Journal of the Institute of Petroleum, Vol. 31, No. 263

V o l. 31. N o . 263. No v e m b e r 1945.

T H E I S O T H E R M A L A N D A D I A B A T I C C O M P R E S S I B I L I T I E S O F O I L .

B y A. Ca m e r o n, P h.D .

Su m m a r y.

T h e w o rk p u b lish ed on b o th th e iso th erm a l a n d a d ia b a tic com pressibilities of oils is co n sid ered a n d av era g e c u rv es a re giv en fo r b o th com pressibilities over a ran g e of te m p e ra tu re s . T h e ra tio iso th e rm a l/a d ia b a tic co m p ressib ility e q u als g a m m a th e ra tio of th e specific h e a ts, w hich is show n to be 1-135 fo r m in e ral oils.

In t r o d u c t i o n.

Th e com pressibility of oils is a subject th a t is becoming of increasing interest to in d u stry as working pressures become higher and higher.

High-pressure oil hydraulic system s a n d direct-injection heavy oil engines are th e tw o m ain types of equipm ent where a knowledge of it is necessary.

In th e first case th e com pressibility is m ainly isotherm al, and in th e other it is adiabatic. I t does n o t appear to be generally appreciated th a t th e values of these tw o quantities are different though th e y are related by y, th e ratio of th e specific heats.

I t was th o u g h t profitable to collect all th e d a ta published on this subject, as th ese are ra th e r scattered. I t is hoped th a t th e references which have been found and are given here are substantially complete.

Pu b l i s h e d Da t a. (i) Isothermal Compressibility.

Alm ost all th e w ork w hich is published on th e com pressibility of oils is concerned w ith th e isotherm al com pressibility, and generally deals w ith pressures up to 1500 atm ospheres.

All th e high-pressure isotherm al compressibilities have been correlated in a paper b y Dow and F in k ,1 to which reference should be m ade. These authors have shown th a t, w ithin th e experim ental error an d th e experi­

m ental pressure range, all th e results can be fitted on to a single curve.

This curve can be approxim ately represented by an equation of th e ty p e P ~ Po(! +

where p == density of th e oil a t pressure p and tem p eratu re t,

p0 = density of th e oil a t atm ospheric pressure and tem p eratu re t, a an d (3 are constants for th e given tem p eratu re t.

The com pressibility can be obtained from this equation by m eans of th e relation x ( — ) , whence it can be seen th a t a t atm ospheric pressure

dp V P o- •

th e com pressibility equals a. The quadratic equation for p/p0 is satis­

facto ry in th e m edium and high-pressure range, b u t, as Dow and F ink indicate, a t low pressures th e equation is only approxim ate, an d so th e

i i

422 CAM ERON : T H E ISO TH ERM A L AND

first differential is som ew hat uncertain. T he atm ospheric com pressibility, therefore, w hen derived from th e equation, is n o t v ery accurate.

V ery accu rate w ork was carried o u t b y Jessu p 2 of th e A m erican B u reau of S tan d ard s, using a glass a p p a ra tu s a n d working a t pressures u p to 50 k g ./sq . cm. a n d tem p e ratu res to 300° C. H e stu d ied th re e classes of oil, four gas oils, tw o spindle oils, a n d four m achine oils. T he viscosities

-IO O 2 0 4 0 6 0 8 0 IOO 120 140 160

TEMPERATURE DEG. CENT.

Fig. 1.

of th e gas oils were a b o u t 5 centistokes, th e tw o spindle oils h a d th e same viscosity, 3l-7 cs., a n d th e m achine oils averaged 75 cs. These were all ta k e n a t 100° F .

E ach group was found to have ab o u t th e sam e com pressibility, which increased w ith increasing te m p eratu re. T he th ick er oils were fo u n d to have slightly sm aller com pressibilities th a n th e th in n e r ones, b u t th e differences were n o t very large. H e found th e com pressibility of th e tw o heavier oils was sensibly co n stan t over th e pressure range 0 -5 0 k g ./sq . cm.

u p to 100° C.

A D IABATIC C O M PR E SSIB IL IT IE S OF OIL. 423 The m ean values of th e compressibilities of th e spindle an d m achine oils a t various tem p eratu res up to 150° C. are p lo tted in Fig. 1 an d are m arked

“ Iso th erm al.” The tw o crosses, a t 25° an d 40° C., are m ean values worked o u t from th e results of Dow 3 a n d Dow an d Fenske 4 for th e range 0-50 kg./sq. cm. F rom th e isotherm al curves in Fig. 1 it is possible to read off th e com pressibility a t any tem p e ratu re up to 150° C. F o r higher te m p eratu res Je ssu p ’s original w ork m u st be consulted.

(ii) Adiabatic Compressibility.

Parsons an d Cook 5 stu d ied b o th th e adiabatic and isotherm al com­

pressibilities of a num ber of liquids, an d am ong th e m was a cylinder oil.

The ap p a ra tu s used was a large direct plunger m achine, and th e authors give a ra th e r elegant m ethod for finding th e change in volum e o f th e steel container w ith pressure. The in terp retatio n of th eir results is n o t a t all certain. T hey used a large volum e of liquid—2000 c.c.—and apparently added a dead volum e correction of 36-6 c.c. on to th e m ean volum e- pressure curve (curve 2 m inus th e m ean of curve 1 on p. 336 gives a constant difference of 36-6 c.c.). Using th is dead volum e correction th e volume pressure curve for a “ Cylinder Oil ” can be constructed from their curve 4. The value of th e com pressibility is, however, very m uch lower th a n th a t found b y any o th er investigator. Parsons an d Cook give a very interesting calculation for th e change of tem p eratu re on adiabatic compression or expansion, an d this will be discussed la te r in th e theoretical section.

