Chemical Instrumentation


Chemical Instrumentationpubs.acs.org/doi/pdf/10.1021/ed037pA705?src=recsysThe gas pressure at the burner can be read wit...

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Chemical Instrumentation 5. Z. LEWIN, New York University, Washington Square, New York 3, N. Y. excited in the former Hame; several dozen elements e m be detected by use of the latter flame.

T h i s series of articles presents a survey ofthe basic principles, characteristics and llmitaticns of those instruments which find important applications in chemical work. The emphasis i s on commercially available equipment, and approximate prices are quoted to show the order of magnitude oj cost of /he uarious types of design and construction.

precision. An example of the type of needle valve commonly used for adjusting the gas flow is shown in Figure 2. The vent, through which the preasure drop occurs from the high to the l o s pressure side of the linc, is controlled in size by the movement (insertion or retraction) of a spring-loaded needle that responds to the adjustment of an external knob by the operetor. The gas pressure a t the burner can be read with adequate sensitivity on a Bourdon-type gauge, or can be measured on s. Ventori-type flow meter (Figure 3).

10. Flame Photometers The great surcess of photoelectric shsorptiometerfi and spectrophotometers in improving the speed, ease and precision of analysis of solutions prompted the development, notably since 1945, of similar instruments for Hame analysis. The commercial Hame photometers currently available are essentially comparable in design to the photometric instruments that have been described in preceding articles of this series, and arc distinguished principally by the fact that special means are employed to produce a Heme, introduce a specimen into the flame, and collect the radiation from the Hame for photometric measurement. Since the flame servcs as the source of radiation, and since tho intensity of the light from a Hame is far more difficult to control and reproduce than is the intensity of s, tungsten-filament lamp, special problems romo to the fore in the design of Ramo phot,ometers that were not encountered, or were relat,iv~lyunimportant in other types of photometers.

aerosol is introduced into the flame a t a slow and constant rate. The intensity of the light produced by the thermal excitation of tho sample is very sensitive to the concentration of the aorosol and it,s rate of introduction into the flame. I t is also very sensit,ive to tho temperature of the Ritme, which depends npon the rate of oxygen How into the hurner, as well as upon the composition and rate of the combuatible Rlel. I t is, therefore, evident t h a t s, number of features of mechanical design plays, erucid d o in determining the performance of this type of innt,rument. That is, in this category of chemical devices, the optical and electronic design have to conform to the requirements of t,he ssmple handling system, rat,her than vice versa, as n.aa the case wit,h xbsorptiometers and spectrophotometers. The proper ternpcmtnrc of the flame in a Hame photometer depends npon the nature of the element whose radiations are to be measured. The alkali elements are excited moat readily; the alkaline earths require higher temperatures; and other elements, such as the transition metals, require still higher temperatures. In general, the Hame should he as cool as p o s sible far the particular element being m e a s wed; i.e., hot enough to eanse copious emission of the charartorintic radiation, but not so hot as to given high background of radiation from other elements or from the fuel and it,^ combustion produrts.

Figure 3. Venturi-type preuvreor Row gouge. The atomization of the sample e m be accomplished in a variety of ways, several

Detail of construction of o precision C, control knob; 5, spring; M, membrane; 1, lever; V, vent.

Figure 2. Figure 1. Basic design of the light source unit in a Rome photometer.

needle volve.

The characteristics of the sample handling system are of critical importance in flame photometers. The fundamental features of this system are shown sehematically in Figure 1. A burner is fed by a eontrolled How of fuel and oxygen (or air) while thc sample, in the form of a. dilute solution, is atomized, and the resulting

The fuels that are generally used for flame photometers itre natural gas, propane, hydrogen, and acetylene. The flame temperatures available with those fuels range from 1700 to 1800°C for natural gas burned in compressed air, to 3100-3200°C far acetylene hurned in pure oxygen. Only sodium and potassium are Volume

Figure 4. Atomizer designs. A, Suction feed; 8, Gravity feed; C ond D, Concentric tube.

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Chemical Instrumentation of which :Lrts r h o n r in Figure 4. 'I'ht. ensmtisl prinrilh, in all rases, is t o cnoac i~ gas atrcnm to How rapidly past a frcc liquid surfsw. The rapid How creates n region of rrduacd pressure perptwdiculur t o the gas stremn (Uwnuulli's priuciple), catlfiirlg thr liquid to rive up into the Kas stream, where it ix dispersed into minute droplrts ilnd carried >!way as an a e r ~ s o l SpnLy. I t is importmt in Hamc phnt,omctry that the- n c ~ m o lpurticlss be titul,lv, and of small muugh rise that they do nut lnyrr out, ronlrsrr, or aggregate before reaching tho Hnme. Tllr following dosigns are vmployed: ( I ) the atomizer chamber is separate from the Ijurner, :md Lhe g i ~ ~ strmm carries the aerosol from the onc to the othrr; h w v y droplets s~.ttle out bcfort. rwrhing t h ~ burrier . and :rrr :rllawrd to drain nay; ( 2 ) the atomizer chamber is hr:~tnl,so that the aolvmt is immediatrly vaporized from the aerosol droplets, leaving :r w r y fine solid aerosol to lx swept into the Hame; (3) the :~eroxulis hlonn out of the i~tomizerchamber into a curved tnlre that leads t o the imrner; the r~trvadpath cuusca centrifugal wpitri~tioo of bhr lnrgrr dropletr from the smnllrr ones, with the fnrmcr depositing on the tulw ni~IIsnnd dubiring awiry; (-1) tlw atomisation oerurs directly into t h ~ k w n w , so t h s t all aemsol partirlcs rntrr thp Hamr a t tht. iustnut they m a formed. 111 t,lw mr;~surr.mcnt of thp rudintiort from the Hnmr, two lmsir photom~terdcr

