INDUSTRIAL a n d ENGINEERING CHEMISTRY
A N A LYT IC A L E D IT IO N
H A RRISO N E . H O W E, E D IT O R » IS S U E D S E P T E M B E R 15. 1941 « V O L . 13, NO. 9 > C O N S E C U T IV E NO. 18
C h em ica l F a c to r s in D e t e r m in a t io n o f W a te r in I n s u la t in g O i l ...
R. N. Evans, J. E. Davenport, and A. J. Revukas 589 L ab oratory C o n d e n s e r ...Milton T. Bush 592 Q u a n tita tiv e D e t e r m in a t io n o f D iss o lv e d O x y g en .
Paul F. Sharp, David B. Hand, and E. S. Guthrie 593 S p e c tr o c h e m ic a l A n a ly s is o f T r a ce E le m e n t s in
F e r tiliz e r s . Z i n c ...Robert T. O'Connor 597 E xtra ctio n o f C a r o te n e fr o m P la n t M a te r ia l . . . .
L. A. Moore and Ray Ely 600 R apid D e t e r m in a t io n o f P h o s p h o r u s in F erro-
m o ly b d e n u m a n d i n C a lc iu m M o ly b d a te . . . . Louis Silverman 602 D e te r m in a tio n o f Z ir c o n iu m in S t e e l ...
Walter G. Hayes and Edward W. Jones 603 S tu d y o f F erric T h io c y a n a t e R e a c t i o n ...
Charles A. Peters and Chester L. French 604 A n a ly sis o f P e t r o le u m O il-S o lu b le S u lfo n ic A cid
S oap s . . F. M. Archibald and E. L. Baldeschwieler 608 C o lo r im etric D e t e r m in a t io n o f Ir o n w it h K o jic
Acid _ ...M. L. Moss with M. G. Mellon 612 C h em istr y o f M e n h a d e n O i l ...
W. H. Baldwin and W. B. Lanham, Jr. 615 R ed u cin g P o w e r o f S ta r c h e s a n d D e x t r i n s ...
F. F. Farley and R. M. Hixon 616 P o te n tio m e tr ie D e t e r m in a t io n o f M e r c a p ta n s in
A q u e o u s A lk a lin e S o lu t io n s . Miroslav W. Tamele, Lloyd B. Ryland, and Vanan C. Irvine 618 S u lfa m ic A cid M o d ific a tio n o f W in k le r M e th o d for
D issolved O x y g en . Stuart Cohen and C. C. Ruchhoft 622 P olarograp h ie D e t e r m in a t io n o f A sco r b ic A cid . .
Mary Mann Kirk 625 S im p lified , W a te r -J a c k e te d , F r a c tio n R e ce iv er . . .
R. S. Towne, E. E. Young, and L. T. Eby 626
D e te r m in a tio n o f P ero x id e V a lu e s fo r R a n c id ity in F is h O i l s ...Maurice E. Stansby 627 C o lo r im e tr ic D e t e r m in a t io n o f L ead b y D ip h e n y l-
c a r b a z i d e ...T. V. Letonoff 631 T h e T a c k m e te r , a n I n s t r u m e n t fo r A n a ly z in g a n d
M e a s u r in g T a c k ...Henry Green 632 A p p a r a tu s a n d M e th o d s fo r P r e cise F r a c tio n a l -
D is t illa t io n A n a ly s is . . . . Walter J. Podbielniak 639 M u ltip le E le c tr o d e S y s t e m fo r P o t e n t io m e t r ie T i
tr a tio n S t u d ie s . H. A. Frediani and Wm. B. Warren 646 S h a p in g L a th e fo r G r a p h ite E le c tr o d e s U s e d in
S p e c tr o c h e m ic a l A n a ly s is ...
K. R. Majors and T. H. Hopper 647 A p p a r a tu s for H ig h -S p e e d S t i r r i n g ...
Avery A. Morton and Donald M. Knott 649 P o te n tio m e tr ie T it r a t io n S t a n d A s s e m b l y ...
Louis Lykken and F. B. Rolfson 653 M a g n e tic S tir re r for U se in C u p T y p e o f M o is t u r e -
T r a n s fu s io n A p p a r a t u s ...
Horace K. Burr and Alfred I. Stamm 655 M IC R O C H E M IS T R Y :
S y s te m a tic Q u a lita tiv e O r g a n ic M ic r o a n a ly s is . . Herbert K. Alber 656 M e a s u r in g M i c r o s c o p e ...
G. H. Wagner, Grant C. Batley, and W. G. Eversole 658 R e m o v a l o f N itr o g e n O xid es in S e m im ic r o d e te r
m in a t io n o f C a rb o n a n d H y d r o g e n ...
Philip J. Elving and Wilbur R. McElroy 660 M O D E R N L A B O R A T O R IE S:
A m e r ic a n T o b a c c o C o m p a n y R e se a r c h L a b o r a to r y Claiborne E. Brogden 664
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25,300 copies of th is issue p rin ted . C o pyright 1941 b y A m erican C hem ical Society.
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4 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 13, No. 9
Please forward to me a copy of the new “ PYREX” Laboratory Glassware Catalog, N o. L P 2 1.
Name---P osition____________________
Company o r Institution _
Address_____________________________________________________________ _ _ _ _ _ _ _ _ City--- State---
Please place my name on your regular mailing list O
“ Pyrex” accurate 6o° Fluted Funnels are the fastest.
Prove it to your ow n com plete satisfaction! Try them out in your laboratory and see h o w they com pare w ith others. W hen you’ve d on e that, you, too, w ill say “ Pyrex fluted funnels are fastest.”
T h e reason these funnels w in out in actual com parison is because they provide practically double the effective filtering area o f ordinary funnels. T hat’s the secret o f their faster filtering speed. Further
more, they give a c lo se paper fit— som eth in g that’s hard to get w ith sm aller-angle funnels.
U se “ Pyrex” accurate 6 o ° funnels, and you’ll be pleased not only w ith their speed but also with the lo n g service they give. T hey are the only funnels made o f Balanced Glass; thus, they’re m echanically strong, chem ically stable, and thermally resistant.
R im s and stem s are fire-polished, too, to assure greater strength and lo n g er service.
A ll Pyrex brand funnels— fluted and plain— are n ow available from your laboratory supply dealer at new lo w prices, as listed in Catalog LP21.
“ P Y R E X " is a registered trade-m ark a n d indicates m anufacture by C O R N I N G G L A S S W O R K S ♦ C O R N I N G , N . Y .
S E N D F O R T H E N E W C A T A L O G N O W
A N A L Y T I C A L E D I T I O N 5
as a m atter of r e c o r d ..
