INDUSTRIAL a n d ENGINEERING CHEMISTRY
AN A LYTICA L EDITION
W A L T E R J. M U R P H Y , EDITOR ISSUED JU N E 17, 1943 VOL. 15, NO. 6 CO NSECUTIVE NO. 12
Editorial A ssistant: G . G l a d y s G o r d o n M anuscript A ssistant: S t e l l a A n d e r s o n M ake-up Assistant: C h a r l o t t e C . S a y r e
B. L. C l a r k e
T . R . C u n n i n g h a m
Advisory Board G. E . F. L u n d e l l M . G. M e l l o n
R. H . M ü l l e r
H . H . W i l l a r d
D e t e r m i n a t i o n o f P r e c is io n o f A n a ly tic a l C o n tr o l M e t h o d s ... Raymond F. M oran 361
T u r b i d i m e t r i c D e t e r m i n a t i o n o f S m a l l A m o u n t s o f C h lo r id e s . E. N. Luce, E. C. Denice, an d F. E. A kerlund 365
C o lo r In d e x . L ig h t- C o lo r e d P e t r o l e u m P r o d u c t s . . I. M. Diller, J. C. Dean, R. J. D eG ray, an d J. W . Wilson, Jr. 367
D e t e r m i n a t i o n of I o d in e i n T e tr a io d o p h e n o l- p h t h a l e i n ...
Sam uel W einer, Byron E. Leach, an d M ary Jane Bratz 373
D e t e r m i n a t i o n o f M o n o a lk y l E t h e r s o f E th y le n e G ly c o l . . . H arold W. W ern er an d Jam es L. M itchell 375
C o lle c tio n a n d E s t i m a t i o n o f T r a c e s o f F o r m a l d e h y d e i n A ir . . F. H. G oldm an an d H erm an Yagoda 3 77
F u r f u r a l ( C o rr e s p o n d e n c e ) . . V anderveer V oorhees 378
D e t e r m i n a t i o n o f S m a l l A m o u n ts o f T e l l u r i u m i n H ig h - L e a d a n d T in - B a s e A l l o y s ...
Ralph A. S chaefer 379
M ix e d S o lv e n t E x tr a c tio n Jam es H. W iegand 380
D e t e r m i n a t i o n o f H a lo g e n s i n O r g a n ic C o m p o u n d s Robert R. U m hoefer 383
D e t e r m i n a t i o n o f C h lo r id e i n B a u x it e - S u p p o r te d A n h y d r o u s A lu m i n u m C h lo r id e C a ta l y s ts . . . .
W . A. La Lande, Jr., Heinz H einem ann, an d W . S. W . M cC arter 385
D e t e r m i n a t i o n o f I r o n i n P r e s e n c e o f C h r o m i u m a n d T i t a n i u m w i t h J o n e s R e d u c t o r ...
F. S. G rim aldi, R. E. Stevens, a n d M. K. C arro n 3 8 7
E x tr a c tio n of A sc o rb ic A cid f r o m P l a n t M a t e r ia ls . . J. D. Ponting 3 89
A p p a r a tu s fo r P u r i f ic a t io n o f H y d r o c a r b o n s b y R e c r y s ta ll iz a t io n ... John Lake Keays 391
F i l t r a t i o n C y lin d e r . R. J. D eG ray an d E. P. Rittershausen 392
M IC R O C H E M IS T R Y :
S e m im ic r o a n a ly s is of S a lin e S o il S o lu t io n s . . . . R. F. Reitem eier 393
E s t i m a t i o n o f S u l f o n a m i d e s ...
S. W. Lee, N. B. H annay, an d W. C. H and 403
M i c r o d e te r m i n a ti o n o f M e r c u r y i n O r g a n ic C o m p o u n d s ... H. William Eckert 406
M ic r o d e te r m in a tio n o f A rse n ic i n B io lo g ic a l M a te r i a l ... James A. Sultzaberger 408
M u s ta r d G a s i n A i r ... W illiam Rieman III 411
M i c r o d e te r m i n a ti o n o f M a g n e s iu m w i t h P o la r o - g r a p h ... C hristopher C arru th ers 412
R e p r o d u c ib ility o f W e ig h in g s M a d e o n M ic r o c h e m ic a l B a la n c e s (A. C . S. C o m m i t te e R e p o r t)
C lem ent J. R odden e t al. 415
D e te c tio n o f G o ld i n P l a t i n g . . . . M elvin L erner 416
T h e A m erican C h em ical S o ciety assu m es no re s p o n sib ility for th e s ta te m e n ts a n d o p in io n s a d v an c e d b y c o n trib u to rs to its p u b lic a tio n s.
29,500 copies of th is issu e p rin te d . C o p y rig h t 1943 b y A m e ric a n C h em ical S ociety.
P u b l i c a t i o n O ffice : E a s t o n , P c n n a . E d i t o r i a l O ilic c : 1155 1 6 th S t r e e t , N . W ., W a s h i n g t o n , D . C .
T e l e p h o n e : R e p u b l ic 5301. C a b l e : J i c c h c m ( W a s h in g t o n )
P u b lish e d b y th e A m e ric a n C h em ica l S o ciety , P u b lic a tio n Office, 2 0 th &
N o r th a m p to n S ts., E a s to n , P e n n a . E n te r e d as seco n d -class m a tte r a t th e P o s t Office a t E a s to n , P e n n a ., u n d e r th e A c t of M arch 3, 1879, as 24 tim es a y e ar. In d u s tr ia l E d itio n m o n th ly o n th e 1 st; A n a ly tic al E d itio n m o n th ly on th e 1 5 th . A c ce p ta n c e fo r m a ilin g a t special r a t e of p o stag e p ro v id e d fo r in S ectio n 1103, A c t of O c to b er 3 , 1917, a u th o riz e d J u ly 13, 1918.
A n n u a l s u b sc rip tio n r a te , In d u s tr ia l E d itio n a n d A n a ly tic a l E d itio n sold o n ly as a u n it, m em b ers S3.00, o th e rs $4.00. F o reig n p o sta g e to c o u n trie s n o t in th e P a n A m e ric a n U n io n , 82.25; C a n a d ia n p o stag e, S0.75. Single
A d v c rti« in g D e p a r t m e n t : 332 W e s t 4 2 n d S t r e e t , N ew Y o r k , N . Y . T e l e p h o n e : B r y a n t 9-W 30
copies: In d u s tria l E d itio n , $0.75; A n a ly tic a l E d itio n , $0.50. S p ecial r a te s to m em bers.