In 1927 D. H . A lexander 6 constructed a simple direct plunger ap p aratu s for m easuring th e com pressibility of a fuel oil on which he was th en working. T he change in volum e of th e vessel was calculated theoretically, and he obtained a value of 3-30 X 10—6/lb./sq. in. a t 5018 lb./sq. in. This corresponds to an adiabatic com pressibility of 47-0 X 10 -6/kg./sq. cm. a t 350 kg./sq. cm. The tem p e ratu re was n o t stated , b u t a value of 60-70° F . will probably n o t be fa r out. N o account was ta k e n of leakage, which was, therefore, presum ably considered to have been small.

In 1933 G. H am abe a n d F . N agao 7 published an extended investigation on nine fuel oils, up to 500 kg./sq. cm., an d a t tem peratures from — 4° to 100° C. an d in one case up to 137° C. T he a p p aratu s used was modified from th e design given b y A lexander. Various refinem ents were added, such as tw o therm ocouples inserted in th e oil and m eans for rem oving th e air. T he te x t is in Jap an ese, so it is probably best to give a fairly extended description of it in order th a t th e original can be followed if necessary.

The volum e of oil used was 503 c.c., an d Parsons and Cook’s m ethod for estim ating AB , th e change in volum e of th e container under pressure, was em ployed. In th is case steel balls were used instead of a steel cylinder, an d th e curve of AR against pressure is p lo tte d in Fig. 3 of th e paper. T he change in volume of th e cylinder w ith pressure corresponded to ab o u t 10 per cent, of th e volume change, due to th e compression of th e oil a t an y pressure.

T hey accounted for th e leakage L , which was sta te d to be very small, b y assum ing th a t if th e plunger descended a distance hx on loading and

424 CAM ERON : T H E ISO TH ERM A L AND

cam e back a sm aller distance h% on unloading, th e tru e distance h w ould unloading. T he change in volum e Av of th e steel balls, u n d er pressure, was calculated from th eir atm ospheric volum e v b y th e equation

T h ey also list th e nam es an d characteristics of th e nine oils used (Table I of th e ir p a p e r). O nly th re e can be identified, an d these are Borneo and S araw ak fuel oils. T he densities are given a t 18° C. a n d are ra th e r high, varying from 0-893 to 0-940. T he E ngler viscosities listed a t 20° C. are norm al for fuel oils— i.e., betw een 1J° an d 10°. F in ally , th e carbon, hydrogen a n d sulphur analyses are ta b u la te d .

T heir results are p lo tte d in graphical form , com pressibility against pres­

sure a t ab o u t six dilferent tem p era tu res for each oil w hich were apparently arb itra rily chosen. As th e com pressibilities do n o t m aterially change w ith th e various oils, th e y have been re -p lo tte d here in th e form of average com pressibility-tem perature curves a t different pressures for all nine oils (Fig. 1) an d are m arked “ A d iab atic.” T he pressures (in k g ./sq . cm.) at which th e com pressibilities were m easured a re n o te d on each curve.

A lexander’s resu lt is given as a cross. H am abe an d N agao s ta te th a t for one of th e oils investigated b y th em (Miri fuel oil) th e isotherm al com­

pressibility is 15 per cent, g reater th a n th e ad iab atic, a t 30° C. T hey do no t definitely sta te if th is was a calculated or m easured resu lt, b u t it is probably m easured, as th e y do n o t give a n y th eo retical discussion a t all.

Now, th e ratio iso th erm al/ad iab atic com pressibility equals y, th e ratio of th e specific heats, an d th is can be determ ined. A specim en calculation gives y equal to 1-135, so t h a t th eo retically th e isotherm al should be 13-5 per cent, greater th a n th e ad iab atic. T he curves in Fig. 1 show th a t th e ra tio is 1-20— 1-28 a t 20° C. a n d 1-15— 1-23 a t 60° C., w hich is some­

w h at larger th a n th eo ry . As th e results are from qu ite different workers an d w ith different oils, th e difference is perhaps no g reater th a n would be expected. H am abe an d N agao’s own re su lt for Miri fuel oil fits in 'well.

These au th o rs give d a ta for th e a d iab atic h eatin g of oil on compression.

T heir results, p lo tte d in Fig. 2 of th e ir paper, show a c o n stan t r a te of h e a t­

ing of 1-0° C. for every 100 kg./sq. cm. up to 450 k g./sq. cm. I t will be

a t high pressures used by H am abe a n d N agao m ay be criticized. They calculate th e com pressibility a t a pressure P from th e relatio n

be (hx -f- hz)/2, it being assum ed th e tim e of loading equalled th e tim e of

where m = 1 /Poisson’s ratio ,

P = pressure,

E — Y oung’s m odulus.

show n in th e theoretical section below th a t th is figure is v ery close to the th eo retical value. T he m ethod of working o u t th e com pressibilities of oils

where Vy = volum e of oil a t pressure = 0, V — V -- 99 99 93 = P± 3

A D IABATIC C O M PR E SSIB IL IT IE S OP OIL. 425 whereas th e com pressibility K a t constant tem p eratu re is defined as th e decrease of u n it volum e per u n it increase of pressure—i.e.

F x \d P )t

A t low pressures th is does n o t m ake a n y appreciable difference.

The ad iabatic com pressibility can also be m easured from th e speed of sound in th e m edium . This was done by Suge 8 using sound of a very high frequency, 4940 kilocycles/sec. The values he obtained were higher th a n th e o th er ad iab atic figures. Also, he found th a t in general f a tty oils had a com pressibility 10 per cent, less th a n m ineral oils. The figures he gives are (65-5 ± 2) X 16“6/kg./sq. cm. for m ineral oils and (60-5 ± 1) X 10_6/kg./sq. cm. for fa tty oils. I t is also possible to calculate th e ratio of th e specific heats of an oil from th e isotherm al com pressibility or th e velocity of sound, knowing th e density and th e rm a l expansion. Suge gives d a ta for th e speed of sound in th e paper already m entioned, and th e density of his oils are n o ted in a previous paper.9

We see, th en , th a t from th e d a ta given in Fig. 1 it is possible to read off th e isotherm al or adiabatic com pressibility over a range of te m p e ra ­ tures. F o r th e isotherm al com pressibility only th e low pressure figures are given— i.e., u p to 50 kg./sq. cm. (or 711 lb./sq. in.), as th e high- pressure compressibilities are given by Dow an d F in k .1 T he experim ental range considered b y these au th o rs is : u p to 1500 kg./sq. cm. a t 25° C., to 1000 kg./sq. cm. a t 40° C., an d to 4000 kg./sq. cm. a t 75° C.

