Date: Thu, 29 Jan 1998 23:46:39 +1100
From: Darryl Smedley <dsmedley@ozemail.com.au>
Subject: [17538] Hydrogen Peroxide Seed Sterilisation

For some years now I have been using 3 Volume W/V (10%) Hydrogen
Peroxide (H2O2) to sterilize orchid seed before sowing. Its simple and
easy as its a one-step operation that doesn't require any further
washing of the seed as in chlorine-based sterilization procedures.

It works because the Hydrogen Peroxide kills the nasty contaminates 
then breaks down into water and oxygen after a period. A friend has 
suggested that maybe the H2O2 might only work when it decomposes into 
H2O and O2 and its the nascent oxygen that does the sterilizing - don't 
know.

What I would like to know is how long the process takes to convert from
H2O2 to water and oxygen?

There is a practical reason for wanting to know this as I have found
that seed sown in the evening generally has a lower germination success
rate than seed sown in the morning.

I have speculated that this is because the flasks done at night are
under the the lights for only a short period and are in contact with 
the H2O2 whilst it is still 'active' for several hours until the lights 
turn on again in the morning. Flasks done in the morning are under the 
lights for up to 8 hours before the lights turn off at night. Presumably 
the H2O2 breaks down during this period. (I once used 6 Volume H202 by
mistake and washed and re-sterilized the seeds within 15 minutes. No
good, killed them all stone dead.)

My procedures are that the seed is placed in a test tube and 2.5ml of
H2O2 is drawn up and syringed into the tube. The tube is agitated on 
and off for 5 minutes then the stopper is removed and the liquid and 
seed poured into the flask. The flask usually has a small quantity of 
free water on top of the agar mixture - no more than 0.5 ml. The flask 
is then placed under 36/37 Watt Growlights at about 18-20" from the 
tube to the top of the agar. The flasking room is kept at temperatures 
ranging from 17-27 deg.C.

I'm told that the stabilizer used in H202 is Phosphoric Acid but I 
don't know in what proportion nor whether its presence contributes to 
the sterilization in any way.

I hope some of our 'OLD' chemists can help with this question which is
basicall how long does it take for H2O2 > 2H20 + 02?

Regards & good growing

Darryl Smedley
Oceania Orchids