xii SUMMARIES OF PAPERS I N THIS ISSUE [June, 1974Ionic Polymerisation as a Means of End-point Indication inNon- aqueous Thermometric TitrimetryPart VI. The Determination of ThiolsAlkyl and aryl thiols have been determined in the presence of carboxylicacids and phenols by means of acid - base catalytic thermometric titrimetry.Two titrations are carried out, with acrylonitrile and acetone as the end-point indicators. With the former indicator, thiol groups are not deter-mined, so that the difference between the titration values obtained by usingthe two methods of end-point indication is a measure of the thiol content.The thiol content of 2-mercaptothiazoline, 4,6-dihydroxypyrimidine-2-thiol (2-thiobarbituric acid), purine-6-thiol and 2-mercaptobenzimidazole canbe determined by the same procedure.In the titration of 2-thiohydantoin,4-hydroxypyrimidine-2-thiol (2-thiouracil), 2-mercaptobenzoxazole and2-mercaptobenzothiazole, however, both end-point methods give the sametitration value. These apparently anomalous results can be explained if itis accepted that the last four heterocyclic thiols exist in the thione tautomericform in dimethylformamide solution. Some thioamides also titrate as acids,and differences between titration values obtained by using the two methodsof end-point indication can again be attributed to thione - thiol tautomerism.Thiols can be determined conveniently in amounts down to 0.01 mequiv,ie., about 2 mg of dodecane-1-thiol, with 0.1 M titrants. In instances whenthe acrylonitrile method can be used for the direct determination of the thiolfunction, 0.001 M titrant can be used and the lower level of determination isthen about 0.0001 mequiv.E.J. GREENHOW and Miss L. H. LOODepartment of Chemistry, Chelsea College, University of London, Manresa Road,London, S.W.3.Analyst, 1974, 99, 360-366.Determination of Ammonia Levels in Water and Wastewater Withan Ammonia ProbeThe application of an ammonia probe has been investigated for discretelaboratory measurement of ammonia levels in a variety of waters. The probedisplays a Nernstian response for the range 0.2 to 40 mg 1-1 of ammoniacalnitrogen in a stirred 0.1 M sodium hydroxide solution containing 0.01 Methylenediaminetetraacetic acid. Recoveries of added ammonia from awide range of water samples are satisfactory.Both these recoveries of addedammonia and repeated calibrations of the probe suggest a precision of 4 percent. for ammoniacal nitrogen concentrations greater than 0.4 mg 1-1 and0-015 mg 1-1 for concentrations less than 0-4 mg 1-l; the statistical limit ofdetection is 0-03 mg 1-l. Good agreement is obtained with existing methodsbased on distillation and spectrophotometric measurement for a furtherrange of samples, but the limit of detection and the precision a t low levelssuggest that accurate determination in potable waters would be difficult.The probe can also be used to determine albuminoid nitrogen by takingthe difference between the ammoniacal nitrogen and the total free plusalbuminoid nitrogen obtained by distillation.Values obtained in this wayagree with those obtained by existing methods subject to the precision of theprobe being acceptable.W. H. EVANS and B. F. PARTRIDGEDepartment of Trade and Industry, Laboratory of the Government Chemist,Cornwall House, Stamford Street, London, SEl 9NQ.Analyst, 1974, 99, 367-375XiV SUMMARIES OF PAPERS I N THIS ISSUE [June, 1974The Coulometric Determination of Trace Levels of Sulphur in GalliumPhosphideSulphur present in gallium phosphide has been determined. At 950 "Cgallium phosphide reacts with platinum and hydrogen to form Ga,Pt, andplatinum phosphide (PtP,) , and sulphur is catalytically converted intohydrogen sulphide. The latter is absorbed in an alkaline medium and deter-mined by controlled-potential coulometry at a silver-gauze electrode. Thedetermination of 0.05 pg of sulphur is shown to be possible, the blank valueby this method amounting t o 0.01 pg of sulphur.K. GIJSBERS, L. BASTINGS and R. van de LEESTPhilips Research Laboratories, Eindhoven, The Netherlands.Analyst, 1974, 99, 376-380