Reef Chemistry Question of the Day# 299: New Salt Water Parameters

[Randy Holmes-Farley]

Reef Chemistry Question of the Day 299

You are making a batch of new salt water for a water change, and realize that you overshot your salinity target. You measure the parameters to be:

salinity 36 ppt
pH 8.0
Calcium 455 ppm
Alkalinity 9 dKH
Magnesium 1380 ppm
ORP 255 mV

You then add some additional 0 ppm TDS RO/DI water straight from the RO/DI outlet.

Which of the following parameter changes in the new salt water is impossible as a consequence of that addition?

Pick all that are impossible.

1. Salinity rises
2. pH rises
3. Calcium falls
4. Alkalinity falls
5. Magnesium falls
5. ORP falls

Good luck and Happy Reefing!



Previous Reef Chemistry Question of the day:

Reef Chemistry Question of the Day #298: Where are those trace elements coming from?

Reef Chemistry Question of the Day 298 We all know that organisms from bacteria to whales require many different trace elements for the biochemical processes of life. One of the things we don't always have a good handle on in a reef tank is where they are coming from and where they are going...

www.reef2reef.com

 

[Salty_Northerner]

If I read correctly I'd say number 1

 

[Randy Holmes-Farley]

bump

 

[Raul-7]

1. Salinity rises
2. pH rises
5. ORP falls

Addition of pure water will lower salinity, causes a decrease in pH and ORP to rise.

 

[Cichlid Dad]

1. Salinity rises
2. pH rises
5. ORP falls

Addition of pure water will lower salinity, causes a decrease in pH and ORP to rise.

Agreed

 

[Miami Reef]

For very complicated reasons, pH will rise when adding freshwater to seawater. I know, it doesn’t make sense how adding 7pH water to 8.0pH+ seawater will cause the seawater to rise, but it does.

I know #1 is false, salinity will not rise when adding freshwater to seawater

I don‘t know about ORP. I’m going to guess #5 is false because it wouldn‘t be complete if there was only 1 option that would be false from the list.

Upon further recollection, I think ORP is also false because I never noticed a drop when adding freshwater to the tank. I never notice any change, but I don’t really track ORP very closely.

@Randy Holmes-Farley

 

[Raul-7]

For very complicated reasons, pH will rise when adding freshwater to seawater. I know, it doesn’t make sense how adding 7pH water to 8.0pH+ seawater will cause the seawater to rise, but it does.

@Randy Holmes-Farley

How will it rise if it's devoid of any carbonates?

 

[Miami Reef]

How will it rise if it's devoid of any carbonates?

How will it rise? No idea. Let’s ask Randy.

Here’s a quote by him

FWIW, if you add totally pure fresh water at pH 7 to 35 ppt seawater at pH 8.00, the pH in the seawater rises, not falls, due to the freshwater addition. Common sense loses out to real world chemistry.

 

[Court_Appointed_Hypeman]

How'd I miss this one?

1 Salinity rising
2PH rising
5 mag falls

I know this might be dumb, but I know for sure if I was in this scenario, and was testing, if I did the aqua forest mag, it would read higher because I would be doing 1 less drop during the test following their suggestion. Its so close to that range I bet I would read it higher, at the very least, my test kits would not detect a fall.

However, we all know the mag would not actually go up, but I wanted to throw this answer out there.

Orp idk a dang thing about yet frankly.

 

[findingsimple]

Salinity rises is false and I'm here to learn anything else!

 

[KrisReef]

So many parameters and my head is spinning.

You would have these "problems" if you used NSW.

I'm waiting for the equations to help explain some of the changes,

 

[Fungusamongus]

1, 5?

 

[Borat]

PH can slightly rise or fall, but I think generally it will remain unchanged in this example.. PH does not follow dilution rules..

my choice is 1,5

 

[TangerineSpeedo]

So many parameters and my head is spinning.

You would have these "problems" if you used NSW.

