The article contains sections titled: 1. Introduction 2. pH Indicators 2.1. Theory 2.2. Errors 2.3. Color‐Change Transition 2.4. Classification 2.4.1. Azo pH Indicators 2.4.2. Nitro pH Indicators 2.4.3. Phthalein pH Indicators 2.4.4. Sulfonephthalein pH Indicators 2.4.5. Triphenylmethane pH Indicators 2.4.6. Benzein pH Indicators 2.4.7. Fluorescent pH Indicators 2.4.8. Miscellaneous pH Indicators 2.4.9. Mixed Indicators 2.5. pH Indicator Papers 2.6. pH Indicators for Nonaqueous Solutions 3. Adsorption Indicators 4. Fluorescent Indicators 5. Chemiluminescent Indicators 6. Redox Indicators 7. Metal Indicators 7.1. Volumetry (Complexometry) 7.2. Colorimetry and Photometry 7.3. Structure 7.4. Ligand Classes 7.4.1. Ligands Forming Five‐Membered Chelate Rings 7.4.2. Ligands Forming Six‐Membered Chelate Rings 8. Analytical Test Kits

OF CONTENTS Introduction Kale of nucleation in precipitate formation Methods of precipitation Increase in pH Anion rclease Cation release Reagent synthesis Precipitation from mixed solvcnts Valency change Photochemical action Main group elements Transition elements ,4pplications t o gravimetric analysis Co-precipitation and fractional precipitation Industrial applications Miscellaneous applications Conclusions * Reprints of this paper will be available shortly.* This was drawn t o our attcntion when the paper was in proof.

A method is prcscntecl for the selective extraction of tin(l\') iodicic from sulphuric acid solution into toluene. Tin(I\-) is then returned t o aqueous solution by shaking the toluene cstr-act \\.it11 dilute sodium hydroxide solution. The metal is finally determined h y spcctropliotometric measurement of the colour o f t h e complex formed between tin( and catcchol x-iolet. The method is cliiick, siniplc, sensitivv and highly selcctive.

Three colour reactions that seemed to merit particular attention have been examined. Certain interferences in the determination of ferrous iron with 4,7-diphenyl-1,lO-phenanthroline have been overcome. Some interferences in the determination of ferrous iron with syn-phenyl 2-pyridyl ketoxime are reported, and the application of this reagent to the determination of iron in glass sand has been confirmed. The colour reaction between ferric iron, ethylenediaminetetra-acetic acid and hydrogen peroxide is recommended for determining larger amounts of iron, particularly in copper and its alloys.

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