V O L U M E 22, NO. 3, M A R C H 1 9 5 0 A test of the rate of distillation of nicotine from an aqueous solution (in Table IV) showed that complete recovery was att,ained in less than 10 minutes. The longer time required for the distillation of nicotine from dried plant tissue must therefore be due to the time required for nicotine to diffuse through the tissur cells. Nicotine was determined in green tobacco leaves by the sprctrophotometric and silicotungstic acid methods with nicotine ciistillat>esobtained from them with both the modified Griffith and I.O.A.C. steam distillation apparatus. Green leaves were chosen for this comparison because they are difficult to sample and to analyze. A laboratory sample was prepared by comminuting the leaves in a Waring Blendor with acidified water. Aliquots were removed for the nicotine analyses and for moisture determination. The results (Table V) indicate no significant difference hetween the two methods of distillation or methods of analysis. Table VI shows the excellent agreement obt,ained between the results of the spectrophotometric and chemical methods on samples of dried tobacco leaves ranging in nicotine content from less than 1 to more than 8%. Table VI1 shows the results of analyses of tobacco juice, distillates, residues, kerosene extracts, and extract residues, and distillates of ensilaged whole tobacco plants. I n all cases there was excellent agreement, between the spectrophotometric and chemical methods. SURIM 4RY
433 Table VII.
Determination of Nicotine i n Tobacco Products
&laterial
Kicotine Found, % SpectrophotoChemiea? metric method method
Tobacco juice Distillate from tobacco juice Residue in still pot Distillate containing ammonia Distillate from potassium chloride solution Juice stripped with kerosene Kerosene extract Whole plants Ensilage (with phosphoric acid) Ensilage (with lime) Ensilage (with sodium chloride) Ensilage (with potassium chloride;
%
Yo
0.52 0.59 0.20
0.52
0.18
0.58 0.20 0.34
0.50 0.40 0.32 0.37
0.58
0 . is 0.17 0.58 0.20 0.34
0.51 0.38 0.32 0.36
Rathgeb for Some of the spectrophotometric measurements, and of C. Ricciuti for some of the chemical analyses. L I T E R A T U R E CITED
Avens, A. IT., and Pearce, G. W., IND. ESQ. CHEY.,ANAL.ED.. 11, 505 (1939).
Bertrand, G . , and Javillier, hl., Bull. sci. pharm. Tom., 1 6 , 7 (1909).
Brice, B. d..and Swain, bl. L., J . Optical SOC.Am., 35, 582 (1945).
Chapin, R. M.. U. S. Dept. Agr., Bur. Animal Ind., Bull. 133, (1911).
.\ rapid, sensitive met hod of nicot>ineanalysis is proposed wtiich
gives results concordant, with the official A.O.A.C. silicotungstiv acid method. Kornicotine, if present, is determined as nicotinr by both methods. The spectrophotometric method is unaffected hy ammonia or alkali salts in the test solution. When the method is used in conjunction with a modified Griffith and Jeffrey nicotine distillation apparatus, the time required t o determine nicotine by the spectrophotometric method is only 20 to 30 minutes, as compared xvith 24 to 48 hours rcquired by the A.O.A.C. silicotungstin acid method.
Garner, W.JT.. Bacon, C. IV., Bowling, J. D.. and Brown, I). E., U. S. Dept. Agr., l'ech. Bull. 4 1 4 , 3 5 (1934). Griffith, R. B., and Jeffrey, R. S . ,ASAL.CHEM.,2 0 , 3 0 7 (1948). Kissling, R.. U. S. Dept. Agr.. Bur. Cheni., Bull. 107, rev.. 32 (1912); 2. anal. C h m . , 21, 64 (1882).
Markwood, L. K.,J . Assoc. Ofic. Agr. Chemists, 22, 427 (1939). Ogg, C. L., Willits, C. O., and Ricciuti, C. R., . 4 w a ~ .CHEM.. 22, 335 (1950). P f y l , B., and Schmitt, O., Z . 1-ntersuch. Lohensm.. 54, 60 (1927).
Sn-ain, M. L., Eisner, A., Woodward, C . F., and Brice, B. A , . J . d m . Chem. Soc.,71, 1341 (1949).
Toth, J., Chem.-Ztg., 25, 610 (1901). Wolff, IT, A., Hawkins, XI. A , , and Giles, \V. E., J . Biol. Chem.. 175, 8 2 5 ( 1 9 4 8 ) .
4CKNOWLEDGRIENT
The authors grarefully acknoivledge the assistance of -1 Eisner in furnishing the purified nicotine preparation, of .Jane
R h c m v m July 9, 1949. Presented before the DiviJion of Analytical and Uicro Chemistry at the 116th Meeting of the ~ I E R I C A NC H E Y I c h L SOCIETY, .Atlantic City, S . ,I. Report of a study made under t h e Rwearch a n d Marketing Act of 1916.
THE RARE EARTHS Spectrophotometric Estimation of Certain Rare Earth Elements THERALD MOELLER AND J. CALVIN BRANTLEY', University of Illinois, Urbana, 111.