I'm waiting for the equations to help explain some of the changes,

But don't you add Alk and raise your salinity to your NSW?
Well, on the Temperate tank I don't, because NSW is correct for that.
On the tropicals I raise both.

 

[Randy Holmes-Farley]

And the answer is…

Pick all that are impossible.

1. Salinity rises
2. pH rises
3. Calcium falls
4. Alkalinity falls
5. Magnesium falls
5. ORP falls

Salinity rising is obviously impossible by adding fresh water.

Calcium, alkalinity, and magnesium fall by simple dilution.

pH rises a small amount because bicarbonate is a weaker acid as salinity declines. That is, the salts in seawater encourage bicarbonate to come apart into H+ and carbonate. When there are fewer ions present by dilution with the fresh water, fewer bicarbonate ions break into H+ and carbonate, and less H+ means pH is higher.

ORP in seawater tends to move oppositely to pH. Since the pH rises, the ORP will fall. The reason is complex, but nearly everyone tracking pH and ORP sees it in their tanks. Essentially, H+ and OH- are involved in the redox reactions that determine ORP. I’ll post a section from my ORP article in a second post with more details.

 

[Randy Holmes-Farley]

ORP and the Reef Aquarium - Reefkeeping.com


The theoretical relationship between ORP and pH

One of the complications of ORP is that the measured value can sometimes depend on pH. Whether ORP does depend on pH or not, and to what extent, is determined by the exact redox reactions that are involved in controlling the ORP in that solution. There have been equations proposed that purport to "correct" ORP for changes in pH, giving a new parameter, sometimes called rH. This parameter was proposed in the 1920's by W. M. Clark.7 One form of this correction is shown below:

rH = mV / 29 + (2 x pH)

and sometimes a correction for changes in oxygen concentration is thrown in:

rH = mV / 29 + (2 x pH) + [O2]

where [O2] is the concentration of O2 in ppm. The use of rH, however, presupposes a detailed understanding of the reactions involved, and is simply wrong for general use (as shown below). In a book8 that he published 40 years after his initial publication, Clark stated:

"At this point the author must confess to the introduction of rH. He conceived that there might be occasions when it would be convenient to speak of relative oxidation-reduction intensity without having to specify both potential AND pH...
...Unfortunately both the original intent and the obvious limitations have been overlooked by many who have converted their potentials for SPECIFIC SYSTEMS to rH numbers...
...In brief, rH has become an unmitigated nuisance."​

Nevertheless, many people still use rH. Since it is imbedded in many articles relating to aquarists, it is worth understanding where the pH dependence comes from, and why it is not always the same.

As an example of a solution where the redox is not pH dependent, take a solution of Fe++ and Fe+++ in water, with no other redox active species. In that case, the ORP is exactly determined by the relative concentration of the two iron species, and is unchanged with pH.

Fe+++ + e- —> Fe++

Specifically, the defining equation here is:

The main thing that is clear from this equation is that the ORP is independent of pH, and only depends on the relative concentrations of Fe++ and Fe+++.

The easiest way to think of the lack of pH dependence here is to recognize that neither H+ nor OH- participate in the reaction at all. So changing the pH has no direct impact on the reaction.

For many reactions where oxygen is an important participant, however, that is not the case:

O2 + 4H+ + 4e- —> 2H2O

In this reaction, H+ does participate. Consequently, the oxidizing power is related to pH. As H+ is raised (by lowering pH), the reaction is driven to the right. One way to think of this is by LeChatlier's Principle where increasing the concentration of one species drives the reaction to the other side. In this case, lowering the pH increases the oxidizing power of the oxygen, and consequently raises the ORP. This result is the basis for the development of rH for many systems.

It is beyond the scope of this article to go into the detailed mathematics behind the pH dependence of ORP measurements, but Pankow does cover such issues in great detail in Aquatic Chemistry Concepts.9 For our purposes, an important result is that the magnitude of the change in ORP with pH depends entirely on the number of H+ involved in the reaction per electron. In the case of the Fe+++/Fe++ situation, this value is zero. For the oxygen/water reaction, the value is 1.0. The standard definition of rH assumes that this ratio is exactly 1.0. Consequently, it may not apply to many redox reactions that take place in aquaria.

