For an ammeter making a liquid rocket engine you really only have three choices of oxidiser. Those are Nitrous oxide, Peroxide and Liquid oxygen.
Peroxide is probably my favourite storable propellant. Granted I haven't actually used any liquid propellants yet, but if I had I am sure peroxide would be my favourite. As a starting point it doesn't cause cancer. Liquid oxygen would be lovely and although the performance is higher so is the cost. The actual cost of the propellant is quite cheap (don't have any figures for how much it would cost in Australia, but in america its about .1-3$/kg depending on the quantity you are after) I haven't tried to get any, but I can imagine it would be a mountain of paperwork and explanations. Anywhoo I am getting a tad carried away, I could write for a long time about advantages and disadvantages of propellants but what i really wanted to talk about was methods of concentrating peroxide.
Freezing and separating water ice from peroxide (ice freezes 0.4k higher)
I wont go into great detail about the first two but quickly freezing would work theoretically but will be too tricky to implement. Vacuum distillation would be ideal, but the equipment will be too costly.
Sparging will be very slow, but relatively safe
Sparging involves bubbling dry air through the peroxide. As the vapour pressure of the water in the peroxide solution is higher it will evaporate at a higher rate than peroxide. Although you will loose some peroxide, but will be 1/10 that of water.
The peroxide also needs to be filtered as it has stabilisers in it, which will foul catalysts. This can be done using a ion exchange resin.
The tricky part I see is getting a source of very dry air. A method I immediately thought of was using silica gel. This is how most equipment that needs a small source of dry air gets it. The problem is air has a alot of water, and sparging needs a a lot of air. The solution is a regenerative desiccant dryer. Its basically two vessels with desiccant. Only one is used to dry air, while the other one regenerates. This is done by simply done by heating up the desiccant so the water evaporates out. A controller monitors the humidity of the output air and switches, and regenerates automatically. I am looking at making one of these now. I will also need a source of oil free air (cant use a compressor). I wouldent imagine the head loss through the dryer would be too high, peobally in the order of 2-10PSI. A car turbo would work well. More about this later.