JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

Effect of Alkali and Alkaline Earth Chlorides on the Anode Temperature of the dc Arc*

BERT L. VALLEE AND RALPH E. THIERSBiophysics Research Laboratory of the Department of Medicine, Harvard Medical School and

Peter Bent Brigham Hospital, Boston, Massachusetts

Temperatures of graphite anodes have been measured for the dc arc in helium and argon by means of arecording thermistor pyrometer when pure chlorides of the alkali and alkaline earth metals are present assamples. Anode temperatures and gap voltages were depressed by all of the samples. The data for heliumindicate a correlation of anode temperature to the boiling point of the sample salt and the lowest excitationpotential of the cation. Correlation was also observed between the boiling point of the salt and its rate ofvolatilization. Temperatures in argon are lower and correlation between anode temperatures and eitherboiling points or excitation potentials was not observed. However, rates of volatilization were related to theboiling points and excitation potentials of the samples. These effects are large enough to account for thepoor precision of the dc arc as a spectrochemical source and show the physical-chemical basis for the use of"spectroscopic buffers."

TEMPERATURES of graphite anodes of the dcT arc in noble gas atmospheres have been reportedpreviously.",2 Anode temperatures, gap voltages, andburn-off timest have now been measured with anodescontaining pure chlorides of the alkali or aklaline earthmetals as samples. Each salt affected these threeparameters of the arc characteristically, but differencesbetween salts were much greater in helium than inargon. In both gases certain of the effects were relatedto the boiling points and the excitation potentials of thesamples.

Previous studies allow the inference that the tem-perature of the anode is a significant variable in thebehaviour of the dc arc. Under some conditions thelines of the elements appear in the spectrum in order ofincreasing boiling points of the compounds in the samplemixture. This phenomenon has been used to estimate theunknown boiling points of several compounds.',4 Whenmagnesium oxide, silver oxide and other compoundsmixed with carbon powder were arced, the spectra ofthe magnesium and silver appeared simultaneously.Silver boils at 2170C, magnesium oxide at 2800C,and magnesium at 1102'C. Therefore, the anodetemperature at which carbon reduces magnesium oxideto the more volatile magnesium metal was taken to be2170'C.4 Control of the anode temperature by variousmeans has been studied to increase the precision of thedc arc.5' 6

The spectral lines of certain elements which occuras traces in a biological matrix have been shown to bemuch more intense when the arc is operated in argon

* These studies were supported by grants-in-aid from theResearch Corporation, New York, and the Rockefeller Foundation,New York.

IB. L. Vallee and M. R. Baker, J. Opt. Soc. Am. 43, 817(1953).

2B. L. Vallee and M. R. Baker, J. Opt. Soc. Am. 46, 77(1956).

t "Burn-off time" is defined as that time which is required tovolatilize the sample completely.

'D. Richardson, Proc. 5th Summer Conf. Spectroscopy andApplications 64-70 (1937).

40 . Leuchs, Spectrochim. Acta 4, 237-251 (1950).6B. J. Stallwood, J. Opt. Soc. Am. 44, 171-176 (1954).6 N. W. H. Addink, Spectrochim. Acta 5, 495-499 (1953).

than in helium or air. Accompanying this increasedintensity is a prolongation of the volatilization time,and a relationship between these factors wassuggested. 8

There is little information about the temperature ofthe anode itself in the dc arc, and the effect of samplesupon it, although there is good evidence that withpure graphite the temperature of the anode spot is thesublimation temperature of carbon, about 3500C. 9

EXPERIMENTAL

The details of the arcing chamber, thermistorpyrometer, and recorders have been described.2 Highestgrade analytical reagents or Johnson Matthey "Spec-pure" salts were employed as samples. Of the verticalgroups in the periodic table, the alkali metal andalkaline earth metal chlorides were selected for thiswork. There were several reasons for this choice. Thesesalts are all available in extremely pure form; theirphysical constants are known well and more completelythan those of other salts 0 ; and the alkali metal chloridesdo not undergo decomposition, reduction, or carbideformation in the electrode.