A
],THOUGH the quantitative determination of one rare earth element in a mixture containing others is of con.iidemble importance, the striking similarities in properties which characterize these element4 in a given state of oxidation have precluded the development of purely chemical procedures except in those cases--e.g., cerium, europium-where changes in oxidation state may be effec+ed Physical or physicochemical approaches are, therefore, essential. Methods involving deterniination of average atomic 15 eight or magnetic susceptibility are ordinal ily limited to binary mixtures and are of doubtful accuracy. However, procedures that depend upon evaluation of arc or spark spectra, upon measurement of absorption spectra, or upon neutron absorption are more widely applicable and do not involve material alteration or destruction of the sample. Of these, procedures involving absorption spectra are the most widely adaptable and probably the most generally useful because of ease of measurement through readily available and convenient instrumentation. The absorptlon spectra of many of the I
Present address, Laboratory, Linde Air Products Co., T o n a v a n d a . V Y
tripositive rare earth ions are characterized by complex system of comparatively sharp bands a t very definite wave lengths in the ultraviolet, visible, and near infrared. RIany of these bands in the visible have been used for many years for the qualitative detection and semiquantitative estimation of certain of the elements, but really reliable quantitative procedures involving them have been developed only within the past few 5 ears. Use of absorption spectra for quantitative determinations of the rare earth elements has been reviewed very completely 1 ) ~ Rodden (19). Briefly, early procedures involved dilution until :i given band showed the same intensity, when viewed with a direct vision spectroscope, as given b>-a standard (1, 3); dilution until a given band disappeared ( 4 , 2.5); relation of band nidth 10 length of absorption path through use of standards ( 2 ) ; and application of spectrophotometric technique ( 1 3 ) . Partridge and Rodden (15)emp1o:ed a filter photometer for the successful determination of neodymium in admixture with praseodymium m d samarium, but were unable to obtain satisfactory result z For the latter two elements,
434
ANALYTICAL CHEMISTRY
In two important papers, Rotiden (19, 20) summarized absorption spectra measured with a Coleman AIodel 10s spectrophotometer for nitrate solutions of the majority of the tripositive rare earth elements over the range 3500 to 10,000 A . Beer's law was found t o apply reasonably well over broad concentratio~i ranges, and both the method and suitable wave lengths for the quantitative estimation of a number of the rare earths in mistures were outlined. Itodden's general procedure has since been used by a number of other workers (6, 7 , 9-11, 23, 26) and has proved both rapid and generally accurate. In conjunction with the general investigations being carried out in this laboratory, rapid determinations of individual rare earth elements in c.omplex mixtures became necessary. Although Rodden's method, as published, was generally useful, it, n x s apparent that conip1ic:ttions due to mnre cvniplete resolution of absorption peaks by the more refined instrumentation at the authors' disposal, limitations of spectral rangc i n Rodden's original studies, and incomplete Itnowledge of the effects of various other inns upon absorption intensities all indic:tted the need for further furidamental studies. Ariv~rdingly, the absorption spectra of some eleven of the tripositive rare earth ions, ecandium, yttrium, and thorium were investigated over the spectral range 2200 t'o 10,000 A . , using chloride, nitrate, acetate, and in many cases perchlorate solutions. In establishing an analytical procedure based upon these spectra, rhoice of bands free from interferences, adherence of absorption t,o Beer's law, effects of slit a i d t h on band resolution, and rffccts of added anions were all considered. The present paper sunimaiizes these results and outlines an optimum spectrophotometric procedure which estends and complements that of Rodden. APPARATUS AND MATERI4LS
All spectrophotometric measurements were made with a Beckman quartz spectrophotometer, Model DU, using fused silica cells with an optical light path of 1.000 * 0.005 cm. This instrument is constructed so that the slit width can be varied from 0.01 to 2.0 mm., giving a nominal band width ranging from 2 to 40 A., depending on the wave length used. At any given wave length, this nominal band width can be varied by altering the sensitivity of the electrical system of the instrument. The only modification made was to cement a slide rule indicator glass (black line up) above the wave-length scale to decrease parallax and invrease the accuracny of the wive-lcngth settings.
Table I. Alaterial
KISO(sod)s
Compositions and Sources o f Alaterials Employed Inventory so.
.... . .
YIOS
YT-15
La203
LA40
Th(S0i)r
.
.... . .
C~(NO~)~.~NII&O . .I PrsOir
PR-L1
NduO:
SD-33
Ern108
SM-35
EU203 Gd203
GD-5
En03
ER-1-2s
TmzOJ
Th1-5-R3
YbzOa
YB-la
LUlOi
LU-4R-11
.. , , . , ,
Composition Siiectroscopic standard Atomic weight 88.7 Free from rare e a r t h
SOUIce Adsin IIilger, Ltd.
