[Procyon]

I'm trying to implement a H2S (hydrogen sulfide) measurement for thermal water in an in situ sample collection by an environmental microbiology and microbial ecology laboratory. I need to stabilize the H2S in the water sample to measure the H2S levels later.

Due to the higly volatility of H2S, I'm evaluating (and understand the theory of) the methods of blue methylene or diamine; but I don't know the practical limitations for an in situ sampling neither what is the proper container for water with volatile H2S.
.

[Stepan]

Find the corresponding EPA method, it will tell what bottle to use and how to preserve

[Procyon]

I found the protocol, but I'm still wondering if the preservation of H2S using Zinc acetate must be done in situ or can be done in the lab.

If the zinc acetate must be added in situ, I think we must use a container with a septum or can be done just opening the container after it has been cooled to 4°C.

[Stepan]

Usually you add concentrate of Zinc acetate into clean container beforehand, and fill it up to the lid with the sample on field. Seal it, and bring back to the lab for testing. 

[Procyon]

The situation is that is recommended that the samples must be cooled to 4-6°C to avoid volatilization of H2S, before adding Zinc acetate.

Considering that the themal water is at 40-50°C in a pond;  I don't think that adding zinc acetate beforehand would be a nice idea.

[Stepan]

Usually collect the sample at the source temperature, and cool it down as a sample. If you believe that this is critical, you can cool the water down as you collect it using a PTFE or glass made cooled line between you source and jar or bottle with a preservative

[Stepan]

By the way, H2S is not "too" volatile. I believe is concentration drops not due to evaporation but rather via oxidation, bacterial consumption and secondary reactions like H2S + SO2 reaction.

[Procyon]

I was thinking of using an exetainer by submerging it into the water and close it underwater. Later the Zn acetate and NaOH (to increase pH to 9) would be incorporated with a syringe.

Another possibility (as is recommended to put Zn acetate and NaOH before the water sample) is getting the water sample with a syringe, but it doesn't seem practical for 100 ml of water sample.


I don't know the importance of the temperature.

[Stepan]

You can get a 100 mL syringe, but you do not want you sample to contact with steel. 

[Procyon]

I was thinking in taking the water sample with a 15 ml syringe (without the steel point) and quickly deposit the sample to a 15 ml falcon tube, which already has an excess of zinc acetate and NaOH.

Does this method has a failure am I missing? Is there a kit to make some kit to facilitate the use of a blue methylene method (sulfaver-like)?.

For the analytical part of this method, I'm not sure which N,N-Dimethyl-p-phenylenediamine to use. The EPA protocols use N,N-Dimethyl-1,4-phenylenediamine oxalate (http://www.sigmaaldrich.com/catalog/product/aldrich/409758?lang=es&region=CL), but a spectrophotometric quality is available.(http://www.sigmaaldrich.com/catalog/product/fluka/07750?lang=es&region=CL).


On a similar topic: to measure the sulfate levels, do the water samples require some treatment in situ? To do the calibration curve for turbidimetry, I'm planning to use Sulfaver kit, but I don't know which kit to use http://www.hach.com/quick.search-quick.search.jsa?keywords=sulfaver.

[Procyon]

Is important for me to know the half-life of a zinc acetate + NaOH 6N solution at 25°C and 4°C.
(how long can maintain its reaction efficiency to fix sulfide?).

 As I'm giving a N number of 50 ml falcon tubes with the solution to the person who is going to get samples in situ (to "simplify" his work).

[Procyon]

Question:

For the blue methylene method is recommended N,N-Dimethyl-1,4-phenylenediamine oxalate or just N,N-Dimethyl-p-phenylenediamine (without oxalate)? Does the difference affect the measurements? Does the oxalate make it more soluble or gives a longer half-life of the reactive? It is VERY important to know as there are differences in prices and depends on stock availability.

Another question is if the level of hydration of certain compounds affect the calibration curves or alters the results ? For example protocols mention using Na2SO4 (unhydrated) and NaS2.9H2O for calibration curve; but here in the lab we only have Na2SO4.10H2O and NaS2.7-9H2O.

[Procyon]

For the blue methylene method is recommended N,N-Dimethyl-1,4-phenylenediamine oxalate or just N,N-Dimethyl-p-phenylenediamine (without oxalate)?
Does the presence or ausence of oxalate affect the measurements?
Does the oxalate make it more soluble or gives a longer half-life of the reactive?

It is VERY important to know as there are big differences in prices and depends on stock availability.