Answer

To make the Biuret solution, our supplier uses 300ml of 10% Sodium hydroxide in 1 litre of the final solution. 10% NaOh is 2.5M, so 300ml of this made up to 1 litre  will be 0.75M of NaOH. Or, as a percentage, this will be 300ml of 10% solution made up to 1 litre giving approx 3% of NaOH.