M erck & Co. Inc. is a N e w Jersey corporation. It is an independent co m pany, ow ned by approxim ately 2,6 0 0 stockholders, m ost o f w hom are citizens o f the U nited States and reside in the U nited States. In fact, less than 5/l00ths o f 1 per cent o f the stock is held outside o f this country. T h e com pany is c o n trolled and m anaged by U nited States citizens.
M erck & Co. Ltd., o f M ontreal, Canada, is a w h olly-ow n ed subsidiary o f M erck & Co. Inc. W ith this exception, Merck & Co. Inc. is n ot connected, directly or indirectly, w ith any firm o f similar name.
A lthough there are a number o f other com panies bearing the name "M erck”
in various parts o f the w orld, n on e o f these com panies is in any w ay connected w ith M erck & Co. Inc. or M erck & Co. Ltd.
M erck & Co. Inc. and M erck & Co. Ltd. are d evotin g their full energies to the service o f their many custom ers, including the U nited States G overnm ent, the Canadian G overnm ent, and the British G overnm ent under the Lend-Lease Act.
T h is statem ent is published as a matter o f record and in justice to all con cerned.
MERCK & CO. Inc. ¿ k a,,u/act,,r .¡,,9 ce /< M RAHWAY, N. J .
N E W Y O R K • P H I L A D E L P H IA • S T . L O U IS
I n C a n a d a : Merck & Co. Ltd. • Montreal and T oronto
iÿt'ne c€ /e m ic a fó foal t / e &icfocáátcná erne/ ffïn c e 4 8 4 8
★
6 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 13, No. 9
GENERAŁ © ELECTRIC X-RAY CORPORATION
2 0 1 2 JA C K S O N t lV D . CHICAGO# III.« U. S . A*
P le a s e » e n d m e c o m p le te in f o r m a tio n a b o u t th e G -E XRD X-Ray D iffractio n U n it a n d its a p p l ic a t io n to a n a ly ti c a l p r o b le m s .
N a m e .
j
.
P o sitio n .___________________________________________ .
C o m p a n y ,__________ __________________________________ ___________
(P fease attach to , o r write on , your co m p a n y le tte rh e a d )
■ S I G N a n d M A I L , T O D A Y - T ’S no se cre t th a t fre q u e n c y c o n tro l o f m any o s c illa tin g
e le c tric a l circu its e m p lo y e d in v ita l d e fe n s e a p p a ra tu s — ra d io tra n sm itte rs a n d re c e ive rs , a ir c r a f t beam in d ic a to rs , a irc ra f t a n d su b m a rin e d e te c to rs — d e p e n d s u p o n q u a rtz crysta ls.
N o se cre t e ith e r, is the fa ct th a t h a ir-lin e a c c u ra te m e a su rin g m ethods to ch e ck the e le c tric a l a x e s o f the crysta ls a re o f e xtre m e im p o rta n c e in the m a n u fa ctu re o f th ese c o n tro l units. U sin g the G - E X R D U n it, m a n u fa c tu re rs h a v e fo u n d in x -r a y d iffra c tio n a s a tis fa c to ry m eth o d o f a n a ly z in g un cu t c rys ta ls a n d d e te rm in in g the p r o p e r d ire c tio n o f cu ttin g . X -R a y d iffra c tio n d o e s the jo b fa s te r, fa r m ore a c c u ra te ly th a n e v e r b e fo re , a n d co n trib u te s a v a lu a b le s a v in g o f b o th time a n d m a te ria l.
T h e XRD G o n io m e te r A s s e m b ly illu s tra te d b e lo w is on e of s e ve ra l ty p e s o f h ig h ly e ffic ie n t instrum ents used b y quartz c ry s ta l m a n u fa ctu re rs. T h e G - E X R D U n it is d e s ig n e d for p re c is io n re s e a rc h a n d c o n tro l a n a ly s e s . It e m b o d ie s the sa fe ty, c o n v e n ie n c e , f le x ib ilit y , e a se o f o p e ra tio n , and a d a p t a b ilit y th a t a re r e q u ire d f o r e ffe c tiv e u tiliza tio n of the x - r a y m e th o d .
M o d e r n ,p ro g re s s iv e a n a ly tic a l la b o ra to rie s h a ve been'quick to r e c o g n iz e th e G - E X R D U n it as a d e p e n d a b le "pro ble m s o lv e r." If, in y o u r la b o r a t o r y , y o u h a v e a p ro b le m th a t has y o u stum ped, w h y n o t d o th is : U s e th e c o n v e n ie n t coupon to re q u e s t fu ll in fo rm a tio n a b o u t th e G - E XRD U n it an d its a p p lic a tio n to y o u r p ro b le m . T h e se rvice s o f o u r X-Ray D iffra c tio n L a b o r a to r y s ta ff a re y o u rs f o r th e asking;
a d d re s s y o u r re q u e s t to D e p a rtm e n t R39
7
H O S K I N S P R O D U C T S
E L E C T R IC H E A T T R E A T I N G F U R N A C E S • • H E A T I N G E L E M E N T A L L O Y S • • T H E R M O C O U P L E A N D LEAD W IR E • • P Y RO M ETERS • • W EL D IN G W IR E • • H EA T R ESISTA N T C A S T IN G S • • EN A M ELIN G fIXTURES • • S P A R K P LU G ELECTR O D E W IR E • • S P E C IA L A L L O Y S O F N IC K EL • • P R O T EC T IO N TUBES
allow
possible Chromel wire. For exam ple, the 1 1 0 -V wire is . 0 7 2 ' d ia ., for 1 10 and 220 it would be .0 4 5 r .
IN S U L A T IO N ^
The insulation is 4 1/* ' thick all around. The furnace is well m ade in a ll respects. W e d o n ’t build a " c h e a p " fur
n ace, b ecau se a ch eap fur
n ace ca n ’t b e g o o d . Hoskins Furnaces are cheaper In the long run.
Hoskins electric la b o ra to ry furnaces d e liv e r the g o o d s, month in and out. Th eir Chrom el Units are dura ble— almost b e yo n d belief.
W e d o n ’t invite y o u to abuse these furnaces, but if y o u must force them through an emer
g e n c y , y o u ’ ll see what a g o o d factor of safety has been built into them. Hoskins Furnaces are d e pe nd able . W rite to yo ur de aler or us. . . . Hoskins M a nufacturing C o m p a n y, D etro it, M ic h .