N o claim s c a n b e allow ed for copies of jo u rn a ls lo s t in th e m ails un less su ch claim s a re receiv ed w ith in 60 d a y s of th e d a te of issue, a n d n o claim s will be allow ed for issues lo st as a re s u lt of insu fficien t n o tic e of ch an g e of ad d ress. (T e n d a y s ’ a d v a n c e n o tic e re q u ire d .) “ M issin g from files”
c a n n o t be a cc e p te d as th e re a so n for h o n o rin g a claim . A d d ress claim s to C h arle s L. P a rso n s, B u sin ess M a n a g e r, 1155 16th S tre e t, N . W ., W ash in g to n . D . C ., U . S. A.
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. 15, No. 6
CENTRAL SCIENTIFIC COMPANY
S C I E N T I F I C I N S T R U M E N TS ( j N ( Q L A B O R A T O R Y A P P A R A T U S Rtc u * pat off.
NEW YORK TORONTO CHICAGO BOSTON SAN FRANCISCO
V V T E e n th u sia stic a lly in tro d u c e a n ew " P h o te lo m e te r ,” n o t
” to s u p p la n t b u t to b e a c o m p a n io n in s tru m e n t to N o . 1 2 3 3 5 C e n c o -S h e a rd -S a n fo rd " P h o te lo m e te r .” T h e n ew type is sm a lle r in size a n d is k n o w n as I n d u s tria l T y p e B -2. I t is d e s ig n e d to ta k e e c o n o m ic a d v a n ta g e o f p ro d u c tio n in q u an tity a n d u tm o st sim p lic ity o f c o n s tru c tio n w ith o u t sacrifice o f d e s ira b le features. Its sen sitiv ity a n d accuracy a re sufficient fo r m o st la b o r a to r y d e te rm in a tio n s .
T y p e B-2 is a c o m p a c t filter p h o to m e te r fo r c h e m ic a l analyses in th e ro u tin e o r c o n tr o l la b o ra to ry . M o ly b d e n u m , tita n iu m , v a n ad iu m , o r m a n g a n e se in steel; le a d , c o p p e r, ir o n , o r vita
m in s in fo o d s a re a few ty p ical d e te rm in a tio n s to w h ic h th is type le n d s itself.
I t is a b a rrie r-la y e r in s tru m e n t c o n s is tin g o f b asically a lo w v o ltag e lig h t so u rc e , a n a d ju sta b le lig h t a p e rtu re , a th re e -c o lo r filter h o ld e r , re c e p ta c le s fo r tu b u la r o r re c ta n g u la r a b s o r p tio n cells, a s in g le p h o to e le c tr ic cell, a n d a sen sitiv e c u rre n t m e a s u rin g in s tru m e n t w ith a 2}/¡“ scale. T h e scale re a d s fro m 0 - 1 0 0 in 50 d iv isio n s.
T h e b a sic p a rts a re m o u n te d in a n a ttractiv e p la s tic case fo r c o n v e n ie n t m a n ip u la tio n a n d r e a d in g . A c o n s ta n t v o ltag e tra n s fo rm e r to su p p ly c o n s ta n t in te n sity o f lig h t w h e n o p e ra te d o n a c o n tro lle d freq u en cy p o w e r lin e is fu rn ish e d w ith th e
115 vo lt A C in s tru m e n ts .
N o . 1 2 3 4 6 " P h o te lo m e te r ” In d u s tria l T y p e B-2 in c lu d in g a p a c k a g e o f ( 1 2 ) N o . 1 2 3 4 4 G T u b u la r A b s o rp tio n C ells.
N o ...: ... A C F o r v o lts ... 115 A C 6 D C P r i c e ... $ 1 1 0 .0 0 $ 1 0 0 .0 0
TYPE B-2
S E T T I N G T H E P A C E I N C H E M I C A L P U R I T Y S I N C E 1 8 8 2
1 B a k e r A d a m s o n C Á & w t t c a / s
SOUU-i.iv/~ - - rmdini!
p la tin g in p la tin g w id e aP P “ h e a r in g s , s to r- a lr p la n c m o lo r c tc . S ig iu fi-
oge battery d lahor
r e s u lt fr o m its «- to d ay '•
AND
N e w York C ." R A id e /s Di vi si on of G E N E R A L C H E M I C A L C O M P A N Y , 4 0 Re ct or St.,
Technical Service Offices: A tlanta • Baltimore • Boston • Bridgeport (C onn.) • Buffalo • C harlotte (N . C .) Chicago • Cleveland • Denver • D etroit • Houston • Kansas City • Milwaukee • M inneapolis
New York • Philadelphia • Pittsburgh • Providence (R . I .) • St. Louis • Utica (N . Y .) Pacific Coast Technical Service Offices: San Francisco • Los Angeles
Pacific Northw est Technical Service Offices: Wenatchee (W ash.) • Yakima (W ash.) In Canada: The Nichols Chemical Company. Limited • M ontreal • T oronto • Vancouver
June 15, 1943 A N A L Y T I C A L E D I T I O N
PIKIIIIICTIIIII LINIiSi
A m erica over, th e accent is on ever-increasing p ro duction! F ro m spraw ling, busy assem bly lines in ship y ard s . . . p lan ts . . . factories, w ar m a te ria l is m oving off p ro d u c tio n lines in surging volum e . . . A m erica is “ deliv erin g tlie goods!” Yet w ith all the pressing need for m ore and m ore w ar m a te ria ls — quality still rem ains th e im p o rta n t factor.
D epen d ab le reagen ts are v ital . . . becausc th ey h elp m a in ta in th is q u ality an d accuracy !
In m any lead in g p lan ts th e country over,
B ak er & A dam son R eagents are th e standard for lab o rato ry control. L abo rato ries know th ey can rely up o n th e h ig h ly uniform q u ality of B&A R e a g e n ts . . . count on th e ir d ep e n d ab ility an d p u r
ity . . . because accuracy, h ig h q uality and u n i
form ity are “b u ilt” into each B &A R eagent th ro u g h 60 years of experience.
W h e th e r yo u r la b o ra to ry re q u ires reagents for special analyses or for ro u tin e testing . . . select B & A Reagents to do the job right!
AQUEOUS PROCESSING SOLUTION
H EATING STEAM AND CONDENSATE
TO BOILER
AQUEOUS PROCESSING SOLUTION
S ig n allin g C o n d u c tiv ity C o n tro lle r in sta lle d on a ste a m re tu rn from processing v a t to feed -w ater su p p ly .
A Slogan for A ll Americans
J r l. Ad N -93-163(3)
I f you w ant a continuous, au to m atic record of the p u rity of the condensate, in addition to an auto
m atic signal and the operation of a dum p valve, we recom m end N o.
33111 M icrom ax Signalling R e
corder. T his in stru m en t can be read from a considerable distance, and its ch a rt records are 106/s"
diam eter. I t requires no more a tte n
M icro m ax C o n tro llin g R e c o rd e r used b y th e C ro s* ett (A rk .) L u m b e r C o., on a w a te r line.
tion th an does a tem p eratu re re
corder.
