This invention pertains generally to the field of chemical synthesis and purification, and more specifically to methods of synthesizing and purifying certain 3,7-diamino-phenothiazin-5-ium compounds (referred to herein as “diaminophenothiazinium compounds”) including Methylthioninium Chloride (MTC) (also known as Methylene Blue). In one embodiment, the e//i method comprises the steps of in order: a thiosulfonic acid formation (TSAF) as illustrated below, wherein the reaction is carried out in the presence of a thio sulfate and an oxidizing agent that is or comprises persulfate an oxidative coupling (OC); and a ring closure (RC). The resulting compounds and compositions comprising them (e.g., tablets, capsules) are useful in methods of medical treatment and diagnosis, etc., for example, tauopathies, or Alzheimer's disease (AD).