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flows throueh a c e n t r i f .. t i d wnarator t o sift. cçihcavy particles before reaching the burner. The latter is surrounded by a

Chemical Instrumentation glass chi~nney,and by an aluminum refleetor, t o reduce the flow of heat to the r& of the inst.mmcnt. The optical design is based upon the double beam principle, with photomdtiplier tubes viewing the two beam3 through appropriate &Item. The filters %r? of the multilayer interference type, with a bandwidth of 8 millimicron^ or less, and peak trmsmi8aion of 70-85%. The output,s of the two photomultipliers are opposed, and the difference signal is read out directly on a micro%mmeter. The fuel is manufact~~rcd gas, or propane-butme, and a source of compres8ed air delivming 1 d m a t 8-12 psi is nwmmry. The mpmdueibility is +0.5% (av.), a minimam sample of 5 ml is r e qt~imd,and thc sensitivity for sodinn is 0.1 ppm.

dmgmm UI Ihgurc 10 The a t o m m r < h:mhcr xaq heated, so that the aernsol

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droplets dried to fine solid particles before entering th? burner assembly. The burner consisted of a water-cooled oylindrieal hxrrel, with a perforated plate a t top, to produce a broad uniform flame the light of which would completely fill the entrance slit of the spectrophotometer. This design wan dropped around 1950 in favor of the combined atomizer-burner first deaerihed hy Weiehselbaum and Varney, in which the aerosol forms directly in the Hame. Diagrams of this unit nrr givrn in Figure 11. Thr rdstionrhip

Figure 11. Principle of construction of Beckman combined otornirer-burner. left, polladivm sample feed tube, remainder of unit gloss; Right, ,711-metal ~ ~ n r t r u ~ t i ~ n .

of the burner-atomizer to the rest of thc instrument is shown in Figure 12. Thv sample, in s. small beaker on a movabk stand, swing8 into position below the atomizer so that the sample feed tube is dipping into the liquid. The reduced prrsanre rreated hy the gas flow sucks the liquid sample up into the Hame. The Hame attachment unit for a Modrl DU spectrophotometer consists of the atomizer-hurner, pressure controls, maunting hardware and housings (price: $540). The fuel is either hydrogen or acetylene, in order to give a hot enough flame to e r c i k the radiations of the several dozen elements that can be detected with the high resolution monorhromator. Continoous operation requires 8 cubic feet of oxygen and 5 cubic f w t of acetylene or 20 eubir feet of hydrogen per hour. The pressure at the burner is 12-20 psi for thc oxygen. Reproducibility e m be ss good as +0.3%; ~ a m p l rsize is 1-3 ml; sensitivit.? for sodium is 0.01 ppm, using n photomultiplier dctrrtor on the sprrtmphotom~ter. The company also has produceds. directreading flame photometer that is a camplete instrument intended only for the analysis for sodium and potassium. This is an ahsalute reading instrument; the parameter read out on the mioroammeter on the instrument panel is calibrated by means of standard solutions of sodium or potassium salts. Because of the serious interferences caused by many substances, the standard should have a composition as (Continued n paye A 7 f f )

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Chemical Instrumentation close as poasihlp to that of tho unknowns. This model is no longer being mannfnctured.

Figure 12. Beckmon Flame Amchmenf, rhoring relotionship between atomizer-burner I l l , rmnple positioning lever 121, condensing (31, a n d burner housing (41.

Perkin-Elmer A rehrmatir diagram of the design of the Model 5Y F h m e Photometer of the Prrkin-l.:lmer Corp., ?;orwalk, Connecticut, is ahowl in Figure 13. The sample is poured into tht, inlet funnel, ia atomized, and the aerosol is swept into the burner. The ent,ranrr slit of x two-prism monochromator looks dirrrt,ly a t the center of the flxmr just xlmvr the bop of the hurnar. Light from the Hxme is dispersed into r spectrum and divided hy s. beam splitting mirror so that part fall8 on n fixed exit slit and part on 3 movitblc one. Each slit has it,s own photoclertric cell and amplifier wit,h independent. gain controls lor mch. As a direct restling photometer, the gain control on thr int,crnnl standard amplifipr is set s t mro. For use as a ratio photompter to compare the light falling on the internal standard cell with that fsllinp on the other, the gain control an the internal standard amplifier is a d j u ~ t e duntil its signal exactly halnneaa that of the unknown. Fpatorw that :rr not shown in the figlrrr are the rotating spctor shutter and synchronous rrrtifirr needed for t h ~nr amplification and the connection from thp element seler.tor and wavelength scale to. the moving slit for retting on the emissiou lines of the dimerent elements. Additional amplification not shown in the figure makes

