Reef Chemistry Question of the Day #92 Hydrogen Sulfide

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #92

Hydrogen sulfide is most likely present in which of the following forms in seawater at pH 8.2?


A. H₂S
B. HS^-
C. HS⁺
D. S^--

Good luck!




























.

 

[beaslbob]

depends on whether or not your tank stinks.

 

[baitrunner]

B. Hs^-

 

[tonizzy22]

I dont have a clue.

 

[tonizzy22]

Is it related with dosing epson salt?

 

[Randy Holmes-Farley]

Nope.

 

[Cory]

I'm going to guess A h2s. But probably this is wrong haha.

 

[Nano sapiens]

H2S predominately, with lower amounts of HS- and S--

 

[Randy Holmes-Farley]

And the answer is...B. HS^-


This has more:

Hydrogen Sulfide and the Reef Aquarium by Randy Holmes-Farley - Reefkeeping.com

from it:

Hydrogen sulfide is fairly soluble in water. At 1 atmosphere H[SIZE=-1]₂[/SIZE]S partial pressure, one gram dissolves in 242 mL of freshwater at 20°C, forming a solution that has a concentration of about 4,000 ppm hydrogen sulfide. This solution is slightly acidic (pH about 4.5) because, like the water molecule, hydrogen sulfide can ionize to release H[SIZE=-1]⁺[/SIZE]:

H₂O ßà H⁺ + OH^-
H₂S ßà H⁺ + SH^-


​

Hydrogen sulfide is much more prone than water to this ionization, because the larger sulfur ion can spread out the charge to a greater extent and hence is more stable than an oxygen ion. Pure water with a pH of 7 contains equal amounts of H[SIZE=-1]₂[/SIZE]S and HS[SIZE=-1]^-[/SIZE] (hydrosulfide ion), while in order for H[SIZE=-1]₂[/SIZE]O to have equal concentrations of H[SIZE=-1]₂[/SIZE]O and OH[SIZE=-1]^-[/SIZE], the pH needs to be above 14. At pH values above 7, HS[SIZE=-1]^-[/SIZE]dominates. At very high pH (above 11), the HS[SIZE=-1]^-[/SIZE] may ionize again to form a sulfide ion (S[SIZE=-1]^--[/SIZE]).

HS^- ßà H⁺ + S^--


​

This ionization is actually somewhat controversial. Literary sources give values for its pKa between 12 and 14, but more recent data suggests that it doesn't ionize until the effective pH is higher than 14 with lower values previously reported being erroneous due to oxidation during the experiment.

Figure 3. Speciation of hydrogen sulfide as a function of pH in freshwater.​

The distribution of the different forms of hydrogen sulfide is shown in Figure 3 (above) as a function of pH in freshwater (assuming the second pKa is about 12). Table 1 shows the relative ratio at pH 8.2 in freshwater. I could not find exact values for seawater, but ions usually form slightly more easily in seawater, so its proportion of H[SIZE=-1]₂[/SIZE]S at any given pH is likely lower. These forms are in rapid equilibrium, so any given S atom will convert between all of these forms many, many times each second.

[SIZE=-1]Table 1. Speciation of hydrogen sulfide in fresh water at pH 8.2[/SIZE]​
[SIZE=-1]Species[/SIZE]​	[SIZE=-1]Relative Concentration[/SIZE]​
[SIZE=-1]H[/SIZE][SIZE=-1]2[/SIZE][SIZE=-1]S[/SIZE]​	[SIZE=-1]0.06[/SIZE]​
[SIZE=-1]HS-[/SIZE]​	[SIZE=-1]0.94[/SIZE]​
[SIZE=-1]S--[/SIZE]​	[SIZE=-1]0.00016[/SIZE]​

Clearly it is the HS[SIZE=-1]^-[/SIZE] form that predominates in reef aquaria, but the other forms are very important. Only the H[SIZE=-1]₂[/SIZE]S form volatilizes and is detected as a foul odor. It is also likely the neutral H[SIZE=-1]₂[/SIZE]S form crosses cell membranes and enters organisms to cause potential toxicity (as is the case for ammonia, with NH[SIZE=-1]₃[/SIZE] v. NH[SIZE=-1]₄⁺[/SIZE], where it is the neutral NH[SIZE=-1]₃[/SIZE] form that crosses membranes and causes toxicity).

On the other hand, even though the S[SIZE=-1]^--[/SIZE] form is only a small fraction of the total, it is also very important as it is this form that precipitates with metals to form the black deposits characteristic of hydrogen sulfide formation in sediments. Even though it is only a small fraction of the total at any given instant, all of the sulfide can be removed from seawater by precipitation as metal sulfides if sufficient metals are available. An analogy is that only a small fraction of the water in a reef tank is inside a pump at any given instant, but over time, all of the water can pass through it and be sent to a different part of the system.

I have used this process to my advantage in some lab experiments I did years ago. After passing hydrogen sulfide gas from a cylinder through a chemical reaction, I sent the excess bubbles into a very high pH solution (> pH 14), trapping the sulfide in the water as S[SIZE=-1]^--[/SIZE]. This solution could be disposed of by a waste disposal company much more easily than if I had somehow tried to collect the gas itself.