F or fu rth er inform ation, see C a ta log N -95-163(l), on th e Signalling C ontroller, or C atalog N-95-163, on the M icrom ax C ontrolling R e
corder.
LEEDS 4. NORTHRUP COMPANY, « 2 0 STENTON AVE., PHI LA.,
LEEDS & N O R TH R U P
MEASURING INSTRUMENTS • TE LE M E TE R S • AU TO M ATIC CONTROLS • H EAT-TREATIN G FURNA
WANT TO IM M OBILIZE THIS L E A K ?
E N G I N E E R I N G C H E M I S T R Y Vol. 15, No. 6
ure, continuously, the electrolytic conductivity of the w ater, and open a dum p valve in the line when a leak is indicated by a rise in conductivity.
T he instru m ent em ployed m ay be either of the following:
If the operation of a signal light and the opening of the dum p valve are all you require when a pipe springs a leak, we recom m end No.
4S50 Signalling C on du ctivity Con
troller. T his in stru m en t has no m oving p arts except a relay; re
quires practically no m aintenance;
gives protection a t m inim um cost.
I N D U S T R I A L A N D
T o keep contam inated condensate from sneaking into a boiler’s tubes and sabotaging steam production, we can supply an instru m en t which will h alt the p otential dam age at its source, and hold it immobilized while a m aintenance pipe-fitter repairs the leak and thus ends the contam ina
tion.
T h e m ethod em ployed is to meas-
June 15, 1943 A N A L Y T I C A L E D I T I O N 7
M E R C K & CO.«, InC. ty/lan ttfa ctur-iny S/emi4t6 R A H W AY, N . J .
P lease se n d m e th e follow ing c h a rts: N a m e ... ACS-6-43
□ S e n sitiv ity of Q u a lita tiv e R eactio n s C o m p a n y ... P o s itio n ...
□ P erio d ic C h a rt of th e E le m e n ts S t r e e t ...
□ S e n sitiv ity C h a rt C ity ... S ta t e ...
The United States N avy’s “battlewagons,”
“tin cans,” and “pig boats”— fam iliar and affectionate terms used by the fighting men of our fighting sea forces— actually are precision machines, designed by skilled engineers, and painstakingly built of the finest materials.
To produce the thousand fold of intricate, even delicate, p arts th a t make an efficient fighting ship, requires highly accurate and scientific control of the raw materials which go into its construction. Such control has its birth in research, analytical, and metallur
gical laboratories, where skilled scientists test and regulate the quality of every raw
material used in building our mighty fleet.
Chemistry has helped to make America’s naval force second to none!
We are proud of the p a rt th a t M erck Labo
ra to ry Chem icals played in th is g re a t achievement.
Chemicals which are destined for use in such a painstaking task must, of necessity, possess superb quality themselves. The rigid control exercised over Merck Reagents in our Ana
lytical Laboratories makes certain th a t they will always be fine tools for precision mea
surements.
8 I N D U S T R I A L Ä N D E N G I N E E R I N G C H E M I S T R Y Vol. 15, No. 6
E L E O T .R -IC
V
F U R N A C E / F U S E D
June 15, 1943 A N A L Y T I C A L E D I T I O N
PROVIDING ACCURACY ASSURANCE
Every Parr Calorimetric Thermometer is especially d esigned, produced and calibrated under specifi
cations of extrem e exactness to incorporate the highest available guality of material and workman
ship.
Each thermometer furnished with a Parr Calorime
ter has a com plete correction chart drawn from test data.
This is just one of the many provisions made by the Parr laboratory m en to provide manufacturers with precision Parr apparatus.
S e n d f o r f u r t h e r i n f o r m a t i o n .
INSTRUMENTED.
M O L I N E I L L .
PARR BOMB CALORIMETERS and CHEMICAL TESTING APPARATUS
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. 15, No. 6
CONSOLIDATED Engineering Corporation announces a new instrument
and techniques for the analysis of gas and liquid mixtures.
S P E C I A L F E A T U R E S
M ASS SPECTRO M ETER
l HE Consolidated M ass Spectrom eter and pertinent a n a ly sis techniques h a v e been developed to p rovide an accurate/ rap id method of a n a ly zin g sim ple or com plex m ixtu res. As a result of unique design featu res of this instrum ent and of techniques developed b y Consolidated, m ixtures containing a s m an y as 15 or m ore components can be a n a ly ze d w ith speeds u n attain ab le before.
FU N C T IO N T h e M a s s S p e c t r o m e t e r b rea ks m olecules of the m a te ria l intro
duced into charged fragm en ts, o r ions, in a m anner dependent on the structure and composition of the m olecule. The ions are then sep arated into b eam s, each beam containing ions of a certain m ass num ber.
The beam s in the sequence of th e ir mass num bers strike a collector w h e re th ey surren d er th e ir charges. The resu ltan t cur
ren ts a r e am plified , and a perm anent autom atic record of the m ass spectrum is o btain ed . The composition of a m ixture m a y be q u a n tita tiv e ly determ ined from its m ass spectrum .
U S E S T h is d e v e lo p m e n t p r o v id e s a m arked im provem ent in the a n a ly sis of charge stocks and fee d stream s used in the m anufacture of synthetic rubber and high octane av iatio n g aso lin e , thus pro
viding a sup erio r m eans of control per
mitting more efficient p lan t o peratio n s.
In Research it p rovides unlim ited oppor
tunities fo r investig atio n of problem s arisin g in d evelopm ent of new products.
The Consolidated Mass Spectrometer is made available to W ar Industries on a basis which provides ample protection in future developments in this field. . . W rite for particulars.
CONSOLIDATED ENGINEERING CORPORATION
Herbert Hoover, J r ., President
1 2 5 5 E A S T G R E E N S T R E E T • P A S A D E N A , C A L I F O R N I A
> Analysis of 15-20 sam
ples can be obtained with one instrument in an eight- hour day.
> Results are practically independent of variable factors due to the human element.
> Results can be computed in such a way as to be self-checking.
> 1 /10 c.c. of sample is usually adequate for an analysis.
> The instrument may be ad ju sted fo r o p eratio n over a molecular weight range from 1 to 250.
> Units are designed for convenience of installation and operation.
> M a n y au to m atic f e a tures result in ease of operation.
> A utom atic p ro te ctive circuits insure against ac
cidental damage.
> Conservative electrical design assures continuous operation.
( j ~ua
yâ n ie.e o f I n f i s - l b i e
.{,)u a li.t.y
KIMBLE GLASS CO M PA N Y--- v i n e l a n d , n . j .