Th e o r e t i c a l. (1) Adiabatic Heating.-

Parsons and Cook have w orked out a relation for j , th e adiabatic heating of a liquid w ith increase of pressure, in term s of th e coefficient of therm al expansion and th e specific h e a t a t constant pressure.

This relation is given as

fd T \ T V A JG p where T = absolute tem perature,

V = volum e per u n it mass,

A = coeff. of th erm a l expansion a t constant pressure, Cp = specific h e a t a t constant pressure,

J = m echanical equivalent of heat.

A an d Cp are know n for oils a t atm ospheric pressures only, an d so this relation is som ew hat restricted in its use.

Parsons an d Cook tested th is relation for w ater and obtained satisfactory agreem ent. Using H am abe an d N agao’s results it is possible to te s t it w ith oil. Their experim ental result for was 0-010° C ./kg./sq. cm. a t 28° C.

Now, tak in g T as 301° abs., A = 0-0007/° C.,

Cp = 0-45 for m ineral oils,

P0 = 0-9, whence F = 1-11 X u n it m ass.

= 0-012° C ./kg./sq. cm., w hich agrees satisfactorily w ith th e experi- V op/

4

,

m e n tal result.

H yde 10 found t h a t A was th e sam e for f a tty an d m ineral oils, b u t CL was 10 per cent, larger for f a tty oils, a n d so for th ese oils th e value of

0/77

(g- ) will be 10 per cent, sm aller th a n for m ineral oils.

(2) R atio o f Specific Heats.

N ow it is possible to find y th e ratio of th e specific h ea ts from the isotherm al com pressibility or th e velocity o f sound in oil, a n d Cp, p0 a n d A .

A relatio n for Cp — Cv th e difference of th e specific h e a ts is given by B irtw istle,11 p. 73 :—

T V A

2

ri __ n r w - K J

T V A 2

whence 1 — y = X J C ...^

where Cp an d Cv = specific h eats a t co n sta n t pressure an d volum e respec­

tively,

K = isotherm al com pressibility.

Now TJ, th e velocity of sound in a m edium , is given by U

2

=

K ?o As we are considering u n it m ass Fp0 = 1, a n d th u s

. . T A

2

U

2

....

y(y — !) = c j - ... (u) T hus y can be o btained from eq u atio n (i) or (ii). Considering eq u atio n (i) first, an d tak in g K equal to — 64-0 X 10“6 cm .2/kg. a t 25° C. (negative sign as ^ is negative) an d A = 0-0007/° C .; if th e d en sity of th e oil == 0-9, so F, th e volum e per u n it m ass, = 1-11 c.c. an d Cp = 0-45 cal./gram , th e n :—

1 _ y = _ 0-1346, an d y = 1-1346.

Now Cp for f a tty oils is 10 p e r cent, larger th a n for m ineral oils, so y will be 1-123.

This value of y should check w ith th e value of y o b tain ed from equation (ii), using th e velocity of sound. Suge 8 gives a value for th e velocity of sound in liquid paraffin. F ro m his earlier p ap er 9 th e density of liquid paraffin is 0-88 a n d th e tem p eratu re a t w hich his m easurem ents 426 CAM ERON : T H E ISO TH ERM A L AND

ADIABATIC C O M PB E SSIB IL IT IE S OF OIL. 427 were carried out is 18° C. I t is necessary to assum e th e results of th e earlier pap er can be applied to th e later one for th e density an d tem p era­

tu re of th e oil.

A will be ta k e n as 0-00070/° C. and Cp as 0-45. This gives a value of y equal to 1-121. This value will n o t be so accurate as th e other value o btained from th e com pressibility.

Now y also equals th e ra tio of th e isotherm al/adiabatic com pressibility (Birtw istle, p. 7 3 ), an d th is provides an o th er experim ental m ethod by which it can be obtained.

Ac k n o w l e d g m e n t s .

This w ork was s ta rte d in th e D ep artm en t of Colloid Science, Cambridge, under Professor E . K . R ideal, F .R .S ., whose interest an d help th e au th o r would like to acknowledge. I t was continued a t th e A dm iralty G unnery E stablishm ent, Teddington, an d published b y th e k in d perm ission of th e S uperintendent an d th e D irector of Scientific Research, A dm iralty.

T he author-w ould like to th a n k Mr. F . Ursell for some useful discussions on th e therm odynam ics of th e subject, and also L ieut. Mills, Queen’s R oyal R egim ent, for m ost useful assistance in th e tran slatio n of th e Japanese.

References.

1 R . B . D ow a n d C. E . F in k , J o u r. A p p l- P h y s ., 1940, 11, 353.

2 R . S. Je ss u p , B u r. S ta n d . J . R es., 1930, 5, 985.

3 R . B . D ow , J o u r. W ash. A ca d . S c i., 1934, 24, 516.

4 R. B . D ow a n d M. R . F e n sk e, I n d . E n g . Chem., 1935, 27, 165.

5 C. A . P a rso n s a n d S. S. Cook, Proc. R o y. Soc., A ., 1911, 85, 332.

6 D . H . A lex an d er, T ra n s. In s t. M a rin e E n g ., 1927, 39, 366.

7 G. H a m a b e a n d F . N ag ao , J o u r. Soc. M ech. E ng. J a p a n , 1933, 36, 518.

8 Y . Suge, S c i. P a p ers I n s t. P h ys. Chem. R es. T okyo, 1938, 34, 1244.

9 Y . Suge, B u ll. I n s t. P h y s . Chem. R es. T o kyo , 1932, 11, 877.

10 J . H . H y d e , R eport L u b n . In q u ir y Committee, D .S I .R . , L o ndon, 1920; also Proc.

R oy. So c., A ., 1920, 97, 240.