The samples were ground with an agate mortar andpestle for 30 minutes. Hygroscopic salts were groundin a "dry box," and all samples were stored in smallglass vials over "Drierite" desiccant (anhydrousCaSO4). The alkaline earth chlorides were left at 120°Cin an atmosphere of anhydrous hydrogen chloride gasfor 24 hours prior to storage over desiccant. Theelectrodes were stored over desiccant and loaded withsample immediately before use.

Samples were arced in helium and argon, at 8.0amperes current and with a gap length of 6.0 mm. Theelectrodes were 3.1 mm in diameter, 63 mm in length,

7 B. L. Vallee and S. J. Adelstein, J. Opt. Soc. Am. 42, 295-299(1952).

8 S. J. Adelstein and B. L. Vallee, Spectrochim. Acta 6, 134-138(954).9 H. Kohn and M. Guckel, Z. Physik 27, 305-357 (1924).10 Selected Values of Chemical Thermodynamic Properties

National Bureau of Standards (U. S.) Cir. No. 500 (U. S. Govt.Printing Office, Washington, D. C., 1952).

83

FEBRUARY, 1956VOLUME 46, NUMBER 2

B. L. VALLEE AND R. E. THIERS

TIME IN SECONDS

FIG. 1. Anode temperature and gap voltage records ofsodium chloride arced in helium.

and the anode cups were 1.6 mm in diameter and 4.0mm deep. The cathode had a flat end. Throughout eachmeasurement, the length of the gap was kept at 6.0 mm,monitored by an image of the electrodes projected ontoa screen. The temperature of an anode segment 4.5mm from the tip of the anode was measured.

After insertion of the electrodes, the chamber wasevacuated with a vacuum pump and filled with eitherhelium or argon. It was then re-evacuated twice andrefilled to ensure the absence of air. The pressure in thechamber was maintained at 2 mm greater than atmos-pheric by a slow flow of gas, which was allowed toescape continuously through a glass capillary, the tipof which was 2 mm below the surface of a mercury pool.To start the arc, the electrodes were touched together;

then quickly drawn 6.0 mm apart as measured by theprojected image. The current was set at 8 amperes bymanual adjustment of the rheostat. The arc wasmaintained until the sample was completely volatilizedand long enough thereafter to ascertain establishmentof a new equilibrium.

RESULTS

Figure 1 shows a typical record of temperature andvoltage as a function of time obtained with the stripchart recorders when sodium chloride was arced inhelium. The sudden rise in temperature and voltage(at 47 seconds in Fig. 1), occurs when the color of thearc changes from that due to the sample salt,-redwith lithium, yellow with sodium, etc.,-to that due tothe pure graphite. The presence of the sample depressesboth the anode temperature and the gap voltage; itsvolatilization results in a rise of anode temperatureand gap voltage to values characteristic of pure graphite.The temperature of the anode slowly rises as the sampleburns away, but as long as some sample is in the anode,the voltage remains almost constant. The temperatureat the end of the burn-off time was taken as the "anodetemperature," and the average voltage during thistime was called the "gap voltage." The records arereproducible and characteristic of the salts and gasesemployed.

Table I states the anode temperature, gap voltages,and burn-off times obtained when replicate samples ofthe alkali and alkaline earth chlorides were arced inhelium and argon. The presence of the salts lowers thegap voltage and the anode temperature to a degreewhich depends strongly on the individual salts. Inhelium the alkaline earth chlorides show higher anodetemperatures and voltages than the alkali chlorides.

TABLE I. Anode temperatures, gap voltages, and burn-off times of various compounds.

