[Ballistic]

I was looking for some kind of preservative that could be added to stock starch solution so I don't have to keep making fresh batches.

(small strands of filamentous non aerobic bacteria / mould appear after a few weeks)

A bit of googling shows me that some chemical indicators use Sodium Azide however this substance is restricted from sale to the public.

Does anyone know if many stock solutions which have a small amount of ethanol added is to do with preservatives or simply to do with solubility?

Any help appreciated.

[Arkcon]

The amount of ethanol needed to insure microbes won't grow is likely so high as to interfere with your application.

I guess, because you haven't said what your application was.

Maybe some people have shared online their recipes for 'soluble starch' and will list their preservative.  Other than the trace of sodium azide.  Which a laboratory worker can buy.

Sigma-aldrich sells small bottles of starch solution, it seems there's no preservative.  I guess people just keep batches clean, use promptly, and when their experiment is done, just throw away excess.

[Ballistic]

It's only for use in simple G CSE testing for starch with Iodine, but the students take the kits out over the summer and there's no telling how many weeks before the experiment is done.

[Arkcon]

The USP (and other pharmacopeia) specify 0.01% by weight of red mercuric iodide.   Yikes.  And some places do sell that.  Some sort of aromatic hydrocarbon would probably inhibit microbes at a very low level, but you'll have to search Google deeply for this random trivia.  Small batches.  Autocalved.  Use promptly. WHy does the experiment take weeks to perform? And so what if there's some mold growth in solution?  Its just an indicator.  You will still see the iodine color.  Uless you've allowed it to grow into a mushroom.

[Babcock_Hall]

What about filter-sterilizing the solution into a sterile tube?  It might work for soluble starch solutions.

[Ballistic]

What about filter-sterilizing the solution into a sterile tube?  It might work for soluble starch solutions.

Probably a bit expensive for what I'm doing just using what I have at hand.

Some sort of aromatic hydrocarbon would probably inhibit microbes at a very low level, but you'll have to search Google deeply for this random trivia.  Small batches.  Autocalved.  Use promptly. WHy does the experiment take weeks to perform? And so what if there's some mold growth in solution?  Its just an indicator.  You will still see the iodine color.  Uless you've allowed it to grow into a mushroom.

Thanks I will look into that.
I tried treating the HDPE tubes with UV light but the light is scattered and ineffective. Can't autoclave.

No, no mushrooms I'm afraid. There isn't mush-room for them to grow in 10ml tubes!

[wildfyr]

What about triclosan (or related compounds)? Its being phased out for hand washing stuff for good reason, but its a good biocide and shouldnt interfere with your experiment.

Another thing you could add is a quaternary ammonium compound like benzalkonium chloride. Very inert and a terrific biocide. Found in pharmeceuticals as a preservative, and dirt cheap.

[Ballistic]

Thanks for the tip on triclosan. I can see now that one of my suppliers is offering Sodium Azide. However, it's only by going through problems like this that I pick up a broader knowledge. Sometimes you completely miss something. For example, I though the UV idea would solve all my problems very easily, but not so and I've learnt all about UV.