How to Prepare Bromcresol Green pH Indicator

Bromocresol green, also called 3′,3′′,5′,5′′-Tetrabromo-m-cresolsulfonphthalein, is a pH indicator that is frequently used in both analytical chemistry and biology laboratories. . In the chemistry laboratory it is used as an indicator in acid/base titrations, while in the biology laboratory it is used to demonstrate the activity of certain enzymes.

This is one of several pH indicators of practical importance and often in use. For this reason, it is important to know how to correctly prepare this solution so that, when used, it presents color changes of consistent intensity and, in addition, in the appropriate pH range for its multiple applications.

What is a pH indicator?

The pH indicators are solutions of weak acids or bases that have very different intense colors in an acid medium and in an alkaline medium. That is to say, they are substances whose conjugate acids or bases have a markedly different color in solution from the respective base or original acid, so they serve to indicate, visually, the pH range of a solution.

color change interval

The color that each indicator presents in a solution is determined by the proportion of indicator molecules that are ionized with respect to those that are not. That is, it is determined by the degree of ionization. This, in turn, is governed by the acid-base balance of each indicator and by its acidity or bacisity constant, as shown below:

The equilibrium constant for this reaction (K _a ) determines the concentrations of all species at equilibrium as follows:

• When the pH of the solution is equal to the pKa of the indicator (pKa is the negative logarithm of the acidity constant, K a ₎ , then both species, the acid and its conjugate base, are at the same concentration.
• If the pH is lower, then the color of the protonated indicator will predominate.
• If the pH is greater than the pKa then the color of the ionized indicator will predominate.

For this reason, the pKa is the determining factor for the selection of a pH indicator for each application.

Bromocresol green as a pH indicator

In the case of bromocresol green, its pKa has a value of 4.7, so the color change from an acidic to a basic medium is in a range of pH values ​​that goes from 4.4 to 5.0. This indicator presents an intense blue color when it is ionized, ie in a basic medium . On the other hand, when it is fully protonated, ie in an acid medium , it shows a yellow color .

Structure of bromocresol green

Bromocresol green has the molecular formula C ₂₁ H ₁₄ Br ₄ O ₅ S with a molar mass of 698.01 g/mol. Its molecular structure is presented below:

As you can see, it is an organic compound with two phenol groups that can ionize, thus acting as an acid. This compound can be synthesized by bromination of cresol purple which has basically the same structure but without the 4 bromine atoms. However, it is common to prepare the indicator from the already synthesized free acid, which is a light brown crystalline solid. Also, the solution can be prepared from the sodium salt which is easier to dissolve in water. This is also a crystalline solid, but with a dark green color due to the presence of the conjugate base.

Bromocresol Green pH Indicator Uses

• It is used as an acid-base indicator for strong acid and base titrations.
• It is used as an acid-base indicator for weak acid titrations.
• It is used as a pH indicator in different bacterial cultures to highlight the fermentation processes that acidify the medium.

Preparation of bromocresol green indicator solution

Now that we know what it is and what it is used for, we will see how to prepare the bromocresol green pH indicator solution.

Two different procedures for its preparation are presented below, one from the acid and the other from the sodium salt. However, in both cases the result will be the same: 100 mL of a 0.05% solution.

Preparation of the indicator from pure bromocresol green

For this preparation you will need the following reagents:

• Bromocresol green (light brown crystalline solid).
• Sodium hydroxide (NaOH) solution 0.1 M.
• 95% V/V ethanol.
• Deionized water.

You will also need the following laboratory materials and instruments:

• Analytical balance.
• 1 mL graduated pipette
• 20 mL graduated pipette
• 100 mL volumetric flask

Procedure

1. 50 mg (0.0500 g) of solid bromocresol green is weighed on an analytical balance and transferred to the 100 mL volumetric flask.
2. Using a 1.0 mL graduated pipet, transfer 0.72 mL of 0.10 M sodium hydroxide solution to the flask containing the bromocresol green and shake to mix and allow for the neutralization reaction. A sharp color change from light brown to deep dark green should be observed.
3. Using the 20 mL pipette, 20 mL of 95% ethanol is transferred to the flask containing the mixture and shaken until completely dissolved.
4. Water is added to the flask up to the 100 mL volume mark.
5. Finally, it is covered and shaken to homogenize the solution.

Preparation of the indicator from the sodium salt of bromocresol green

If the sodium salt is used instead of bromocresol green, the neutralization step with sodium hydroxide is not necessary since the salt is already soluble in water.

In this case, you will only need:

• Bromocresol green sodium salt (dark green crystalline solid).
• 95% V/V ethanol.
• Deionized water.

You will need the same laboratory materials and instruments except for the 1.0 mL graduated pipette.

Procedure

1. 51.6 mg (0.0516 g) of the bromocresol green sodium salt are weighed on an analytical balance and transferred to the 100 mL volumetric flask.
2. Using the 20 mL pipet, transfer 20 mL of 95% ethanol to the flask and shake until the salt is completely dissolved. A dark green solution should be obtained.
3. Water is added to the flask up to the 100 mL volume mark.
4. Finally, it is covered and shaken to homogenize the solution.

sensitivity test

It is important to carry out a preliminary test of the indicator to ensure that it turns color in the proper pH range, and that the colors are intense enough to be detected with the naked eye. For this, the following test is carried out:

1. Add 0.2 mL (approximately 4 drops) of bromocresol green indicator to 100 mL of degassed, deionized water in a beaker. The result should be a blue colored solution.
2. The color should change to yellow with the addition of not more than 0.2 mL (4 drops) of 0.02 M hydrochloric acid, as this produces a solution with a pH of 4.40).

References

Bromocresol green safety data sheet (nd). Retrieved from https://www.fishersci.com/store/msds?partNumber=B3835&productDescription=BROMOCRESOL+GREEN+ACS+5G&vendorId=VN00033897&countryCode=US&language=en

Bromocresol Green sodium salt – Product Specifications (nd). Retrieved from https://www.sigmaaldrich.com/specification-sheets/108/559/B1256-BULK________SIAL_____.pdf

Choudhary, A. (nd). Preparation of indicator solutions. Retrieved from https://www.pharmaguideline.com/2010/09/preparation-of-indicator-solutions.html

Acid-base titrations. (2020, October 30). Retrieved July 10, 2021, from https://espanol.libretexts.org/@go/page/1916