Expt. Group I-Alkali metal chlorides Group II-Alkaline earth metal chloridesNo. LiCI NaCi KCI RbC1 CsCl MgCl2 CaC12 SrC12 BaC12 No sample

Anode 1. 1170 1340 1230 1030 940 1340 1340 1100temp. in 2. 1320 1320 1230 990 980 1340 1350 a 1500IC 3. 1210 1320 1280 1010 970Gap 1. 31 29 29 32 28 33 43 34

Helium voltage 2. 32 31 29 32 26 33 43 503. 33 32 32 33 30

Burn-off 1. 25 43 42 30 10 85 20 20time in 2. 28 56 36 29 13 50 20 ...seconds 3. 31 57 33 39 18

Anode 1. 1100 1040 1100 1050 1090 1090 1070 1140 1170temp. in 2. 1100 1070 1100 980 1090 1110 1140 1140 980 1250IcGap 1. 20 17 18 17 19 20 20 21 21

Argon voltage 2. 20 17 18 17 17 20 21 20 21 22Burn-off 1. 75 77 82 63 31 400 190 680 400time in 2. 77 137 77 46 43 365 910 800 160 ...seconds

Boiling pt.in C (10) 1382 1465 1407 1381 1300 1418 unknown unknown 1189Excitationpotential ineV 1.8 2.1 1.6 1.5 1.4 2.7 1.9 1.8 1.6Trouton'sconstant in 26.0 28.0 27.6 26.8 27.6kilocaloriesper mole per K

£ Sample consistently ejected from electrode.

84 tVol. 46

Februaryl956 EFFECT OF SOME CHLORIDES ON ANODE TEMPERATURE

The data for the former in helium are sparse, however,because barium chloride and calcium chloride almostinvariably formed a solid lump which was ejected fromthe anode cup after 10 or 15 seconds of arcing, andtherefore gave no values.

The alkali metals exhibit a definite intra-group effect.In helium, the anode temperatures of different alkalichlorides vary over a range of 400C and the volatili-zation times from 10 to 50 seconds. The voltage ispractically constant, however. These temperatures arerelated to the boiling points of the sample compounds,and to the lowest excitation potential of the samplecation. Figure 2 (a) shows the relationship between theaverage anode temperature observed and the boilingpoints of the samples. Figure 2(b) shows the relation-ship between anode temperatures and the excitationpotentials of the samples.

In argon, no such relationship was found (Figs.3 (a) and 3 (b)).

14 00 r HELIUM ATMOSPHERE

o 1300

I 1200

4I: 1100wa.

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Excitation Potential in eV

1300 161400 1500BOILING POINT IN' C

2

00

FIG. 4. Relationship between burn-off times and (a) boilingpoints, (b) excitation potentials of alkali metal chlorides arced inhelium. r is the correlation coefficient.

120

aso 100z0ILL 80Ul)

z

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IL 40UL.

, 20at

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CSCI

Excitation Potentiol in eV

1300 1400 1500BOILING POINT IN OC

FIG. 2. Relationship between anode temperatures and (a)boiling points, (b) excitation potentials of alkali metal chloridesarced in helium. r is the correlation coefficient.

ARGON ATMOSPHERE

a

.2 tA 1.6 .8 2.0Excitation Potential in eV

1300 1400 1500BOILING POINT IN0 C

NaCI

KCIRbCI

LICI

CICI

-2.2

'O1 600

FIG. 5. Relationship between burn-off times and (a) boilingpoints, (b) excitation potentials of alkali metal chlorides arced inargon. r is the correlation coefficient.

a0 0 0 b

0 00

KCLICI

N

R0 0

L ;.2 14 t I P I 2.0 2.I.2 1. Excitation Potintialt

800 1300 1400 1500 16BOILING POINT IN C

Figures 4(a) and 4(b) show the average bum-offtimes in helium plotted against the boiling points of thecompounds of the element and the excitation potentials.Figures 5(a) and 5(b) show similar data for argon. Inall figures the lines show the regression of the propertiesof the ordinates on those of the abcissas.

Ionization potential of the cation, sample meltingpoint, heat of fusion, specific heat, and heat of formation

act of the compounds were not found related to the voltage,bCI anode temperature, or burn-off time. The anode

temperatures and bum-off times were not related toheats of vaporization of the samples when expressed in

,0 calories per total amount arced, but were related whenthe heat of vaporization per mole of sample was used.

O

FIG. 3. Relationship between anode temperatures and (a)boiling points, (b) excitation potentials of alkali metal chloridesarced in argon.