# W h en the unit wears out, a ll yo u need to b u y is the Chromel wire itself, which comes as a co il.
# Y o u install the Chrom el w ire m erely b y w rapping it around the g ro o ve d muffle.
O This furnace can be h ad, e q u ip p e d with a small inexpensive pyrom eter and cou ple .
# The slidin g d o o r saves heat. T o remove a crucible y o u d o n ’t need to open the d o o r a ll the w a y .
• The turns o f the Chrom el c o il are " c o n densed” to compensate for heat losses at the d oor.
# These furnaces are a v a ila b le as muffle, carbon combustion and small crucible type s, g o o d for 1850-2000° F.
8 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 13, No. 9
Exchequer W ine Gallon was a standard measure when Queen Anne ruled.
STANDARD FOR ANALYSIS
A p p aratu s of th e u tm o st se n sitiv ity is u sed for th e scien tific control w h ic h m a k es M allin ck rod t A n a ly tica l R e a g e n ts conform to exacting, predeterm in ed stan d ard s of purity. T h e s e ch em ic a ls m eet, in every w ay, th e h ig h req u irem en ts of th e a n a ly tica l ch e m ist in gravim etric, gasom etric, colorim etric, or titrim etric work.
S E N D F O R C A T A L O G U E O F M A L L I N C K R O D T A N A L Y T I C A L R E A G E N T S A N D O T H E R C H E M I C A L S F O R L A B O R A T O R Y U S E .
A l w a y s S p e c if y R e a g e n t s in M a n u f a c t u r e r ’s O r i g i n a l P a c k a g e s
M A L L I N C K R O D T C H E M I C A L WORKS
S T . L O U I S • P H I L A D E L P H I A • M O N T R E A L
C H I C A G O • N E W Y O R K • T O R O N T O
9
V I N E L A N D , N. J
• C H I C A G O
10 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 13, No. 9
A .H .T . CO. S P E C IF IC A T IO N
BEAKER R IN G S f o r WATER BATHS
BEAKER R IN G S, A .H .T. Co. Specification, of H R -108 H eat-R esistin g R ubber. For attachm ent to glass, m etal or porcelain beakers to form a rem ovable supporting flange so th at th ey can be immersed in water or steam baths a t variable depths to control or accelerate evaporation. Evaporations can be made in situ at a rate which compares favorably with that obtained w hen using porcelain dishes. See J. A. Scherrer,
“ Rubber Beaker R ings for Accelerating Evaporation on Steam B ath ,” Industrial and Engineering Chemistry, A nal. E d.,
Vol. 5, N o. 1 {Jan. 1 5 ,1 9 3 3 ), p. 22.
Rings have rectangular cross section and are made of the same composi
tion as HR-108 Rubber Stoppers and will withstand continued use directly on metal or porcelain tops of steam or water baths without apparent change in shape or size.
Sizes listed below, when wetted, slide readily over most types of beakers with capacities of from 200 to 1000 ml, but the outside diameter of the beaker, as well as the inside and outside diameters of the ring and inside diameter of water bath opening, must be noted carefully w'hen ordering rings because of wide variation in shapes and makes of both beakers and bath openings. Rings should be selected with inside diameter slightly smaller than the beaker but they have considerable elasticity and can be 2146-A. attached without difficulty, even though the difference in diameter is as
much as Vie-ineh.
2146-A. B eaker Rings, of H R -108 H eat-R esistin g Rubber, as above described.
Inside diameter, inches... 2Vj 27/ s 3‘/< 3 l5/i«
Outside diameter, inches... 3J/< 4J/is 45/s 5!/<
Thickness, inches... ’/s Vs ‘/i V*
Fits Griffin beakers, ml... 250_____________ 400____________ 600____________ 1000 E a ch ... 13 .15 .21 .29 Code W ord...1 tjpy Atjuo Alkac Atkim
10% discount in lots of 12, one size only 20% discount in lots of l/+4i assorted sizes
W A X
A .H .T . CO.
HEAT RESISTANT
WAX PEN C ILS, H eat R esistant, A.H.T.
Co. Consisting of high m elting point wax, w ith superior, uniform, sm ooth writing qualities, enclosed in hard, rolled paper finished in the sam e color as the wax.
For writing on glass, porcelain, etc., but also suitable for skin marking. Markings will withstand hot air or steam pressure sterilization and the heat of boiling flasks, etc., w ithout spreading or blurring.
W hen used on porcelain and fired at a low red heat, m arkings w ith the Brown pencil
penetrate the glaze and becom e perm anent. 7964 ^
7964-D . P en cils, W ax, H ea t R esistan t, A.H .T. Co., as above described, packed in cartons of one dozen of a color.
Color... White Blue Black Red Yellow Brown
E a ch ... 15 .15 .15 .15 .15 .15
Code Word Lyxce Lyxcq Lyxcs Lyxdm Lyxdo Lyxgi
10% discount in carton of one dozen, one color only.
20% discount in lots of 72 \ , , , • carton units 25% discount in lots of 1U i ’
A R T H U R H. T H O M A S C O M P A N Y
R E T A IL — W H O LE SA LE — E X P O R T
LABORATORY APPARATUS AND REAGENTS
W E S T W A S H IN G T O N S Q U A R E , P H IL A D E L P H IA , U. S . A . Cable Address, “Balance” Philadelphia
INDUSTRIAL a n d ENGINEERING CHEMISTRY
A N A L Y T I C A L E D I T I O N
P U B L IS H E D B Y T H E A M E R IC A N C H E M I C A L S O C I E T Y • H A R R IS O N E . H O W E , E D I T O R
Chemical Factors in the D eterm ination o f Water in Insulating Oil
A New Electrical Method
R . N . E V A N S , J . E . D A V E N P O R T , a n d A . J . R E V U K A S C o n s o lid a te d E d is o n C o m p a n y o f N ew Y o r k , I n c ., B r o o k ly n , N . Y .
T h e p r e se n c e o f s m a ll q u a n t itie s o f w a ter in in s u la t in g o il se r io u sly im p a ir s it s u s e fu ln ess in e le c tr ic a l e q u ip m e n t. T h e n e e d for a n a c c u r a te a s w e ll as rap id m e th o d for its d e te r m in a tio n c a n n o t b e o v ere m p h a sized . T h e p r e s e n t p ap er d escrib es t h e ex p e r im e n ta l m o d ific a tio n s o f th e c o m b u s
tio n p ro ced u re d ir e c te d to w a rd lo w e r in g th e h y d r o ca rb o n c o r r e ctio n . T h e in flu e n c e o f te m p e r a tu r e i n t h e rem oval o f th e w a ter from o il a n d t h e lim it a t io n s o f t h e G rig- nard p ro ced u re are e x p e r im e n ta lly d e m o n str a te d . A n e le c tr ic a l m e th o d is briefly d escrib ed a n d i t s a p p lic a tio n t o a field d e te r m in a tio n o f w a te r is p o in te d o u t.
