J O H N W . BERRY, D A V I D G . C H A P P E L L , a n d R. B O W L I N G B A R N E S Stamford Research Laboratories, A m e ric an Cyanam id C om pany, Stamford, Conn.
A n im proved flame ph otom eter, em plo yin g a dual optical system, has been devised. W ith this instrument the internal standard prin
c ip le often e m p lo y e d in spectral analysis may be used. Since b y this m ethod lig h t intensity ratios are m easured rather than absolute lig ht intensities, the disturbing effects caused b y gas and air pres
sure fluctuations, b y the presence of foreign ions and molecules, and b y viscosity differences are considerably reduced. The con
struction and perform ance of this photom eter are discussed in detail.
I
N A recen t p ap er ( i) a rap id analytical technique for th e dete rm in atio n of sodium and potassium in aqueous solution w as presented. A n in stru m en t known as th e flame photom eter w as described, an d th e an aly tical procedure explained. Briefly, th e m eth o d consists of atom izing an aqueous solution of th e m etal in to th e base of a gas burner, w hereupon the vapor is carried into th e flame and ignited. T he light arising from the flame ch aracteristic of th e elem ent being determ ined is filtered free of o th e r ra d ia tio n an d is b ro u g h t to fall upon a photocell. By m easuring th e in te n sity of th e light produced w ith solutions of know n concentration an d preparing a calibration curve of in
ten sity versus concentration, th e m etal content of other solutions m ay subsequently be determ ined by m aking use of the curve.
T h e prim e requisite of successful flame photom etry by th is m eth o d is th e estab lish m en t of constant atom ization and burning conditions in th e in stru m en t. Considerable investigative work led to a design of in stru m e n t which would fulfill this condition w ith reasonable satisfaction. In the flame photom eter previously described, these problem s h ad been solved to obtain an average accuracy of ± 3 % of th e to ta l elem ent p resent in th e sample in a single determ in atio n . T h e errors m ade were found to be alm ost entirely random — i.e., equal num ber positive and negative— and it could be assum ed th a t th ey came entirely from m om entary v ariations of lig h t in ten sity in the flame.
I n th e early course of th is work, however, it was noted th a t an erro r far m ore serious in n atu re th a n th is instrum ental error could arise w hen excessive am ounts of certain ions were intro
duced in to solutions containing sodium or potassium for ex
am ple, th e presence of 1% sulfuric acid in a solution containing 100 p.p.m . of sodium reduced the light em itted by th e flame by some 15% . I t w as fu rth e r noted th a t acids, salts, or indeed alm ost a n y foreign molecule, sim ilarly reduce the apparent so
dium co n ten t of solutions as analyzed by the flame photom eter.
T h e m ost obvious m an n er in which to correct this ty p e of error is to com pound th e sta n d a rd solutions used in calibrating the in
stru m en t in such a m an n er th a t these stan d ard s contain q u an ti
ties of th e interfering molecules in proportions sim ilar to those q uantities contained in th e solutions to be analyzed. T h is pro
cedure has been adopted, and in applications where one can ac
curately predict the composition of th e solutions su b m itted for analysis, it has been satisfactory.
I t is apparent, however, th a t such a procedure is n o t altogether convenient, since it requires th e com pounding of a series of sta n d ard solutions for each ty p e of unknow n w hich th e laboratory m ust analyze. In oth er cases, where th e chemical composition of unknow ns m ay vary considerably from one sam ple to the next, th e procedure is n o t feasible. F o r these reasons, a m ethod was sought w hich would elim inate as n early as possible th e effect of foreign molecules upon th e q u a n tita tiv e determ ination of the alkali m etals.
