when problem arises with the pH in the CAS medium. The pH value is the critical point when you are going to prepare the CAS medium. If I am correct, HDTMA is also in composition of CAS. I suggest you to prepare Petri dishes that contain CAS and a common solid medium, for instance KB.
Firstly you should pour the common solid medium. Once it is solid, you should cut and remove from the Petri dish a hal part of the medium. Then you will pour melted CAS medium in this empty section of the Petri dish.
You will streak the bacteria that you want to evaluate on the common solid medium. The siderophore will move in the solid and determin the change in the colour of CAS.   _{Your Comment}