I
N R E C E N T years several chemical procedures have been advanced which claim specificity for th e estim ation of water in organic liquids.The Fischer method (<?), worked on extensively in this country by Smith, Bryant, and Mitchell {12), depends on the oxidation of sulfur dioxide by iodine in the presence of water. The latter authors first worked out the estimation of water by acetyl chlo
ride, which was adapted by Clark to the determination of water in transformer oil. Because of the color of an insulating oil and the probable presence of interfering substances in the original oil sample, it appears likely th at in any chemical procedure the water first must be removed from the oil, as, for example, in the pro
cedure described by the authors (4). The chief difficulties with the Fischer method and the acetyl chloride method in their ap
plication to oils are the estimation of peroxides in the former method and the estimation of volatile acidity in the latter, two additional methods involve the use of a-naphthoxydichloro- phosphine (0), which is not specific to the OH group of water,
®nd of benzoic anhydride (11). The Grignard reagent has also been suggested by Larsen (7) as a possibility for determination of traces of water in oils.
In the m ethod (4) which the authors have described, the volatile carbonaceous m aterial and th e free water, after being trapped out by th e use of a Dewar flask containing solid car
bon dioxide, were carried into a combustion furnace. The
products of combustion and th e free w ater were absorbed in microchemical absorption tubes and weighed as carbon di
oxide and gross water. T he n et w ater was obtained b y ap
plying an empirical correction ratio of water to carbon di
oxide.
The accuracy of th e m ethod depends on the use of th e cor
rect ratio of w ater to carbon dioxide and th e weight of carbon dioxide obtained in each experiment. Some knowledge of th e former was obtained by continuing th e experiment for 1 liter of nitrogen after th e w ater weight had fallen to a mini
mum. The la tte r factor was greatly reduced by a change in procedure which is described in this paper. The ratio adopted for th e mineral oil type of transform er oil was 0.3 and for the noninflammable synthetic oil 0.2. Carbonaceous m aterial vaporized from a m ineral oil which would be retained by the solid-carbon dioxide tra p would be expected theoretically to have a w ater-carbon dioxide ratio of between 0.2 and 0.5.
The higher th a n theoretical ratio (0.1) for th e synthetic non- inflammable oils m ay be due to partially chlorinated com
pounds below C2„H„C1„ or to organic im purities absorbed in the operation of th e transform er.
A p p a r a tu s
T he modifications in th e ap paratus were directed tow ard the reduction of th e am ount of carbonaceous m aterial entering the combustion furnace. I t was found th a t th e m agnitude of the correction was greatly reduced b y means of an oil scrubber in the train where preferential solution of the oil vapors took place. The change in procedure for th e deter
mination of water in oil is apparent from a description of the new section of th e ap paratus as shown in Figure 1. This sec
tion replaces D and F in Figure 1 of (4). (In 5, page 301, in the description of the apparatus, “ cell E ” should read “ cell D ").
One hundred milliliters of oil were introduced into G through the serum rubber stopper and stopcock D after the residual water in the train had been removed. Approximately 10 ml. of Apiezon oil (an oil of low volatility used in high vacuum prac
tice) were also added to cell B. Purified nitrogen gas in small bubbles formed by the porous fritted disk, A, carried the water from the oil sample into the solid carbon dioxide trap, E. During this period, scrubber B was maintained a t a temperature of 100° C. When the removal of water was complete, the Dewar flask was removed and the nitrogen was directed around the oil sample cell hy means of the three-way stopcock G. In this manner the
590 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 13, No. 9
F i g u r e 1. I n t e r c h a n g e a b l e A p p a r a t u s f o r D e t e r m i n i n g W a t e r i n I n s u l a t i n g Oil
A . Porous fritte d disks B. Apiezon oil cell C. Oil sam ple flask
D. Serum ru b b e r sto p p er a n d stopcock E. D ry ice tra p
F. E x tern al sp iral coiled h e ater G. T hree-w ay stopcock
trapped water in E, together with any volatile products, was carried through scrubber II, which was maintained a t room tem
perature, into the combustion furnace. An external coiled heater, F, prevented frothing in either B or C. When the oil was of the halogenated type a spiral tube containing silver oxide maintained at a temperature of 400° C. was put in place a t the exit end of the combustion furance. As is shown below, the tem
perature of the oil sample must be considered in connection with the removal of water. A safe maximum temperature would ap
pear to be the average operating temperature of the oil. One may also be guided by the condition of the oil as revealed by chemical tests. Unless otherwise stated, a new mineral oil or the average nonflammable synthetic oil was heated to 105° C.
during the passage of 2 liters of nitrogen, whereas for a used mineral oil the passage of 5 liters of nitrogen was carried out at 50° C. or at room temperature.
E fficien cy o f A p iezo n S cru b b cr in D im in is h in g C arbon D io x id e C o r r e c tio n
In Table I is illustrated the reduction of the carbon dioxide correction by th e use of th e Apiezon-oil scrubber. When the oil scrubber was m aintained a t an elevated tem perature, it was apparently ineffective in retaining carbonaceous m aterial (experiments 3 and 3A). However, when th e scrubber was m aintained a t room tem perature it lowered greatly th e quan
tity of oil vapor entering the combustion furnace. The adopted carbon dioxide-water correction apparently would have enabled one to obtain the correct results (experiments 3,
Ta b l e II. Ef f e c to f Ti m e o f He a t i n ga n d Te m p e r a t u r e o n Re m o v a lo f Wa t e r f r o m Sa m p l e 6 ( 1 0 C Oi l) ”
Oil Sam ple
E x p t. T em p era- W eight of G ross N et,
N o. tu re Sam ple N j H tO CO2 HjO
0 C. Grams Liters P . p. m . P . p. m. P . p. in.
34 4 (con
tinued) 4 (con
tinued) 125 12525 25 125 125
56 87 87 57
11.5 4 17 4 additio n al
59 99 29 34 112
26 31 20 1717
51 90 23 10729
18 ad d itio n al 34 11 31 T o ta l w ater found in E x p t. 4 167 1 S crubber of A piezon oil a t room tem p eratu re.
3A, A-16, and A -l) w ithout th e scrubber even where large am ounts of carbon dioxide were obtained. T he correction due to the solubility of w ater in th e Apiezon oil is negligible, as was indicated by continuing th e experim ent until an addi
tional liter of nitrogen had passed through th e train (experi
m ent 3A).