N E W Y O R K • C H I C A G O . P H I L A D E L P H I A • D E T R O I T • B O S T O N • I N D I A N A P O L I S • S A N F R A N C I S C O
June 15, 1943 A N A L Y T I C A L E D I T I O N
I n m e e t in g t h e d e m a n d s o f w ar p r o d u c t i o n n o w a n d i n a n t ic i p a t in g t h e n e e d s o f c iv ilia n lif e a ft e r v ic to r y c o m e s , I n d u s t r y ’s la b o r a to r ie s a re serv ed by
K I M B L E L A B O R A T O R Y G L A S S WA R E
« ■ e s **KS K K
< EXAX> <^N0RMAXS / \
B L U E X u.
L 1 N T M M C - U S A* T.M . R E G . U S A.
U s in g s ta n d a r d i t e m s w h e r e v e r p o s s ib le m a y b e im p o r t a n t i n m i n im iz in g d e la y s . C o n s u lt le a d in g L a b o r a to r y S u p p ly H o u s e s t h r o u g h o u t t h e U n i t e d S t a t e s a n d C a n a d a a b o u t t h e K im b le p r o d u c t s y o u r e q u ir e .
12 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. 15, No. 6
A
complete set of mated A.R.L.-DIETERT Spectrographic Equip
ment as illustrated increa
ses production and assures material specification control in minutes instead of hours. Send today
for catalogue No. 128, MODERN ANALYSIS.
A P P L I E D 4 3 3 6 SAN
i
R E S E A R C H L A B O R A T O R I E S F E R N A N D O RD., G L EN D A LE , CALIF.
m l .
H A R R Y W. D I E T E R T ~ ~C O . 9330 ROSELAWN AVE.. DETROIT, MICH.
USE SPECTROGRAPHIC EQUIPMENT
TO ANALYZE YOUR WAR MATERIALS
June 15, 1943 A N A L Y T I C A L E D I T I O N 13
PYREX
T h e P y rex b ran d K jeld ah l F lask , fa b ri
cated from P yrcx b ra n d B alanced C h em i
cal Glass N o. 774, is b u t one o f th e m ore th a n 2700 P y rex L a b o ra to ry W are item s co n trib u tin g to lab o rato ry , achievem ent.
C o r n i n g
— — m e a n s — —
Research in Glass
L o c k e d i n e a c h g r a i n o f w h e a t is a precious substance— nitrogen— which is essential to the health and well-being of the human race.
Scientists in the laboratories of America’s food industries tirelessly seek the hidden secrets locked inside these health- giving grains so that we may have more nutritious flour and cereals.
Laboratory research, too, makes possible better, faster- acting fertilizers to invigorate the earth, to speed plant growth and to increase the production of vital food-stuffs.
The Pyrex brand Kjeldahl Flask, because of its low expan
sion and high chemical stability, successfully meets the se
vere conditions encountered in nitrogen digestion and distil
lation. Pyrex Kjeldahl Flasks have strong tool-finished necks of uniform taper to insure accurate stopper fit. This finish in
creases the strength and minimizes the possibility of break
age in handling and use, particularly when inserting stoppers.
Corning Research pledges itself to supplying constantly improved laboratory glassware. Consult your laboratory
supply dealer for complete inform ation on any requirement. “ Y O U H A V E D O N E A G O O D J O B O F S E N D IN G G L A S S TO W A R ”
P Y R E X* >* » ° L A B O R A T O R Y W A R E
" P Y R E X ” a n d " V Y C O R " a re r e g is te re d tra d e - m a rk s a n d in d ica te m anufacture b y
C O R N I N G G L A S S W O R K S . C O R N I N G , NE W Y O R K
14 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. 15, No. 6
A.H.T. CO. SPECIFICATIO N
G AST PO R TA B LE ROTARY AIR B LA S T AND SUCTIO N A P P A R A T U S
R O TA R Y A IR BLAST AND S U C TIO N A PPA R A TU S, GAST P O R T A B L E , A .H .T . CO.
S P E C IF IC A T IO N . A qu iet, air cooled, m o to r d riv en u n it com plete w ith v a c u u m an d pressu re gauges, a n d th e rm a l overload circu it breaker. S u itab le for con tin uou s o p eratio n a t p ressures n o t exceeding 20 lbs., or for in te rm itte n t use u p to 30 lbs.
C onsisting of a pu m p w ith cast iron ro to r fitted w ith fo ur com position vanes revolving in a p re cision m achined housing of special alloy iro n w hich is connected d irec tly to a 1/6 h. p., 1725 r. p. m.
m otor. T h e p u m p ro to r is a n extension of, an d in teg ra l w ith, th e sh a ft of th e m oto r. T h e com p lete a p p a ra tu s is m o u n ted on five ru b b e r feet a n d is eq uipp ed w ith ca rry in g h andle, air filters an d oil tra p a tta c h e d d irec tly beh ind th e ta p e re d ru b b e r tu b in g connections a t th e in let an d o u tlet, dial ty p e vacu u m gauge 2 inches d iam eter, g ra d u a te d from 0 to 30 inches of m erc u ry in in te rv a ls of 1 inch, pressure gauge 2 inches diam eter, g ra d u a te d from 0 to 50 lbs. in in te rv a ls of 1 lb., au to m a tic pressure release valve a d ju ste d a t 30“ lbs., an d bleeder petcocks for re g u la tin g th e pressure a n d v ac
u u m to th e re q u irem en ts of th e w ork in ten d ed w ith in th e lim its described below.
T h e com bined filter, muffle a n d tr a p on th e pressure side is enclosed in cast iron an d is supplied w ith a cartridge w hich can be rem oved for cleaning o r replacem ent. T h e filter on th e vacuum side is a com bined oiling a n d air filtering device. I t is enclosed in a screw neck glass reservoir in w hich th e oil level can be observed a t all tim es.
F o r in te rm itte n t use th e p u m p can be o p erated up to 30 lbs. pressure, or higher, if th e safety valve is re a d ju ste d in th e lab o rato ry b u t, for continuous use, th e m axim um pressure m u s t n o t exceed 20 lbs.
Speed, r. p. m ... 1725
M axim um pressure, lbs. p er sq. in ... 20 to 30 C u. ft. of free a ir (atm ospheric pressure) p er m inute, a p p ro x im a te ly ... 1.3 Cu. ft. of free a ir a t 30 lbs. pressure (for 10-m inute p e rio d s)... 0.95 N um ber of b la st lam ps (M .I.T . ty p e or e q u a l)... 4
M axim um vacuum , inches of m ercu ry ... 27
Pow er consum ption, w a tts ... 250
N et w eight, lbs... 32 Code 1033-G. G ast P o rtab le R otary Air B last an d Suction A pparatus, A .H .T. Co. Specification, as above described, W ord
com plete w ith pressure an d vacuum gauges, a n d therm al overload circuit breaker, filters, carrying
handle an d 10 ft. cord w ith snap sw itch a n d plug. F o r 110 volts, 60 cycles, a .c ... 33.50 Abxes 1033-H. D itto, b u t for 110 volte, d.c... 44.50 Abxhm 1033-J. D itto, b u t for 220 volts, 60 cycles, a.c... 34.75 Abxik 1033-K. D itto, b u t for 220 volts, d.c... 44.75 A b xji
I o % d i s c o u n t i n lo ts o f J , a s s o r t e d
A R TH U R H. T H O M A S C O M P A N Y
R E T A I L — W H O L E S A L E — E X P O R T
LABORATORY APPARATUS AND REAGENTS
W EST W ASHINGTON SQUARE, PH ILA D E LPH IA , U. S. A.