DISCUSSION

It has been shown that the temperature of the anodespot is equal to the boiling point of the anode

85

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B. L. VALLEE AND R. E. THIERS

material.9 ' Heat is lost to the anode mainly by conduc-tion'2 and secondarily by radiation and convection.However, the spread in the anode temperaturesobserved with the alkali metals is too great to beexplained by these facts. Even 4.5 mm from the tipof the anode-the point where the temperature wasmeasured-a difference of 340TC was noted betweenNaCl and CsCl, while their boiling points are only1650 apart.

The sample, therefore, has a major effect on theanode temperature. With pure graphite electrodes, achange of the gap from 4 to 8 mm alters the anodetemperature by 130TC in helium and a change incurrent from 6 to 9 amperes alters it by 1000C (2);but the addition of cesium chloride to the anode lowersits temperature by 540TC. This is an even greatereffect than that of changing the surrounding gas fromhelium to argon.

The magnitude of variations of temperature due tothe composition of the sample, is adequate to accountfor the variability generally associated with the dcarc as a spectrochemical source. The addition of anexcess of one salt to all samples as a "spectroscopicbuffer" probably eliminates the large variations inanode temperature from sample to sample, and thewidespread use of buffers attests to the effectivenessof this measure.

For compounds which follow Trouton's rule, theboiling point is proportional to the heat of vaporizationin calories per mole. Trouton's constants for the alkalimetal chlorides are listed in Table I and show that theyadhere closely to this rule. Since the boiling points andburn-off times of the compounds and the anode tem-peratures show high degrees of correlation, the molarheat of vaporization should also be related to the anodetemperature, as was observed.

The constancy of the temperature in argon, and itsindependence of sample boiling point or excitationpotential are in strong contrast to its variability inhelium and its relationship to those properties of thesample. These relationships provide a possible explana-tion for the increase in the intensity of the lines of themore volatile elements in argon which was noticed

11 W. B. Nottingham, Phys. Rev. 28, 764-768 (1926).12 H. G. McPherson, Phys. Rev. 66, 357 (1944).13 L. H. Ahrens, Spectroclhetical Analysis (Addison-Wesley

Press, Inc., Cambridge, 1950).

when trace elements in a biological matrix were arcedin noble gases.7 In argon the anode temperature isapparently independent of the boiling point of thesample and estimation of the temperature at the anodetip, based an the data of Table I, yields values betweenthe boiling points of the volatile elements and theelements of the biological matrix-mostly sodium andpotassium. The volatile elements are probably vaporizedinto the arc unaccompanied by large quantities ofalkali metals. This condition is conducive to efficientexcitation of these elements, for the presence of alkalimetal is known to suppress their lines.'4 This process,occuring during the much longer volatilization timesprevailing in argon along with the resonance effectpreviously described," may account for the markedenhancement of lines observed in the presence of thisgas. When a mixture of salts is arced in helium, thevolatile elements boil off due to the rapid rise in anodetemperature, which in turn results from the rise ineffective boiling point of the sample. The matrixelements, therefore, enter the arc quickly and theelements are excited inefficiently during the shortvolatilization times prevailing in helium.

CONCLUSION

The temperature of the anode in the dc arc has beenshown to depend not only on the gas surrounding thearc, but also on the sample in the electrode. In helium,it is related to the boiling point and excitation potentialof the sample and the variation between the effects ofdifferent samples is as great as that caused by changingthe ambient gas. No such relationship appears in argon,a fact which may explain the enhancement of the linesof lower boiling elements in that gas.

The burn-off time for samples in the anode is alsorelated to the sample boiling point and excitationpotential in both helium and argon.

The gap voltage is generally higher for the alkalineearth chlorides than for the alkali chlorides, and isalmost constant from element to element within thelatter group.

The authors take pleasure in acknowledging thetechnical assistance of Mr. James F. Munafo and thehelpful advice of Mr. M. R. Baker.

4 W. R. Brode and D. L. Timma, J. Opt. Soc. Am. 39, 478-481(1949).

"5Baker, Adelstein, and Vallee, Anal. Chem. 27, 320 (1955).

86 Vol. 46