Shown below are some typical reactions that also take place in aquaria. First, the oxidation of acetic acid to carbon dioxide, again with one H+ per electron (this reaction typifies many reactions involving neutral organic materials):

2CO2 + 8H+ + 8e- —> CH3COOH + 2H2O

but if the same reaction proceeds with acetate, the reaction is:

2CO2 + 7H+ + 8e- —> CH3COO- + 2H2O

and the ratio of H+ to e- is no longer 1.0, but is now 0.875.

For the various reactions of the nitrogen cycle, we have ratios that vary from 1.0 to 1.33:

NO2- + 7H+ + 6e- —> NH3 + 2H2O

NO2- + 8H+ + 6e- —> NH4+ + 2H2O

NO3- + 2H+ 2e- —> NO2- + H2O

N2 + 6H+ + 6e- —> 2NH3

N2 + 8H+ + 6e- —> 2NH4+

The iodide/iodate reaction fits the 1.0 ratio:

IO3- + 6H+ + 6e- —> I- + 3 H2O

Some other redox reactions that have other ratios:

MnO2 + 4H+ + 2e- —> Mn++ + 2H2O

SO4-- + 10H+ + 8e- —> H2S + 4H2O

SO4-- + 9H+ + 8e- —> HS- + 4H2O

So if one really wants to understand how ORP would change with pH, one would have to know what the species are in aquaria that control redox. If it is a mixture of species, then the end result will come back as a complex averaging of the different reactions involved. Unfortunately, the species involved have not been clearly defined for seawater. In aquaria, which vary considerably in the concentrations of many redox active species, the situation is even more complicated.

The empirical relationship between ORP and pH in aquaria

While understanding the details of the theoretical relationship between pH and ORP is complicated, measuring it for a single aquarium is fairly easy. Figure 1 shows simultaneous plots of pH and ORP values over the course of several days in the aquarium of Simon Huntington. Clearly, the measured ORP and the pH are on exactly opposite cycles, as one would expect from a system where reactions involving oxygen are important (and as is shown by rH).

 

[KrisReef]

But don't you add Alk and raise your salinity to your NSW?
Well, on the Temperate tank I don't, because NSW is correct for that.
On the tropicals I raise both.

Yup, the 5 gallon bucket takes about a tablespoon (or two?) of baking soda to get the dKh into the 7-8 range for the reef. Mg has always seemed low as well, but I'm not certain that isn't more of a testing error issue vs a real thing? I run the salinity low, so no adjustment is needed in my lazy method.

 

[KrisReef]

ORP and the Reef Aquarium - Reefkeeping.com


The theoretical relationship between ORP and pH

One of the complications of ORP is that the measured value can sometimes depend on pH. Whether ORP does depend on pH or not, and to what extent, is determined by the exact redox reactions that are involved in controlling the ORP in that solution. There have been equations proposed that purport to "correct" ORP for changes in pH, giving a new parameter, sometimes called rH. This parameter was proposed in the 1920's by W. M. Clark.7 One form of this correction is shown below:

rH = mV / 29 + (2 x pH)

and sometimes a correction for changes in oxygen concentration is thrown in:

rH = mV / 29 + (2 x pH) + [O2]

where [O2] is the concentration of O2 in ppm. The use of rH, however, presupposes a detailed understanding of the reactions involved, and is simply wrong for general use (as shown below). In a book8 that he published 40 years after his initial publication, Clark stated:

"At this point the author must confess to the introduction of rH. He conceived that there might be occasions when it would be convenient to speak of relative oxidation-reduction intensity without having to specify both potential AND pH...
...Unfortunately both the original intent and the obvious limitations have been overlooked by many who have converted their potentials for SPECIFIC SYSTEMS to rH numbers...
...In brief, rH has become an unmitigated nuisance."​

Nevertheless, many people still use rH. Since it is imbedded in many articles relating to aquarists, it is worth understanding where the pH dependence comes from, and why it is not always the same.

