S. W . F e r r i s a n d J. M . M c I l v a i n , T h e A tla n tic R efining C om p any, P hiladelp hia, P a.
M
: ZOO P IS O Ï? ISO
y i n
a.
I '
O 150
< MO p a: OO o O 120
— „
_
y— /
— / ^ SAMPLE 1
1 I 1 1
10 20
DEPTH
3 0 4 0 OF SAMPLE
3 0 60 I (nn) Fi g u r e 1 . Re v e r s a l i n Ap p a r e n t Tr u e Co l o r w i t h Di f f e r e n t De p t h s
o f Oi l s
E T H O D S used in this country for assigning numerical color values to petroleum lubricating oils may be grouped as follows:
C l a s s 1. A given thickncss of oil is m atched against one of a set of glass or liquid standards, prim arily on the basis of hue.
In th is class fall the Lovibond (7) and th e N ational Petroleum Association (1) scales.
Cla s s 2. A variable thickness of oil is m atched against a
standard color disk, as in the m ethod initially described by Del-bridge (5) and later applied in the Tag- R obinson1 colorime
ter (13)] or against a fixed depth of liquid standard, as in the m ethods of Rogers, Grimm, and Lemmon (10) a n d o f W e ir , H o u g h t o n , a n d Majewski (12). In a t least one of these (12) t h e m a t c h is m ade on th e basis of e q u a l i t y of bright
ness. A variant of m ethods of this class consists in matching a fixed thickness of th e oil sample against a variable thickness of oil standard, as in t h e m e t h o d o f Parsons and Wilson (9). The color values d e r i v e d f r o m the m ethods of class 2, w ith the exception of th e Tag-Robinson colorimeter, have been called “ true colors.”
Cl a s s 3 . T he fraction of light absorbed by a certain thickness of oil is determ ined photom etrically. Examples of this class are the instrum ent proposed by C o x (S) which uses a sectored flicker disk, an d th a t of Story and K alichevsky (11) in which the dis
tance between sam ple and illum inant is varied and the inverse- square law employed. A pparatus in which the relative trans
mission of unknow n and standard oils are indicated by the re
sponse of photoelectric cells have been developed by several of the instrum ent companies. In all the methods cited except one (S) w hite light has been used as illum inant.
While several of these methods have been useful in the specification of color for sales work, from the viewpoint of the refiner each has more or less serious shortcomings. In order to serve him best, a color scale must meet six specifications:
1. Samples should be accorded values in agreement w ith ordinary visual inspection.
2. Values should be additive in th e sense th a t th e color Cm, of a m ixture of two oils having colors C„ and C \ will be given by th e equation
c„ _ +
PjL*
(!)“ 100
where F„ and Vt, are th e respective percentages (by volume) of th e oils whose colors are Ca and Ct,.
3. A pparatus, standards, an d color values should be repro
ducible in an y laboratory.
4. Color values should be based on fundam ental rather th an arbitrary' units.
5. The m ethod should be usable in routine work.
6. Consistent color values should be obtainable on oils ranging from light finished products to the darkest ta rs and bottoms.
1 T h e S a y b o lt c h ro m o m e te r f o r m e a s u rin g th e co lo r of g ao lin e s a n d n ap h * t h a a f a l l s i n t h i s c la s s .
T h e m e th o d s of class 1 fail to m eet specification 2. (F o r exam ple, a m ix tu re of equal p a rts of tw o finished oils from th e sam e sto ck , w hich h a v e resp ectiv e N . P . A . colors of 4 a n d 8, m a y h av e a n N . P . A. color of 6.5, w hereas E q u a tio n 1 w ould req u ire a n N . P . A. color of 6 fo r th e m ix tu re . S to ry a n d K alich e v sk y (11) show t h a t th e L ovibond scale lead s to oil colors w hich are fa r from a d d itiv e in th e sense of E q u a tio n 1.) T h e m eth o d s of class 2 w hen re stric te d to m a tc h in g a v ariab le th ick n ess of oil a g a in s t a fixed th ick n ess of one s ta n d a rd , a s p rescribed in th e m o s t recen tly p u b lish ed ex
am ple (12) of th e class, yield color v alu es w hich sa tisfy speci
fications 1, 2, a n d 6, b u t do n o t m e e t specifications 3 a n d 4, a n d could be b e tte r w ith reg ard to specification 5. T h e m e th ods of class 3 fail to m e e t e ith e r specification I (3) o r specifica
tio n 2 (11). (T h e m eth o d of Cox is open to o b jectio n because of his choice of b lu e lig h t as illu m in an t, r a th e r th a n because of h is p h o to m etric a rran g em en t.) W ie n photocells are used w ith w h ite lig h t as illu m in a n t i t is w ell to re m e m b e r t h a t th e response cu rv e of th e cell m a y v a ry m ark ed ly from t h a t of th e n o rm al eye (th e so-called v isib ility cu rv e). O 'B rien
(8) h a s show n th a t filters can be devised for a t le a st one ty p e of photocell, such t h a t th e response curve of th e co m b in a
tion is in sa tisfa c to ry a g reem en t w ith t h a t of th e eye.
F i c u r e 2. V a r i a t i o n o f A p p a r e n t T r u e C o l o r o f D a r k O i l s * M a t c h e d a g a i n s t V a r i
o u s D e p t h s o f S t a n d a r d
* 0.1 to 2.0 per cent solution in ethylene dichloride.
T h is p a p e r p re se n ts ex p erim en tal d a ta to show how a color scale fo r p etro leu m lu b ric a tin g oils w as developed, w hich m eets all six of th e above specifications. A com plete d e scrip tio n of th e final m eth o d is also given, w ith exam ples of d e te rm in a tio n s on a larg e n u m b e r of oils.
Ex p e r i m e n t a l
One of th e m o s t serious ob jectio n s to th e tr u e color m eth o d of d eterm in in g oil colors is th e change in a p p a re n t tr u e color w ith th e n a tu r e o f th e s ta n d a rd oil (10) o r w ith th e d e p th of sta n d a rd a g a in s t w hich th e m a tc h is m a d e . F ig u re 1 show s th e re su lts of a n e x p erim en t in w hich tw o oils of n e a rly th e sam e N . P . A. colors w ere co m p ared in th e D u b o sc q colorim eter, using w h ite lig h t. (T hese a n d su b s e q u e n t 23
24 A N A L Y T I C A L E D I T I O N Vol. 6, No. I
Ja n u a ry 15, 1934 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 25
26 A N A L Y T I C A L E D I T I O N Vol. 6, No. 1