Developments and Current Problems in Inorganic Chemistry
Fisher Award Symposium Honoring James I. Hoffman, Division of Analytical Chemistry, 135th Meeting of the American Chemical Society, Boston, Mass.. April 1959
The EvoIution of Certified Reference Materials JAMES 1. HOFFMAN National Bureau o f Standards, Washington,
,The discussion traces the development of reference materials through the early days when the buyer was content to ask for a material that matched a satisfactory material that he now had, down to the present highly developed standard reference materials that are currently furnished by the National Bureau of Standards as a regular service. It also shows how similar movements have developed in other countries, notably Great Britain, and how trade and chemical processes in this country have become dependent on these standard reference materials. The latter part of the discussion shows how these valuable reference materials depend on progress and development of analytical methods, and then, in turn, how these reference standards play an important part in the development of new methods. New methods of analysis make possible new standards, and new standards aid in the development of more new methods. The development of spectrographic standards i s included and the newer methods of separation of niobium and tantalum in the presence of titanium are discussed.
HEX the early cave man sent his son out to procure a weapon fur the defense of his priniitir-e home, he probably told the son to fashion the new weapon after the old one in the father’s hand; or, it is equally probable that the fathcsr or the son suggested some improwment or modification of the old onc. The fact remains. lion-ever, that tlic w a p o n in hand served as a refcrcnc.c niatcrial foi the next one. Dovn through the agcs the spice collector, the taster of footis. and the wheelwight p:itterned rlic.11 products after niatcmils that had prc~iously hcen found satisfactory. Tli(>qe rrfcrencc material- did not l ~ c a i a company designation nor a Sational Burcau of Standards nuniber,
1934
ANALYTICAL CHEMISTRY
D. C.
but the!. scmwl in a crude n a y as models or to communicate the requirements of thc consumer to the producer. Kor can it l ~ csaid that such reference materials arc obsolcte in our modern age. Even now it is convenient to request a product equivalent to another product. K h a t analytical chemist has not said. “Use a No. 42 Whatman or equivalent filter?” The escliangc of goods by referencc to other goods of it similar kind scrved very ~ 1 as1 long as sniall tliffrrenees were of littlt conscqiiencc, hut sooner or later small diffcrcnccs in composition of a metal or an ore were bound to come into play, There was little danger that a cabbage nould be furnished instead of a turnip or a cat instead of a dog, hut there still was the possibility of substituting a pole-cat for a harmless tabby-cat. Thus strickr definition of refrrence materials brcanie necessary, and this could usually be best accomplished by defining tlic coniposit,ion of the material. Composition is the most meaningful property of a material to the analytical chemist; yet, until rather recently, reference materials implied more than composition. This was not always recognized. For example, Swedish iron meant the purest commercial form of iron made, but it has its origin in high grade iron ore and other pure starting materials, and therefore by implication the term SI\-edish iron included origin and msthod of nianufacture as d l as coniposition in its definition. To many the stipulation that construction shall be of wrought iron means that a very high class of materials is demanded, but to the analytical clicniist tvrought iron is a fairly dirty product. It is a ferrous material, aggregated from a solidifying mass of pasty particlcs of highly refined nietallic iron \vitli which a minutely and uniformly distributed quantitjof slag i,s incorporated without subsequent fnsioii ( 2 ) . The slag contcnt
varies bctn-ern 1 and 3%. It is apparent therefore that “wrought iron” as a reference material con\-eys a t least three ideas-appearance. composition, and method of manufactureand none of thew is very exactly d ~ fined. On the other hand. sterling silver, coinage silver, and 24-carat gold can serve as reference niat,erial* for the analytical chemist’ because t h q . contain exactly dcfinctl prrccntages of silver and gold. The fact that a certain app,cnranct is implied docs not inipair thcii valui~U P a rrfwence material for USE b>. the. :iiialytical chcmist. Of course. the dpgrev to n-Iiic.11 thew materials conform to stcrling. ooinagc,. or 24-carat m i s t tic k~ioivn. TRANSITION F R O M CRUDE W O R K I N G STANDARDS TO CERTIFIED STANDARDS