Illinois htock (6) Lindsay Light and Chemical Co. Atoniic weight ])ur- Lindsay Light and ItY Chemical Co. Analytical reagent C; . 1: r e d P r i c k quality Riiiith Chemicni Co. 93% PraOll, 2% Sdu- Lindsay Light and 0 8 , 5 % La201 Chemical Co. Free from other rare Illinois stock earths Free from other rare Illinois stock ( 9 2 ) earths ~ t o m i weiglit c purity IT. S . k c c o y ( 8 ) Atomic weight rjiir- Illinoiq stock ( 1 4 ) ity
98% Erz03, 1% 1102Oa and TmzOa, balance YuO3 8 8 4 TmzOs 11% L k h , 1% h 0 r Atomic weight purity 95% Lu203, 4.7% TmuOs, 0.3% Yhr01
Illinois stock Illinois s t o c k (10) Illinois stock (16) Iilinoi3 stock ( 1 0 )
The wave-length scale was calibrated frequently against the 5460.7 A . emission line from a mercury vapor lamp, as recommended by the manufacturer. -4 tungsten lamp was employed foi readings above 3200 A. and a hydrogen discharge lamp for readings at lower wave lengths. The silica cells were thoroughly cleaned with nitric acid before each use. They matched exactly in the visible range of the spectrum, but because they did not have the same transmission characteristics in the ultraviolet, cell corrections were determined and applied for readings in this region. Purified rare earth materials were obtained for the most part from the stocks of the Universitv of Illinois. The europium material used was a pure sample kindly provided some years ago by H. X. JIcCoy. The praseodymium material was an analyzed sample obtained through the courtesy of the Lindsay Light and Chemical Company. The erbium material was a sample that was purified by the ferrocyanide procedure (11). Because of the lack of a completely independent chec-k, its evact purity was unkno~vn. The value given is based upon comparison with other sampleq of known compositions. The conipositions and sources of the materials used are summarized in Table I. Other chemicals nerr of analytical reagent qualitv or better. ABSORPTlOV SPECTRA
Chloride, nitrate, and acetate solutions were prepared by slurrying weighed samples of freshly ignited rare earth metal oxides with water, adding the desired arid in slight excess, and narming until dissolution occurred. The solutions were then evaporiitcd to dryness either on a steam bath or with an infrared heater, the residues were taken up in n.:rtcr, and the process was rcpeated to remove excess acid. The final, acid-free solutions n w e diluted to such volumes that c*onrc.ntr;ttions equivalent to 25 grams of rare earth metal per liter ivere obtained. Cerium (11:) chloride was prepared by treating a viater solution of cerium (11) ammonium nitrate with concentrated hydrochloric acid and evaporating to dryness. Dissolution of the residue in concentrated hydrochloric acid and evaporation, repeated twice, gave R white, crystalline product. This material was dissolved in water (with the aid of 2 drops of concentrated hydrochloric acid per 100-ml. volume) to a solution containing 0.1276 gram of cerium per liter. Yttrium chloride solution Tvas prepared from the ignited oxide by the procedure used for the rare earth elements. Scandium and thorium chloride solutions were obtained in the same fmhion, except, that the freshly precipitated hydrous hydroxides xere used. Perchlorate solutions were prepared by slurrying the weighed oxides Kith water, heating, antl adding the calculated quantity of 10% perchloric acid in small increments; a few minutes were allowed to elapse after each addition to permit the slow reactions to occur. Excess of the rather difficult to remove acid was thus avoided. Concentrations were agitin equivalent to 25 grams of metal per liter. Ahsorption spectra of thesc solutions were measured against distilled water as a comparison blank over the range 2200 to 10,000 A . Readings were made every 20 A . through the ultraviolet and up to 6000 A. and ever!- 50 .%. from 6000 to 10,000 A. A-here absorption bands were found, readings were made at) narrower wavc-length intervals to estahlish the wave lengths of peak xbsorpt ions as precisely as permitted by the instrument. Whcre the original solution was too concentrated to give accurate log Io/I values, it was diluted to the necessary concentration. No attempt was made to control temperature accurately, for measurements showed the effects of small temperatore variation to be too small to be of importance. The temperatures during readings ranged from 25' to 34" C. Absorption curves were then plotted as wave length (A) in Angstroms versus k , the absorption coefficient, where I; is defined by the Beer-Lambert expression as k = -log I , l I Cl
In this esprcssion, c is concentration i n gram6 of metul per liter, I the light path ( = 1 cni.), antl I , anti I are the intensities of the incident and transmitted light, respectively. Expression of c in moles per Iiter would give corresponding molecular absorption coefficients ( e ) . Spectm for chloride, nitrate, acetate, and perchlorate solutions were essentially similar, although the nitrate absorption band in the region of 3000 A4.obscured any rare earth bands at wave lengths below 3400 A. The positions of absorption bands in chloride, nitrate, and perchlorate solutions of a given cation werc ncnrly identical. Hon-ever, i n :ic.et:rte solutions of neo-
V O L U M E 22, NO. 3, M A R C H 1 9 5 0
435
Absorption spectra hale been investigated oier the range 2200 to 10,000 A. for eleven of the tripositive rare earth elements, scandium, yttrium, and thorium, in aqueous solutions of the chlorides, nitrates, acetates, and perchlorates. Specific data are presented for the chlorides (and erbium perchlorate) of those ions which show absorption in the ultraviolet, visible, or near infrared. Data are also given for the effects of slit width on band resolution, for the effects of concentration upon absorption, for
I 500
-
I
I
I
I
I
CEROUS CHLORl DE
-
wave lengths of bands free from interferences by other rare earth metal ions, for sensitivities of d e tection at these wave lengths, and for the effects of added ions. Based upon these valires, a spectrophotometric procedure adaptable to the estimation of praseodymium, neodymium, samarium, europium, thulium, and ytterbium to within "1% is outlined. Information relative to the estimation of gadolinium and erbium, where larger errors are encountered, is also presented.
measured over essentially the same range by Kremers ( 7 ) . Spectra given by Stewart ( 2 4 ) for perchlorate solutions of the cerium earths are also in excellent accord with the authors' observations. SELECTION OF ABSORPTION BANDS SUlTABLE FOR SPECTROPHOTOMETRIC ANALYSIS
From the analytical point of view, selection of bands free from interference by other rare earth metal ions would be desirable. I t is also important that the bands choscri be the strongest in the given spectrum, in order that maximum sensitivity be obtained. Examination of the spectra given in Figures 1 to 9 shows that many of the strongest bands are subject to greater or lesser interference by other rare earth metal ions. In most cases, however, the interfering ions may be determined with accuracy and corrections applied. Such corrections become important only when the interfering ion is present in larger concentration than the ion being determined.
