pH indicators are chemical compounds that change color in response to variations in hydrogen ion concentration, allowing for the qualitative and semi-quantitative determination of pH in various solutions. The functionality of these indicators is based on acid-base equilibria and their ability to exhibit distinct colors when existing in varying protonation states. When choosing a pH indicator, you must consider both the target pH range and the specific application needs, such as titrations, biochemical assays, and industrial processes. The acid-base pH indicator reference table from Alfa Chemistry contains information about common pH indicators and their transition pH ranges as well as their color changes.
| Indicator | pH Transition Range | pKa | Acidic Form (Low pH) Color | Basic Form (High pH) Color | Common Applications |
| Cresol Red (Step 1) | 0.2 – 1.8 | 1 | Red | Yellow | Strong acid detection |
| Malachite Green (Step 1) | 0.2 – 1.8 | 1.3 | Yellow | Green | Dye applications, bacterial staining |
| Thymol Blue (Step 1) | 1.2 – 2.8 | 1.7 | Red | Yellow | Acidic pH testing, dual transition use |
| Methyl Yellow | 2.9 – 4.0 | 3.3 | Red | Yellow | Acid-base titrations |
| Methyl Orange | 3.1 – 4.4 | 3.5 | Red | Yellow | Strong acid titrations |
| Bromophenol Blue | 3.0 – 4.6 | 4.1 | Yellow | Blue | Protein electrophoresis, pH monitoring |
| Congo Red | 3.0 – 5.0 | - | Blue-Violet | Red | Histological staining, acid-base indicators |
| Methyl Orange in Xylene Cyanol | 3.2 – 4.2 | - | Grey-Violet | Green | Screened indicator for titrations |
| Bromocresol Green | 3.8 – 5.4 | 4.7 | Yellow | Blue | Biochemical pH assays, industrial pH testing |
| Methyl Red | 4.4 – 6.2 | 5.1 | Red | Yellow | Microbiological tests, acid-base titrations |
| Methyl Purple | 4.8 – 5.4 | - | Purple | Green | pH determination in chemical solutions |
| Bromocresol Purple | 5.2 – 6.8 | 6.4 | Yellow | Purple | pH monitoring in biological systems |
| Azolitmin (Litmus) | 4.5 – 8.3 | - | Red | Blue | General pH indicator for qualitative testing |
| Bromothymol Blue | 6.0 – 7.6 | 7.3 | Yellow | Blue | Buffer systems, respiration studies |
| Phenol Red | 6.4 – 8.0 | 8 | Yellow | Red | Cell culture pH monitoring, water testing |
| Neutral Red | 6.8 – 8.0 | - | Red | Yellow | Microbiological and histological stains |
| Cresol Red (Step 2) | 7.2 – 8.8 | 8.5 | Yellow | Reddish-Purple | Dual-range pH testing |
| Naphtholphthalein | 7.3 – 8.7 | - | Pale Red | Greenish-Blue | Specialized acid-base indicators |
| Thymol Blue (Step 2) | 8.0 – 9.6 | 9.2 | Yellow | Blue | Dual-function pH determination |
| Cresolphthalein | 8.2 – 9.8 | - | Colorless | Purple | Strong base titrations |
| Phenolphthalein | 8.3 – 10.0 | 9.5 | Colorless | Red-Pink | Standard indicator for neutralization reactions |
| Thymolphthalein | 9.0 – 10.5 | 9.2 | Colorless | Blue | Alkaline titrations |
| Alizarin Yellow R | 10.1 – 12.0 | 11.2 | Yellow | Red | Textile dyeing, high-pH detection |
| Indigo Carmine | 11.4 – 13.0 | - | Blue | Yellow | Redox indicators, high pH monitoring |
| Malachite Green (Step 2) | 11.5 – 13.2 | - | Green | Colorless | Dye chemistry, pH-dependent bacterial staining |
Mechanism of pH Indicators
Most pH indicators are weak acids or bases that undergo a reversible reaction in response to pH changes. Their color change arises from structural modifications due to protonation or deprotonation. The general dissociation reaction of an indicator (HIn) can be represented as:
where:
HIn is the protonated (acidic) form of the indicator, exhibiting one color.
In- is the deprotonated (basic) form, displaying a different color.
The equilibrium constant for this reaction is expressed as:
The color transition occurs at a pH close to the pKa of the indicator, which defines the pH range over which the color shift is observable.
Applications of pH Indicators
- Acid-Base Titrations
Titration procedures use pH indicators extensively to identify equivalence points during acid-base reactions. Methyl orange and phenolphthalein serve as visual endpoints in strong acid-weak base and strong base-weak acid titrations, respectively.
- Industrial and Environmental Monitoring
Quality control in chemical manufacturing as well as wastewater treatment and food processing relies on the use of pH indicators by various industries. Environmental scientists use bromocresol green and bromothymol blue as tools for tracking pH changes in both soil and water samples.
- Biochemical and Medical Applications
Biochemical assays and respiratory gas analysis rely heavily on pH indicators, while cell culture media monitoring also requires their usage. In cell culture media, scientists use phenol red to monitor pH fluctuations, which ensures cells maintain optimal growth conditions.
- Microbiological and Biological Studies
Microorganisms release metabolic byproducts that are either acidic or alkaline to change their environmental pH levels. Bacterial differentiation assays and enzyme activity studies utilize acid-base indicators methyl red and bromophenol blue.
pH indicators serve as essential instruments for analytical chemistry and biological research as well as environmental monitoring and industrial processing. The careful choice of pH indicators is fundamental for achieving precise pH measurements across different applications. The reference table helps users select suitable indicators according to both the desired pH range and specific experimental needs.
Alfa Chemistry provides a table of pH values for common acids and bases for your reference: pH Scale of Common Acids and Bases. We also provide a chart of the strengths of the most common acids and bases for your reference:Chart of Acid and Base Strength.
