I am performing an experiment where I am spiking my product with various % of CTAB. I have made stock solution of 2% (w/v) CTAB in 40 mM NaCl. I also need to make sure that I maintain the NaCl conentration at 40 mM. The product that the stock solution is being added to doesn't have any NaCl in it to begin with. I know how to calculate the amount of 2% CTAB stock solution to obtain my final CTAB concentrations but am at a loss for how to maintain the NaCl at 40 mM in all samples. Example: for 1% CTAB in 1 mL of product, I add 1 mL of 2% CTAB stock solution but by doing this I am reducing the NaCl concentration to 20 mM. I can add more NaCl to the mixture but then I end up reducing the CTAB concentration. Can someone please help???