There came a time. Iiowver, wrien it became nccessary to fis the composition of certain niaterials closely because the properties of the materials depended on their composition. The properties of cast iron depend, among other things, on the content of combined carbon and graphite, and thc hardenability of a steel may be materially changed by tlic presence of very small amounts of boron. Tht. effect of small amounts of boron on nuclear reactions is well knoivn. Likewise very accurate knorrledge of coniposition is demanded in high-pricctl niaterials such as ferrotungsten. Those concerned with the propert’ies of inaterials of construction or their sales value saw conipelling reasons for certified standard substances. so that all could refer to a coininon base. Consequently nianafacturers-for example, steel companies-and possibly some consumers, prepared materials which they analyzed carefully and used as standards. Later, trade associations siicli as the American Foundrynien’s Association (now Society) prepared niid analyzed standards for their group. Many priT-ate laboratories undoubtedljhac1 sonic carefull!- malyzctl material
t h a t served as a htandard for their own product. Some readers remember when iron wire was the standard for permanganate solutions. However, the variation that was bound to exist between the standards used by associations and the various laboratories had the effect of making i t desirous t o have some central agency certify a particular reference material. The Kational Bureau of Standards finally became that agency in this country, and a little history of this development is in order. CERTIFIED REFERENCE MATERIALS AVAILABLE
National Bureau of Standards. I n 1905 the American Foundrymen’s Association turned over to t h e Chemistry Division of t h e S a t i o n a l Bureau of Standards its project on t h e standardization of four types of cast iron. These were re-analyzed b y chemists a t the bureau and in industrial and commercial laboratories. T h e y were then issued as Kational Bureau of Standards standard samples. It very soon became apparent from the numerous inquiries received that thc preparation of other materials for t h e steel industry should be included, and the issue of various types of steel was begun. Since this modest beginning, the demand for new standards has never ceased, and at the prcwxt time a coriderable numher of spectrographic as well as chemical standards are iricluded. Currently more than 600 qtandard reference materials, including metals, ores, chemicals, and other materials, are available (9). The standard reference materials program a t the Sational Bureau of Standards is described by Bright (3) who gives a more detailed discussion of the bureau’s standard materials program, Since Bright’s article was published the following iniportant new composition standards have been issued:
TWOhigh temperature alloys (Co, Mo, S b , Ta. IT), three lithium ores (spodumene, petalit?, lepidolite), one bronze, two alumina silicon refractories, two titanium-base alloys, eight steels for oxygen and nitrogen content, 52 spectrographic standards, 13 uranium isotopic standards, seven phosphors, and il number of radioactivity standards The spectroscopic standards include carbon and low alloy steels. stainless steels, tool steels. and zinc-base alloy. The uranium isotopic standards are used in transactions involving reactor fuels and contain 0.5, I , 1.5, 2, 3, 5, 20, 35, 7 5 , 80, 85, 90, and 93% of uranium-235, respwtively. They are available only to the U. S. Atomic Energy Commission’s contractors and licensees. For this occasion only those standards that are of use to the analytical chemist
are directly pertinent, but it is interesting to note that, in addition to the composition standards, the list includes samples certified for density, refractive index, heat of combustion, melting point, viscosity, radioactivity, rubber compounding, turbidity, and color. The standards that supply the needs of the analytical chemist are the composition standards, which comprise between 80 and 90% of the total with respect to number of units issued. The composition standards are further divided into two classes for convenience -namely, chemical standards and spectrographic standards. The spectrographic standards fill a n important need in analytical laboratories and greatly simplify the work \There many analyses of the same kind of materials are made. However, the chemical work of the analytical group a t the Bureau of Standards charged with preparation of standards is greatly increased because practically all spectrographic standards are analyzed b y chemical methods to establish their composition. -4 few of the standards such as the nickel oxides are prepared by careful synthesis, but, even here, the raw materials need the touch of the analytical chemist. The preparation of a standard reference material a t XBS is a fascinating story, but one which would be too lengthy for this occasion. summary of the main steps, however, should prove interesting. The selection of the particular standard to be prepared comes first. There are plenty of choices, because there are always many more demands for different types of materials than can be met. Certain