E ffect o f T e m p e r a tu r e o n R e m o v a l o f W ater fro m O il
T he experim ental results on m any oil samples indicated th a t if an oil sam ple were heated while the w ater was being removed into th e solid carbon dioxide trap high results were obtained which depended in m agnitude on the tim e of heating.
This effect was more evident in th e m ineral oil than in the nonflammable synthetic oil. F urtherm ore, the used trans
former oils exhibited this effect to a greater degree th a n the new oils. In T able II, th e results on a sample of used trans
former oil (mineral oil type) are given. W ie n experiments 1 and 2 are compared with experim ent 3 (Table I I, last column) the m agnitude of th e error involved is clearly illustrated.
Therm al decomposition of oxygenated compounds was re
sponsible for th e form ation of the additional water, although the residual dissolved oxygen gas in the sample m ight have played a minor p art. In experim ent 4 after 17 liters of nitro- gen had passed through th e oil sample a t room tem perature a very large increase in w ater content occurred when the tem
perature of th e oil sam ple was raised to 125° C. Further heating yielded com paratively little additional w ater, indi
cating th a t oxygenated compounds were present in a limited am ount. T he completeness of removal of w ater a t room tem perature was indicated by th e fact th a t th e 4-liter run (experim ent 3) yielded very little less th a n the 17-liter run (experim ent 4). T hus by continued heating a t 125° C. for a period involving th e passage of 22 liters of nitrogen, an oil sample actually containing 30 p. p. m. of w ater was shown to behave as if 105 p. p. m. of w ater were present.
Ta b l e I . Ef f i c i e n c y o f Ap i e z o n Sc r u b b e r in Di m i n i s h i n g t h e Ca r b o n Di o x i d e Co r r e c t i o n
T em p e ra tu re
Oil Oil G ross N et,
E x p t. N o. Oil T y p e scru b b er
° C.
sam ple
° C.
HjO Mg.
CO*
Mli.
H*0 P. p. m.
C 0 2 P. p. VI.
II iO P . p. tn.
3 Used 10C'1 25 125 3.5 S 1 .3 5 33 13 29
3 A Used 10C 110 125 4 .7 5 S .00 62 105 31
3A (contin
ued) 1 ad ditional liter N i
0 .1 4 0 .4 3
A -l 5 New 10C A bsent 25 3 .3 4 5 .8 0 108 187 52
A -15-A New 10C 25 25 2 .1 4 0.9 1 50 21 43
A -l N . S. 0.*> Absent 125 11.57 9 .5 5 116 96 97
A -16 N . S. O. 25
* M ineral oil ty p e of tran sfo rm er oil.
•> N onflam m able sy n th etic oil.
125 3 .8 7 0 .5 8 102 15 99
B eh a v io r o f G rign ard R e a g e n t in D eter
m in a tio n o f W a ter in Oil T he application of Grignard reagent to the determ ination of w ater in oils showed th a t after m aking allowance for the acid content deter
mined on a duplicate oil sam ple th e residual active hydrogen content of th e oil sample is too large to be neglected. The term “ residual ac
tive hydrogen” m ay be conveniently applied to th e active hydrogen other th a n th a t of water and acids. Alcohols, amines, phenols, and acids too weak to be titra te d m ay be considered to be th e source of the residual active hy
drogen.
A N A L Y T I C A L E D I T I O N 591
Ta b l e III. Co m p a r i s o n o f Re s u l t s o f Giu g n a r d a n d Co m b u s t io n Me t h o d s
(Single oil sam ple a t 25° an d 125° C.)
C om bustion Tem perature of
Oil Sam ple
N e u tra li
zation N um ber
,--- G r i g n a r d ---*
M ethane Evolved C alculated'1
F ound Corr.t» W ater W ater
E q u iv a
lent m ethane 0 C. 1\ p. in. P . p. VI. P. p. in. P. p. VI. P. p. VI.
25 0 .3 6 22CC 123 69 10 21
125 D istillate
R esidue 0 .0 4
0.31 95
175 67
92 38
52
T o ta l 159 90 30 54
® lH iO =» 2CH<- A cid ity ded u cted using relation 1(H) « IC II4.
& Blank d educted. A cidity ded u cted using relation 1(H) = IC H4. e G rignard reactio n carried o u t in a m ixture of 30% eth e r and 70% 10C oil.
It was first observed th a t the character of the oil sample (petroleum origin or chlorinated aromatic) influenced greatly the yield of th e reaction. This phenomenon has been described by H ibbert and co-workers (.S’). I t could be mini
mized by m aintaining a large ratio of isoamyl ether to the oil sample. An a tte m p t was made to analyze with the Grig- nard reagent the two fractions formed in the combustion procedure. T he m aterial which was condensed in the solid carbon dioxide tra p (cell 2) is referred to in Table I I I as the distillate and th a t which remained in the oil cell (cell 1) as the residue. In a separate experiment, the acidity of the dis
tillate and residue was determ ined by titratio n by means of a capillary syringe w ithout removing either from the analyti
cal train. The results bring out several points of interest.
The large increase in w ater resulting from its removal from the oil sample a t an elevated tem perature (30 p. p. m. as com
pared to 16 p. p. m.) confirmed the results reported in Table II.
The m ajor portion of the acid for this particular mineral oil sample was nonvolatile. In experiments with non
flammable synthetic oil carried out in a similar manner, the entire acid content remained in the residue. The residual active hydrogen appeared to a limited extent in the distillate (cf. 54 and 67 p. p. m. m ethane). After due allowance was made for the influence of the solvent and after the m ethane equivalent to the acid had been deducted, there was an ap
parent increase in th e active hydrogen content of the oil sample as a result of heating to 125° C. (cf. 123 with 67 + 92
= 159 p. p. m. m ethane). This discrepancy (159 — 123 = 36 p. p. m.) was in good "agreement with the difference be
tween the calculated m ethane from the water obtained a t 125° and 25° C. (54 - 21 = 33 p. p. in.). Except for th e possibility of oxida
tion a t the elevated tem perature due to dis
solved oxygen initially present in the oil sample, a probable explanation of the anom aly was re
lated to the facts th a t ketones m ay enolize a t the elevated tem perature and th a t alcohols m ay split off water which under optim um conditions would yield two moles of m ethane as compared to a yield of but one mole of m ethane from the original alcohol.