(Continued on page A7241

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Chemical Instrumentation tho find output large enough t o rcxd on a meter. This instrumcnt can he used pither for nbsoluto measurements, ealibrnt,ing the readings with standard mlutions, or for mtio mcssnroments, using lithium chloride as intcrnal standard. The fnel for thc hurncr may he propsnc or nrrtylenc, doponding upon whethrr an1.y Ns and I i are to he determined, or other elements as well. Reproducibility is *2%; simple size is 5 ml: scnsit,ivitv for sodium is 0.03 wm. The Model 52 has heen supcr.sodcd by the lModel 14fi (ahout 81000), the Imie clefiign of which is similar, hut which has scvcral imprcvcmcnts. Thc mnst significant is an improvod atomizer-lxirner, shown in Figure 11. The atomizcr uses a straight capillary that is easily rlcnned, and x centrifugal soparator is inclrtdcd bebwccn atomiarr and hurner. Thc lattcr is mado of stninlcss stcel. Sample size can hc as small as 2 ml.

Process and Instruments A simple, high-quality flame photometer dcsiened for sodium and notsssium analv-

Figure 13.

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Schematic diagrorn of the Perkin-Elmer Model 52 Rame photometer.

Chemical Instrumentation

Figure 14. Design of the Atomizer-Burner unit in the Perkin-Elmer Model 1 4 6 Flame photometer.

(Model 1B: 81100). I t is n dorhle beam instrument, employing harrier-lnwr photoc e l l ~(hence no external power supply is required), and the fuel is propane. The atomizer is unique in consisting c f n stainless s t r d i n k t tuhe with stainlras steel

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Figure 15. Proces m d Instruments Model 1 8 flame photometer.

spray nevdlc, thc position of which can hv precisely :!rl/~~aterlhy a manunl control. The needlc ran lbc removed for cleaning and m e t to its previous position very conveniently. This instnnnmt is shown i n Figure 15.

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Chemical Instrumentation Other Manufacturers Many othcr flame photometprs m e commercially nvailnble that possess charactcristics similar to those described in connection wit,h the instruments t,rerted above. These include: Ccleman Model 21 Flame Photometer. uses natwal or artificial gas; vacmtm phototuhe photometer; price, 3500. Norelco Flame Photomet~rof Philips IGlretronie~, Mount Vernon, Iiw York; double-beam, harrier-layer phatorells; for either sbsr~lutoor int,crnal standard measurement; galvanometer rend-out; price, $800. Unicam SP.900 Flame Sp~ctrophotomotor; usos any gas from propane to hydrog m ; high-rcsolution monochromat,or and ac-chopped electronic nmplifier nibh galvanometer read-out for ver)- high sensitivity and 8tahilit.v; available in U. S. through Wilkens-Anderson Co., Chicago 51, Illinois.

Bibliography BARNES,R. B., RICHARDSON, D., BERRY, J. W., A N D HOOD, R. L., "Flame Photometry, a Rapid Analytical Proeedore," I&EC, Anal. Ed., 17,605 (1945). BERRY,J. W., CHAPPELL, 1). G., AND BARNES,R. B., "Improwd hlct,hod of Flame Photometry," {hid., 18, 111 (1946). BILLS,C. E., ~ ~ C D O N A F.L(i., D , SIEDER1 2 . C., MEIER,W., A N D SCHWARTZ, "Reduction of Error in Flame Photometry," Anal. Chem., 21, l O i 0 (11149). BURRIEL-MARTI, F., .AND RAMIREIMUNOZ,J., "Flnme Photometry," Elsevicr Puh. Co., Princeton, N. J., 1957. I ~ A N , J. A,, "Flame Phatometr.~," MoGraw-Hill Book Co., Nen York, 1960. DIAMOND, J. J., A N D BEAX,L., "Improvements in Flame Photometric Determination of Sodium in Portland Cement," Anal. Chem., 25, 1825 (1953). Fox, C. L., "Stahlo Internal Standard Flame Photometer for Potassium :tnd Sodium Analysis," ibid., 23, 137 (1951). GILBERT, P. T.,JR., HAWES,R. C., AND BBCKMAN, A. 0 "Beckmm Flame Photometer," ibid., 22, 792 (1950). HERRMANN, R., "Flamm~nphotometrie," Springer-Verlag, Berlin, 1956. WW~TE, J. U., "Precisian of a Simple Flame Photometer," 4nal. Chcm., 24, 394 (1952). Ned: Zqfrared Speelron~efers.

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