11 G. B irtw istle , T h e P rin cip les o f T herm odynam ics. C am bridge, 1925.

428

S P E C T R O S C O P I C A N A L Y S I S .

A P P L IC A T IO N O F T H E U L T R A -V IO L E T A B S O R P T IO N M E T H O D TO T H E A N A LY SIS O F M IX T U R E S CON­

T A IN IN G AROM ATIC H Y D R O C A R B O N S.

B y R . R . Go r d o n, M.A., P h.D ., a n d H . Po w e l l, Ph.D .*

St j m m a e y.

T h e p rin cip les, a p p a ra tu s , a n d te c h n iq u e in v o lv e d in th e q u a n tita tiv e a n aly sis o f 2-, 3-, a n d 4 -co m p o n en t m ix tu re s o f a ro m a tic h y d ro c a rb o n s b y th e u ltra -v io le t a b so rp tio n m e th o d a re d iscu ssed in d e ta il. A m e th o d h a s b e en ev o lv ed w h ich is n o t d e p e n d e n t o n co m p a riso n b e tw e e n th e u n k n o w n m ix tu re a n d s y n th e tic b len d s.

T h e m ix tu re s d e a lt w ith a re : (a) b e n z e n e -to lu e n e , (6) th e C g a ro m a tic s e th y lb e n ze n e, ortho-, m eta-, a n d p a r a - x jle n e , (c) ortho-x.ylene a n d isopropyl- benzene.

T h e a cc u ra c y a tta in a b le is c alc u la te d , a n d is illu s tra te d b y th e a n aly sis o f a n u m b e r o f s y n th e tic m ix tu re s . T h e d e p en d e n ce o f th e e rro r o n th e co m ­ p o sitio n o f t h e m ix tu re is g ra p h ic a lly illu s tra te d , a n d th e m a x im u m possible e rro r in th is m e th o d o f a n a ly sis is discussed.

Th e need of th e petroleum in d u stry for m ore sensitive m ethods of analytical control has been explained in a previous p a p e r,f in which infra­

re d absorption m ethods of analysis hav e been described. T hese methods are applicable to a wide range of problem s and, indeed, are so sensitive t h a t care is necessary to avoid over-com plexity in th e m ix tu re to be analysed. A t present, however, th e q u a n tita tiv e application of th e infra-red m ethod is lim ited b y th e num ber of p u re hydrocarbons w hich hav e been exam ined, m ixtures o f m aterials having boiling points n o t exceeding 120° C.

being th e present lim it. M any in d u strially valuable hydrocarbons have higher boiling points th a n this, n otable am ong th ese being th e arom atics.

A m ethod of simplifying th e an aly tical problem b y using th e ultra-violet region of th e spectrum has been evolved.

T he principles involved are relatively simple, an d m a y be sum m arized as follows.

T he arom atic hydrocarbons, unlike th e paraffins a n d n a p h th en e s with which th e y are associated in straig h t-ru n benzines, n a p h th a s, a n d sa tu rated synthetic m aterials, absorb u ltra-v io let ra d ia tio n in th e w ave-length range 2400-2900 A. (1 A. = 10~8 cm.). T he am o u n t of ra d ia tio n absorbed is not constant over th e range o f w ave-lengths, so t h a t a curve of radiation absorbed versus w ave-length shows a series of peaks a n d valleys. Among th e lower-boiling m em bers of th e arom atic series these curves are different for each hydrocarbon. T hus, for exam ple, benzene gives a d iffe r e n t curve from th a t of toluene, an d p-xylene a different curve from t h a t of o-xylene. These differences form th e basis of th e an aly tical m ethod described.

* A n g lo -Ira n ia n Oil Co., L td ., R e se arc h L ab o ra to rie s, S u n b u ry -o n -T h a m e s E n g la n d .

t J . In s t. P etrol., 1945, 31 (259), 191.

GORDON AND PO W ELL : SPECTROSCOPIC ANALYSIS. 429 One m ethod of analysis m ight be based on a com parison of th e absorp­

tion due to th e sam ple w ith th a t due to m ixtures of know n composition- This, however, would be an extrem ely laborious m ethod, p articu larly in th e case of m ixtures containing four or five arom atic hydrocarbons, and it is necessary to ad o p t a sim pler procedure, derived th u s :

L am bert and B eer’s laws of th e absorption of rad iatio n sta te th a t th e am ount of rad iatio n absorbed by successive layers of th e absorbing m aterial is a definite fraction of th e radiation falling on each layer, an d is also proportional to th e num ber of absorbing molecules in th e radiation p ath . This m ay be expressed in m athem atical sym bols as—

« where I

0

is th e in ten sity of rad iatio n incident on th e absorbing m aterial,

I is th e in ten sity of th e rad iatio n emerging after traversing a thickness t (cm.) of th e absorbing m aterial,

C is th e concentration of absorbing m aterial in gm .-m ol./litre, E is a constant for a given m aterial a t a given w ave-length.

T he q u a n tity E is called th e m olecular extinction coefficient, while th e expression log j is referred to as th e optical density and bears th e sym bol

“ d .” E is th u s th e optical density corresponding to an absorbing th ick ­ ness of 1 cm. an d a concentration of 1 gm .-m ol./litre.

