Kith the exception of an occasional determination, the analytical results for the recoveries using calcium or magnesium appear to be eqiially good, regardless of xhether pure calcium and magnesium mixtures alone or other mixtures were analyzed.Figure G s h o w the titrimetric curves obtained at 660 mp for varying concentrations of calcium between 1.25 and 5.0 mey per liter of spinal fluid. The intersection of the tangent to the steep ascending slope of the titration curve to a line parallel to the abscissa and tangent to the upper portion of the cuive indicates the end point.A >imilar study ip shoxn in Figure 7 , where increasing concentrations of concentration of magnesium in the range of 1.05 to 4 20 meq. per liter of spinal fluid are titrated a t 660 mp. The end point is graphically determined in the same manner a.; io1 calciiim. A method has been devised for the photometric determination of small amounts of chloride in water. The method is based on the displacement of thiocyanate f r o m mercuric thiocy-anate by chloride ion and the subsequent reaction of the liberated thiocyanate with ferric iron to form the colored complex [Fe(SCN)] ++, which is measured either visually or in a spectrophotometer. Concentrations of chloride as low as 0.05 p.p.m. can be determined. HE literature on the determination of chloridesisvoluminous.T \Yhether present as a required constitumt or as an impuiity, the chloride ion is usually determined by either gravimetric or volumetric methods. The oldest and the classical method is the gravimetric, in which the chloride ion is evaluated as silver chloride. ..inother method frequently employed is the volumetiic, several variations of which are available. The Volhard method, oiiginated by Carpentier ( 7 ) , described by T'olhard (38), and later improved by Lundbak (65) and others (26), is more accurate than the Mohr method ( 2 7 ) .A comparatively recent method is the mercurometric method, which was developed in 1933 by Dubskj. and Trtilek (9, 10).Diphenplcarbohydrazide v a s used as an indicator in the titration n-ith mercuric nitrate. Other 13-orkers (1, 5 , 8, 20, 62, 29,32, 54) later adopted this method with some modifications. ill1 these methods, homeever, are not always suitable for the detei mination of micro quantities of chloride.The present investigation was the result of a need for a simple colorimetric method for the determination of less than microgram quantities of chloride in condensate. Luce, Denice, and ilkerlund ( 6 4 ) determined small amounts of chloride turbidimet-1 ically. This method, however, lacked the required precision.Other methods (2, 4, 6, 14, 15, 17, 81, 28, 55, S9) for the determination of small amounts of chloride either required special apparatus or lacked the desired simplicity.The method presented here is a modification of that proposed b! . Vtsumi (36, 37) and followed up by Imasaki (16). This modified procedure has been greatly improved and broadened in ita application. The use of ferric perchlorate instead of ferric ammonium sulfate eliminates a ...