___________ I t was concluded th a t the results of the Grignard test when expressed as evolved m eth
ane m ay be used only as an index of the stage of oxidation of an insulating oil. Based on experi
m ental results with the microgravimetric procedure devel
oped in this laboratory, it was considered extremely uncertain to ascribe th e evolved m ethane to any particular chemi
cal reactive group or compound after an acidity correction had been applied. Results of even greater uncertainty m ay be expected in the estim ation of th e Grignard added. The establishm ent of an oxygen balance by means of the results of the Grignard test did not appear to be of general applica
tion in the light of experimental work.
RP.m.HjO
Fi g c r e 3. Re l a t i o n b e t w e e n El e c t r i c a l Re s i s t a n c e o f Hy g r o m e t e r a n d Wa t e r Co n t e n t o f In s u l a t i n g Oil s
592 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 13, No. 9
Ta b l e IV. So l u b i l i t yo f Wa t e ri n Ne w In s u l a t i n g Oi l sa t Ro o m Te m p e r a t u r e
T em p era --- HîO -
tu re of C lark 's
Oil Oil values
E xpt No, T y p e Sam ple CO* G ross N e t a t 25° C.
° C. P . p. m. P . p. m. P . p. m. P . p. m.
A-17 10C 25 7 46 44
A-15-a 10C 25 21 50 43 *75
A-15-b IOC 125 32 57 47
A-16-a N . S. O. 25 6 103 102 Î2Ô
(Pyranol)
A-16-b N . S. O. 125 15 102 99
,,,
A-12 5314° 125 7 54 52 70
° M in eral oil ty p e of high-voltage cable oil.
Oil sam ples were p rep ared b y sh ak in g w ith a n excess of w a ter a t 50° C.
a n d cooling to room te m p eratu re. T h e sa tu ra te d 10C oil was rep eated ly d e can ted , th e N . S. O. was rep eated ly d raw n off in a sep a ra to ry funnel, a n d th e 5314 oil w as centrifuged.
S o lu b ility o f W a ter in I n s u la t in g O ils a t R o o m T e m p e r a tu r e
I n a paper by C lark (2) the solubility of w ater in new P yranol, transform er oil, and high-voltage cable oil was given. In connection w ith checking th e precision of analyti
cal trains, th e authors prepared stock saturated solutions of th e oils with respect to w ater a t room tem perature, which for purposes of comparison m ay be considered to be between 20°
and 25° C. The saturated values determ ined together with C lark’s values are listed in Table IV.
E le c tr ic H y g r o m e te r
The need of an apparatus for th e determ ination of w ater in oil which would operate satisfactorily in th e field is very ap
parent. W ith this in mind, th e electric hygrom eter described by D unm ore (3) was adapted to a procedure which removed th e w ater from the oil a t a tem perature not greater th a n th e norm al operating tem perature of th e oil. In Figure 2, a schematic drawing of th e ap paratus is shown.
The oil was introduced by syringe through E, spraying into the evacuated chamber through the porous fritted disk, A. Dur
ing this operation, the stopcock above D remained closed and the
cold trap, C, was surrounded by a bath of dry ice. Stopcocks F and G were then closed and the contents of the trap and coil raised to some convenient temperature. The resistance of the film was measured either by a General Radio megohm bridge, as extended by Balsbaugh et al. (/), or a modified Jones and Joseph bridge (10) with a cathode ray tube as a null detector with three stages of amplification.
In Figure 3 is shown a plot of th e logarithm of th e resistance against th e w ater content of the oils as determ ined by the combustion procedure. The d rift in th e resistance readings is illustrated by th e insert curves. T he change in resistance with tim e, although of a larger m agnitude a t th e higher measured resistance, in term s of w ater content, is approxi
m ately the same as a t the lower m easured resistance. The resistance readings on th e m ain curve were taken in each case after a 5-minute delay period.
Experim ents are in progress on th e simplification of the hygrom eter construction and th e selection of a more rugged alternating current resistance bridge of a suitable range. T he m ethod shows every promise of working out satisfactorily in th e field.
A c k n o w le d g m e n t
T he electrical bridges described in this paper were con
structed in this laboratory under th e direction of W. F.
D avidson, director of research.
L ite r a tu r e C ited
(1) Balsbaugh and Oucley, I n d . E n o . C h e m ., 31, 318 (1939).
(2) Clark, Elec. Engr,, 59, 433 (1940).
(3) Dunmore, Bur. Standards J . Research, 23, 701 (1939).
(4) Evans, Davenport, and Revukas, I n d . E n o . Chem., A n a l . E d ., 11, 553 (1939).
(5) Ibid., 12, 301 (1940).
(6) Fischer, Angew. Chem., 48, 394 (1935).
(7) Larsen, I n d . E n o . C h e m ., A n a l . E d ., 10, 195 (1938).
(8) Lieff, Wright, and Hibbert, J . A m . Chem. Soc., 61, 865 (1939).
(9) Lindner, Z. anal. Chem., 86, 141 (1931).
(10) Luder, J . A m . Chem. Soc., 62, 89 (1940).
(11) Ross, J . Soc. Chem. Ind., 51, 121T (1932).
(12) Smith, Bryant, and Mitchell, J . A m . Chem. Soc., 57, 841 (1935);
61, 2407 (1939); 62, 1, 3504 (1940).
Pr e sen t ed before th e C onference on E lectrical In su latio n , W ashington, D . C.
A Laboratory Condenser
M IL T O N T . B U S H , V a n d e r b ilt U n iv e r s ity S c h o o l o f M e d ic in e , N a s h v ille , T e n n .
C
OOLING agents more effective th a n ta p w ater are often needed in the laboratory condenser. In th e ordinary straig h t condenser it is not convenient to use ice or other such refrigerant. The single-spiral condenser, suitable for downw ard distillation, does n o t usually have sufficient capacity for refluxing.
T he double-spiral condenser shown was designed prim arily to fill th e need of an ice condenser which could be used for b oth downward distillation and refluxing. T he coils are made from Pyrex glass tubing 9 mm. in inside diam eter, and are carefully wound so th a t there are no trap s in which liquid can accum ulate.
I t is apparent from the figure th a t ta p w ater, ordinary ice, or solid carbon dioxide can be used as cooling agents.
This condenser has been par
ticularly useful in this laboratory during th e summer, when the tap w ater is often above 30° C. It has been very effective in the prepa
ration of such products as acetalde- hyde, hydrogen cyanide, and diazo- m ethane. T he ascending coil is an effective barrier against th e evapo
ration of condensate.
T h e s p e c im e n s h o w n w as fabricated by the Scientific Glass A pparatus Company, Bloomfield, N. J.
Quantitative D eterm ination o f D issolved Oxygen
Ascorbic Acid Oxidase Method
P A U L F . S H A R P , D A V ID B . H A N D , A N D E. S . G U T H R IE C orn ell U n iv e r sity , Ith n c a , N . Y.