C able A ddress, “ B alance,” P h iladelphia
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 I S H E D B Y T H E A M E R I C A N C H E M I C A L S O C I E T Y • W A L T E R J . M U R P H Y , E D I T O R
D ete rm in a tio n o f th e P recisio n o f A nalytical C ontrol M ethods
R A Y M O N D F . M O R A N , W e s tv a c o C h lo r i n e P r o d u c t s C o r p o r a t io n , S o u t h C h a r le s to n , W . V a.
For th e in t e llig e n t c o n tr o l o f p la n t op era
tio n s a n d p r o d u c t q u a lity , i t is e s s e n tia l t h a t t h e p r e c isio n o f t h e a n a ly tic a l co n tr o l m e th o d s b e q u a n tita tiv e ly k n o w n . T h e u s e o f s t a tis tic a l r e a so n in g b a sed o n t h e s ta n d ard d e v ia tio n h a s b e e n fo u n d a p p lica b le.
T h e a n a ly tic a l m e th o d is first te s te d u n d er id e a l c o n d itio n s to fin d t h e h ig h e s t p reci
sio n o f w h ic h th e m e th o d is cap a b le. I f th is p r e c isio n is ju d g e d h ig h e n o u g h , th e m e th o d is t h e n te s te d in r o u tin e p r a c tic e for a year t o d iscover t h e v a ria b ility u n d er ro u tin e la b o r a to ry c o n d itio n s . A n LJJ2 (lim it
o f u n c e r ta in ty ) ca n th e n b e c a lc u la te d w ith in w h ic h r o u tin e a n a ly se s m a y b e gu a ra n tee d . T h e average o f d u p lic a te d e te r m in a tio n s m a d e a t th e s a m e tim e d oes n o t r e s u lt in as m u c h im p r o v e m e n t as m a y b e th e o r e tic a lly c a lc u la te d , e v id e n tly b e ca u se th e r e s u lts are n o t tr u ly ra n d o m . N o rm a l co n tr o l m e th o d s w ere fo u n d to h a v e 1.5 to 2.5 as m u c h v a ria tio n u n d e r r o u tin e c o n d itio n s as th e s a m e m e th o d u n d e r th e b e st c o n d itio n s . T h e u se o f th is m e th o d o f c r itic ism h a s proved a v a lu a b le to o l in th e a u th o r ’s lab o ra to ries.
I
T IS a primary concept of nature th at no one physical measurement is exact. Only those values th at are accepted by definition are free from deviations in the last significant figure. The determination of the composition of any sample even by the best known technique is similarly influenced by the inability to measure weights, volumes, colors, chemical equilibria, etc., with exactitude. Practically all analytical methods contain enough small constant errors to make them somewhat empirical. I t follows that some differences will be obtained between individual analytical results from the same sample even if the best possible tech
nique and instrumentation are used.
When the conditions th at obtain in commercial control laboratories are considered, the above concept becomes much more important, since the variations between personal ana
lytical techniques, solutions, apparatus, and surrounding conditions are certain to influence the precision of an ana
lytical method to a considerable extent. This is true even if the method is followed exactly as written; a departure from standard instructions or error in judgment would give further deviations from the truth.
Since these variations are known to exist, they are a con
stant threat to good commercial operation or product quality until they are quantitatively determined. An attem pt to control a plant operation within 0.05 per cent of a standard value with a method precise to but =*=0.5 per cent would ob
viously fail, yet similar situations frequently occur in industry
because the precision of the analytical method is not known, There also exists a tendency on the part of many engineers, sales personnel, and even chemists to treat a single analytical result as an exact quantity and to make decisions therefrom th at would be unjustifiable if the significance of the result were known.
After an extensive program of development and refinement of analytical methods, this company was faced with the deter
mination of how well the routine control laboratories were following these new methods. Several years were spent in check sample work in which standard samples were run in duplicate by one laboratory and attempts were made to check these results in another laboratory. Although considerable information was obtained, a great many disputable differ
ences arose that could not be traced to any assignable cause.
One reason for this disagreement was found to be the use of the “average deviation” as a criterion, since any variations greater than the criterion were treated as poor analyses.
In commercial analytical practice speed is often essential for good control, even if accuracy and precision are sacrificed.
I t is often expedient to employ nontechnical personnel as analysts or testers for routine determinations. Any method for determining the precision of routine results must be able to evaluate these personnel factors.
The procedure described in this article was developed in the summer of 1940 and has been used throughout the company with satisfaction since that time.
361
362 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. 15, No. 6
T a b l e I. P r e c i s i o n ' o f M e t h o d u n d e r B e s t C o n d i t i o n s M e th o d : SC -11-3 of 6-27-41 fo r specific g r a v ity of c a rb o n tc tra c h lo rid e b y
p y c n o m c ter) T e s t N o. Specific G ra v ity
a t 2 5 ° /4 ° C. d d* (X 10»*)
1 1.5 8 4 9 - 0 . 0 0 0 1 6 256
2 1.5851 + 0 .0 0 0 0 4 16
3 1.5853 + 0 .0 0 0 2 4 576
4 1.5853 + 0 .0 0 0 2 4 576
5 1.5850 - 0 . 0 0 0 0 6 . 36
6 1.5849 - 0 . 0 0 0 1 6 256
7 1.5847 - 0 . 0 0 0 3 6 1296
8 1.5851 + 0 .0 0 0 0 4 16
9 1.5852 + 0 .0 0 0 1 4 196
10 1.5851 + 0 .0 0 0 0 4 16
Av. >=* X \ « 1.58506 , 2d* » 3240 X 10l°
J E Í =
T 10 0 .0 0 0 1 8 *= on
L U aw - 1 .5 8 5 0 6 * 3 (0 .0 0 0 1 8 )
V w 1.58-189 to 1.58523
L U i 3 (0 .0 0 0 1 8 )
0 .9 2 3 0 .0 0 0 5 9
P rior C o n cep ts
A number of investigators (2,3,5,6) have indicated that the variations of an analytical method may be treated by statisti
cal calculations. Power (6) found th at over 100 determina
tions had to be made before the variability of a micromethod for carbon could be accurately estimated. Informative as such an extensive investigation might be, such a study in a commercial laboratory would be difficult to justify for eco
nomic reasons. The two routines given below use 10 samples and 24 samples to obtain the precision and the results have been satisfactory from the author’s point of view. Al
though some of the calculations have the surface appearance of complexity, the most complicated operation is the extrac
tion of square roots.
