Why is phenolphthalein and methyl orange not used as indicator in dictator of calcium
Both MO and phenolphthalein are indicators of pH of a solution; i.e., both are acid/base indicators. The calcium/EDTA titration is not an acid/base titration. The Ca/EDTA titration is a complexometric one.
Phenolphthalein and methyl orange are not suitable indicators for the titration of calcium ions because they undergo color changes at pH values that are different from the pH range typically involved in calcium titrations.
Phenolphthalein is a commonly used indicator for acid-base titrations, where it goes from colorless to pink in the pH range of about 8.2 to 10.0. However, calcium titrations typically involve a higher pH range, around 12 or above, where the phenolphthalein indicator remains pink. This means that the color change of phenolphthalein will not occur within the pH range of the calcium titration, therefore making it ineffective as an indicator.
Similarly, methyl orange is another acid-base indicator that transitions from red to yellow in the pH range of approximately 3.1 to 4.4. Unlike phenolphthalein, methyl orange undergoes a color change at a much lower pH range. In calcium titrations, the pH is usually well above the range where methyl orange changes color, making it unsuitable for detecting the endpoint of the titration accurately.
To titrate calcium ions, other indicators like eriochrome black T (EBT) or calmagite are commonly used. These indicators undergo color changes within the desired pH range for calcium titrations and ensure accurate determination of the endpoint.
Phenolphthalein and methyl orange are not suitable indicators for the determination of calcium ions due to different reasons:
1. Phenolphthalein: Phenolphthalein is a pH indicator that changes color in a specific pH range. It is typically used for acid-base titrations, as it changes from colorless to pink in the pH range of 8.2 to 10.0. However, calcium ions do not cause a significant change in pH, so phenolphthalein is not appropriate for calcium ion determination.
2. Methyl orange: Methyl orange is another commonly used pH indicator that changes color in a different pH range compared to phenolphthalein. It changes from red to yellow in the pH range of 3.1 to 4.4. Like phenolphthalein, methyl orange is not suitable for the determination of calcium ions because calcium ions do not cause a significant pH change within this range.
To accurately determine calcium ions, specific indicators such as complexometric indicators, like Eriochrome Black T (EBT) or Calcon, are used. These indicators form stable complexes with calcium ions, resulting in a color change that can be detected visually or instrumentally. These indicators have been specifically designed to be more sensitive and selective towards calcium ions, providing more accurate results in calcium determinations.