CALCEIN (fluorescein complexone) has been developed by Diehl and Ellingboel as an indicator for calcium. It is analogous to o-cresolphthalein complexone2 (called "Phthalein Complexone" by Anderegg, Flaschka, Sallmann and Schwar~enbach~) . Calcein is more sensitive than murexide to calcium ions, but its use in titrations is similar. In neutral solutions it gives a green fluorescence, which changes on adding alkali to a pinkish orange with some residual fluorescence. In the presence of calcium, strontium, barium and magnesium ions the fluorescence is intensified and persists in alkaline solutions. The fluorescence due to magnesium is only extinguished when enough alkali is added to precipitate magnesium hydroxide completely.The colour change on titrating calcium with ethylenediaminetetra-acetic acid (EDTA) above pH 12 is from a bright green fluorescence to a pinkish orange and in good diffuse daylight this change can be seen clearly A suitable indicator mixture consists of 0.2 g of calcein, 0.12 g of thymolphthalein and 20g of potassium chloride ground together to a fine powder. About 10 mg of this powder are required for a titration volume of 50 ml. At this concentration the green colour is less obviously fluorescent and the residual fluorescence at the end-point does not obtrude. Under the best conditions the addition of less than 0.1 ml of 0.01 N EDTA solution to a volume of 50 ml gives the complete colour change from green to purple. As with niurexide and Eriochrome black T, an excess of indicator must be avoided. Magnesium may also be titrated by using a calceinthymol blue mixture if the pH is not high enough to precipitate the hydroxide, but the end-point is much inferior t o that of o-cresolphthalein complexone or Eriochrome black T.As none of the indicators so far suggested [murexide, calcein, 2-hydroxy-l-(2-hydroxy-4sulpho-l-naphthylazo)-3-naphthoic acid41 is insensitive to magnesium, the titrations of calcium It is much improved, however, if thymolphthalein is added.

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