The criterion by which the precision of the method is to be expressed is of considerable importance. I t may be mathe
matically proved th at the standard deviation, <r, is the most accurate measure of dispersion about an arithmetical mean.
Any multiple of cr may be selected according to the limits of precision desired.
Since the author wished to express the precision as the limits within which an analytical result could be practically guaranteed, the value ±3cr was chosen. If the variations are distributed about the mean according to chance and the determinations are representative, the average ±30- should include 997 out of 1000 determinations (1,7). This is equiva
lent to one result outside the limit in a year’s routine of daily analyses and was thought to represent the precision in the best possible manner for commercial practice. The investi
gator must use some judgment before any unreserved guar
antees are given, since these characteristics of cr are strictly true only for a system of chance causes; but the viewpoint adopted was that all variations are due to chance until the cause becomes known. By intelligent usage of the data obtained from these calculations, variations due to other than chance causes may be detected and subsequently eliminated.
Although the accuracy of a method is of great importance, a general procedure for its determination cannot be described.
When a standard sample made from knowrn amounts of the desired ingredients can be prepared, a definite measure of accuracy may be detennined, but such samples are sometimes impossible to create with certainty because of the nature of the sample. Most commercial methods are more or less empirical in nature and are accurate by definition, their purpose being to enable two or more interested parties to obtain substantially the same results on the same material when analyzed independently. All the author’s methods were checked by whatever convenient means were possible,
such as the use of standard samples, synthetic knowns, checks with standard methods, etc., and if the limit of uncertainty of the average (described below) bracketed the known result, the method was said to be “accurate” in a qualitative sense.
L im it o f U n c e r ta in ty u n d e r th e B e s t C o n d itio n s (££/i)
The highest precision that can be expccted from an analyti
cal method is the precision as shown by the best available technician working under the most closely controlled condi
tions. The following procedure was prescribed as standard for obtaining a measure of this precision:
P rep are a sy n th etic m ix tu re t h a t exactly reproduces th e com m ercial m aterial an d co n tain s a know n a m o u n t of th e desired in gredient, or select a rep resen tativ e hom ogeneous sam ple of the com m ercial m aterial. Store enough m aterial for a t least fifty analyses in containers t h a t will p re v e n t an y change in com position for a t least one year.
Select a n an a ly st w ho is well acq u ain ted w ith th e m ethod, p ref
erab ly th e one who developed th e m ethod.
P rep are fresh reagents, stan d ard iz e all solutions, an d calibrate all a p p a ra tu s before m aking th e determ in atio n s.
R un te n analyses as closely to g e th e r as possible u n d er th e m ost favorable conditions. C alculate all results to one m ore place th an
is generally rep o rted . _
C alculate th e arith m etical average, X t, o f th e te n analyses.
C alculate th e sta n d a rd d ev iatio n of th e results by th e use of the following eq u atio n :
trio /zd*
\ 10
w here o-io = sta n d a rd deviation from th e average show n by the 10 results
d = individual deviation of each resu lt from th e average C alculate th e lim it of u n c e rta in ty of th e average, L U a,-., b y the following eq u atio n (1):
LUa,. = Xl * 3<r‘°
V Io
C alculate th e lim it of u n c e rta in ty of th e m ethod u n d er th e best conditions, L U i, as follows (1):
L U i = * 3trio 0.923
Table I indicates the method used for calculation.
If the sample is of known composition and the method is accurate, Xi =*= LUav. should bracket the known value. If the result differs, judgment has to be used to decide whether the method should be improved until it is accurate or a standard correction should be applied to all results. If the concentration is not known, the accuracy cannot be deter
mined unless the method is “accurate by definition” as previously mentioned.
Since the LU\ is a measure of the ultimate precision of the method as written, a decision as to the commercial applicabil
ity of the method can be made. If the indicated precision is high enough, the method is ready for test under routine condi
tions, but if it is found too low the method must first be im
proved, since the test under routine conditions is almost certain to show an even lower precision because of the addi
tional sources of variation introduced.
L im it o f U n c e r ta in ty u n d e r R o u tin e C o n d itio n s (L U %)
If all the above conditions have been satisfied, the method may be tested under routine conditions by the following pre
scribed procedure:
U se th e sam ple on w hich th e L U \ te s t w as m ade.
H av e th e sam ple analyzed in a ro u tin e m an n er b y a ro u tin e technician, p referably to g eth er w ith ro u tin e p la n t sam ples of a
June 15, 1943 A N A L Y T I C A L E D I T I O N 363 sim ilar n a tu re . I f possible, th e technician should n o t know the
prior results.
H av e th e sam ple analyzed in duplicate each m o n th for one y ear, using as m an y different ro u tin e technicians as possible.
As soon as th e m o n th ly analyses h ave been com pleted, com p are th e resu lts w ith th e average an d L U i o b tain ed durin g th e first te s t. I f th e in d iv id u al results are w ithin X i + L U allow th em to sta n d as rep resen tativ e. If th e results are outside these lim its, im m ediately check th e technique, reagents, etc., so th a t a n y d ev iatio n from th e m eth o d will be discovered before th e de
tails slip th e tech n ician ’s m ind. I f an objective difference in technique is found, h av e th e analyses rerun by th e sam e tech n i
cian b u t in th e correct m anner. I f no assignable cause is found, allow th e resu lts to rem ain uncorrected.
W hen th e 12-m onth accum ulation o f 24 analyses has been com pleted, average th e 24 results to o b tain A': . Also calculate th e average o f each m o n th ’s duplicate tests.
C alculate th e s ta n d a rd d ev iatio n of th e 24 individual tests, an, an d th e sta n d a rd deviations of th e 12 m o n th ly averages, 0"12.
C alculate th e lim it of u n c e rta in ty of th e average as follows:
LU*. = 3ff;<
V 2 4
C alculate th e lim it of u n c e rta in ty o f th e m ethod ( /) :
LUi = *3 au 0 .9 6 8 Table II illustrates these calculations.
