluent and the alk if the salt water made from it.

 

[whybenormal]

I don’t believe it is anything except test error. There is no type of 0 ppm tds ro/di that will make seawater ten times more acidic than normal.

As I've checked and rechecked the pH with 5 different pH meters/tests, including the one that my husband uses in his commercial boiler business, I'm fairly confident that at least his professional one is correct.
My issue I'm trying to solve is how both the water from our RO/DI system and our tap when mixed with Red Sea Salt (in the appropriate amount weighed out in grams, and the tap water was solely for testing purposes, I don't put tap water in my tank) still measured 7.1 pH.
When the appropriate amount of salt was mixed with bottled water it measured 8.3 pH.

Just to check calibration I tested my high pH water, 9.7pH, and checked it in borax, 9.2 (It doesn't do 2 decimal places)

Today I went to my favorite LFS and bought some of their RODI water, taking with me a measured amount of salt for my jug of water, and my pH meter.
We mixed it up on the spot and tested 8.3.

What could be wrong with my water?

 

[arking_mark]

Please mix up a batch of saltwater and aerate a cup of it outside with an aitstone for 1hr.

Report back pH and Alk

 

[Randy Holmes-Farley]

As I've checked and rechecked the pH with 5 different pH meters/tests, including the one that my husband uses in his commercial boiler business, I'm fairly confident that at least his professional one is correct.
My issue I'm trying to solve is how both the water from our RO/DI system and our tap when mixed with Red Sea Salt (in the appropriate amount weighed out in grams, and the tap water was solely for testing purposes, I don't put tap water in my tank) still measured 7.1 pH.
When the appropriate amount of salt was mixed with bottled water it measured 8.3 pH.

Just to check calibration I tested my high pH water, 9.7pH, and checked it in borax, 9.2 (It doesn't do 2 decimal places)

Today I went to my favorite LFS and bought some of their RODI water, taking with me a measured amount of salt for my jug of water, and my pH meter.
We mixed it up on the spot and tested 8.3.

What could be wrong with my water?

did you check the alkalinity after mixing? I suggested that earlier.

If you aerate it, does it rise?

 

[Randy Holmes-Farley]

Did you make a repeat batch of the low pH water using the ro/di and the Red Sea salt?

I am still not seeing how any fluid that would come out of an ro/di at pH 7.1 can mix with sea salt mix and remain at pH 7.1.

 

[stoney7713]

What about testing the pH before the DI resin?

 

[TokenReefer]

Since Red Sea is supposed to mix up at 8.1-8.3 I thought this would be a good move.

How does Red Sea stat the PH their salt mixes to? It depends on outside variables. Could you possibly be referring to alkalinity? Sorry I'm a little confused...ignore me if you wish

 

[EeyoreIsMySpiritAnimal]

I have

To check salinity I use my refractometer.
I have 3 different pH meters, 2 can be calibrated and I have the solution and do. The other is my Seneye. I also have the API test kit.
The TDS out of the RODI was 6 vs out of the tap at 109.

May or may not be the cause of your pH issue, but if tap TDS is 109, the TDS after the membrane should be no higher than 5 or 6 (depending on the specific membrane you use) assuming the correct water pressure. Once the water is filtered through DI resin, TDS should drop to close to 0.

How recently have you changed the sediment and carbon filters? If they are relatively new, I suspect you need to change the membrane and/or it's not working correctly.