Table 11. 2MM
2400
A
-
2800
3200
Figure 1. Absorption Spectrum of Cerium (111) Chloride Solution
dymium, samarium, europium, and gadolinium, certain bands were displaced to longer wave lengths. Spectra for chloride and perchlorate solutions agreed in all cases where measurements were made, except with erbium. Erbium chloride solutions did not give reproducible absorption values, although erbium perchlorate solutions gave the same results even after standing for several months. Because of the greater utilities of chloride solutions and the slrnplicity of handling them, only the spectra of such solutions, plus that of erbium perchlorate, are included in Figures 1 to 9. Lack of purified samples of terbium, dysprosium, and holmium precluded measurement of their spectra. Scandium, yttrium, lanthanum, lutecium, and thorium ions showed no absorption over the range studied and are, therefore, omitted. For the other materials, only the regions of absorption are included. If, therefore, the complete spectrum is not given-e.g., for cerium chloride, ytterbium chloride-it is understood that the omitted portion contains no absorption bands. The spectrum of praseodymium chloride has been corrected for the neodymium absorption shown by the samples. The spectra given in Figures 1 to 9 are in excellent agreement with those obtained by Prandtl and Scheiner ( 1 7 ) by photographic means. Certain of the weaker bands reported by these authors do not appear in the present data because of the inability of the Beckman instrument to detect them. Excellent agreement is also noted between the authors' spectra and those given over more limited ranges by Rodden (19, 20) and those
Salt Used
PrCh Pr(X0a)s NdCls Nd(C1Oda Kd(N03)3 Kd(C~Ha0z)z SmClr Sm(N0a)a Sm(CzH30n)l EuCla Eu(K0a)r Eu(CzHs0g)r GdClr Gd(CzHs03s Er(Cl0i)r TmCla Tm(C10di YbCls Yb(N0a)s Yb(ClO4)a Yb(CzHn03z
Wave Length
A. 4445 4445 5218 5218 5218 5230 4020
4020 4026 3939 3945 3947 2728 2729 3792 6825 6826 9760 9750 9750 9720
Spectrophotometric Data hIolecular Absorption Coefficjent e
9.85 9.20 4.33 4.30 3.88 3.80 3.14 3.28 3.68 2.92 2.05 1.92 2.34 3.28 4.00 2.58
2.49 1.94 1.91 1.96 1.99
Absorption Coefficient
k 0.0700 0.0654 0,0300 0.0298 0.0269 0.0263 0.0210 0.0218 0.0244 0,0192 0,0135 0,0126 0.0149 0,0209 0.0239 0.0152 0.0147 0,0111 0.0110 0.0113 0.0116
Slit Width
Mm. 0.025 0.025 0.013 0.013 0.013 0.013 0.044 0.044
0.044 0,029 0.053 0.053 0.205 0,205 0.030
0.018 0,018 0.026
0.026 0.026 0.026
hIinimum Sensitivity 1-Cm. Cell' 0.metal/ 1OOml.
0.143 0.156 0,333 0.336 0.372 0.380 0.480 0,458 0.410 0.520 0,743 0.793 0.670 0.479 0.419 0.659 0.880
0,901 0.912 0.885 0.864
The ivave lengths summarized in Table I1 are those which are most useful for analytical determinations. Because cerium is more accurately determined by oxidimetric means, a value for it is not given. These recommended wave lengths compare well with those listed by Rodden (19,dO) but are more precisely determined. The same may be said with regard to the values given by Spedding et al. (M), except that these workers prefer the 740 and 795 mp bands for neodymium and the 521,653, and 975 mp bands for erbium. Although these neodymium bands are both
ANALYTICAL CHEMISTRY
436
K -
a03
001 '
I 6000
l
l
6400
I
( 6800
7200
7600
I 8000
8400
8800
9200
96OC
IC030
ACANGSTR O M S )
Figure 2.
Absorption Spectrum of Praseodymium Chloride Solution
Figure 3.
Absorption Spectrum of Neod) inium Chloride Solution
more intense than the recommended 5218 A. band, they are also more easily affected by excess anions. The recommended band for erbium is the most intense one in the useful spectrum. Data are included for acetates and nitrates in certain instances as well as for chlorideb and perchlorates. The minimum sensitivity values given in Table I1 are the concentrations required to give a log Io/I reading nE 0.100. A list of interferences is included in T a b l e 111. I n t w o cases-interference of the 5230 A. erbium band with the 5218 A neodymium band anti of the 6800 A. neodymium band nith the
V O L U M E 22, NO. 3, M A R C H 1 9 5 0
I
j
I
1
6500
bIG0
i
I
I
6900
437
I 1300
Figure 1.