factors such as difficulty in milling, r;elting, or grinding a material may prevent or delay the preparation of a desirable standard. It is obvious that a white cast iron would not be chosen if means of reducing it to chips were not available; nor would a turbine alloy be chosen if methods of analysis were lacking, unless prolonged research was permissible. The relation of availability of methods of analysis to the evolution of standards !vi11 be discussed later. Homogeneity and permanence play a n important role in the certification of reference materials. The starting material bhould be homogeneous. but this is not sufficient. Segregation may occur in powdered samples if the particles are not all of nearly the same size. For example, in the preparation of a ferrotungsten standard it n a s found that the material that passed a Xo. 100 sievf: and was retained in a So. 200 sieve contained 80.270 of tungsten, whereas the finer material that passed through a KO.325 sieve contained only 70.4y0 of tungsten. T o be of maximum value to the analyt-
ical chemist, a knowledge of a reference material’s permanence must be available. The composition may change by such processes as absorption of carbon dioxide or water, contamination by parts of the container, loss of water, or oxidation. One standard gained weight by absorption of moisture during the hot damp Washington summers and lost it again in the dry atmosphere prevailing in winter. KO harm was done because analyses were referred to the dry basis (constant weight a t 105” C.). I n one winterto-summer cycle the moisture content went from 0.36 to 0.647, in standard phosphate rock 56a. A review of all the things that might go wrong in the preparation and maintenance of a standard might deter the less bold from ever attempting the program. Few appreciate all the consideration that has been given to the issuance of every standard. Human fallibility must be considered. There is always a possibility of mislabeling, mixing samples, or using unclean containers in the handling of the material. I n spite of all these possibilities, in the entire history of the program (55 years) the bureau has had only one complaint of mistaken identity. Only two samples, a highaluminum ferrovanadium and a manganese metal, were withdrawn because they changed in composition owing to slow oxidation. Only one analysis, where a sniall amount of lithium was counted as sodium, required modification after the sample was issued. This is a most enviable record and should command the respect of all analytical chemists. The integrity of these standards and their universal acceptance is closely associated n ith the names of W. F. Hillebrand, G. E. F. Lundell, and H. A. Bright. To these must now be added those of Bourdon F. Scribner and Robert E. Michaelis for the prominent part they are playing in the establishment of spectrographic standards. Through wise management and insistence on high standards in the preparation and analysis of these S B S reference inaterials a high prestige became attached to the privilege of working on these standards or furnishing materials for them. Chemists ask for the privilege of cooperating in the analytical work, and no analyst has ever been paid in money for his work on the final standard as issued. He felt fully compensated in having his name on the certificate of analysis as a cooperating analyst and in getting experience in precise analysis. Many of the metal companies have gone to much trouble and expense in preparing suitable homogeneous materials and have furnished thousands of dollars worth of metal free of charge because VOL 31, NO. 12, DECEMBER 1959
.1935
of their desire to have their materials included in the certified list of standard reference materials. Cooperative Standards. T h e National Bureau of Standards has never been able to keep abreast of all t h e demands for standard reference materials. This has been true especially with respect t o spectrographic standards, because it is this area t h a t lends itself readily t o instrumentation. High-speed spectrocheniical methods have been extensively adopted by producers and consumers of metals, and carefully prepared standards have become essential for calibration. The preparation and issuance of zinc-base alloy standards illustrate a desirable type of cooperation between companies and the bureau. On learning late in 1952 that General Motors Corp. intended to prepare a series of zinc-base standards for use within the corporation, the bureau suggested that the program be expanded to provide nationally available standards for these alloys. The suggestion was adopted, and there was close cooperation in all phases of the work from the planning through the final analj& and certification. Six such standards here issued with certification for Cu, 1-11, hIg, Fe, Pb, Cd, Sn, Cr, hln, Xi, and Si. After the ivork was finished, the samples n ere divided betn eeii General hIotors Corp. and XBS. These are no)! available as