THYMOL BLUE: CHARACTERISTICS, PREPARATION AND APPLICATIONS - BIOLOGY - 2024

The thymol blue is a substance of organic nature recognized for its utility as a pH indicator in chemical laboratories. It is also known by the name of thymol sulfonaphthalene and its chemical formula is C ₂₇ H ₃₀ O ₅ S.

It has two turn intervals, one located in the acidic zone and the other located in the alkaline zone. At pH below 2.8 the color of the indicator is red. Immediately afterwards is the first turn interval, located between 1.2 and 2.8 where it turns brownish yellow.

Colors of thymol blue indicator at different pH. Source: LHcheM edited image.

This color remains stable until reaching pH 8, where the second turning interval is located, between 8.0 to 9.6, turning to blue-violet. At present it is being used for very interesting applications, such as in the creation of a chemical sensor (optode) impregnated with thymol blue that works thanks to the sensitivity of this substance to changes in pH.

Thymol blue must be handled with biosafety instruments, as it is a substance that is irritating to the skin and mucous membranes. In case of direct contact, splashing, ingestion or accidental inhalation, first aid procedures should be applied and a doctor is sought.

Regarding risk and toxicity, this substance is classified as a compound with risk grade 1 for the 3 parameters (health, flammability and reactivity). The numbering 1 indicates that the existing risk for the three aspects mentioned above is slight. However, it is considered a corrosive substance.

characteristics

The most outstanding characteristics are that thymol blue is a compound that has a crystalline appearance and is brownish-green in color.

Water cannot be used to prepare an aqueous solution of this pH indicator, because it is not soluble in this solvent. In this case, ethyl alcohol or dilute alkaline solutions can be used.

Thymol blue has a characteristic odor. Its molecular mass is 466.60 g / mol, it has a melting point that ranges between 221-224 ° C, while its density is 1.19 Kg / L.

Preparation

After being prepared, it should be stored between 15 ° C-20 ° C temperature. Below are several preparation recipes.

Thymol Blue pH Indicator Solution

Weigh out 0.1 g of thymol blue and dissolve in 2.15 ml of a 0.1 molar sodium hydroxide solution and 20 ml of ethanol (95%). Subsequently add water to complete 100 ml.

Ethanolic solution of thymol blue pH indicator

Weigh 0.1 g of thymol blue in 100 ml of ethanol (95%), and then filter if necessary.

Thymol Blue Indicator Solution with Phenolphthalein

Prepare a mixture of 2.2 ml of 0.1 molar sodium hydroxide and 50 ml of ethanol (95%) and dissolve there 0.1 g of previously weighed thymol blue. Make up the volume with water up to 100 ml.

Subsequently, take 3 volumes of this solution and mix with 2 volumes of a phenolphthalein solution.

Yamada Gauge

In certain titrations a mixture of some acid-base indicators can be used, which is called "Yamada indicator" in honor of its creator. This indicator can be prepared as follows.

- Weigh out 0.05 g of thymol blue, 0.125 g of methyl red, 0.6 g of bromothymol blue and 1.0 g of phenolphthalein.

- Dissolve in 1 liter of ethanol. The solution will present a strong red color, which must be neutralized with a few drops of a 0.05 mol / L NaOH solution until it turns green.

- Place about 500 ml of water and add the coloring. Then, dilute in 2 L of distilled water. This combination offers the following color twists:

- Red pH ≥ 0 - ≤ 3

- Yellow: pH> 3 pH ≤ 6

- Green pH = 7

- Blue ≥ pH 8 - <11

- Purple: pH ≥ 11 - ≤ 14

Toxicity

Thymol blue produces a slight irritation of the skin and the ocular mucosa in direct contact. It is also harmful if ingested or inhaled. In the case of contact with skin and mucous membranes, it is recommended to wash the affected area with plenty of water. An emollient cream can be placed on the skin to relieve irritation.

In case of ingestion and inhalation, immediate medical assistance should be sought by going to the nearest health center.

In this sense, the NFPA (National Fire Protection Association) classifies this substance with a health, flammability and reactivity risk of 1. This means a low risk, for all 3 aspects.

Applications

Thymol blue has various uses, but its main use is as a pH indicator in acid-base reactions.

At the time of titration with thymol blue, if it was prepared with alcohol, it must be borne in mind that a drop of the alcoholic solution tends to spread and spread more easily than aqueous indicators. Therefore, there is a risk that the indicator comes into contact with different media prematurely.

Citric acid titration

Citric acid can be titrated using a 1N alkali; and as a pH indicator, the use of thymol blue is recommended.

Thymol blue used to make an optode (chemical sensor)

An optode (optical chemical sensor) based on the thymol blue pH indicator has recently been created. This pH indicator was chosen because it has two ranges of change, one at acidic pH and one at alkaline pH.

The methodology was proposed in order to detect CO ₂ in the flow injection analysis system (FIA) and spectrophotometric detection.

To do this, the researchers chemically fixed the thymol blue pH indicator on a group of bifurcated glass optical fibers through a process called silanization in organic media. Subsequently, the conditions are created for CO _{2 to} be formed using buffered hydrogen carbonate plus phosphoric acid.

The CO ₂ formed diffuses through a polytetrafluoroethylene membrane, being directed to the detection cell, where the optode is located, which in this case contains a substance that is sensitive to changes in pH.

The optode captures the substance to be measured, being able to colorimetrically mark the reactions produced there.

The described method obtained satisfactory results, very similar to those obtained by potentiometry when CO ₂ was determined in mineral water samples.

The optode made from borosilicate and thymol blue fiber optics had a short response time and a long service life.

Other apps

Gabriel et al. Studied the dependence of the molar absorptivities of thymol blue with temperature and salinity and its application in the spectrophotometric determination of pH in estuarine waters.

The investigation showed that there is a linear dependence of molar absorptivities with respect to temperature between 5 ° C and 30 ° C, and with salinity only for ₅₉₆ ε (I ^2-).

References

1. Sotomayor M, Raimundo J, IRohwedder J, Oliveira G. (2010). A pH optode based on thymol blue: application to determination of CO2 using flow injection analysis system. Eclectic Chemistry, 35 (2), 33-43. dx.doi.org
2. Gabriel M, Forja JM, Rubio J and Gómez-Parra A. Dependence of molar absorptivities of thymol blue on temperature and salinity: Application to the spectrophotometric determination of pH in estuarine waters. Science. 2005; 31 (1b)
3. Pharmaceutical guidelines. Available at: pharmaguideline.com
4. Wikipedia contributors. "Thymol blue." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 28 Mar. 2018. Web. 23 May. 2019.
5. Sánchez C., Francisco J., Cesteros, Carlos, & Katime, Issa A. (2006). Use of an infrared probe in situ to monitor esterification reactions. Engineering and Research, 26 (1), 5-14. Retrieved May 24, 2019, Available at: scielo.org.