If, therefore, as in th e in stru m en t used for this work and described later, the value of “ d ” a t a given w ave-length is known, and th e value of E a t th e same w ave-length has been determ ined using th e pure absorbing com ponent, th e value of C— th e concentration of th e absorbing substance—- m ay be calculated.

In a m ixture containing more th a n one absorbing com pound th e optical density observed a t an y wave-length is th e sum of th e optical densities due to th e individual com ponents a t th a t wave-length. T h a t th is statem en t is tru e for hydrocarbons is borne out by th e results quoted later.

Therefore in a m ixture containing tw o arom atics, if th e optical density is m easured a t tw o wave-lengths, tw o sim ultaneous equations are obtained, e.g.—

d 1(obs.) = t(E

1

^C

1

^{+ E}

2

^{G2) 1}

d2(obs.) = t(E

1

^'C

1

^{+ E}

2

^{C2) j}

where d ± and cl

2

are th e optical densities observed a t w ave-lengths Xx and

\ ; E x an d E

2

are th e m olecular extinction coefficients of th e tw o arom atics a t Xv a n d E x' and E

2

are th e m olecular extinction coefficients a t X2.

These tw o equations m ay be solved to give C

2

^{and C}

2

—th e concentrations of th e tw o arom atics in gm. m ois./litre. W hen th ree arom atics are present together, th e determ ination of optical densities an d molecular extinction coefficients a t th ree w ave-lengths is necessary, and in this case three sim ultaneous equations have to be solved and so on.

The m ethod of analysis discussed, therefore, involves th e determ ination of th e m olecular extinction coefficients of each pure arom atic a t a series

430 GORDON AND P O W E LL : SPECTROSCOPIC A N A L Y SIS.

of wave-lengths, a n d th e use of these values in th e exam ination of unknow n m ixtures of th ese substances.

Ex p e r i m e n t a l. (a) A pparatus.

The a p p a ra tu s used com prised a H ilger m edium q u artz spectrograph an d a Spekker photom eter. This is a sta n d a rd a p p a ra tu s for w ork of this n a tu re , an d is described fully in th e m ak ers’ publications. A brief descrip­

tio n of th e photom eter is included here.

The Spekker p hotom eter is a device for th e q u a n tita tiv e m easurem ent of th e in te n sity of absorption of solutions. I n it th e ra d ia tio n is split by q u artz rhom bs in to tw o beam s w hich pass, th e u p p er th ro u g h th e solution being exam ined, th e lower th ro u g h a blan k cell filled w ith solvent. These tw o beam s are th e n b ro u g h t to a focus on th e slit of th e spectrograph, the resulting p hotographs appearing as a p air o f spectra in close juxtaposition.- I n th e beam passing th ro u g h th e b lan k cell is placed a variable aperture op erated b y a d ru m on w hich is a scale giving th e logarithm s of th e area ratio s of th e fixed to th e variable a p e rtu re . T he d ru m is th erefo re calibrated in optical densities.

W hen eq u a lity of photographic blackening a t a n y w ave-length is observed betw een th e tw o halves of each spectrum pair, th e n th e in te n sity has been cu t dow n b y th e variable ap ertu re to th e sam e e x te n t as i t has b y the solution. T he am o u n t of th is red u ctio n is know n from th e d ru m reading, an d hence a q u a n tita tiv e m easure o f light a b sorption is obtained.

Since th e in ten sity of absorption varies considerably a n d in a n irregular m anner w ith w ave-length, it is necessary to m ake exposures over a range of d ru m readings in order to be certain of covering th e optical densities at a series of w ave-lengths.

T he source of u ltra-v io let rad iatio n em ployed is a high-tension spark operating betw een tu n g ste n steel electrodes a t 15,000 volts, a 0-005 gF.

condenser being connected in parallel w ith th e electrodes. T he use of the hydrogen discharge lam p as an a lte rn a tiv e source will form th e subject of a subsequent paper.

To p re v en t interference w ith radio an d o th er sensitive ap p aratu s, due to th e high-frequency spark, th e p h o to m eter p a rt of th e equipm ent is enclosed in a double steel box, th e space betw een th e boxes being filled w ith glass-wool as a sound insulator. These tw o boxes are earthed independently, an d it is advisable to keep all leads to th e in stru m en t and e a rth lines as sh o rt as possible. A tte n tio n should also be p aid to the direction of th e axis of th e in stru m en t, if it is to be o p erated n ear other sensitive ap p aratu s. A filter in an e a rth ed m etal box is interposed between th e m ains supply and th e transform er feeding th e spark.

Ilfo rd Z enith p lates hav e been em ployed th ro u g h o u t th e presen t work, an d these have been developed in a m axim um c o n trast developer for a sta n d a rd developm ent tim e of 1 m inute.

T he spectroscopic solvent em ployed has been a m ix tu re of C 7 a n d C

8

paraffins or alkylate, w hich has been given tw o washes of 1 2 | per cent, b y vol. of 96 per cent. H 2S 0 4. This washing is followed b y a soda wash in an all-glass a p p a ra tu s, a n d th e p ro d u ct is distilled in a 40-plate, eyelet-

GORDON AND PO W ELL : SPECTROSCOPIC A N ALYSIS. 431 packed column a t 10 to 1 reflux ratio. The cu t representing 10-80 per cent, of th e charge to th e still is used as solvent.

(b) Procedure.

The d eterm ination of arom atic hydrocarbon contents by th e u ltra ­ violet spectroscopic m ethod depends largely on th e in terp retatio n of the photographic plates obtained, and in p articu lar on determ ining th e points a t which equal blackening occurs of th e tw o halves of each p air of spectra produced b y th e photom eter. This is an a rt which is acquired by experience, an d it has been found th a t an unskilled operator requires about six weeks’ practice before he becomes proficient. The com parison is m ade using a low-power microscope (magnification ab o u t X 4) for viewing purposes.