As an example of a solution where the redox is not pH dependent, take a solution of Fe++ and Fe+++ in water, with no other redox active species. In that case, the ORP is exactly determined by the relative concentration of the two iron species, and is unchanged with pH.

Fe+++ + e- —> Fe++

Specifically, the defining equation here is:

The main thing that is clear from this equation is that the ORP is independent of pH, and only depends on the relative concentrations of Fe++ and Fe+++.

The easiest way to think of the lack of pH dependence here is to recognize that neither H+ nor OH- participate in the reaction at all. So changing the pH has no direct impact on the reaction.

For many reactions where oxygen is an important participant, however, that is not the case:

O2 + 4H+ + 4e- —> 2H2O

In this reaction, H+ does participate. Consequently, the oxidizing power is related to pH. As H+ is raised (by lowering pH), the reaction is driven to the right. One way to think of this is by LeChatlier's Principle where increasing the concentration of one species drives the reaction to the other side. In this case, lowering the pH increases the oxidizing power of the oxygen, and consequently raises the ORP. This result is the basis for the development of rH for many systems.

It is beyond the scope of this article to go into the detailed mathematics behind the pH dependence of ORP measurements, but Pankow does cover such issues in great detail in Aquatic Chemistry Concepts.9 For our purposes, an important result is that the magnitude of the change in ORP with pH depends entirely on the number of H+ involved in the reaction per electron. In the case of the Fe+++/Fe++ situation, this value is zero. For the oxygen/water reaction, the value is 1.0. The standard definition of rH assumes that this ratio is exactly 1.0. Consequently, it may not apply to many redox reactions that take place in aquaria.

Shown below are some typical reactions that also take place in aquaria. First, the oxidation of acetic acid to carbon dioxide, again with one H+ per electron (this reaction typifies many reactions involving neutral organic materials):

2CO2 + 8H+ + 8e- —> CH3COOH + 2H2O

but if the same reaction proceeds with acetate, the reaction is:

2CO2 + 7H+ + 8e- —> CH3COO- + 2H2O

and the ratio of H+ to e- is no longer 1.0, but is now 0.875.

For the various reactions of the nitrogen cycle, we have ratios that vary from 1.0 to 1.33:

NO2- + 7H+ + 6e- —> NH3 + 2H2O

NO2- + 8H+ + 6e- —> NH4+ + 2H2O

NO3- + 2H+ 2e- —> NO2- + H2O

N2 + 6H+ + 6e- —> 2NH3

N2 + 8H+ + 6e- —> 2NH4+

The iodide/iodate reaction fits the 1.0 ratio:

IO3- + 6H+ + 6e- —> I- + 3 H2O

Some other redox reactions that have other ratios:

MnO2 + 4H+ + 2e- —> Mn++ + 2H2O

SO4-- + 10H+ + 8e- —> H2S + 4H2O

SO4-- + 9H+ + 8e- —> HS- + 4H2O

So if one really wants to understand how ORP would change with pH, one would have to know what the species are in aquaria that control redox. If it is a mixture of species, then the end result will come back as a complex averaging of the different reactions involved. Unfortunately, the species involved have not been clearly defined for seawater. In aquaria, which vary considerably in the concentrations of many redox active species, the situation is even more complicated.

The empirical relationship between ORP and pH in aquaria

While understanding the details of the theoretical relationship between pH and ORP is complicated, measuring it for a single aquarium is fairly easy. Figure 1 shows simultaneous plots of pH and ORP values over the course of several days in the aquarium of Simon Huntington. Clearly, the measured ORP and the pH are on exactly opposite cycles, as one would expect from a system where reactions involving oxygen are important (and as is shown by rH).

Great, I've memorized this and just in case I will add a bookmark!
Thanks Randy for the great puzzles and quizes to help keep us on top of these chemical situations.