Actually, to make another reply... The reason you are getting the negative number is because peroxide is only sold industrially as a 30% solution. Therefore, it is impossible to add HF (which comes as a 49% solution max) to any amount of peroxide, and get a 30% solution. If you look at the assumptions made in the program, you can see the concentrations that were assumed. If you have access to a higher concentration of peroxide, than you will have to add it as an "Other" or modify the code. Thanks again, David