Since th e procedure followed is th e basis of this and following papers, a detailed exam ple is given of th e determ ination of th e molecular extinction coefficients of benzene. This p articu lar case is described because the absorption spectrum of benzene is very well defined, and th e in terp retatio n of th e photographic p late is relatively simple.

Ta b l e I .

P h ysica l Properties o f P u re A rom atics.

C om pound. B .P .

° C .

F .P .

°C . d f . „20 n B • P u rity .

P e r cen t.

B enzene 80-20 5-48 0-8792 1-5012 99-98

T oluene 110-85 - 9 4 - 9 7 0-8669 1-4968 99-83

E th y lb e n ze n e 135-95 - 9 5 - 0 2 0-8672 1-4961 99-6

m -X ylene 139-15 - 4 8 - 0 5 0-8644 1-4971 99-98

p -X y le n e 138-35 13-21 0-8610 1-4957 99-96

o-X ylene . : 144-45 - 2 5 - 3 1 0-8803 1-5054 99-92

K oP ropylbenzeno . 152-45 - 9 5 - 8 9 0-8619 1-4915 99-9

A solution of benzene (for properties see Table I) in arom atic-free solvent is prepared by weighing a sm all q u a n tity of th e form er an d diluting it to a know n volum e w ith th e solvent. (In a p articu lar case 0-7060 gm. benzene was m ade up to 50 m l.; 4 ml. of th e resulting solution were th e n m ade up to 50 ml. w ith th e same solvent, a n d finally 10 ml. of th e second solution again m ade u p to 50 ml. w ith solvent. This procedure gave a diluted solution containing 0-2259 gm. benzene per litfe equivalent to 0-002896 gm .-m ol./litre, a n d avoided th e use of large volumes of solvent.)

T he n e x t step is to m ake a series of exposures on th e p late b y which to m easure th e zero error of th e in stru m en t— i.e., th e difference betw een the am ounts of rad iatio n passing along th e tw o optical paths. This is done before th e cells are placed in position, an d a series of exposures is m ade on th e p late corresponding to know n small variations from th e fully open position of th e variable aperture. Thus, in th e exam ple being discussed, which is illu strated in th e photographic reproduction shown in Fig. 1, the three pairs of spectra shown im m ediately above th e wave-length scale were m ade for zero correction purposes. T hey correspond to variable a p e rtu re drum readings (optical densities) of 0-05, 0-10, and 0-15, respec-

Ta b l e I I .

432 GORDON AND P O W E LL : SPECTROSCOPIC A N A L Y SIS.

D etails o f E xposures M ade in D eterm ining the E xtin ctio n Coefficients of Benzene.

V a riab le a p e rtu re . E x p o su re . P o s itio n o f p la te o n d iv id e d scale.

D ru m read in g . Seconds. (1 D ivisio n = 1 m m .)

0-30 2-0 15

0-35 3 0 18

0*40 3-0 21

0-45 4-0 24

0-50 5 0 27

0-60 4 0 30

0-70 5-0 33

0-80 6 0 36

0-90 6-0 39

1 0 0 7 0 42

1 1 0 7-0 45

1*20 8-0 . 48

1*25 8-0 51

1-30 8-0 54

1-35 10-0 57

1-40 1 0 0 60

1-50 1 0 0 63

1-55 1 2 0 66

1-60 14-0 69

1-65 1 4 0 72

1-70 1 6 0 75

1-75 1 6 0 78

Ta b l e I I I .

R esults o f E x a m in a tio n o f P hotographic P la te E xp o sed fo r D eterm ination of E xtin ctio n Coefficients o f Benzene.

A bsorption cell le n g th = 2 cm . Zero erro r of p h o to m eter = + 0-11.

V ariable ap e rtu re drum reading (optical density).

Corrected optical den sity ( = drum

read in g m inus 0*11).

W ave-lengths * (i..) a t w hich th e p airs of sp ec tra h av e eq u al intensities.

Molecular extinction coefficient.

E.

0-30 0-19 2422 2630 32-8

0-35 0-24 2424 — 2455 --- — 2525 2530 2570 2585 2628 41-4

0-40 0-29 2450 2465 2478 2515 2525 2530 2566 2598 2625 ^— 50-1

0-45 0-34 2427 2438 — 2479 — 2508 2538 2564 2601 2622 58-7

0-50 0-39 2428 2438' — 2480 — 2506 2539 2564 2601 2621 67-3

0-60 0-49 — — — — — 2500 2540 2562 — 2-620 84-6

0-70 0-59 — 2435 — 2482 — 2497 2540 ' 2559 — 2615 101-9

0-80 0-69 2495 2541 2557 2603 2611 119-1

0-90 0-79 2492 2541 2556 2604 2606 136-4

1-00 0-89 — — — — 2484 2490 2542 2555 2605 — 153-7

1-10 0-99 — — -7— — — 2490 2542 2554 2605 — 170-9

1-20 1-09 — — ---- — 2485 — 2543 2550 — — 188-2

1-25 1-14 — — ---- — 2487 — — 2549 — — 196-8

1-30 1-19 — — ---- — — — 2544 2549 — — 205-4

1-35 1-24 ^{— — ---- — — —} 2544 ^{— — ___} 214-1

1-40 1-29 ^{— — ---- — — — —} 2546 ^{— ___} 222-7

1-50 1*39 ^{— — --- — — — —} 2548 ^{— ___} 240-0

1-55 1-44

1-60 1-49 — — --- — — — — — — — —

1-65 1-54 — — ---- — — — — — — ___ ___

1-70 1-59 ___

1-75 1-64 —

* As recorded on w ave-length scale in in stru m en t.

GORDON AND PO W ELL : SPECTROSCOPIC ANALYSIS. 433 tivoly, and constant exposures of 1 second. E xam ination of these spectra shows th a t a t a drum reading of 0-10 th e upper h alf is a trifle darker th a n th e bottom half, whereas a t a drum reading of 0-15 th e reverse is the case, so th a t th e tru e zero lies between these two figures. Interpolation by a skilled operator gives a value of 0-11.