If the limit of uncertainty of the average includes the known value, the method may be called accurate. If the concen
tration is not known but the method is “accurate by defini-
T a b l e I I . P r e c i s i o n o f M e t h o d u n d e r R o u t i n e C o n d i t i o n s (M e th o d : SC -11-3 of 6-27-41 fo r specific g r a v ity of c a rb o n te tra c h lo rid e b y p y c n o m e te r. A v erag e of 10 an aly se s fo r L U i t e s t =» 1 .5 8 5 0 6 , L U i =*
*=0.00059)
M o n th j J a n u a r y F e b r u a ry M a rc h A p ril M a y J u n e J u l y A u g u st S e p te m b e r O c to b e r N o v e m b er D e c e m b e r
■Grand a v e ra g e ®* X j *» 1.5 8 5 0 3
.S ta n d a rd d e v ia tio n (in d iv id u a l d e te rm in a tio n s ) = an
D e v ia tio n of D e v ia tio n of
Specific In d iv id u a l D u p lic a te
G ra v ity fro m G ra n d A v erag e fro m
a t A v e ra g e of A v erag e, G ra n d a ly st 2 5 ° /4 ° C. D u p lic a te s di A v e ra g e, di
1 1 .5 8 5 0 - 0 . 0 0 0 0 3
1 1.5851 1 .5 8 5 0 5 + 0 .0 0 0 0 7 + 0 .0 0 0 0 2
2 1 .5 8 4 8 - 0 . 0 0 0 2 1
2 1.5 8 5 0 1 .5 8 4 9 0 - 0 . 0 0 0 0 3 - 0 . 0 0 0 1 3
1 1.5 8 4 6 - 0 . 0 0 0 4 3
1 1.5 8 4 6 1.5 8 4 6 0 - 0 . 0 0 0 4 3 - 0 . 0 0 0 4 3
o 1.5851 + 0 .0 0 0 0 7
2 1.5851 1 .5 8 5 1 0 + 0 .0 0 0 0 7 + 0 .0 0 0 0 7
3 1.5 8 5 3 + 0 .0 0 0 2 7
3 1 .5 8 5 3 1.5 8 5 3 0 + 0 .0 0 0 2 7 + 0 .0 0 0 2 7
4 1.5 8 4 9 - 0 . 0 0 0 1 3
4 1 .5 8 5 0 1 .5 8 4 9 5 - 0 . 0 0 0 0 3 - 0 . 0 0 0 0 8
5 1.5 8 4 9 - 0 . 0 0 0 1 3
5 1 .5 8 4 8 1.5 8 4 8 5 - 0 . 0 0 0 2 3 - 0 . 0 0 0 1 8
3 1.5 8 5 4 + 0 .0 0 0 3 7
3 1.5 8 5 4 1 .5 8 5 4 0 + 0 .0 0 0 3 7 + 0 .0 0 0 3 7
3 1.5 8 5 2 + 0 .0 0 0 1 7
3 1.5853 1 .5 8 5 2 5 + 0 .0 0 0 2 7 + 0 .0 0 0 2 2
2 1.5 8 4 7 - 0 . 0 0 0 3 3
2 1.5851 1 .5 8 4 9 0 + 0 .0 0 0 0 7 - 0 . 0 0 0 1 3
2 1 .5 8 5 0 - 0 . 0 0 0 0 3
2 1.5 8 5 0 1 .5 8 5 0 0 - 0 . 0 0 0 0 3 - 0 . 0 0 0 0 3
6 1.5 8 4 9 - 0 . 0 0 0 1 3
6 1.5851 1 .5 8 5 0 0 + 0 .0 0 0 0 7 - 0 . 0 0 0 0 3
V:
Sdl24 0.00022T 1 2
3<T24 V24 1
■Standard d e v ia tio n (d u p lic a te a v erag es) «■ an
JLimit of u n c e r ta in ty of g ra n d a v e ra g e =* 1.58503 ***
L im it of u n c e r ta in ty of m e th o d u n d e r ro u tin e c o n d itio n s = =*=
0.00022
0.00021
1 .5 8 4 9 to 1.5 8 5 2
3 cm 1=0.00068 = L U i
0 .9 6 8
H a tio of im p r o v e m e n t b y d u p lic a tio n . L U t 0.00068
au
aiz 0.00021 1 .0 5
R a tio of
L U i 0.00059 1 .1 5
tion” , the limits of uncertainty of the averages under both LU 1 and LUi conditions must overlap for approval of the method. The LUi result is the objective figure of the whole study and represents the precision that may be expected from the method under routine conditions. Since 997 out of 1000 determinations can be expected to represent the truth within ±LUi, this result may be used to judge whether the method is precise enough for routine control and is a measure of the range within which analyses are to be trusted. If the LUi is judged small enough, the method is approved for routine but if it is too large for good control, it must be re
placed or amended.
By comparison of the LUi with the LUi, a measure of the amount of possible improvement in routine work is obtained.
Since the LUi was obtained under ideal conditions, the LUi may be brought closer to the LU\ value by further education of routine personnel, intensive standardization of technique or equipment, closer temperature control during seasonal variations, etc., but it might be found th at the analytical method possesses inherent characteristics that allow personal variation that cannot be standardized without extensive modification of the method. The comparison of the stand
ard deviations shown by the individual analyses and the dup
licate averages represent the amount of improvement that may be obtained by the averaging of two or more analyses.
For all these possibilities that may confront the investigator, the LUt and LUi are excellent tools for guidance.
D isc u s s io n o f R e su lts
The analytical method reported in Tables I and II rep
resents a simple technique th at is of major importance to the author’s laboratories, since it is a sensitive test for the com
position and purity of chlorinated hydrocarbons and their mixtures. This determination is made in a 25-ml. pyc- nometer containing an accurate thermometer. The pycnome
ter is filled with the sample and allowed to expand to near equilibrium temperature in a balance room. The contents are adjusted to the mark, the temperature is read, and the pycnometer is weighed. The indicated specific gravity is then corrected to 25°/4° C. by means of the appropriate factor.
The LUi test (Table I) indicated that this method would give results within ±0.0006 of the mean in 997 out of 1000 trials. By comparison with plant control and finished prod
uct specifications, this indicated precision was found satis
factory. The average and limit of uncertainty of the average check closely with the 1.5850 a t 25°/4° C. characteristic cal
culated from reported data for carbon tetrachloride (4).
Accordingly, the method was judged ready for the LUi test.
The LUi test (Table II) indicated th at the method was also accurate in routine work, since the average agreed very closely with the previously obtained figure. The L Ui was found to be
=*=0.0007, which was only slightly higher than the LUi of
±0.0006 and represents very good agreement for routine work. The method was therefore judged satisfactory for all routine laboratory work.
The standard deviation of the averages of the duplicate determinations in this study was most enlightening. If the deviations were purely random, the averaging of two inde
pendent analyses should have decreased the variations by a factor of V 2 or 1.41. The actual improvement as shown by the ratio of standard deviations was only 1.05. This ratio indicated that duplicate analyses made a t the same time are not truly random but are rather influenced by slight varia
tions in technique or surrounding conditions.