I
I
I
I
i
ei 00
1100
I
A (ANGSTROMS)
i
t
i
I
6900
9300
-~
t
9100
.Absorption Spectrum of Samarium Chloride Solution
-
05 -
-k
04 -
-
03-
-
02 -
I
01 -
3400 1
3000
22110
2630
&nI
L , '
-
4200
3800
4600
5000
-
5400
I 5800
(A ANGSTROMS)
Figure 5 .
i h r o r p t i o n S p e c t r u m of Europiirnl (:hloride Soliitiori
6825 A. thulium band-such interference is unimportant because, except in natural minerals or in synthetically prepared samples, the simultaneous presence of neodymium and erbium 01 thulium is unlikely. The correction formulas which may be applied for these interferences are discussed in a later section of this paper.
Table 111. Salt Used
PrCSa
NdCls
8mCla EuCla Tm(ClO4)s Y b (CS0i)s
Interferences with Bands Chosen for Analytical Purposes
Wave Length, A.
k
4445 5218 4020 3939 6825 9750
0.0700 0.0300 0.0210 0.0192 0.0147 0.0111
Interfering Ion Sin+-' Er-" Eu-Srn-7Nd- Er-
k of Interfering I o n 0.0012
0.00998 0.0006 0.00101 0.00240 0 00529
EFFECT OF SLIT WIDTH
For it given absorption band, the first measurements were always made x i t h the sensitivity control a t three turns from the extreme counterclockn.ise position. The slit width a t some ivave length was then recorded, so t h a t readings could be duplii,ated if desired. The slit width KW then increased by a factor of 2 by decreasing the sensitivit?. anti the band chosen for analytiwl purposes was measured again. .I third measurement wu;i riiade with t,he slit width decreased by x factor of 2 from the origirial value. The k values were cdculated for all slit xidth viilries. Constancy iri k was taken as indicative of complete rrsolution of the band. Although complete resolution of each hand was desirable, it, was not essential for a given material as long as the k value a t a knoxn slit width could be duplicated. The slit width listed for each wave length in Table I1 is that at, which complete resolution was obtained, or a t which k was
ANALYTICAL CHEMISTRY
438 reproducible. The spectra of tripositive europium, erbium, and samarium are all affected by changes in slit width, and the bands are not completely resolved at any slit width obtainable with the Beckman instrument. For these materials, X: values are reproducible a t the slit widths listed in Table 11. The spectra of the other ions, with the exception of gadolinium, in chloride and perchlorate solutions are unaffected by any obtainable slit width changes. Gadolinium salt solutions presented unusual cases. At the ordinnry operating slit width of the instrument (0.330 mm. at 2730 .4.),such solutions showed only a broad, diffuse absorption band centering a t 2730 .1. A very slight decrease in slit width resolved this band into two bands. Accordingly investigations were extended using a technique which gives minimum slit width a t a sacrifice in sensitivity. The sensitivity control was set at the counterclockwise limit, and the “check” switch was turned to 0.1 on the scale. The instrument was then balanced against the solvent by setting the density scale to zero and bringing the needle to zero through adjustment of the slit width control. The optical density was then read by introducing the sample into the light beam and balancing the instrument with the density scale control. As shown in Figure 6, the original band was thus split into sis relatively sharp bands. The wave lengths characterizing these bands are in excellent agreement with those given by Prandtl and Scheiner ( 1 7 ) . -4t 2730 A., the slit width obtained was 0.080 mm.. corresponding to a nominal band width of 1.0 to 1.5 A. ADHERENCE TO BEER’S LAW
Rigorous adherence of the absorption bands of the rare earth metal ions to Beer’s Ian has often been questioned (18-21, 8 6 ) . Ilowever. the system:itic studies of Rodden (19, $0) and the reports of others (6, 7 , 9-11, 23, 26) indicate that deviations at lower concentrations a t least are not marked for most materials. It was essential to this investigation to obtain further information on Concentration effects. A solution of the salt being studied was prepared in such concentration as t o give a log Io ’I reading between 0.800 and 1.000. This solution was then diluted in a stepwise fashion, spectral readings being made a t appropriate wave lengths a t each dilution, until, as a loner limit. :L solution was obtained for which
005-
n-
1
I-
I
I
I
I
I
I
I
I
t
Figure 6.
Absorption Spectrum of Gadolinium Chloride Solution
Ia/I lay betncen 0.100 and 0.200. These upper and lower limits were chosen arbitrarily because of the enhanced accuracy of the instrument in this range. In no cases were concentrations in excess of 72 grams of metal per liter used, and in the majority of the cases the upper limit was 25 grams. In this fashion, chloride solutions and erbium perchlorate solutions were investigated systematically. Plots of log l o l l against concentration were made to establish the independence of absorption coefficient, k , and concentration. Beer’s law requires the existence of a linear relation between log Ia/I and concentration. Data summarized in Table I V indicate clearly that over the concentration ranges investigated Beer’s law is obeyed within 1% at the absorption peaks of analytical importance for all the materials studied except gadolinium. With gadolinium, deviations up to * 10% xere noted. This is undoubtedly due to the difficulty of resolving the absorption in the vicinity of 2730 A . and the consequent inaccuracy in spectrophotometric readings. However, i t is apparent that, with this exception, estimation of the rare earths by means of absorption spectra data is feasible. EFFECTS OF OTHER I O N S
Because of the difficulty of removing excess acid through evaporation, it was of interest to determine whether accurate
-
-
K -
003-
>001-
1
1
0.02 d
-
005-
I
-
-
439
V O L U M E 2 2 , NO. 3, M A R C H 1950 Table 11'. Rare Earth Salt
Beer's Law Determinations
Wave Length A.