regular NUS standards. Similar cooperation prevailed in the preparation and certification of four samples of high-temperature alloys for optical emission and x-ray fluorescence anal? sis, six stainless steels, two nickel oxide standards, and eight ingot iron and lom-alloy steel standards. The last mentioned are certified as spectrographic standards for C, hIn, P, Si, Cu, Ki, Cr, V, hlo, Sn, Ti, B, As, \V, Zr, Kb, and Ta. The mere mention that all the values on the certificates had to be determined b y chemical analysis will make a n experienced analytical chemist shudder. Yet this had to be done in the bureau’s analytical laboratories. A statement b y Michaelis ( 8 ) concerning these standards shows the magnitude of cooperation that exists in this work. These standards were prepared with the generous cooperation of industrial and government groups. The melting and casting was done at the Kava1 Research Laboratory and the fabrication from ingot to final size a t the Republic Steel Corp. The base metal was furnished b y the Armco Steel Corp. and the U. S. Steel Corp., and alloy additions were provided by several industrial and government organizations. Grants from the American Iron and Steel Institute greatly accelerated the NBS program for preparation of the standards. Finally, several outstanding analytical laboratories cooperated in
1936
0
ANALYT!CAL CHEMISTRY
the extensive analytical program for certification: American Cast Iron Pipe Co., Armco Steel Corp., Bethlehem Steel Co., Crucible Steel Co. of America, Electro Metallurgical Co., General Xlotors Corp., Timken Roller Bearing Co., U. S. Steel Corp., and the National Bureau of Standards. Further information on spectrographic standards with its cooperative aspects is given by Michaelis ( 7 ) . The company standards referred to are not issued to compete with NBS standards but to supplement the areas not covered by XBS. National Physical Laboratory Standa r d s (1). T h e National Physical Laboratory of Teddington, England, has issued twelve composition standards (cast iron and steel), b u t it has never attempted to cover t h e field of standards as evtensively as t h e Iiational Bureau of Standards has. These standards are mentioned in K P L annual report for 1916-17, and nine of thein are still available. They were prepared in cooperation with the British Iron and Steel Institute Committee for AIetnllographj-, Chemistry, and Physics. Some time ago a desire \vas expressed in Britain for a scheme whereby analyzed samples prepared by a private firm or individual could receive a British Standards Institution mark which would guarantee the reliability of the standard. XPL cooperated by offering to act as a n independent analyst and to inspect marked samples, if required, to confirm that they corresponded to the samples originally independently analyzed. Except for a series of hlazak spectrographic standards, only one analyzed sample has received this mark, and the plan has not been put into effective action. However, the formal position of N P L is that any new standard analyzed sample bearing this mark would have their approval, whereas a standard not having this mark enjoys only the authority of the manufacturer. Other Standards. It seems natural t h a t t h e success achieved by N B S reference materials should stimulate other agencies in this country and in other countries to p u t out their own standards either as verbtures for profit or as a scientific service. KO serious competitors for profit have appeared in this country, because in most cases the establishment of a new standard is a time-consuming and costly operation. The distribution of certified hydrocarbons by the American Petroleum Institute is a notable example of such a n operation as a scientific service. The nearest approach to the K’BS standard reference materials program is that of the Bureau of Analyzed Samples, Ltd., Newham Hall, Rliddlesbrough, England, under the name of British
Chemical Standards and Spectrographic Standards. The movement was started in 1916, by Ridsdale & Co., and the method of operation and form of certificates of analysis follow the pattern set by the NBS. During their early years the Bureau of Standards cooperated in the analytical work. Their composition standards include cast irons, carbon and alloy steels, ferroalloys, slags, ores, refractories, non ferrous alloys, pure reagents, and spectrographic standards. The writer has seen French and Russian standard steels but has made no attempt to ascertain the present status of these reference materials. Both N P L and the Bureau of Analysed Samples, Ltd., permit their standards to be handled by supply houses, whereas the Kational Bureau of Standards prefers to deal directly with consumers of its standard reference materials and does not desire to distribute them through dealers for resale, The KBS,however, will supply standards to a dealer who serves as a n ordering agency for its clients, but in this case the ultimate consumer is to be named. The analyses given for the various standard reference materials of composition discussed vary from a complete analysis to certification for only one element. At the National Bureau of Standards the tendency is toward a complete analysis, because the position is taken that the more that is known about a standard the more useful it is. For example, it is known that a special absorption train is required in the determination of c a k o n in high-sulfur steels whereas a simple train will suffice if sulfur is in the low rapge of 0.01 to 0.03%. It is therefore useful to know both the sulfur and carbon content I
