782
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
out such corrections, t h e error may amount t o 7 t o I O parts in 1000, t h e figure for chromium appearing too low b y this amount. Even when sodium phosphate is used t h e correction is necessary. It has been t h e custom in this laboratory t o prepare a blank b y reducing t h e chromate solution with SO2 in hot solution, after which i t is boiled gently for I j min. while a stream of CO2 is passed in. The solution is then cooled, diluted t o t h e proper volume and permanganate added gradually t o t h e first permanent change in color. During t h e reduction material reducible by S O 2 other t h a n chromates must be carefully excluded and t h e presence of halogens is t o be avoided also because of t h e action on t h e permanganate. Another method of finding this blank consists in adding a quantity of standard ferrous sulfate solution t o t h e solution of chromic salt, which upon titration will be found t o have required a n amount of permanganate too large b y t h e amount of t h e correction. The end-point obtained has a color which is described as blue by some and as gray b y others. I t appears much earlier t h a n t h e first pink color and i t is a more definite point. This method of obtaining t h e blank is somewhat questionable because t h e color of t h e solution of chromic salts is noticeably different from t h a t which results from t h e reduction of chromates by ferrous sulfate. If all titrations, except t h e titration of sodium oxalate with permanganate, are made on t h e electrometric apparatus, better results are obtained. On this apparatus one may titrate, ( I ) permanganate and ferrous sulfate, ( 2 ) chromate and ferrous sulfate, a n d (3) chromate, using a n excess of ferrous sulfate with respect t o t h e chromate, completing t h e titration with permanganate a n d ferrous sulfate. I n working with different combinations of oxidizing and reducing reagents on this apparatus, we have noted certain characteristic differences a t t h e end-point. Allowance must be made for this if full advantage is t o be taken of t h e accuracy of which t h e instrument is capable. I n titrating a chromate solution t h e addition of ferrous sulfate almost invariably produces what Forbes describes as a n anomalous rise. I n titrating a permanganate solution, this phenomenon does not appear with quite t h e same regularity and is always‘ less marked. I n titrating chromate, t h e change in potential is always abrupt when t h e end-point is reached, b u t with permanganate t h e addition of as many as 0.2 t o 0.4 cc. of ferrous sulf a t e after t h e disappearance of t h e pink color often causes only a gradual change. If now permanganate be added drop by drop t h e return of t h e original potential is delayed until one or two drops cause a marked change. One or two additional drops of permanganate are now sufficient t o produce a pink color. From this point t h e titration of t h e permanganate with ferrous sulfate closely resembles t h e titration of chromates, in t h a t no change occurs until t h e end-point is reached when t h e change is abrupt. I t . has occurred t o US t h a t in adding ferrous sulfate gradually t o a n excess of permanganate, a suspension of manganese dioxide is built up which requires a n appreciable excess of ferrous sulfate for its reduction. On titrating back with permanganate, either t h e condition is overcome,
Vol. 9 , No. 8
or i t is so much less marked as t o have no appreciable effect upon t h e behavior of t h e electrodes. I t is our custom t o a d d ferrous sulfate in excess until there is a marked change of potential. Permanganate is then added gradually until t h e original potential is reached which existed immediately before t h e fall in potential began or until t h e further addition of two or three drops causes no further change. T h e dropwise addition of ferrous sulfate then gives a sharp and definite end-point. I n connection with t h e analyses given in this paper I O O cc. of t h e solution of chromate containing I g. Cr in a liter was diluted t o 1000 cc. A solution of ferrous sulfate of equivalent strength mas similarly diluted. When these solutions were compared their relation was found t o be unchanged. These solutions were of such strength t h a t 0.1 cc. corresponded t o 0.001 per cent of Cr in a I g. sample of steel or t o o . o o o j per cent in a sample weighing 2 g. T h e samples were prepared for titration exactly as described elsewhere.‘ I n t h e table below, we give analyses for chromium on samples of steel issued by t h e Bureau of Standards. The column marked “Certif. Value” contains t h e percentage of chromium given on t h e certificate: i t is not known what accuracy is claimed for these figures, b u t i t is probably not greater t h a n 0.003 or 0.004 per cent. I n t h e case of Sample 33, one analyst reports 0 . 1 1 and another 0.12. ANALYSES OF STANDARD SAMPLES OF STEELFOR CHROMIUM CONTENT SAMPLE PER CENTCHROMIUM SAMPLE PER CENTCHROMIUM B. of S. GramsCertif. Found B. of S. Grams Certif. Found Eo. Taken Value Electr. Titr. No. Taken Value Electr. Titr. 10b ..... 2 0 . 0 0 5 O.OOi(2) 33.. 2 0.11 0.113(5) 2 0.007(2) and 0 . 1 2 3 0.007 (0) 2 0.113(5) 0.007 ( 7 ) 3 0.113(3) 3 16a 2 0.008 0.009(5) 3 0 . 1 13 ( 7 ) 2 0 . 0 0 9(5) 19a 2 0.08 0.076(5) 34.. 2 0.01 0.011(5) 0.076(7) 2 0 . 0 1l ( 5 ) 2 0.076(5) 3 0 . 0 1l ( 3 ) 3 3 0.076(3) 3 0.01 l(5)
..... .....
