The contamination of precipitates by substances that are normally soluble under the conditions of the precipitation is an important factor in the evaluation of reagents used m gravimetric methods of analysis. The problem of coprecipitation is generally associated with the use of inorganic precipitants, but with increasing use of organic reagents in gravimetry some knowledge of the relative tendencies of organic and inorganic precipitants to coprecipitate seems highly desirable.
A general discussion of sources and types of error possible in various uses of organic reagents for inorganic analysis has been presented by WEST~. In the discussion of gravimetric methods it was pointed out that a survey of accepted procedures failed to disclose infomlation concerning coprecipitation phenomena, an exception to this statement being the work of RYAN AND FAINER on the gravimetric determination of palladium with x:ro-phenanthroline2 in which IS disclosed that platinum metals other than palladium fail to yield precipitates with the reagent under test conditions. The presence of such metals during the determination of palladium, however, leads to slightly high results. A study dealing specifically with coprecipitation has been made by HECK, HUNT, AND MELLON~. Their study represents the first research on this type of error in the application of organic reagents to be published. This survey involved a comprehensive study of the coprecipitation phenomena of nitron nitrate and their conclusion was that only neglegible error could be traced to these effects.
The apparent failure of common procedures employing organic reagents to indicate the existence of coprecipitation can be attributed to a number of possibilities. In some cases there seems to be no doubt but what the organic compounds have superior characteristics. The work of MELLON and his coworkers cited above substantiates this, and there are good theoretical grounds for believing that certain types of coprecipitation would be minimized due to complexities of bonding, spatial restrictions, and steric hindrance.
Because of the real lack of facts concerning the coprecipitation problem in the use of organic reagents, a study seemed warranted which would survey the Roferonccs $. 565.
PH.