INTERDEPENDENCE BETWEEN DEVELOPMENT OF STANDARDS A N D DEVELOPMENT OF METHODS
Preliminary requirements in the production of a standard reference niaterial have already been mentioned. The material to be used as a composition standard must be stable, homogeneous, and in a form convenient for the intended use. -4s a sulfur standard a steel is usually proirided in thc form of chips or drillings; as a standard for oxygen it is preferably in the form of a bar. Ore or glass samples are usually provided in the form of fine powders. However, the most important requirement in the development of a composition standard is the method of analysis. It has become customary in this country and abroad to subiect coniposition standards to cooperative analysis by a number of laboratories. Practically all XBS standards are sent to
~
analytical chemists outside the bureau, with the request that they use methods known to yield accurate results. Sometimes methods are furnished, sometimes the analyst is requested to use a n y method t h a t he deems satisfactory, and sometimes a n entirely satisfactory method does not exist. It is obvious that grave difficulties are encountered in the issuance of a standard reference material if methods for its analysis are not available. Irons and steels represent a class of materials for which satisfactory methods of analysis have long been available; methods for the analysis of certain refractory and ceramic materials are in existence, but the results olrtained by their use stili leave soirietliiiig t o be desired; until very recently alloys containing titanium, niobium, and tantalum could not be put out as standards because accurate nlethods of analysis were not available. A few examples of the relation of avuilability of methods to the issuance of a standard referlmx material should be of interest. The first duty of the bureau is to certify the actual composition of the standard, regardless of the method of analysis-that is, enough work must be done a t XBS or elsewhere to make sure that the values given on the certificate are the best attainable. Wide latitude is allo\ved the cooperating analyst in the choice of a method. It does not suffice to say that a value of 30.007, PzOs was obtained b y Metbpd d and 30,20y, by l l e t h o d B. It is fie b u d e n of the bureau t’o determineA\vhat the correct result is. This is sometimes done a t great cost in time and money, but the record of NBS standard ,reference materials previousiy cited seems ta have made these expcnditures worth while. Fortunately, after satisfac+,ory methods of preparing and analyzing a standard reference material have been developed, the renewal of this same standard is usually a simple matter For sever:d years prior to 1923 there was considerable agitation’vn the part of the fertilizer industry for a standard phosphate rock. -4properly prepared quantity of material was provided by then?, arid its analysis and issuance were undertaken by the Sational Bureau of Standards. Cooperation in the analysis by laboratories outside the bureau yielded the results shown for S o . 56 in Table I. It was impossible to get agreement among the cooperating analysts, and much work was done on synthetic mixtures and pure phosphates which convinced the bureau that the average value for Pz06was about 0.57, high. There r a s no doubt that the correct value \vas different from the most frequent values reported. A certificate giving the recommended value, 31.337, of PzOs, was issued in
Table
I.
~~
~~~~~
Cooperative Analysis of Phosphate Rocks 56 and 5 6 0
KO.56 Sample sent to cooperators Filial certificate printed First results for Y*Os(gravimetric, weighing as ,Ilgd’,O?), 7’
1-25-1937 4-4-1939
31.22
32.93 32.33 33.15 33 20 32 83 32.90 32.90 32 33 to 33.26 32.90 0.21 0.21
31 77.
Av. Spread Recommended value Av. dev. from mean Av. dev. from recommended value
KO.56a
4-17-1923 5-20-1927
31.91 32.29 31.95 31.66 31.80 31.22 to 32.29 = 1 07 31.33 0.25 0 . 5 1 (high)