A fter th e zero has been determ ined, th e absorption cells (a stan d ard cell length of 2 cm. is used) are filled w ith solution an d solvent respectively.

T hey are th en placed in position in the Spekker photom eter, th a t con-

W A V E L.E N O .TH - ANGSTROM UNITS

Fig. 2.

U L T R A -V IO L E T A B S O R P T I O N S P E C T R U M O P B E N Z E N E .

taining th e solution being upperm ost. The ap p aratu s is now ready for th e analysis.

The following cycle of operations is carried o u t :

1. A djust p late to proper position as shown by th e scale on th e rack- and-pinion device.

2. A djust th e variable-aperture recorder drum to th e required value.

3. Switch on spark for th e required exposure time.

The cycle of operations is repeated for a series of drum readings covering th e whole scale as shown in T able II .

434 GORDON AND P O W E LL : SPECTROSCOPIC A N A LY SIS.

I t should be n o te d th a t as th e ap ertu re in th e p h o to m eter is reduced th e exposure tim e has to be increased correspondingly to give an im age of reasonably co n sta n t den sity on th e developed plate. T he exposure tim es given in T able I I ap p ly in a general w ay to all m ixtures, b u t m ay require to be shortened or increased, depending on th e in ten sity of th e sp ark line to be used in m easurem ent.

A photograph of th e developed p late o b tain ed is reproduced in Fig. 1.

E a ch p air of spectra is now exam ined u n d er a low-power m icroscope to determ ine a t w h at w ave-lengths th e intensities o f th e u p p er a n d lower

" W A V E L E N G T H - A N G S T R O M U N IT S Fig. 3.

U L T R A -V IO L E T A B S O R P T I O N S P E C T R U M O P T O L U E N E .

spectra are equal. These points of eq u a lity of blackening are m ark ed on Fig. 1. T hus, as shown in Fig. 1, a t a d ru m reading of 1-00 th ese are equal a t w ave-lengths of 2485, 2490, 2542, 2554 an d 2605 A. an d a t no o th er wave-lengths. T able I I I gives th e d a ta o btained a t all settin g s of the variable ap ertu re, an d th e m olecular e x tin ctio n coefficients calculated from equation (1). T he ex tin ctio n coefficient/w ave-length relatio n is shown in F ig. 2.

I t should be n o te d th a t th e tru e values of th e optical densities are o b tain ed b y su b trac tin g th e zero erro r from all v ariab le-ap ertu re drum readings. In doing this, th e zero error a t th e a c tu a l w ave-length to be

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F I G . 1 .

P L A T E E X P O S E D T O D E T E B M I N E TBDE M O L E C T JL A B E X T I N C T IO N C O E F F I C I E N T S O F B E N Z E N E .

[To face Trans., p . 434.

434 GORDON AND P O W E LL : SPECTR O SCO PIC A N A L Y SIS.

I t should be n o te d t h a t as th e ap e rtu re in th e p h o to m eter is r e d u c e d th e exposure tim e has to be increased correspondingly to give a n im age 01 reasonably co n sta n t d e n sity on th e developed plate. T he e x p o s u r e tim es given in T able I I ap p ly in a general w ay to all m ixtures, b u t m a y require to be shortened or increased, depending on th e in te n sity of th e sp ark line to be used in m easurem ent.

A photograph of th e developed p late o b tain ed is r e p r o d u c e d in Tig- 1.

E ach p a ir of sp e ctra is now exam ined u n d er a low-power m icroscope to determ ine a t w h a t w ave-lengths th e intensities of th e u p p er a n d lower

Fig. 3.

U L T R A -V IO L E T A B S O R P T I O N S P E C T R U M O P T O L U E N E .

sp ectra are equal. T hese p o in ts of eq u ality of blackening are m arked on Fig. 1. T hus, as shown in Fig. 1, a t a d ru m reading o f 1-00 th e se are equal a t w ave-lengths o f 2485, 2490, 2542, 2554 a n d 2605 A. a n d a t no other w ave-lengths. T able I I I gives th e d a ta o b tain ed a t all settin g s of the variable ap ertu re, an d th e m olecular ex tin ctio n coefficients calculated from eq u atio n (1). T he ex tin ctio n coefficient/w ave-length re la tio n is show n in F ig. 2.

I t should be n o te d th a t th e tru e values of th e op tical densities are o b tain ed b y su b tra c tin g th e zero erro r from all v ariab le-ap ertu re drum readings. I n doing th is, th e zero error a t th e a c tu a l w ave-length to be

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1-75 E x ­ posure (secs.).

16

1-70 16

1-65 14

1-60 14

1‘55 12

1-50 10

1-40 10

1-35 10

1-30 8

1-25 8

1-20 8

M O 7

1-00 7

0-90 6

0-80 6

0-70 5

0-60 5

0-50 4

0-45 4

0-40 3

0-35 3

0-30 2

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P L A T E E X P O S E D T O D E T E R M I N E T H E M O L E C U L A R E X T IN C T I O N C O E F F I C I E N T S O F B E N Z E N E .

[To face T r a n s p . 434.

GORDON AND PO W E LL : SPECTROSCOPIC A N A LY SIS. 435 exam ined m ust be used. This zero error is usually co n stan t over a very wide range of w ave-lengths, b u t m ay be appreciably different a t th e two extrem es of th e range. H ence th e procedure followed b y some workers of determ ining th e zero error w ith th e absorption cells already in position and relying on absence of absorption a t long w ave-lengths (and constancy of zero error over th e full wave range) is n o t to he recommended.

W A V E L E N G T H - A N G S T R O M U N IT S .

Fig. 4.

U L T R A -V IO L E T A B S O R P T IO N S P E C T R U M O P E T H Y L B E N Z E N E .

