DETERMINATION OF MAN$ ANESE AS SULPHATE AND B$ THE SODIUM BISMUTHATE METHOD By William Blum CONTENTS Im n u m n n s m n ns 1 . porta ce of acc rate a ga e e deter i atio u s in v m m s 2 . So rce of error gra i etric ethod ur s in v u m m s 3 . So ce of error ol etric ethod THE B IS M UTHATE METHO D . Ou tline of m ethod H st m t . 2 . i ory of e hod n n z n s u . 3 . Sta dardi atio of ol tio (a) Discu ssion o f m ethods ‘ (b) Evidence b ased o n redu ction and reoxidation P n m s and s u ns 4 . reparatio of aterial ol tio (a) Water . (d) Potassiu m perm anganate (f ) Stab ility of perm anganate solu tions (g) Manganou s su lphate . (h) Sodiu m ox alate . s— b (i) Ferrou s su lphate solu tion Sta ility . (j) Nitric acid . (k) B ismu thate . ‘ (l) F erric nitrate (m ) Use of weight b u rettes Standardiz ation of perm anganate with sodiu m ox alate Standardiz ation of perm anganate With manganou s sulphate (a) Standardi z ation of m anganou s su lphate solu tions (b) Efie ct of conditions u pon the standardiz ations with m anga nou s su lphate - ( 1 ) F errou s su lphate permanganate ratio . m n m n n i m n (2 ) A ou t of a ganese prese t n a deter i ation . v u m m s n n (3 ) Acidity , ol e , ti e of ta di g , etc (4) Use of phosphoric acid (0) Prob ab le cou rse of reactions mm (d) Conditions reco ended . Agreement of valu es derived from sodiu m ox alate and m anganese su lphate Analysis of pu re permanganate crystals Analy sis of m anganese ores . mm Su ary . B u lletin of the B u reau of S tandards [VOL J I. INTRODUCTION 1 P C C C $ . IM ORTAN E OF A URATE MAN ANESE DETERMINATIONS In spite of the large number o f methods in use for the deterrnina , hi e ff nd tion of t s important element , r sults by di erent methods a i i diff erent chem sts seldom show satisfactory agreement . Wh le differences of as much as a f ew per cent of the manganese present have little commercial significance in iron and steel containing 1 o f ur per cent or less manganese , the highest possible acc acy is demanded in the analysis of high - grade materials such as manga Of hi un nese ore and ferromanganese , w ch large amo ts enter into m at co merce , prices dependent upon the results of analysis . For exam e 1 1 1 m, imports of manganese ore by this country in 9 n 1 8 2 1 1 86 1 6 . amou ted to 7 5 long tons , valued at $ 79 It can readily be seen that a constant error of 1 per cent in the analyses of such material may cause a considerable diff erence in the total amount paid for the ores . That constant errors of such magni tude are possible with our present methods will be shown in this paper . C OF IN $ V C H 2. SOUR ES ERROR RA IMETRI MET ODS Even with the greatest care the gravimetric results are not r o f necessarily accu ate , due not alone to the possibilities losses by solubility of precipitates ; and o f too high weight due to con tam inatio n from vessels or reagents ; but also to uncertainty in f the composition o the precipitates as weighed . The three forms in which manganese is most commonly determined gravimetri Mn M P Mn m 0 n O O . cally are 3 4 , , z 7 and S 4 It is generally ad itted r o f f that the fi st these is unsatis actory , as the composition depends directly upon the temperature o f the ignition and the nature o f e r the atmosphere surrounding the precipitat . Even unde care 1 fully regulated conditions Raiko w and Tisch l<1o w could not 2 obtain results which agreed to better than 1 part in 0 0 . $ ooch 1 t 3 5 . em . Z . xo r 1 1 r. Ch g , , p 3 ; 9 3 m .) Determination of M anganese and Austin 2 have shown that the composition of manganese pyrophosphate depends upon the content of ammonium salts and ammonia , and the temperature , volume , and method of pre ti cipita o n of the manganese ammonium phosphate . Even under “ o hi i in the conditi ns w ch they recommend , the r errors amounted s 1 o f ome cases to per cent the manganese present , and were in hi t e . r gen ral too gh The me hod can not , therefore , be conside ed f r hi n satis acto y for ghly accurate work , and certai ly not for obtaining a known amount of manganese to serve as a primary standard . Experiments described in this paper have led to i the conclusion that manganous sulphate , obta ned under proper $ n r hi i co ditions , is the most accurate fo m in w ch th s element can be weighed , both in gravimetric analysis and in securing a known o f a amount mang nese . O E OF ERROR IN V L C M ETHODS 3 . S URC S O UMETRI The difli cu lty of securing a known amount of manganese to serve as a primary standard has hindered the accurate investigation of the great number of volumetric methods which have been pro r posed . In most cases they have been tested by compa ison with r methods , gravimetric or volumet ic which had not been shown to hi u r . be int insically accurate T s fact , together with the us al de pendence o f the results of such methods upon the precise conditions of operation , has led to the publication of a large number of contra i to r d c . y papers upon these methods For example , the Volhard method and its various modifications has been the subject of over 0 i 5 investigations since its publ cation in 1 8 79 . It is generally admitted that the results by this method are low unless an em irical ndi p factor dependent upon the co tions of operation , is em re ployed , though some investigators have obtained theoretical lt un f su s d . V o der certain con itions In iew the above situation , it is highl y desirable to find some method which will yield results o f knownaccuracy , which are not closely dependent upon the exact o f hi r conditions operation , and w ch may se ve to test other methods . ’ Am i . 6 . Sc . 2 18 8 . J , , p 33 ; 9 - B u lletin. o th e B u reau o tandards o 8 f f S [V l . THE B IS M U HA E M E H II . T T T OD 1 L H . OUT INE OF MET OD Of various methods considered , the bismuthate appeared most f i promising , and has been ound to entirely fulfill the above requ re ments . In this method the manganese in nitric acid s olution is o f oxidized with sodium bismuthate , which the excess is removed by filtration through asbestos . To the resulting permanganic o f f r acid is added a measured excess e rous sulphate solution , which is then titrated with permanganate of known strength f and of known ratio to the errous solution . The investigation therefore resolved itself into a study o f the methods of standardiz ing the permanganate employed in the final titration and the influence o f the various conditions o f operation upon the results obtained by the bismuthate method . 2 H $ H . ISTOR OF MET OD The method as originally prepared by Schneider 3 depended upon the use o f bismuth tetroxide as the oxidizing agent and titra dr tion o f the permanganate acid with hy ogen peroxide . In this 4 f Cam re do n 5 orm the method was employed by p , Mignot and s Reddro 7 di jaboulay . p and Ramage mo fied it by employing hi di f f sodium bismuthate , w ch was more rea ly obtained ree rom r chlorine , and suggested filtration of the pe manganate acid di On o f rectly into the hydrogen peroxide . account the instability 3 o f the latter reagent Ibbotson and B rearley replaced it by ferrous i n h i f ammonium sulphate , w ch orm the method has been since 9 B di f o r used , being described in detail by lair , whose rections this hi method are generally followed in t s country . 3 P 269 2 1 888 D l . 2 . i . ng o y J , , p 4 ; 4 h 6 1 8 8 R ev im Ind u st 9 . 0 . C . , p 3 ; 9 5 Ann h im al 5 1 2 1 0 0 . C . An , p . 7 ; 9 . 5 R e v en c im 6 1 1 1 0 . g . h , p . 9 ; 9 3 7 m 6 7 68 1 8 . e . S o c . 2 . J Ch , , p ; 9 5 8 w 84 2 1 0 1 .