We believe this t o be t h e most sensitive a n d accurate method known for t h e determination of small amounts of chromium in complex solutions. Of substances ordinarily present in steel, only vanadium interferes. RESEARCH DEPARTMENT THEMIDVALE STEELCOMPANY ’ PHILADELPHIA
THE PHOTOMICROGRAPHY OF PAPER STRUCTURE By MILLARD B. HODGSON Received May 16, 1917
I n t h e s t u d y of t h e ultimate structure of paper, much valuable information can be obtained b y photomicrographs of cross-sections of t h e paper stocks. Some published accounts have appeared from time t o time illustrating t h e matting together of fibers in paper stocks, b u t little has been done in t h e direction of a more intimate s t u d y of t h e subject, a n d t h e possibility of d a t a t o be obtained from photomicrographs of cross-sections of paper has scarcely been touched upon, one reason for t h e lack of work in this last direction probably being t h e difficulty of making cross-sections of paper. 1
LOC.cit.
Aug., '"'7
1 ' N E J0i'R.V.A L O F I N U l i S 7 ' K I . ! L : I S D E N G I N E E R I N G C H E M I S T R Y
PLC. II-PLAX
PAP,:.
P l O . lV-%ROWN
In some recent work on the penelration of various materials into paper stocks t h e writer found it necessary to develop a technique for initking sections of paper stocks as thin as 0.05 mm. Considerable difficulty isas experienced in obtaining satisfactory results, but t h e following method was finally adopted: T h e particular microtome used was one made b y the Spencer Lens Company, of Buffalo, with a micrometer adjustment, enabling cuts from 0.001 t o o . o j o nim. t o be made. The paper of which a section is desired is mounted between two pieces of gelatine-coated film, ordinary Kodak S C film being used, the gelatine being moistened t o cause it to adhere to t h e paper, and t h e paper,
wanPPirc
FAPLK
783
IThr above iuiciophofogiaphs have becir reduced to threbhfthr (s/s) the original size]
held betivcen thc picces of film, is then placed between 1.wo pieces of moderately dry castile soap, the latter being placed in t h e chuck of t h e microtome with the paper edge normal to t h e razor edge. T h e use of the gelatine is vcry important as it forms a firm but slightly re,silient binder for the paper and prevents the tearing of t h e surface fibers. T h e microtome lilacie should be sharpened on a n Arkansas stone moistened with kerosene. No type of commercial razor hone or strop should be used as the s a w t o o t h edge peculiar t o such sharpening is fatal t o t h e cutting of good sections. Great care should be taken of t h e microtome since this is a determining factor in obtaining good or bad
sections. The niorement slioiild bt. kept sniootli at tlw perpendicular at a n angle of .io" toward the source ;ill times and t h e entire mechanism carefully scrccned of illumination wliicli is p l a ~ i lbetrveen and slightly from dust. T h e Madas must be kept sharpened t o 1,elow them. 'The distances of t h e backgrounds an exact edge, t h a t is. until unilcr fifty dirtmeters from the zero point ox-m t h e source of illumination of magnification t h e dctccta1,ic flaws iirc ncgligible. are read on scalcs cngrave(1 on t h e track, t h e scales In cutting. t h e micro adjustment is i-cnrling in milliineters from o t o 300. T h e centers mni. cut, the action is started slowly, t h c r u t off "specimen" heing guided with the fingcr t i p or i i 1,it nt curliiig. A good st:irt being assured, t h e finish of t h e cut is made i n a iiuii.k. stcady A b o u t ten or twenty of such c u t s arc toland ii f e w good ones selected for niounting \VI i the aid of a pocket microscope anil ii sinall pair rs. T h c specimens arc then muiinteil on $1. .