Summary. The estimation of molybdenum and tungsten when present together has proved difficult, as is indicated by the large number of methods put forward for their determination. 1 Many of the methods given are for one or other of these elements only, a note often being added that the presence of the other invalidates the method. The estimation of molybdate and tungstate in the mineral powellite has been briefly described by one of the authors (A. A. M.) :2 molybdate and tungstate were precipitated as oxinates (Mo,W)O~(CgHsON)2 and the molybdate (MoO~") determined by measurement of the optical density at 230 m~ of a solution containing molybdate and tungstate. The amount of tungstate present was then estimated by difference. The property of the molybdate ion, MoO~", to absorb strongly in the ultra-violet has been further investigated and it has been found that, if a wave-length of 240 m/~ is chosen and the proper precautions taken, the MoO~" ion can be estimated without significant interference by the W04 '~ ion. Methods are given for the analysis of the common molybdate and tungstate minerals, together with results obtained during the course of this investigation.T HE analysis of molybdates and tungstates resolves itself into three parts: the separation of the anions from the cations, the estimation of the cations, and the estimation of the anions.The methods used to separate the anions from the cations and to estimate the cations depend on the nature of the nfineral but all methods yield a solution of sodium molybdate and sodium tungstate sensibly free from the mineral cations, and the method of analysis of the anions is the same for all minerals, a Experience has shown that, provided the conditions set out below, under scheelite, are followed, molybdate and tungstate can be precipitated together quantitatively, by means of oxine.A solution of sodium molybdate containing about 10 ppm MoO a shows no appreciable absorption above 275 m~. At wave-lengths less than this the absorption increases very rapidly but does not attain a maximum within the limits imposed by the instrument used (Unieam SP 500) : with similar concentrations of tungstate the absorption begins at much lower