The w ave-length scale provided w ith th e in stru m en t cannot be assumed accurate, b u t th ere is no reason to calibrate this scale for th e present purpose, provided th e sam e spectrum lines are used on every occasion.

The wave-length scale m ay th e n be used as a rough indication of the spectrum lines.

(c) Molecular E xtinction Coefficients.

Molecular extinction coefficient w ave-length curves for benzene, toluene, ethylbenzene, m -xylene, p-xylene, o-xylene, a n d ¿sopropylbenzene, d eter­

436 GORDON AN D PO W E LL : SPECTROSCOPIC A N A LY SIS.

m ined in an exactly sim ilar m anner to th a t of benzene described above, are reproduced in Figs. 2 -8.

These arom atic hydrocarbons were pure specimens p rep ared b y th e Chemical R esearch Section of th e A nglo-Iranian Oil Co., L td ., a n d th e ir physical properties are d etailed in T able I.

W A V E L E N G T H A N G S T R O M U N I T S .

Fi g. 5.

U L T R A -V IO L E T A B S O R P T I O N S P E C T R U M O F p a f a - X Y L E N E .

An a l y t i c a l Me t h o d. (a) Benzene and Toluene.

T he absorption spectra o f benzene an d to lu en e are show n on a single g raph in Fig. 9. F ro m th is it is evident t h a t th e differences in absorption a t 2548 A. an d 2686 A. are well m arked, a n d th ese tw o w ave-lengths are therefore chosen as th e “ k ey ” w ave-lengths for analysis. As explained above, th e optical density a t a given w ave-length is th e sum of th e optical densities due to th e com ponents, so th a t applying equations (2) we arrive a t th e following equations using th e ex tin ctio n coefficients show n in Table IV

240 C

1

+ 171 C

2

= § a t 2548 Â.

¿j

10 C

1

^{+ 240 C}

2

= ^ a t 2686 Â.

GORDON AND ROW ELL : SPECTROSCOPIC ANALYSIS. 437

Ta b l e IV .

M olecular E xtin ctio n Coefficients o f P u re A rom atics at the

“ K e y ” W ave-Lengths.

W av e-len g th , Â. 2548. 2615. 2678. 2686. 2708. 2725. 2745.

A rom atics :

B enzene . 239 8

T oluene 171 — .— 240 — — —

E th y lb e n ze n e — 226 — — 65 18 10

m -X y len e . — 202 —- — 131 275 125

p -X y le n o . — 315 — —- 257 193 679

o-X ylene . — 243 — — 215 154 51

iso P ro p y lb en zen e — — 160 — 24 — ---

where C \ an d C

2

are th e concentrations of benzene and toluene respectively in gm .-m ol./litre.

Fi g. 6 .

U L T R A -V IO L E T A B S O R P T IO N S P E C T R U M O P m e i a - X Y L E N E .

K K

The facto r 2 corresponds to th e cell length of 2 cm.

T he results of analyses of six synthetic m ixtures following th is procedure are given in T able V.

Ta b l e V;

438 GORDON AND PO W E LL : SPECTROSCOPIC A N A LY SIS.

R esults o f A n a ly ses o f B in a r y B len d s o f B enzene a n d Toluene.

B len d No.

B en zen e, w t. p e r cen t. T oluene, w t. p e r cen t.

A c tu al. O bserved. A c tu al. O bserved.

1 60-5 59-5 ± 1-5 39-5 38-0 ± 1-5

2 40-5 39-0 ^ 1-5 59-5 59-5 ± 1-5

3 20-2 19-7 ± 1-5 79-8 79-3 ± 1-5

4 4-81 5-29 ± 1-5 95-19 93-3 ± 1-5

5 79-52 78-74 ± 1-5 20-48 22-0 ± 1-5

6 95-28 96-70 ± 1-5 4-72 5-2 ± 1-5

W A V E L E N G T H - A N G S T R O M U N I T S .

F ig . 7.

U L T R A -V IO L E T A B S O R P T I O N S P E C T R U M O E O r t A o - X Y L E N E .

(b) C

8

Aromatics.

The absorption spectra of th e C8 arom atics a n d isopropylbenzene are shown on one graph in Fig. 10, from which it is evident th a t, while im p o rta n t

GORDON AND P O W E LL : SPECTROSCOPIC A N A L Y SIS. • 439 differences exist; th e y are n o t so well m arked as we m ight wish, except in th e case of p-xylene, w hich is outstanding. The differences are, however, such t h a t th e y can h e used as th e basis o f a n analytical m ethod. Fig. 10 shows th a t a t a wave-length o f 2745 A. p-xylene has an ex tinction coefficient which is very m uch greater th a n th a t o f in-xylene, th e only o th er arom atic showing an appreciable absorption a t th is w ave-length, an d th a t a t 2725 A.

an d 2708 A. th e m ain absorption is due to m-xylene an d o-xylene respec­

tively. D ue to th e fa ct t h a t in w ell-fractionated cuts o-xylene an d ethyl-

2550 ^{2600 2650} 2100 ^{2150 2800}

W A V E L E N G T H - A N G S T R O M U N I T S . Fi g. 8.

U L T R A -V IO L E T A B S O R P T I O N S P E C T R U M O F is O P R O P Y X B E X Z E N E .

benzene are rarely presen t together in appreciable qu an tities, th e absorption of ethylbenzene a t 2615 A. is also im p o rtan t. I t m u st be em phasized th a t it is n o t essential for each com ponent to have an absorption b a n d greater th a n all th e other com ponents a t some w ave-length for th e analyses to he successful, since we are concerned w ith th e sum of optical densities.

T hus we arrive a t th e following “ key ” wave-lengths :—

in-xylene . . . . 2725 A.

p-xylene . . . 2745 A.

o-xylene . . . . 2708 A.

ethylbenzene . . . . 2615 A.