FAQs
What is murexide used for?
Murexide is a metal indicator for complexometric (EDTA) titrations, most classically the direct titration of calcium at high pH. It forms a colored complex with the metal and changes color sharply when EDTA has removed the last free metal ions, marking the endpoint. It is also used to determine nickel, copper, and cobalt.
Why is the indicator ground with sodium chloride?
Pure murexide is unstable and needed only in tiny amounts, which makes it hard to weigh and dispense reliably. Grinding it with inert sodium chloride produces a stable, free-flowing mixture, so a small measured scoop delivers a consistent amount of dye to the titration. This is the conventional way solid metal indicators are supplied.
At what pH is murexide used for calcium?
Murexide is used for the direct EDTA titration of calcium at high pH, typically around pH 12–13, where magnesium is precipitated as hydroxide and calcium is titrated selectively. The high pH is established with a strong base before adding the indicator and titrating. The color changes from the calcium-complex hue to the free-indicator color at the endpoint.
How much indicator should I add to a titration?
Only a small amount is needed- typically a small scoop or a few tens of milligrams of the NaCl-ground mixture per titration flask, enough to give a clear color without itself consuming significant titrant. Using a consistent small amount each time helps make the endpoint reproducible. Follow your method's specified quantity.
Why does murexide need to be kept dry and dark?
The dye degrades on exposure to light and moisture, which can weaken or shift its color response and blur the endpoint. Keeping the mixture tightly closed, cool, dry, and out of the light preserves its performance. Discard the indicator that no longer gives a sharp, expected color change at the endpoint.
Can murexide be used for metals other than calcium?
Yes. Besides calcium, murexide serves as a complexometric indicator for nickel, copper, and cobalt titrations, forming distinctively colored complexes with each. The specific color change and optimal conditions vary by metal, so follow the analytical method for the metal you are determining, including the required pH and any masking agents.
