Hardness testing should be carried out at a pH of 10.5 or greater. If Iron is present in any great amount, it may become an interference agent to suppress the total hardness found. Most, if not all, Hardness reagent systems have some limited Iron suppression agents contained in their formulations. Remove gross amounts of Iron as an interfering agent by filtration or by precipitation. Metallic ions such as Aluminum, Copper or Zinc can be interfering agents if present in sufficient quantity.