D
ISSOLVED oxygen exerts the controlling role in many processes occurring in biological fluids. T h at this is true of milk has been am ply dem onstrated (Jh 8 ,1 8 , l/t). I t is evident th a t to understand and control these oxidations a knowledge of the dissolved oxygen content is highly important. A simple rapid m ethod for the determ ination of dis
solved oxygen in m ilk is needed, especially a t the present time, in view of the possibility of commercially dcaerating pas
teurized milk to preserve its vitam in C content and prevent the development of th e oxidized flavor.
Methods for determ ining dissolved oxygen fall into several general classes.
The dissolved gases may be removed, the volume of mixed gases measured, and the mixture analyzed for carbon dioxide, oxy
gen, nitrogen, and water vapor. Complete removal of the dis
solved gases is difficult and heating of the liquid in conjunction with reduced pressure is recommended (10). During the time required for removal at the temperatures recommended, some of the oxygen may react with other constituents of the biological fluid. This method is time-consuming and requires rather elabo
rate apparatus.
The amount of dissolved oxygen present in water low in or
ganic matter, and from which certain interfering inorganic sub
stances are absent, can be determined by adding to the water reagents th a t react quantitatively with the dissolved oxygen (3,17). The well-known YVinkler (1, 17) method is of this type.
These methods are not applicable to biological fluids because the reagents are relatively strong, are not specific for oxygen, and react with organic material.
Methods have been described based on the reaction between dissolved oxygen and any one of a scries of reducing agents to produce a colored compound, the amount of which is determined colorimetrically (9, 11,18). Alkalies or acids are used to acceler
ate the reaction.
A quantitative m ethod for dissolved oxygen, based on its reaction with reduced ascorbic acid added to the liquid, is re
ported in this paper. The reaction is completed in 5 to 15 minutes a t 25° C., within the pH range 5.0 to 7.5.
In general, th e literature indicates th a t one atom of oxygen reacts with one molecule of reduced ascorbic acid to form one molecule of dehydroascorbic acid. From th e standpoint of a desirable analytical m ethod the reaction in milk between dis
solved oxygen and reduced ascorbic acid is too slow.
In the authors’ early studies, after adding ascorbic acid, the reaction was catalyzed, by adding 1 mg. per liter of copper as copper sulfate, after which the milk was kept cold for 2 days and the decrease in ascorbic acid was determined. Mixtures of de
aerated and aerated milk were prepared and the composition of the mixtures as determined by oxygen content was found to agree within 1 to 2 per cent with the composition of the mixtures as pre
pared by weighing.
Next, work on the photosensitizing action of riboflavin on the oxidation of ascorbic acid by light in the presence of dissolved oxygen (6) led to the use of blue light as a catalyst. Completely filled test tubes containing a marble were rotated end oyer end before a strong blue light. The reaction between the dissolved oxygen and the reduced ascorbic acid was complete in less than 30 minutes. Although a large number of analyses were made using this catalyst, the absolute values obtained for dissolved oxygen raised doubts as to the quantitative nature of the reaction.
A study of the quantitative reaction between dissolved oxygen and reduced ascorbic acid (5) revealed th at under various condi
tions 1.19 to 1.67 atoms of oxygen were involved with copper as the catalyst, 1.57 to 2.04 with light and riboflavin, 2.00 with alkali, and 1.00 with ascorbic acid oxidase (from cucumber juice or cabbage juice).
As a result of this study ascorbic acid oxidase was adopted as the catalyst and with a 10 to 1 concentrate of the enzyme the reaction was found to be complete in milk in 5 m inutes a t 25° C.
This general m ethod for the determ ination of dissolved oxygen has been under investigation for 3 years and the method using cucumber juice as a catalyst has been in use for more than a year, during which tim e several thousand oxygen determinations have been m ade on milk, water, buffer solu
tions, bacteriological culture media, etc. W ith plenty of equipment conveniently arranged one person can make 30 de
term inations in a half day.
R e a g e n ts a n d A p p a ra tu s
Pr e p a r a t i o n o f Dy e So l u t i o n. Grind about 0.135 gram of 2,6-dichlorophenolindophenol in a mortar, add about 50 ml. of hot distilled water, grind further, and decant through a filter into a 1-liter volumetric flask. Add more hot water to the residue remaining in the mortar and again decant through the filter.
Continue the process until practically all the blue color has passed through the filter, then adjust to room temperature, and make up to 1 liter with distilled water. The use of dye solution more than 2 weeks old is not advisable.
St a n d a r d i z a t i o n o f Dy e So l u t i o n. Weigh accurately ap
proximately 100 mg. of ascorbic acid, place in a 1-liter volumetric flask, and make up to volume with distilled water. Mix thor
oughly and use at once for standardizing. Introduce 5 ml. of this standard ascorbic acid solution and 15 ml. of 0.1 N sulfuric acid into a 200-ml. beaker and titrate at once with the dye solution, using 15-ml. burets graduated to 0.05 ml. Titrate the solution to a light pink color which is permanent for 30 seconds. There is a tendency which must be guarded against to select darker and darker end points. Subtract from this titration the value ob
tained on a blank determination made by titrating a comparable acid water mixture to the same end point. The blank is usually about 0.3 ml. The dye factor in terms of milligrams of ascorbic acid per liter of milk when a 10-ml. aliquot of the milk is titrated is given by the following equation
Factor - mg. of ascorbic acid weighed 2 (standardization titration — blank) The amount of original dye taken should be adjusted or the solution should be diluted to give a factor between 6.5 and 7.0.
Su l f u k ic Ac id So l u t i o n. Prepare a 10 Ar stock solution by diluting 285 ml. of concentrated sulfuric acid to 1 liter. For use, dilute 10 ml. of the stock solution to 1 liter and use approximately 25 ml. of this dilute solution for each determination.
As c o r b ic Ac id Ox i d a s e. Freeze solid two to four medium to large green cucumbers, make 2 or 3 small holes about 0.6 cm.
(0.25 inch) deep in the end of each, and suspend the cucumbers hole end downward in a piece of cheesecloth over a 1-liter beaker.
A t laboratory temperature the cucumbers will thaw and most of the juice (about 500 to 800 ml.) will drain out in 8 to 12 hours.
A little squeezing of the bag will express an additional amount of juice. Most of the enzyme resides in the outer layer of the cu
cumber, and little is present in the seed portion. Therefore the yield of enzyme is not increased appreciably by excessive pressure or extraction of the pulp.
