What is Alizarin Yellow and How to Use It as a pH Indicator?


What is Alizarin Yellow and How to Use It as a pH Indicator?

Alizarin yellow is a yellow-colored azo dye that is commonly used as a pH indicator in various applications. It is also known as alizarin yellow GG, alizarin yellow 2G, metachrome yellow, mordant yellow 1, or C.I. 14030. In this article, we will explain what alizarin yellow is, how it works as a pH indicator, and how to use it in different scenarios.

What is Alizarin Yellow?

Alizarin yellow is an organic compound that has the chemical formula C13H8N3NaO5. It is usually commercially available as a sodium salt, but it can also be obtained as a free acid. Alizarin yellow is made by the diazo coupling reaction of 5-nitrosalicylic acid and aniline. In its pure form, it is a rust-colored solid.

How Does Alizarin Yellow Work as a pH Indicator?


What is Alizarin Yellow?

A pH indicator is a substance that changes color depending on the acidity or alkalinity of a solution. Alizarin yellow works as a pH indicator by changing from yellow to red as the pH increases from acidic to basic. The color change occurs because alizarin yellow exists in two different forms depending on the pH: the yellow form (HAY) and the red form (AY). The equilibrium between these two forms can be represented by the following equation:

HAY <--> AY + H

The equilibrium constant for this reaction is called the pKa, which is the negative logarithm of the acid dissociation constant (Ka). The pKa of alizarin yellow is about 11.0, which means that at pH 11.0, half of the alizarin yellow molecules are in the yellow form and half are in the red form. As the pH decreases below 11.0, more H ions are present in the solution, which shifts the equilibrium to the left and favors the formation of HAY. As a result, the solution becomes more yellow. Conversely, as the pH increases above 11.0, less H ions are present in the solution, which shifts the equilibrium to the right and favors the formation of AY. As a result, the solution becomes more red.

How to Use Alizarin Yellow as a pH Indicator?


How Does Alizarin Yellow Work as a pH Indicator?

To use alizarin yellow as a pH indicator, you need to prepare a solution of alizarin yellow in water or ethanol. The concentration of alizarin yellow does not affect the color change, but it affects the intensity of the color. A typical concentration range is between 0.01% and 0.1% (w/v). You can also add some buffer solution to adjust the pH range of the indicator.

To measure the pH of a sample solution, you need to add a few drops of alizarin yellow solution to it and observe the color change. You can compare the color with a standard color chart or use a spectrophotometer to measure the absorbance at different wavelengths. The absorbance peaks of alizarin yellow are at 257 nm and 362 nm for the yellow form and at 434 nm for the red form.

Some examples of using alizarin yellow as a pH indicator are:

  • In nutrient media preparation, alizarin yellow can be used to indicate whether the media is acidic or alkaline.
  • In histology and stains, alizarin yellow can be used to stain calcium deposits in tissues.
  • In corrosion inhibition, alizarin yellow can be used to monitor the corrosion rate of metals by measuring the change in pH.

Conclusion


How to Use Alizarin Yellow as a pH Indicator?

Alizarin yellow is a versatile and useful

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