In th e usual spectrographic m ethod of analysis it has been common practice to em ploy w h at is term ed an “ in tern al sta n d a rd ” (S) elem ent to reduce th e effect of v ariatio n of th e light source, and o th er disturbing influences, upon th e accuracy of the results obtained. T he m ethod consists of purposely adding to each sam ple to be analyzed a fixed q u a n tity of some elem ent (the in tern al stan d ard ) n o t norm ally occurring in th e sam ple, before bringing th e sam ple to excitation. U pon excitation, light is em itted by b o th th e elem ent being determ ined and th e in ter
nal stan d ard , and th e ra tio of th e intensities of these tw o charac
teristic lights em itted is subsequently determ ined b y photography and densitom etry. T h e principle of th e m ethod is sim ply th a t any change in th e source o r any o th er facto r influencing th e light intensity em itted by one elem ent sim ilarly affects th e internal sta n d a rd elem ent, so th a t th e ra tio of intensities o btained is co n stan t regardless of th e experim ental conditions. N atu rally it is advantageous to choose as an in tern al sta n d a rd an elem ent which bears excitation characteristics as sim ilar as possible to th e elem ent being determ ined.
In attem p tin g to apply th e in tern al sta n d a rd m ethod to flame photom etry, th e choice of a suitable elem ent as a sta n d a rd was ra th e r lim ited, because of th e few elem ents excited a t th e flame tem peratures used in th e in stru m en t. Since th e flame photom eter was prim arily designed as an in stru m en t for th e determ ination of sodium and potassium , it w as desirable, if possible, to choose one of th e alkali m etals as a sta n d a rd ra th e r th a n an alkaline earth m etal. T h e sp ectra of rubidium , cesium, lithium , and in dium were investigated as possibilities. F o r various reasons such as th e in ten sity of light em itted by th e elem ent, th e problem of filtering th e characteristic rad iatio n em itted, an d th e wave
length sensitivity of th e photodetecting device to be em ployed,
Figure 1 . Internal Standard Flame Photom eter
<?, chimney, i t / 1, M 1, mirrors. L 1, L \ N l, N 1, Fresnei lenses. F l , sodium or potassium filters, T*-1, lithium fil
ters. P l , P 1, barrier layer photocells. C, atomizing chamber. G 1, G 1, gas and air pressure gages. K l, K3, gas and air regulator knobs. D , main potentiometer dial. E , compensating rheostat. T , toggle switches. S 1, S'1,
sensitivity adjustment, coarse and fine
a ll except lithium were rejected. T h e only serious objection to th e use of lithium is th e fact th a t th e elem ent is relatively ab u n d a n t an d m ay therefore occur as an im p u rity in certain ty p es of sam ples, especially those of m ineral origin. T h e lig h t em itted b y lithium is sufficiently intense, and th e only line- em itted (6708
A.)
is favorably placed a b o u t e q u id istan t in th e spectrum b etw een th e lines of sodium an d of potassium . T h e problem of■filtering th e th ree rad iatio n s sufficiently free from one ano th er w as accom plished w ith C orning glass filters and a special liquid filte r developed in these laboratories (4) w hich consists of
■cupric chloride dissolved in concentrated hydrochloric acid.
IN T E R N A L S T A N D A R D F L A M E P H O T O M E T E R
T o te s t th e in tern al sta n d a rd m ethod as applied to flame pho
to m e try , it w as necessary to co n stru ct a special in stru m e n t hous
in g tw o light p a th s an d tw o photocells. T h e in stru m e n t b u ilt (F igure 1) w as sim ilar in general design to th e flame photom eter -previously described (/).
