Murexide Test: Procedure, Uses, and Applications

Murexide, as a special indicator, plays a key role in the Murexide test. By observing the color change that occurs after a sample reacts with Murexide reagent, the concentration of the target substance in the sample can be accurately determined, providing a reliable basis for further research on the properties and content of substances.

What is the Murexide Test?

Murexide (NH4C8H4N5O6 or C8H5N5O6·NH3), also known as ammonium purpurate or MX, is the ammonium salt of purpuric acid; its English name is Murexide. The Murexide test is an analytical technique used to detect the presence of caffeine and other purine derivatives in a sample. These compounds do not react to common alkaloid identification tests, such as the Dragendorff reagent. In this test, the drug (to be identified) is mixed with a trace of potassium chlorate and a drop of hydrochloric acid. The sample is then evaporated to dryness, and the resulting residue is exposed to ammonia vapor. Purine alkaloids produce a pink-purple color in this test due to the formation of Murexide (ammonium purpurate, purple in pure form), which gives the test its name.

In its pure form, Murexide is purple, but when it is produced by the reaction of purines with acidified ammonia, it generates various shades of purple and pink.

What are the Uses of Murexide?

Complex indicator (for titration of calcium, copper, cobalt, nickel, manganese, scandium, zinc, etc.). Photometric determination of calcium.

Direct titration: Ca, Co, Cu, Ni

Back titration: Ca, Cr, Ga

Displacement titration: Ag, Au, Pd

Colors of Murexide Indicator when Combined with Metal Ions

• Calcium ion: Pink (pH > 10 NaOH solution)
• Cobalt ion: Yellow (pH 8-10 ammonia buffer)
• Copper ion: Orange (pH ≈ 4 acetate buffer) or Yellow (pH 7-8 ammonia buffer)
• Nickel ion: Yellow (pH 8.5-11.5 ammonia buffer)
• Zinc ion: Pink (pH 5-7 acetate buffer)

Acid-Base Indicator

Murexide is a valuable acid-base indicator, exhibiting different color changes at different pH levels.Murexide color change from yellow in acidic conditions (pH below 4.5) to bright purple in alkaline conditions (pH above 7.5), with a transition occurring between pH 5.5 and 6.5. This color change provides researchers with a clear visual cue to accurately determine the endpoint during titration, helping to determine the concentration of an unknown solution.

Detection of Specific Compounds

Murexide reacts with specific compounds to form colored complexes, which is particularly important for detecting and identifying specific compounds such as calcium and magnesium in biological samples.

Applications and Uses of the Murexide Test

The Murexide test is a color test for uric acid and some other purines. (Solid) samples are first treated with a small volume of concentrated acid (such as hydrochloric or nitric acid) and slowly evaporated; if uric acid is present, the subsequent addition of ammonia (NH3) will turn it purple due to the formation of Murexide. If xanthine or its derivatives are present, they will turn yellow when heated.

What is the Principle of Murexide Test?

The Murexide test is a colorimetric method used to identify purine derivatives, particularly the presence of uric acid. The test involves two steps:

Oxidation

Treat the sample containing purine derivatives with concentrated nitric acid and heat it. This process oxidizes the purine compounds to form a product known as a denatured toxin.

Color Formation

Add ammonia to the oxidized product. If uric acid or other purine derivatives are present, they will react with ammonia to form Murexide, a purple compound. The intensity of the purple color is proportional to the concentration of purine compounds in the sample.

How to Perform the Murexide Test at Home or in a Laboratory?

The Murexide test is a colorimetric test used to detect uric acid and some other purines. Here are the detailed steps:

Materials

• Sample containing uric acid
• Concentrated nitric acid (HNO3)
• Ammonia solution (NH3)
• Porcelain dish or evaporating dish
• Water bath

Steps

Sample Preparation

Ensure the sample is dry and finely powdered.

Oxidation with Nitric Acid

Place a small amount of the sample in a porcelain dish. Add a few drops of concentrated nitric acid to the sample. Gently evaporate the mixture to dryness over a water bath. This step oxidizes uric acid to form a denatured toxin.

Formation of Murexide

Allow the residue to cool completely. Add a drop of dilute ammonia to the residue. The presence of uric acid is indicated by a purple color, forming the purple compound Murexide.

Observation and Interpretation

Positive result: Purple indicates the presence of uric acid. Negative result: No color change or different color indicates the absence of uric acid.

Precautions

• Nitric acid is a strong acid. Handle with care and wear appropriate protective equipment.
• Ammonia solution has a strong odor. Work in a well-ventilated area.
• Other compounds, such as xanthine and its derivatives, can also produce a positive Murexide test, but the color produced may vary slightly.

References:

• Sánchez-Viesca F, Gómez R. On the mechanism of the murexide reaction[J]. World J. Org. Chem, 2019, 7(1): 14-18.
• https://www.oxfordreference.com/display/10.1093/oi/authority.20110803100217202
• https://baike.baidu.com/
• https://es.wikipedia.org/wiki/Wikipedia:Portada
• https://www.britannica.com/