Slit IVidtlr
PrCls
4448
NdCir
A218
Concentration G . metal/liter
log I Q / I
0.025
12.5 6.25 2.80 1.25
0.819 0.410 0.164
0 013
25.0
0.741 0.370 0.148
Mm.
0.082
12.5 5.0
n . 534
8niCla
0.268 0.106
EuCli
3939
0.054
GdCli
2730
0.230
The effevts of individual rare earth ions upon the absorption spectra of eircli other have never been evaluated systematically, probably because the rarities of these materials have precluded their use i n s!mtlietic mixtures in any hut exceptional cases. I n this investigation, a few such synthetic mixtures were studied. I n view of the excellent results obtained during the spectrophotometric analysis of these mixtures, it :tppears that the individual materials exerted little or no effect upon each other, with t h e exception of an increase in the alxmrption value for erbium when ytterbium or thulium ion was present. This does not rule out the possibility of interactions a t higher concwtrations, :LS reported by Quill, Selivood, and Hopltins (f8).
0.450
23.0
n . 224
12.5 24.99 12,495
SPECTROPIIOTOZIETRIC ESTIiMATlON OF RARE EAHTII METAL IONS
0,360 0.171
Based upon the above ohservations, the i'ollowing p r o c d u r r is recommended for the spectrophotometric estimation of rare earth metal ions that shon- ch:ii,actcristic absorption bands. TmC
I
PbClt
6825
0.018
33 91 26,'Jfi 13.48
0.723 0.363 0 I182
9750
0.026
72.43 36.21 18.11
0,800 0.401 0.200
results could he obtained in the presence of unrcniovcd acid or excew of the characteristic anion. Acid-free chloride solutions containing the equivalent of 25 grams of the metal per liter wcw prepared by the technique previously outlined. .4liquots werc: then treatcvl Kith v:irying quantities of standard hydrochloric acid or ammonium chloride solutions, the total rare earth content in a given series being kept constant Iiy appropriate dilution. For each solution, the absorption coefficient \vas then measured at the predetermined analytic,al wave length, the v:ilue for the pure chloride solution being used for coinpttrison. For the cerium earths, addition of chloride ion had no nie:isurable effect. This is shown by the data for neotlj.mium as summarized in Table 1'. Quantit:itive removal of exccss hydrochloric acid is thus not essential to spectrophotometrics estimation of these elements. On the other hand, yttrium carth vhloride solutinns---c.g.. those of erbium, thulium, yttcrl)iuni-were sensitivc to the presence of excess chloride ion. At)sorption coefficicmis varied with both cliloride content and time. Removal of exrcss hydrochloric acid permitted reprodurihle readings with both thulium and ytterl)iuni, i f measurcments ivvre ni:ide within 2-day intervals.
The freshl). ignited, weighed (1 to 3 gwins) oxide sample is slurried with water and dissolved in :t slight excess of hydrochloric (or perchloric) acid. Excess acid is removed by evaporating to dryness, taking up the residue in water, and re-evsporating. The final residue is taken up in water and the resulting solution is diluted to a suitable volume (say 100 ml.). The log Ie/I values are then determined at the wave lengths and slit widths summarized in Table I1 if a mixture is being studied or a t an individual recornmended wave length if but a single element is to he estimated. (These values apply rigidly only to the instrument used by the authors. Anyone using another instrument should obtain his own set of calibration data.) Although in the authors' experience duplication of these wave lengths has been achieved in 95% of the cases studied, the sharp natures of the bands may make peak settings difficult with some instruments. It is recommended that no less than five readings be made in the vicinity of each peak at, slightly differing nave-length settings and that the maximum log T0;T value so obtained be used in subsequent calculations. The quantity of rare earth ion present is then calculated from the measured log T J / I value and the characteristic k value as summarized in T:it)lc 11, using the expression Grams of It++'per liter
=
log
Toll
___ k
use of 1-mi re115 being assumed. In cases where other ions interfere (Table 111), neccsary corrections may be made by calculating log I,/I due to the interfering ion and subtracting this value from the measured log I o / I . This correction factor is obtained by multiplying the concentration of the interfering ion by the absorption coefficient of this ion a t the wave length in question. Thus, to detcrniine the amount of a rare earth, A , present when another one, R , interferes, one uses thc expression Grams of .1 per liter = log Io!I
- (grams of R per liter
X ISH)
~~~
~
kA
Table V. Effects of Chloride Ion on Absorption of Neodymium Chloride Solutions a t 5218 A . A hsorption Coefficient,
Material Added
HCI
k
0.00 0.0397 0.0993 n . 993 9.939
0.000 2.00 4.00 10.00
0 0300 0.0300 n , 0298 0 , moo 0 . moo 0.0300
n , 0300 0,11302 n ,0306
Brbium chloride solutions gave varying absorption coefficients even when free of excess chloride ion. Perchlorate solutions, however, gave reproducible readings a t 3792 A., either when purr or when the mole ratio of added excess perchloric acid to erbium was as much as 4 to 1. For any mixture containing el bium, a perchlorate medium is recommended. Addition of excess perchloric acid was also without effect in other cases.