L ig h t leaves th e flame through tw o rectan g u lar ap ertu res on
•opposite sides of th e chim ney, Q, each beam being reflected in to an
•optical system sim ilar to th a t previously used. T h e lens, L 1 or L l, nearest th e chim ney casts an enlarged im age of th e ap e rtu re on
■•the second lens, N l or N*, filling th is lens as nearly as possible w ith th e rectan g u lar im age produced. T h e second lens casts a
■slightly reduced image of th e first lens upon a rou n d photocell, P l or P \ ju s t filling th e active area. T h is system provides high .light-gathering pow er y e t allows th e photocells to be well rem oved from th e h e a t of th e source. T h e b arrier lay er photocell has been
■retained as th e light-detecting device. Before one photocell is placed th e set of lith iu m filters, w hile sodium or potassium filters
■may be in terch an g ed before th e o th e r photocell. [If desired, s e p a r a te lig h t p a th s and photocells m ay be provided for th e so
dium a n d potassium (m aking a to ta l of th ree optical system s) :and th e desired sodium o r potassium cell sw itched into th e circuit
•electrically. ]
G as a n d a ir supplied to th e b u rn er a n d atom izer, respectively, are regulated an d m easured by ap p ro p riate regulators a n d gages,
■G1 a n d G2. T h e stainless steel hypoderm ic needle atom izing u n it, .and th e conical-shaped atom izing cham ber, C, used in th e pre
vious in stru m e n t were left unchanged.
T h e circuit used for m easuring th e ratio of th e light intensities betw een th e in te rn a l s ta n d a rd a n d th e elem ent being determ ined i s show n in F igure 2. (A ctually th e circuit em ployed is n o t a tru e ratio-m easuring device b u t ra th e r a com pensated circuit.
.At balance, how ever, th e p o ten tio m eter reading o b tain ed is very
I '
Vol. 18, No. 1 nearly pro p o rtio n al to th e ra tio of th e lig h t intensities on th e tw o photocells.) Several of the "b u c k ing circuit” arrangem ents sug
gested in th e lite ra tu re (3) were tried b u t none proved satisfactory a t th e low light levels encountered in th e photom eter. T h e circuit used m ain tain s a resistance of som e 10,000 ohm s across each photocell, w hich seems a b o u t p roper for the dam ping ch aracter
istics of th e galvanom eter. T he 10,000-olun po ten tio m eter, D, was coupled to a 20-cm. (8-inch) circular dial on th e fro n t panel, m aking possible th e reading of th e p o ten tio m eter to w ithin
* 0 .2 % of full scale. A sm all 500- ohm rh eo stat, E , w as added to one en d of th e slide w ire for slight com pensating ad ju stm en ts found necessary from tim e to tim e during operation. T h e box-type galva
nom eter used in th e previous in
stru m e n t (G .E. 32C-245-G9) m ade a satisfacto ry null in d icato r for th e in stru m en t. As form erly, th e g alvanom eter w as connected to th e p hotom eter by jacks.
Also show n in F igure 2 is th e circuit used in th e previous m ethod of flame p h o to m etry . A lthough n o t shown, th e in stru m e n t w as so arran g ed t h a t eith er circuit show n in F igure 2 could be utilized by m erely throw ing toggle sw itches, T . T h e in stru m e n t could th u s be used as previously described— to m easure th e d irect or absolute in ten sity of th e sodium o r potassium w ave lengths, now term ed th e “ absolute m eth o d ” of flame p h o to m etry , o r th e ra tio of light intensities of sodium versus lithium or potassium versus lithium , term ed the “ in tern al sta n d a rd m eth o d ” of flame p h o to m etry . T h is arran g em en t w as very convenient in m aking com parisons of th e tw o m ethods.
No
No OR K C E L L
Figure 2. Circuits of Internal Standard Flame Photom eter
Ratio circuit above, absolute circuit below
A N A L Y T I C A L PR O C E D U R E
T h e procedure em ployed in m aking use of th e in tern al sta n d a rd m ethod is sim ilar in principle to th a t em ployed in th e absolute m ethod, in th a t th e in stru m e n t is first calib rated w ith solutions of know n concentration. A ll th e stan d ard s, however, are p repared to contain a fixed a m o u n t of a soluble lith iu m salt. L ith iu m sulfate h as generally been em ployed for th is purpose, as th e sa lt is ra th e r easily prep ared in a form relativ ely free of sodium and potassium .
