[m00se]

Hello good people. This is my first post, and to be honest, I came here specifically for your expertise. I hope this isn't against a rule I ignored, and if it is, I apologize. I have googled till my calluses have calluses, and not found anything that my tiny high school educated brain can get around. So here is what I'm trying to do:

I want to make a solution of 30% Na2S2O3•5H2O (sodium thiosulfate) dechlorinator for my aquarium. I know that API makes a product called "Tap Water Conditioner" and the MSDS tells us that Na2S2O3•5H2O is at 30.2%, and then they add EDTA 9.8%. I assume (correct me if you think I'm wrong) that the EDTA is there as a preservative.

http://cms.marsfishcare.com/files/msds/tap_water_conditioner_122309.pdf

I want to make 500ml of this product so that I can use 1 drop per gallon to neutralize approximately 1 ppm of chlorine in my tap water.

My rudimentary calculations tell me that to get about 30% solution I would need to mix 150g of Na2S2O3•5H2O with distilled water, and then bring the volume up to 500ml.

Am I close?

Thank you in advance!

[Hunter2]

Yes you are close, but you have to use the mass. 150 g Thiosulfate and 350 g water. The mass will be 500g but not 500 ml. The spacific gravity of the solution is not 1 g/cm³.

But for your purpose its ok to do so how you described.

[m00se]

So it's a straight mass/mass calculation and not a mass/volume type of thing?

[Hunter2]

Yes, solid substances normaly used as % w.w.  Thiosulfate itself is solid not liquid. Its different to alcohol water solution likegin, whiskey,etc. These are volume percentags.

[Borek]

I want to make 500ml of this product so that I can use 1 drop per gallon to neutralize approximately 1 ppm of chlorine in my tap water.

Don't prepare much more solution than you need for immediate use, as dissolved thiosulfate is unstable and slowly decomposes. It is more stable as solid.

[Hunter2]

Well, its quite stable, because used as titration solutions for iodine and also in development machines for photographs in shopping centers..

[Borek]

Well, its quite stable, because used as titration solutions for iodine and also in development machines for photographs in shopping centers..

Just because it is used for these purposes doesn't mean the solution is stable. It is stable enough to be used for titration if the solution was freshly standardized. When used as a component of a developer it is stabilized by additives, besides, it is rarely used longer than a few days. Leaving the solution for several weeks is not a good idea, solution becomes milky (it decomposes producing elemental sulfur). It is possible to slow down decomposition adding small amount of sodium carbonate to raise the pH and some antimicrobial agent, although selecting the latter here can be difficult (remember we are talking about solution that will be added to fish tank).

[Hunter2]

I agree with that. Maybe better would be to have a kind of ion exchanger. Sulfure for fish is also not good I think.

[Borek]

No need to reinvent the wheel, thiosulfate is tested as a safe chlorine scavenger for fish tanks.

At least it is known to be much safer than the chlorine it removes.

[m00se]

Thank you all for this information. I will take the stability of the solution into consideration. Originally I wanted to make this batch up to neutralize a strong household bleach solution that I use to recharge polymer bead resin filtration material for the aquarium. It requires a good long soak in copious amounts dechlor and using the "premium" product I normally use for water changes is costly. I have had aquariums since I was a young boy (50 years ago) and my father was a professional photographer, so naturally we had hypo around. I don't believe in all the hoo-doo that these manufacturers market for what is essentially < .01¢ worth of raw material. There is a trend for municipalities to add chloramine to the water supplies lately, and these marketers have scared plenty of hobbyists into believing that using sodium thiosulfate alone will allow the ammonia/chlorine bond to break, releasing the ammonia into the aquarium. While that is true, the amounts are negligible and are quickly absorbed by plants in the tank. Also considering in my case that the pH of the tank water is well below 7, rendering the ammonia harmless (ammonium at that pH?).

That being said, API Inc. adds "EDTA tetrasodium salt"  in their product at 9.8%. Could they be using it as a preservative?

Thanks again folks - I do appreciate it.

[DrCMS]

That being said, API Inc. adds "EDTA tetrasodium salt"  in their product at 9.8%. Could they be using it as a preservative?

No the EDTA is a chelating agent see http://en.wikipedia.org/wiki/Chelation it's there to mop up any heavy metals before they kill your fish.