Values of iilisted in Table I11 are Cor such c::ilculations. The mutunl interference b e t w e n the 3939 d. band for curopium and the 4020 A. band for samarium, the on]>-strong hands available for these ions, presents a moro difficult problem. Two alternatives are suggestetl. The first involves chemical estimation of europium by oxidimeti,ic means :tnd corresponding correction of the samarium determination. The second, which is simpler and of equal accuracy, involves L: preliminary calculation of the quantity of material present in the larger amount. This quantity is then used to correct the mensured roncentration of the other ion in the fashion outlined above. The resulting concentration of the second ion is then used in arriving at a more accurate value for the first. I n three such calculations, accuracies within 1% are obtained.
A sample calculation follows for a mixture of europium chloride and samarium chloride (see Table VI, Item 3). 1.
Gramsof Sm/litcr
=
0.311 0.0210 ~~
=
14.80
-
-
04
-
-
03
-
-
02
-
-
01
-
-
05
k
; 2200
01
2600
!
I 4200
L l
4600
I
I
MOO
I 5400
I
I 58W
-
7000
6xK)
Figure 8.
2. GramsofEuiliter = 3.
I
I 3800
lJ/ MOO
3000
Gramsof Smlliter =
82W 8600 woo ( X AMSTROUS) ibsorption SpPrtriim of Thulium Chloride Solution 1400
0.281 - (14.80 X 0.001) - - - 12,84 --0.0192
0.311
- (12.82 X 0.0006) = 0.0210
94 00
7800
9000
07 05
, ~ , ~ ~ 04
Data summarized in Table VI indicate the applicability of the general method. Mixtures analyzed were either synthetic O
Table VI. NO.
2"
Representative Analyses of Rare Earth Materials Materials Present
Pr Kd Srn La Nd Srn Srn Eu Er Yb Er Yb Er Yb Er
Trn Yb Nd Srn
b e
Amount Present 0. metaZ/liter 1.62
4.84 9.10
5.65 7.20 6.80 14.54 12.95
Amount Found G . metnl/l t m 1.63 4.86
L l a0I0 I I I 8600 I I I I
~
so00
l ~ l l o y
i X ANGSTROM^
Figure 9.
Absorption Spectrum of Ytterbium Chloride Solution
I) 19
7 15 6 85
14 48 12 84
12.3
12 4
26.3 6.15 13.2
26 2 6 65 13 1
2.46 -17.4
47 2 84 5
3,09 9.40 .i.95
4 80 9 38 i 94
5.02
5 05 a 70
8.69
t
Synthetic mixture. Analyzed spectrophotometrically by C . J. Rodden. Analyzed gravimetrically by oxalate precipitation. -~
~
mixtures prepared from hnor5 n quantities of pure m*terial> or samples for which independent analyses were available as indicated. It is therefore apparent that an accuracy of *l% may be expected for materials discussed in this paper, except gadolinium and erbium. Precision is excellent. The procedure is adaptable to the analysis of a variety of samples. It is particularly applicable to analyses in the presence of scandium, yttrium, lanthanum, lutecium, and thorium, because these ions show no absorption in the range 2200 to 10,000 A. Estimation of these elements must be effected by other means, and cerium is better determined by oxidimetry. For routine Fork in following fractionations or tracing the behavior of an element in a series of reactions, the procedure is invaluable because of its rapidity and accuracy. It must be emphasized, however, that unlike many colorimetric proredures, this one is not adaptable to a mirro srale. The
V O L U M E 22, NO. 3, M A R C H 1 9 5 0
44 1
characteristic absorption exhibited by the rare earth metal ions is not sufficiently intense to permit such an extension. ACKNOWLEDGMENT
Moeller, Therald, and Quinty, G . H . , J . Phys. Colloicl Chem., 54
(13) (14)
hfuthmann, W., and Sttitrel, L., Ber., 32, 2653 (1899). Naeser, C. W., and Hopkins, B. S.,J . Am. Chem. Soc., 5 7 , 2153
(1950).
(1936).
The authors wish to express their appreciation to the Office of S a v a l Research for support received during this investigation. Thev also express appreciation to C. J. Rodden for analyses of certain samples made some years ago in conjunction with another investigation. Receipt of samples from the Lindsay Light and Chemical Company is also gratefully acknonledged. LITERATURE CITED
(12)
'
Brauner, B., Chem. 'Yews, 77, 161 (1898). Delauney, E., Compt. rend., 185, 354 (1927). Friend, J.N., and Hall, D. -4., A n a l ~ s t65, , 144 (1920). Haas, dissertation, Berlin, 1920. Hopkins, B. S.,and Balke, C. It-.,J . Am. Chem. Soc., 38, 2 : < : < 2 (1916).
Kremers, H. E . , doctoral dissertation, University of Illinoi.3, 1944.
Kremers, H. E., private communication. McCoy, IT. N., J . Am. Chem. Soc., 59, 1131 (1937).