R eag en t q u ality lith iu m su lfate is p recip itated as th e fluoride b y a d d in g am m onium fluoride in slig h t excess. T h e lithium fluoride is th en wrashed w ith cold distilled w ater, dried, a n d con
21 v erted back to th e sulfate in platin u m ware by the addition of
concentrated sulfuric acid an d beating. Since th e sodium and potassium fluorides are several times more soluble th a n the lithium salt, the procedure appears satisfactory.
In practice, all sodium stan d ard s were m ade to contain 1000 p.p.m . of lithium , an d am o u n ts of sodium ranging from 0 to 90 p.p.m . (I t w as found by experim ent th a t this q u a n tity of lithium gives an electrical response equal to th e response of 95 p.p.m . of sodium w ith th e p articu lar filters and photocells em
ployed in th e in stru m en t.) T h e standards th u s prepared are successively introduced in to th e instrum ent and in each case the potentiom eter is so ad ju ste d as to cause no current to flow through the galvanom eter. A potentiom eter dial reading is taken for each sta n d a rd a n d a calibration curve is prepared.
T he curve o btained is sm ooth, an d fairly linear (Figure 3).
Figure 3 . T ypical Calibration Curve for Sodium as O b ta in e d b y the Internal
Standard Flame Photometer
g o OCQL 111 h-Z bJ
INT.
1
STD. METHOD
N j
UJOl
15
ABSOLUTE ---t ^ ^ M E T H O D
I
0 5 10 15 2 0 25 3 0 35 4 0
P E R C E N T DECREASE IN GAS PRESSURE
Figure 4 . Effect of V ariatio n o f Gas Pressure
T o determ ine th e sodium content of a sam ple, an aqueous solu
tion is ap p ro p riately p ip etted into a volum etric flask, so th a t upon dilution th e sodium c o n te n t will fall below 90 p.p.m. A suffi
cient q u a n tity of a sto ck lithium sulfate solution is then added to bring th e lithium concentration to 1000 p.p.m. upon dilution.
T h e sam ple th u s p repared when introduced into the instrum ent yields a p o ten tio m eter reading which is readily converted into p a rts per m illion of sodium by use of the calibration curve.
In actu a l operation th e calibration curve remains well fixed a fte r establishm ent. T he sta n d a rd solution containing 0 p.p.m . of sodium an d 1000 p.p.m . of lithium will read near th e lower end of th e m ain p o ten tio m eter scale an d the 90 p.p.m . of sodium sta n d a rd will read n ear th e upper end of the scale. 1» hen em
ploying th e flame p h o to m eter for analysis it will be found wise to check th e u p p er an d low er ends of the scale occasionally w ith th e sta n d a rd solutions. Unless th e flame in some m anner be
comes co n tam in ated w ith sodium (dust in the atm osphere, etc.), th e low er end of th e calibration curve will rem ain well anchored.
th e u p p er end of th e scale, compensation m ay tm e n t of th e secondary slide wire, E, to restore If d rift occurs a t
be m ade b y a d ju stm e n t
th e original reading of th e stan d ard . _
A sim ilar procedure is used in th e determ ination of potassium, w ith th e exception t h a t th e solutions are prepared to contain 200 p.p.m . of lithium . T h is a m o u n t of lithium balances about 9o p.p.m . of potassium . T h e lithium concentrations chosen were such as to balance convenient w orking concentrations of sodium and potassium , b u t m ore or less lithium m ay be added to prepare
a series of stan d ard s covering g reater or lesser ranges of concen
tra tio n of sodium an d of potassium .
T he a m o u n t of sam ple required for a single sodium or potassium determ ination is a b o u t the sam e as th a t required by the previous in stru m en t (I). Sam ples of as little as 3 ml. of solution have been analyzed, b u t a larger q u a n tity of sam ple (say 10 ml.) is preferred. A lthough th e a u th o rs prefer w orking in a concen
tra tio n range of 1 to 100 p.p.m . of sodium or potassium , concen
tra tio n s as low as 0.1 p.p.m . of sodium or 0.5 p.p.m . of p otas
sium have been determ ined. In w orking a t concentrations be
low 5 p.p.m . of sodium o r potassium a special se t of sta n d a rd solutions containing less lithium should be used for calibration of the instrum ent.
