Calmagite
Calmagite is a complexometric indicator used in analytical chemistry for the detection and quantification of metal ions in solution. It is particularly useful in titrations involving calcium or magnesium with ethylenediaminetetraacetic acid (EDTA), where it changes color upon binding to metal ions. In its free form, calmagite can exhibit colors such as blue, red, or orange, but when it binds to a metal ion, it turns wine red[1][6][15].
The chemical structure of calmagite is represented by the molecular formula C17H14N2O5S, and its molecular weight is 358.37 grams per mole[6][7]. The structure includes a sulfonic acid group, which contributes to its solubility in water and its ability to act as an indicator in aqueous solutions[4][5].
Calmagite is often used in conjunction with EDTA because EDTA is a stronger metal binding agent. During a titration, the metal ions are first captured by the EDTA, and once all the free metal ions are complexed, the EDTA then reacts with the metal ion-calmagite complex. This reaction is possible because the formation constant for the metal-calmagite complex is smaller than that for the metal-EDTA complex, allowing EDTA to effectively remove the metal ions from the calmagite, resulting in a color change that indicates the endpoint of the titration[13].
The color change observed at the endpoint corresponds to the reaction where the metal ion-calmagite complex (which is wine-red) reacts with the EDTA to form a metal ion-EDTA complex and a free indicator (which is colorless or blue, depending on the pH)[13].
Calmagite is also used for the determination of magnesium in biological materials and in the assessment of water hardness[3][13]. It is available from various suppliers and manufacturers in different grades, such as indicator grade, and can be purchased in various quantities[4][14][15][16][17].
Citations:
[1] https://en.wikipedia.org/wiki/Calmagite
[2] https://www.drugfuture.com/chemdata/Calmagite.html
[3] https://www.sciencedirect.com/science/article/pii/0003269769901055
[4] https://www.sigmaaldrich.com/US/en/product/aldrich/c204
[5] https://www.researchgate.net/figure/Chemical-structure-of-Calmagite_fig1_324862121
[6] http://www.dubichem.com/product/calmagite
[7] https://pubchem.ncbi.nlm.nih.gov/compound/Calmagite
[8] https://www.medchemexpress.com/Calmagite.html
[9] https://pubmed.ncbi.nlm.nih.gov/5398269/
[10] https://www.riccachemical.com/products/chemical-indicators/complexometric-indicators/1830-%28-r1830000-%29
[11] https://www.researchgate.net/figure/Structures-of-Calmagite-and-Orange-G-in-their-more-stable-hydrazone-forms_fig1_5861730
[12] https://www.tandfonline.com/doi/abs/10.1080/03067319.2019.1691182
[13] https://www.chem.uci.edu/~unicorn/M3LC/handouts/Week7/calmagite.pdf
[14] https://www.spectrumchemical.com/calmagite-reagent-acs-c2362
[15] https://www.thermofisher.com/order/catalog/product/B21482.06
[16] https://www.chemworld.com/Calmagite-p/ca2250-i.htm
[17] https://www.sigmaaldrich.com/US/en/substance/calmagite358373147146