Moeller, Therald, and Kremers, H. E., I S D . ESG. CHEM.,ASAI.. ED., 17, 4 4 (1945). Ihid., p. 798.
Moeller, Therald, and Kremers, H. E., J . Am. Chem.
Soc., 6 6 ,
307 (1944).
(15) Partridge, H. M., and Rodden, C. J., Abstracts of 81st Meeting, (16)
A M E R I C A N C H E M I C A L SOCIETY, Indianapolis, 1931. Pearce, D. W., and Naeser, C. W., with Hopkins, B. S . . Trans.
(17)
Prandtl, W., and Scheiner, K., 2. anorg. allgem. Chem,., 220, 113
Electrochem. Soc., 69, 557 (1934). (1934).
(18) Quill, L. L., Selwood, P. TI'., and Hopkins, B. S.,J . A m . Chem. SOC..50. 2929 (1928). (19) Rodden, C. J., J.Research ~ V a t lB. u r . Standards, 26, 55; (19411. (20) Ibid., 28, 265 (1942). (21)
Selwood, P. W., doctoral dissertation, University of I l l i i i i ~ i ~ , 1931.
(22) Selwood, P. IT., J . Am. Chem. Soc., 52, 1937 (1930). (23) Spedding, F. H., Fulmer, E. I., Butler, T. rl., Gladrow. 1,;. SI., Gobush, M . , Porter, P. E., Powell, J. E . , and Wright. .J. SI., Ibid., 69, 2812 ( 1 9 4 7 ) . (24) Stewart, D. C., University of California Radiation Lahoi a t o r y , Declassified Document UCRL 182 (Sept. 22, 1 9 4 8 ) . ( 2 5 ) Yntema, L. F., J . Am. Chem. Soc., 45, 907 (1923). (26) Young, R. C . . Arch. A . , and Shyne, W.V., Ihid.. 63, 95; (1841). RECEIVED August 8, 1949. Presented in part before the Dirision of Physical and Inorganic Chemistry a t the 115th Meeting of t h e A n E m x s CHEMIC A L SOCIETY, San Francisco, Calif. The preceding conimunication in t h i ? series was published in the Journal of Physical and Colloid Chemistry (1%').
Spectrographic Analysis of Coal and Coal Ash RICHARD G. HUNTER
A ~ A. D
J. W. HEADLEE
West Virginia Geological Suraey, Morgantown, W . Vu. Coal can be analyzed on the spectrograph for per cent ash and composition of ash in a matter of a few minutes, using the total energy method. The composition of the ash so determined can be used to calculate ash softening temperatures. This analysis can be made in sufficiently short a time to control tipple and washing operations for preparation of coal to meet specifications. This spectrographic method can be readily adapted to the analj-sis of rocks, minerals, and inorganic chemicals of all kinds.
S
PECTKOGKAPHIC methods are being developed for quantitatively analyzing all types of rocks and minerals (1-3). The rapidity with which the analyses can be made, the positivc identification of the element sought, and, in numerous instances, the accuracy of the data obtained make this method of analysis desirable. The total energy method (W), in which the sample is burned completely in a direct current arc, was used in this investigation for developing a method for the analysis of ash in coal. Internal standards are not needed in this method. The wide range of volatility of the elements in coal ash gives the total energy method a n advantage over the internal standard method for this type of sample. The internal standard method requires many more standards than does the total energy method; a very few standards suffice for the analysis of several elements in a a i d e range of sample types, using the total energy method. APPARATUS AND METHODS
Tile instruments used are the large Littrow prism-type spectrograph, conventional direct current excit,ation source unit, and a projection comparator densitometer. I11 the analysis of raw coal the samples are ground to 200-mesh and after being thoroughly mixed are weighed directly into a high-purity graphite spectroscopic electrode. Five milligrams of sample are employed. These electrodes are then placed in a small electric muffle for about 10 minutes a t 550" C. to drive off the volatile matter. The spectra of four samples and one stand-
ard sample are then photographed on the same spectroscopirplate. The olectrode holding the sample forms the positive pole of a 220-volt direct current arc dran-ing 12 to 15 amperes. All spectra are modulated a t the slit by a logarithmic step sector, revolving a t 2500 r.p.m. Samples are burned to completion. From this plate t>hedensities of the various spectral lines are read on a densitometer and the following oxides, the major constituents of the ash, are quantitatively determined: silicon, frrric, aluminum, titanium. calci,mi, magnesium, sodium, and potassium. The per cent ash will therefore be the sum total of the above determinations plus the sulfur trioxide, which is discussed below, The standard employed on these tests was a sample of West Virginia coal, C-5. The coal and ash were analyzed by conventional methods in the authors' laboratory. The analysis of this ash sample was also determined on the spectrograph, employing a synt,hetic standard made from oxides and carbonates of known purity (Table I), Tho chemical analysis of C-5 ash was used as a basis for calcularing the spectrographic analysis of the unknoir-n samples. After 70 coal samples had been analyzed, the data were tabulated and the difference between the burning method and spectrographic method ivas determined. On plotting the per cent ash in coal against the plus or minus difference, it was found that the spectrographic method invariably gave high values on low-ash coals and loiv values on coals ivhose ash content was considerably greater than the coal used as a standard. The variation for each element was directly proportional to the ratio of concentration of that element in the standard, versus
