Of the three substances you mentioned—hexacyanoferrate, citric acid, and copper sulfate—copper sulfate is the most dangerous to humans when not handled properly.
Copper sulfate is a chemical compound that can be toxic if ingested or if there is skin contact. Ingesting copper sulfate can lead to symptoms such as nausea, vomiting, abdominal pain, diarrhea, and in severe cases, it can lead to more serious health issues, including liver and kidney damage. Inhalation of copper sulfate dust or fumes can also be harmful to the respiratory system.
Citric acid is generally safe for humans when used in food and beverage products, and it is naturally found in many fruits. It's not considered dangerous in typical usage.
Hexacyanoferrate, also known as Prussian blue, is generally considered low in toxicity and is used in various applications, including art and photography. However, it is important to handle any chemical with care and follow safety precautions.