Master Reef Chemist Certification Question!!! (yes, you want this title)

[nautical_nathaniel]

A. Above 8.1

pH is the measure of the concentration of H+ ions in a solution. When adding pure water to a solution that already has a reduce concentration of Hydronium versus Hydroxide ions (i.e. a basic solution), the resulting solution will have a larger total volume and relatively the same amount of Hydronium ions, thus increasing the overall pH of the solution.

This is all in a perfect environment of course with no gas exchange occurring between the two solutions. In a real world scenario, carbon dioxide will dissolve in pure water forming carbonic acid which will lead to a reduction in pH of the RODI solution since it has zero buffer capacity. Adding the reduce pH RODI solution to the saltwater would cause a reduction in the overall pH of the resulting solution.

 

[Chuk]

Roughly 3:1 dilution
h+ in SW is 10^8.1*1L
and freshwater is 10^7x3L
So log((10^8.1+3x10^7))/4~7.5 which is C

 

[GoVols]

Shooting for a ph of 7.55

Just splitting the difference between 8.1 seawater and the 7.o RO/DI water???

Going with C about 7.5

 

[AdamNC]

C. 8.1 +7.0=15.1 15.1/2=7.55

 

[Mical]

D

 

[120reefkeeper]

A.

 

[reeferfoxx]

A

Edit: I don't want a Master Reef Chemist Title for this question. Thats like saying i'm a doctor for curing a headache with tylenol.

 

[jeffscooby13]

When doing the math, you have to remember pH is logrhythmic not linear. Secondarily, and this is where it gets trippy, the dissociation of -OH and +H is relative to the coligative properties of the solution and is also non-linear. The rough answer, and I did zero math, should be significantly lower because of the decreased coligitave action caused by the reduced ionic load.

 

[NovaReef]

Hi @Randy Holmes-Farley ,
To keep this simple: At high salinity, the relationship between alkalinity and pH is a higher order exponential than at a lower salinity. So, assuming equal alkalinity, or even a little less, there will a much higher value of pH at a lower salinity.

So, I'm going with A!

Explanation: pH is the measure of H+ ions in a solution. So at a lower salinity, there will be a higher concentration of H+ ions than at a higher salinity.
Here is where it would be on the chart I drew in:

Helpful Source:
https://www.bibliotecapleyades.net/ciencia/ciencia_agua19.htm

 

[Koi Obsession]

how about 7.5 for neutral water?

 

[jahnje]

My math says A at 8.2ish. Weird and unexpected. Intuitively I'd like it to be about 7.6. But since ph basically increases with amount of room for hydrogen ions I guess it makes since. By adding water with no acid from co2 in it you're leaving room for all that acid to get sucked in as soon as you take the lid off. At which point in time it would then move to a lower more intuitive ph. I think.

Hmm. Cool.

 

[MnFish1]

I think the Ph will stay th

Let's have some fun with a question that wraps up several different aspects of reef chemistry into a single, very simple (perhaps deceptively simple) question. It's just the pH of a hyposalinity solution. How much easier can it get? [emoji1]

Get it right for the right reason and you too can be certified as a Master Reef Chemist!!!

Suppose you have normal natural seawater of about 35 ppt salinity (specific gravity of about 1.0264), and you want to make hyposalinity water with it to treat a sick fish. You decide you want the salinity to be about 12 ppt (specific gravity about 1.009)

The pH of the normal natural seawater at 35 ppt is 8.1 in perfect equilibrium with your home air.

Assume you have totally perfect RO/DI water with nothing at all in it besides water. No CO2, nothing. You collected it straight from the DI with no gas exchange with the room air. You know that the pH of this water is ~7.0.

You mix the 35 ppt salinity seawater at pH 8.1 with sufficient RO/DI water at pH 7 to get to the 12 ppt salinity target. All mixing and solutions and measurements are at 25 deg C and are done in a closed container.

The pH of the final hyposaline solution before any gas exchange may occur (e.g., no CO2 entering, no CO2 leaving, no evaporation, etc.) is:

A. Above 8.1 [note this says above 8.1, it implies a rise from the level in the salt water]
B. About 7.7
C. About 7.5
D. About 7.3
E. Below 7.0 [note this says below 7, so it implies a drop from the level in the RO/DI water]

Please explain your choice (if you want full credit)

Searching online for help is encouraged (if needed).

Good luck!

pH is a logarithmic scale. Diluting the tank even this much shouldn't have that much effect on the pH. So the answer I think is A. because it is a 'H+ concentration'. The concentration of H+ will decrease with dilution - thus, the pH will increase

 

[JerSaint]

.

 

[Cae]

Id say B. 7.5 because I averaged the difference between 8.1 and 7ph.
Came up with 1.1ph and then halved it to get an average between the two. then added it to 7ph.

 

[Randy Holmes-Farley]

I’m surprised no one has tried to simply measure it (or something similar) and see what happens (though that wasn’t the intent). [emoji3]

Sometimes, experiments are easier than predictions. [emoji23]

 

[NanoCrazed]

I’m surprised no one has tried to simply measure it (or something similar) and see what happens (though that wasn’t the intent). [emoji3]

Sometimes, experiments are easier than predictions. [emoji23]

That thought crossed my mind but I'm too lazy...

 

[themcfreak]

I’m surprised no one has tried to simply measure it (or something similar) and see what happens (though that wasn’t the intent). [emoji3]

Sometimes, experiments are easier than predictions. [emoji23]

I did mix up 5 ml of RO and 5 ml of the water out of my tank (about 8.1) and it looked very close to 8.1 still. But that was in a non controlled, quick and easy, co2 filled test. It was very quick mostly to make sure there were any noticeable drastic changes.

 

[Newb73]

The problem for me is "assume".

In the real world that never works. It may work in the confines of a lab with all variables controlled but a reef tank invokes systems theory and the reality is there is no way to control or factor or even always accurately model complex systems (see previous challenges with climate change modeling). Too many variables in the RODI water, salt mix variations, room for errors in other places like volume and even temp impacting the conductivity reading.

In Health care we don't even bother with that level of mental masturbation based on theory.

If this were a patient we would measure it....hope our lab test was accurate and then have them breath into a paper bag if they were alkalotic or give some bicarb if they were acidotic and re-measure.

We would never just "assume".

That might be reflected in a previous post i made about various ways to measure salinity and mix to a measurement and Randy replying that he would simply mix to the formula and be done....lol.


I know Randy laid out all the needed variables needed to do the calculation, but with chemistry not being my strong suit....I wouldn't even attempt a guess unless there was real world utility.

Of course, this does mean putting probably too much faith in the accuracy of testing procedures.

Okay I'll guess. My guess is that RODI water is still going to somehow tank a PH level.

Ill guess D

My rationale?? Ive seen MY RO/DI water bottom out my ph a bit, in much smaller ratios.

 

[ReefTeacher]

The dilution requires adding about 1.9L of fresh water to every 1 L of saltwater. Since the alkalinity of seawater about 125ppm we are diluting this only by a factor of about 3. therefore our alkalinity is approximately 42.7 ppm. We are well within the approximations of the Henderson-Hasselbalch equation https://en.wikipedia.org/wiki/Henderson–Hasselbalch_equation so the pH stays about the same or choice A.

 

[Hitman]

A

Reading your past articles there will not be CO2 introduction to lower the ph in this controlled test.