A C C U R A C Y
In order to determ ine th e accuracy of th e internal sta n d a rd m ethod of flame photom etry, a series of 100 solutions containing known concent rations of sodium was prepared and analyzed by the m ethod. A sim ilar series was studied in the case of potassium . T hese solutions were prepared from c.p. sodium and potassium chlorides and lithium sulfate prepared as previously described.
T h e average error of a single determ ination of sodium was
± 1 .2 4 % , while for potassium a figure of ± 1 .0 1 % w as obtained.
T h e sign of the error was found to be random . T hese figures show conclusively th e superiority of th e in tern al sta n d a rd m ethod over the absolute, for in a sim ilar experim ent using the absolute m ethod, th e average error of a single determ ination w as show n to be ± 3 % of the am o u n t of elem ent present.
F u rth e r experim ents have been conducted w hich show th e su periority of the in tern al sta n d a rd m ethod w ith respect to com
m on interferences which m ay beset th e absolute m ethod. T hese detrim ental effects m ay be divided into a t least four classes:
effect of v ariatio n of gas pressure, effect of v ariation of air pres
sure, effect of foreign molecules and ions, an d effect of viscosity of th e sam ple.
In order to determ ine th e m agnitude of these various effects, sta n d a rd solutions were prepared and analyzed by bo th th e in
ternal sta n d a rd an d th e absolute m ethod. T hese solutions all contained 50 p.p.m . of sodium (as th e chloride) plus varying am ounts of the interfering substances (except in th e case of th e effect of sodium chloride, in w hich case 50 p.p.m . of potassium was th e concentration chosen). T h e sta n d a rd s used in the evaluation of th e in tern al sta n d a rd m ethod contained, in ad d i
tion, 1000 p.p.m . of lithium .
Figure 5 . Effect of V a ria tio n o f A i r Pressure
In a sep arate series of experim ents, i t was subsequently shown th a t th e percentage error produced by a given q u a n tity of in te r
fering ion or molecule is in d ep en d e n t of th e a m o u n t of sodium or potassium present in th e sam ple over a concentration range of 10 to 100 p.p.m .
INTERFERENCES
G a s Pr e s s u r e. T h e effect of lowering th e gas pressure from th a t norm ally used in th e operation of th e flame p h o to m eter [0.21
22 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. 18, No. 1
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kg. per sq. cm. (3 pounds p er sq. inch) of p ropane] is shown graphically in Figure 4. I t is im m ediately n o ted t h a t th e gas pressure m ay be lowered b y 33% [from 0.21 kg. p er sq. cm.
(3 pounds) dow n to 0.14 kg. p er sq. cm. (2 pounds)] using th e in tern al sta n d a rd m ethod w ith no appreciable change in th e q u an tity of m etal determ ined in a sam ple, w hile a sim ilar change of gas pressure lowers th e resu lt o b tain ed in using th e absolute m ethod by some 12.5%. T h e obvious su p erio rity of th e 'in te rn a l sta n d a rd m ethod should now m ake feasible th e op eratio n of flame photom eters directly from city gas m ains w here v ariatio n s in gas pressure previously m ade such operation v ery questionable.
Ai r Pr e s s u r e. T h a t v ariatio n s of flame photo m eter readings are m uch more dependent upon air pressure fluctuations th a n upon gas pressure changes is im m ediately a p p a re n t from a stu d y of F igure 5. T h e in tern al sta n d a rd m eth o d does n o t com pletely elim inate th e effect of v ariatio n s in air pressure a t an y range.
Ai r Pr e s s u r e. T h a t v ariatio n s of flame photo m eter readings are m uch more dependent upon air pressure fluctuations th a n upon gas pressure changes is im m ediately a p p a re n t from a stu d y of F igure 5. T h e in tern al sta n d a rd m eth o d does n o t com pletely elim inate th e effect of v ariatio n s in air pressure a t an y range.