~.., ji t h e usual micro slides in Canadzi b:tis;im ililut,cil - , ne,I w i t h xylol. Lions :ire easy t o make, of the areas rciiecteil from tlie backgrounds slioiilil l'liotoniicrograplis of sucli t h e best resiilts being obtaine 'y t,hc use of ~ r t l i o ~ h r o -come directly over t h e j o o nim. n i a r l ~when t h e backmatic plates with a ycllou~iilter. In t h e p r grounils are nt t h e ends oi t h e scales. Ahovc t h e light. Standard Orthonon plates >sere used with t,hc K r a i t c n and t h e zero Ipoint is supported a specially uonstruc (; filter, normal development adjustcd to olitain c prism a n d eyepiece so placed as t o rcflect t h e surfaces of t h e backgrounds into adjacent fields. P, Lummersidcrablc contrast being employed. I n soiiic c a t h e illumination of t h e ciimcra system was totally Brodhum prisni is more satisfactory, a s it e1imin;itcs dircct. so that t h e photographs were tnkcn by light the divi&ng line between t h e fields. T h e instrument is set b y placing t h e right-hand transinittctl througii t h r specimen, while in other cases t h e lighting \\.;is partially transmirteil i i n ( l Ijiocl- ;It t h e 3 0 0 mm. inark. The left-hand block is then m o v e d until :i position is found where t h e fields partially reflected from t h e suriace of t h e p:ip~-'"r. Some of t h e results obtained are shown in the illus- appc:zr matched i n luminosity. Five readings should trations. Fig. I, C, shows t h e way in which t h e l x made t o determine t h e ;i\-cragc position. This mark of the pen carrying India ink penetrates tlic a\-erage reading may he considered constant for any paper. Fig. 111, "Lily of t h e Valley" paper, from set of papers, provided t h e position of t h e light remains Brazil, shows t h e long plant fibers. Figs. V anii 'i'I unchanged. The left-hand block is then locked in are sections tlirough a Yelox print: this photoirr;iiAi this position. was taken b y transmitted light so l.hat we sce on it A single sheet of p:Lper of ivhicli tlie translucent first t h e transparent emulsion layer, only ahout 6 p effect is t o be measured is then placed over t h e leftthick; then t h e baryta substratum, which is white h a n d block. Snother piece is placed over t h e other by reflected light but absorbs t h e transmitted light Mock with black velvet hetxeen. T h e black velvet' anil so iippears black, and then finally t h e paper stock absorlx practically all of t h e light nThich the paper t r m s m i t s , t h u s a l l o n h g only t h e light from t h e suritsclf. l h S l r t a N K,,DAK C O X P A N Y face of t h e paper t o reach the corresponding field in K"YeeSTBn, KliW Y o x K
industrial use xhich meets t h c necessities of small cost of construction anii easy inanipulation, a n t l lias t.he
aiiiled ad\-antages of being scnsitivc t o r e r y small differences in translucent cfrict:ts. I t gives strictly eoniparablc results, although perhaps not aIwAntc \-slues. The principle of t h e method is t h a t used i n nixking photometric me:isiircmcnts. Two movable st;ind:iril white hackgrounds ire mounteri on ii track ( s r e Pig. I ) . In order t o avoid surface glare they arc inclined i r o m 3
u. S. m p t . .4g%
Bureau of Chemistry. Cirr. 98, i912.
I n cunsequence this ticlil is t b c darker since tlie otlicr is illuminated by t h e light. reilected fi-om the surface of t,he paper p l u s t h a t reflected b y ?lie standard white background. When t h e former block has heen advanced toward t h e light i t becomes hrigliter and iinally a position is reached wliere t h e iields appear matchcci. T h c distance this block h a s i3irrk YI.IYC~ is runirientir black nnd the fresh rurfrcrr oi the r w g ~ carbonate blocks sufftcicntlg white ioi the sensitiveness of the i n s i r i s ~ mcnf $0come far within the distinction of diBerrncei of tianslucent effect dcsired hy practical rurkers. 2
iiusiiiili