The juice may be concentrated by either freezing or pervapo
ration. In concentration by freezing about half of the water is frozen, the juice is expelled from the ice crystals, and about half of the water is then frozen in the expelled liquid. This proc
ess is continued until about 30 to 50 ml. of the juice concentrate remain. The concentrate is filtered and preserved by a drop or two of toluene or chloroform.
Concentration by pervaporation is more efficient. The ex
pressed juice is placed in cellophane tubing about 1.8 cm. (0.75 inch) in diameter and the tube is looped over a glass rod.
An electric fan is directed toward the loops. Evaporation to one
594 I N D U S T R I A L A N D E N G
Fi g u r e 1. Sp e c i a l Tu b e f o r De t e r m i n i n g Di s s o l v e d Ox y g e nb y As c o r b i c Ac i d- As c o r b ic
Ac id Ox i d a s e Me t h o d
tenth to one fourteenth of the original volume will take about 8 hours. The temperature of the juice remains considerably below room temperature, owing to the cooling effect of evaporation.
The concentrate is filtered and preserved by a drop or two of toluene or chloroform. For each determination 0.1 ml. of the concentrated juice is used.
The concentrate preserved in this way in the dark a t 20° to 25° C. will maintain its activity for about 1 month; if kept cold it remains active for 2 months or more. The enzyme is destroyed very rapidly by shaking. Mercuric chloride, sodium chloride, and glycerol (Vs volume), when used in bactericidal concentra
tion, rapidly inactivate the enzyme. One-half saturation with ammonium sulfate can be used ¡is a preservative, but it is not so satisfactory as toluene or chloroform. The enzyme can be pre
cipitated with an equal volume of acetone and the dried precipi
tate possesses activity.
Co n c e n t r a t e d As c o r b i c Ac id So l u t i o n. Weigh 3.0 grams of i-ascorbic acid and 10 grams of sodium chloride and make up to 50-ml. volume with water. For each determination use 0.1 ml.
of the solution.
Sp e c i a l Tu b e s f o r Ox y g e n An a l y s i s. T he tubes U s e d
for th e oxygen determ ination are illustrated in Figure 1, which is largely self-explanatory.
The tube is so made th at it can be closed at three levels, leaving no air trapped under the stopper after filling and after removing portions for titration. The filled tube with a No. 5 stopper in position A holds about 70 ml., with No. 3 stopper on a glass rod in position B about 45 ml., and with No. 1 stopper on a rod in position C about 20 ml. A large rubber band held over the small end of the tube by a small band holds the stoppers tightly in place. A round piece of wood partly drilled through is used to center and press downward on the glass rods. The upper end of this wooden cap is grooved, so tha the band will not slip off.
In preparation for the taking of the sample the tube arranged as in A is used. A glass marble is used for mixing and is held in place in a depression cut on the top of the No. 5 stopper. The
marble is mounted on the stopper of each tube, so th at it will be in the hand while taking the sample and thus will not be for
gotten after the sample is taken. The sample should not be taken with the marble in the tube, because air may be trapped between the marble and the bottom of the tube.
The authors have over 100 of these analysis tubes which were m ade by a glass blower. A large num ber of tubes was needed in studies of the oxygen content of milk which were carried out in commercial m ilk plants, and for studies on the rate of disappearance of oxygen from milk on holding.
Ti m e- Sa v i n g De v i c e s. The authors’ work has required so m any oxygen determ inations th a t it has been found ad
visable to construct a num ber of devices to simplify and shorten th e tim e required for th e m anipulation. These de
vices include a b attery of 8 b uret holders, beaker holders with pipet drainage support, oxygen analysis tube holder for shaking with spring clamps to hold in stoppers and tubes, a drainage rack, and various racks for holding the analysis tubes.
M e th o d
Co l l e c t i o n o f Sa m p l e. T he special oxygen-analysis tube as illustrated in Figure 1, .4, is used for collecting the sample.
Preferably the milk is made to flow from the vat by a glass or rubber-tube siphon or from a milk line by a petcock and rubber or glass tube. Hot milk can be siphoned or passed through a cold water-jacketed coil and thus discharged cold without ex
posure to air. The marble and stopper are removed from tube .4, the end of the siphon is inserted to the bottom of the tube, the milk is made to flow steadily with undisturbed surface into the sampling tube, and one to two volumes are allowed to overflow.
The siphon is then removed and the marble is placed gently on the surface'of the milk in the analysis tube and released. If the marble is dropped into the milk a row of bubbles of air will follow in its wake. The rounded No. 5 stopper is then inserted. The rounding of the stopper aids in preventing the trapping of air and in expelling the surface milk. The rubber band placed over the stopper holds it under tension, so th at if the milk is cooled further the stopper will follow the surface down and not permit air to be drawn in between the wall and the stopper. The sample is now ready for analysis.
Samples can be taken satisfactorily by means of a specially constructed pipet of about 250-ml. capacity. The pipet should be rather long and narrow, with a stem th a t is long enough to reach to the bottom of the special oxygen-analysis tubes. In sampling, the pipet is slowly drawn full of milk, the end is in
serted to the bottom of the oxyjren-analysis tube, and about two tube volumes of milk are allowed to overflow slowly; the pipet is then withdrawn, the marble added, and the stopper placed in the tube.
De t e r m i n a t i o n o f Re d u c e d As c o r b i c Ac i d i n Mi l k.
T he analysis tube is inverted several times to mix the sample by causing the marble to fall from one end to the other.
Pipet 10 ml. of milk from the upper compartment of the tube into a 200-ml. beaker to which 25 ml. of dilute sulfuric acid have previously been added. Duplicate aliquots may be taken if de
sired. T itrate with the dye solution, using a 15-ml. buret cali
brated in 0.05-ml. divisions. In titrating it will be found that after a small amount of dye is added the solution will assume a pink color. If this color fades on standing, more dye is added.
Continue the addition of dye in portions until a light but definite pink color remains for 30 seconds. This is taken as the end point of the titration. Before calculating the original ascorbic acid in the milk a blank of about 0.4 ml. (12) should be subtracted.
In order to save the time which may be required for the pink color to fade it is convenient to have 6 or 8 burets and titrate 6 to 8 samples a t one time. If one is interested only in the oxygen con
tent, determination of the reduced ascorbic acid present in the original milk may be omitted.
Ad d i t i o n a l As c o r b i c Ac i d Ad d e d. Fresh milk contains only about one third enough ascorbic acid to react with the dissolved oxygen present in ordinary milk; therefore ascorbic acid m ust be added in am ount to be am ply sufficient to react with all the oxygen which m ay be dissolved in the milk.
N E E R I N G C H E M I S T R Y Vol. 13, No. 9
