Sodium hypochlorite 15%

Trust me bleach can easily damage cotton, based on recent experience & in my youth when dyeing clothes…had a pair of jeans basically disintegrate when left in domestic bleach for far too long.
The thing is that he needs to know exactly the concentration of NaClO (that information sometimes is not available in commercial bleach, or it gives a range of 5 to 10%) to prepare six solutions at different concentrations.
Why not just use thick bleach as the max ( label it whatever you want) and then dilute down. Just leave the cotton sitting in bleach for longer. Or, to determine actual hypo content you could do this
We have already ordered the 15% NaClO solution, it should arrive before the 21st. We also considered determining the content of NaClO of commercial bleach (still, we have to go to a supermarket and buy a bottle...), but we considered it would be easier to just order the NaClO. I will have a look at the protocol just in case we need to do a similar experiment in the future.