Table II I gives the over-all results from a number of rep
resentative commercial analytical methods. For each of these seven methods the precision shown by the LUi test
364 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. 15, No. 6 appeared satisfactory. The LUi oi tests 1, 3, 4, and 5 were
low enough for the required closeness of control. Test 2, a determination by a modified Volhard method for chlorides, indicated an LUi that was 2.7 times as large as the LU\ and was considered too variable for good control. This method was re-examined for possible improvement. Tests 6 and 7 were obtained by a refined Engler distillation technique, but the results were found to be influenced by seasonal varia
tions and superheating of vapors in the flask. These methods were also improved to eliminate these assignable causes.
C o m m erc ia l A p p lic a tio n
Because the study is founded on statistical reasoning from the comparatively small number of samples dictated by eco
nomic reasoning, the results are open to some variations in an absolute sense. However, in practice this objection has been found academic, since the study has resulted in defi
nite precision characteristics close enough for purposes of refinement of methods and techniques and to discover within what limits routine analytical results are significant. The
T a b l e I I I . P r e c i s i o n ' T J e t e r m i n a t i o n s o p R e p r e s e n t a t i v e C o m m e r c i a l A n a l y t i c a l M e t h o d s Im p ro v e m e n t
N o. S a m p le A n aly sis L U i L U t L U t / L U i D e te rm in a tio n s
1 5 0 % c a u s tic so d a N a O H , % ± 0 .0 7 8 ± 0 .1 4 7 1 .9 1 .2
2 5 0 % c a u stic soda N a C l, % ± 0 .0 1 0 2 ± 0 .0 2 8 0 2 .7 1 .0
3 5 0 % cau stic so d a F e , p. p . m. ± 0 .7 7 ± 1 .0 0 1 .3 1.1
4 C arb o n te tra c h lo rid e S p. gr. a t 2 5 ° /4 ° C. ± 0 .0 0 0 5 9 ± 0 .0 0 0 6 8 1 .1 5 1 .0 5
5 C a rb o n bisulfide N o n v o la tile re sid u e, % ± 0 .0 0 0 4 ± 0 .0 0 0 8 2 .0 1 .0
6 T ric h lo ro e th y le n e B oilin g ra n g e , ° C. ± 0 11 ± 0 .3 3 3 .0 1 .0 7
7 T ric h lo ro e th y le n e F i r s t d ro p , ° C. ± 0 .0 5 ± 0 .3 0 6 .0 1 .0 7
In the study of some 40 basic analytical methods during the last two years, the following relationships were found from the LU»/LUi ratio:
R atio s of 1 .0 to 1 .5 indicate th a t th e v ariatio n s resu lt m ainly from lim itatio n s of in stru m en tatio n .
R atio s of 1.5 to 2.5 indicate norm al relationship betw een an aly ses m ade u n d er th e b e st conditions b y one m an an d analyses m ade u n d er routine la b o rato ry conditions.
R atio s over 2.5 generally indicate considerable personal and seasonal v ariations th a t m ay be reduced w ith o u t m odification of th e basic an aly tical m ethod.
D u p licate d eterm in atio n s ru n a t th e sam e tim e are generally n o t tru ly random a n d th e averaging of such results does n o t im prove th e precision appreciably. T h e only justification for ru n n ing analyses in duplicate is th e ad d itio n al safeguard afforded against an o u trig h t m istake, such as an e rro r in calculation o r an erroneous weighing.
Most commercial analyses are made on products of ap
proximately constant composition, so that a measure of pre
cision may be expressed in terms of the actual percentage variations on the original sample basis. Whenever several widely different levels of composition exist, it has been found desirable to make complete studies at each level to discover whether or not the extent of the variations is proportional to the amount of ingredient.
P ro ced u re for D e c o m p o sa b le S a m p le s Several cases have been encountered where the year’s study was rendered impossible because of the instability of the samples or attack on the containers. In this case either one of two techniques has been used to approximate the true LU2:
1. If th e sam ple decomposes a t a c o n sta n t ra te , th e results m ay be p lo tte d ag ain st tim e an d an average curve draw n th ro u g h the points. T h e deviations of th e individual resu lts from th is curve are used for th e calculation.
2. If th e sam ple decom poses in an in c o n sta n t m an n er or a t tack s its co n tain er appreciably, or if an approxim ate d eterm in a
tio n of th e ro u tin e precision is desired rapidly, a speedup o f the technique m ay be used by m aking th e L U i tests du rin g one day by as m an y ro u tin e an aly sts as possible. T h is gives only an approxim ation of th e tru e L U i, because v ariatio n s encountered du rin g a y early s tu d y m ay n o t be observed. I t does yield an approxim ation of personal differences.
procedure described is offered as a tool for such investigation and as a standard method for describing precision.
The use of standard samples in the control laboratory has the additional advantages of providing periodical checks on personal techniques and for the education of new labora
tory personnel. Since the basis of comparison is fair, the individual technician is more likely to cooperate than if he is expected to duplicate exactly a single analysis from some out
side source.
These techniques have been in use throughout this com
pany for over two years and have formed the first common bases for describing and comparing precision. They have led to the discovery and subsequent correction of many varia
tions previously not suspected and the quantitative evalua
tion of other deviations th at were known to exist. By the judicious use of L [/2 figures, the men having supervision of plant operation and shipping know within what limits routine analyses are to be trusted. Some similar procedure could well be adopted as standard by industrial analytical chemists to provide a mutual basis of understanding when describing the precision of analytical methods.
A c k n o w le d g m e n t
The writer gratefully acknowledges the assistance of Dwight Williams in the development of the described procedures and the preparation of this paper, and the permission of Westvaco Chlorine Products Corporation for publication.
L ite r a tu r e C ited
(1) Am. Soc. Testing M aterials, "1933 M anual on Presentation of D ata", second printing, 1937.
(2 ) B e n e d e t t i - P i e h l e r , A . A ., In d. En o. Ch e m., An a l. Ed., 8 , 3 7 3
(1936).
(3) Crumpler, T . B., and Yoe, J. H ., “ Chemical Com putations and E rrors", New York, John Wiley & Sons, 1910.
(4) International Critical Tables, 1st ed., Vol. 3, p. 28, Now York, McGraw-Hill Publishing Co., 1928.
(5 ) P o w e r , F. W ., I n d . E n o . C h e m ., A n a l . E d . , 11, 6 6 0 (1 9 3 9 ) .
(6) Power, F. W., “ Application of Modern Statistical M ethods in Chemical Analysis” , presented before Division of Analytical and Micro Chemistry, A m e r i c a n C h e m i c a l S o c i e t y , Memphis, Tenn.
(7) Shewhart, “ Economic Control of Q uality of M anufactured Prod
ucts", New York, D. Van N ostrand Co., 1931.
Pr e s e n t e d b efo re th e D iv isio n of A n a ly tic a l a n d M icro C h e m istr y a t th e lO oth M e e tin g of th e Am e r i c a n Ch e m i c a l So c i e t y, D e tro it, M ich .
