- What it is. This murexide indicator is a dry mixture of murexide ground with sodium chloride, supplied in a 100 g size. It is the classic indicator for titrating calcium with EDTA at high pH, separately from magnesium.
- Chemical identity. Murexide is ammonium purpurate (CAS 3051-09-0, C8H8N6O6, 284.19 g/mol), a purple dye derived from purpuric acid. It forms colored complexes with calcium and several other metal ions.
- How the endpoint works. At pH 12 to 13, murexide bound to calcium is pink to red. As EDTA is titrated in, it removes calcium from the indicator, and at the endpoint the solution turns violet or purple, the color of free murexide at high pH.
- Why high pH separates calcium from magnesium. Raising the sample pH to about 12 with sodium hydroxide precipitates magnesium as magnesium hydroxide, so only calcium is titrated. Subtracting calcium from total hardness, measured with Eriochrome Black T at pH 10, gives magnesium by difference.
- Why it is mixed with sodium chloride. Murexide solutions are unstable and lose sensitivity within hours to days. Grinding the dye with sodium chloride produces a stable dry indicator that keeps well and lets the analyst add a consistent small scoop to each sample.
- The complete calcium titration. Pair murexide with Lab Pro's EDTA Titrant 0.1 N / 0.05 M or EDTA Disodium Solution, 0.1 M (0.2 N). For total hardness, use Eriochrome Black T Indicator at pH 10.
- Calculating calcium hardness. Calcium hardness (mg/L CaCO3) equals the EDTA volume multiplied by its molarity and 100,090, divided by the sample volume in milliliters (calculated from the molar mass of calcium carbonate). Use the standardized EDTA molarity for accurate results.
- Using the indicator. Add about 0.1 to 0.2 g of the mixture to the buffered sample, swirl to dissolve, and titrate promptly, since the endpoint color can fade at high pH. Keep the amount consistent between samples and blanks.
- Where it is used. Murexide is used to determine calcium hardness in drinking water, boiler and cooling water, and wastewater; calcium in milk, minerals, and cement; and in teaching labs demonstrating complexometric titration.
Technical Specifications
| Product | Murexide indicator, ground with sodium chloride |
| Active Indicator | Murexide (ammonium purpurate) |
| CAS Numbers | 3051-09-0 (murexide); 7647-14-5 (sodium chloride) |
| Formula (murexide) | C8H8N6O6 |
| Molecular Weight (murexide) | 284.19 g/mol |
| Diluent | Sodium chloride |
| Endpoint (Calcium with EDTA) | Pink/red to violet/purple |
| Working pH | About 12 to 13 |
| Typical Dose | About 0.1 to 0.2 g per titration |
| Form | Dry powder mixture |
| Stability | Dry mixture stable; solutions deteriorate rapidly |
| Hazard Profile | Low (confirm on SDS) |
| Package Size | 100 g |
Applications / Common Uses
- Calcium hardness: Titrates calcium separately from magnesium.
- Magnesium by difference: Combines with total hardness to calculate magnesium.
- Boiler and cooling water: Monitors calcium in industrial water.
- Food and mineral analysis: Determines calcium in milk, minerals, and cement.
- Teaching labs: Demonstrates complexometric titration.
Storage, Handling & Shipping
Store tightly closed at room temperature in a dry place, protected from light and moisture. Murexide is stable as a dry mixture but deteriorates in solution, so add the powder directly to each sample rather than preparing stock solutions.
- Incompatible materials: Strong oxidizers and strong acids.
- Personal protection: Safety glasses and gloves; the dye stains skin and surfaces.
- Spill response: Sweep up without raising dust and wipe residue with a damp cloth.
- First aid: Eyes: rinse with water for several minutes. Skin: wash with soap and water.
Related Categories & Services
- Shop Buffers, Standards & Solutions for related products in this category.
- Explore General Chemistry and ACS Grade Laboratory Chemicals.
- Vendor Managed Inventory (VMI) Program: Lab Pro can stock, track, and replenish this item on site so it never runs out. Contact info@labproinc.com or 888-452-2776 to set up a VMI assessment.