spite of the over-all average of 31.80%. It was some time before acceptance of this lower value was achieved, but the anall sis in Table I for KO.56a shows that in the intervening ten years better agreement became the rule. Of course, such agreenieiit as is shown for KO. 56a between the average of first results and the reconiniended value is exceptional, but the difference in elapsed time betneen the beginning of the analysis and the issuing of the final certificate is typical of first and subsequent issues of a standard. I n February 1926 in cooperation ~ i t h Committee C-8 of the ASTLI, the NBS undertook to handle the preparation and analysis of three burnt refractories containing approximately 40, 60, and 80% of alumina, rrspectively. Satisfactory methods of analj sis were said to be available, but when the first results obtained by the cooperating analj sts were tabulated, the extreme values shown in Table I1 \I ere indicated. This was a n unsatisfactory state of affairs, and it was difficult for Dr. Lundell to understand why the two constituents, silica and alumina with their simple chemistry, should be so difficult to determine. It is true that the dissolution of these materials is difficult, but after this is accomplished, the chemistry of the silica and alumina determinations should be simple. It id not the purpose of this paper to go into all the causes of the discordant results. These are discussed bj. Lundell and Hoffman ( 6 ) . Suffice i t to say that probably the greatest source of error lay in improper ignition. It should be remembered that furnaces with pyrometric control a t that time were still the exception, not the rule. Many directions for ignition of large precipitates of SiOz and RzO3 still stated, “Ignite to constant weight under good oxidizing conditions in a strong blast.” Considerable research was required on the methods used in the analysis of these refractories, and a paper on the analysis of bauxite and refractories of high alumina content was published ( 6 ) . This, in the opinion of the authors,
=
0.93
was not a paper that contnined a n y radically ncn. niethods but only gave detailed directions and pointed out sources of error. TIE p:ipcr received wide circulation; it n-us reprinted by the Government Printing Office several times and then n-as reproduced by a private firm. It is interesting t o speculate n-ht influence the dissemination of these standard refractories and the methods of analysis had on tlie analytical chemistry of silicates and on standard reference materials. I n the case of phosphate rocks 56 and 56a it was shon-n that in the ten years between the original issue and its renewal quite a n iniprovenient was made in the first results reported. I n the case of the refractories a rcnen-a1 has not been necessary, Iwcause the original supply of material has not been e.uhausted. Hon-ever, a comprehensive cooperative project on the analysis of carefully prepared samples of diabase and granite n’as conducted by the C. S.Geological Survey (22) in 1951, in which it \\-as evident that for similar determinations of silica and alumina the results shon.ed a marked improvement over those obtained in the refractories. For example, the average dcviations of the results from the mean or the extreme deviations were less than half those found in the analysis of the refractories 25 years before. It is recognized that average deviation and extreme values do not define accuracy, but these with further inspection of the tabulated results for the diabase and granite indicate a turn for the better in silicate analysis during the past 25 years. There is no definite assurance that the issuance of the standard refractories (Kos. 76, 77, and 78) by the Kational Bureau of Standards and the publication and dissemination of the methods used in their analysis are responsible for this improvement, but those in charge of the composition standards a t NBS believe that this is so. Among the materials so far discussed there is a notable absence of standards that would serve the analytical chemist in the determination of niobium and VOL. 31, NO. 12, DECEMBER 1959
1937
Table II.
Results Reported in First Analyses of NBS Standard Refractories
Sample 7@ Constitiirnt Si02 A1203
Fed& TiO? CaO IlgO
Hecommended
.iv. of
54.68 3 7 , 67 2.38 2.21 0.27 0.58 1.37 0.38
54.60 37,85 2.52
first results valur, yc reported, yo
KSO Na10 Twelve analysts participated. * Eleven analysts participated. This is a modification of Table I ( 6 ) .
Extreme values,
AV . deviation,
70
Recommended value, %
5 3 . 0 to 55.0 3 7 . 5 t,o 40.8 0 . 8 5 t o 3.69 0 . 1 0 t o 2.70 0 . 1 1 to 0 . 9 5 0.08 to 0 . 7 1 0.87to 1.43 0 09 t o 1.66
0.43 0.76 0.37 0.35 0.21 0.23 0.26 0.53
'20.69 69.97 O.i9 3 . :37 0 38 0.51 2.83 0 53
70
Sample 78b Av. of first results Kstreme values, 07 reported, yo /C 20.73 1 8 . 3 t o 21.6 70.22 60.2 to74.4 1.16 0 . 6 3 to 3 . 3 8 0.17 t,o 3 . 6 0 0 . 2 8 to 0 . 6 5 0 . 0 3 to 0.70 1.84 to 2 . 9 9 0 . 3 2 t o 1.72
AV.
devia t ion, c
0 51 1 68 0 43 0 59 0 09 0 19 0 66 0 47
I'
Table 111.
Determination of Niobium and Tantalum in Titanium Metal by Ion Exchange Method
(First results) WA-75
.Ka4-74
Sh, Analyst
CC
2 01 1 98 :3 2 01 4 1.99 5 1.98 6 1.9; 7 2.04 8 2.02 9 1,900 10 1 .97 11 2.05 12 1.97 Av. 1.99 Av. deviations 0 . 0 3 1
2
T?
Scb ,
1 00 1 00 0 98 0.99 1 .oo 1 .00 0.99 1 02 1.01 0.96 1 .OR 1.01 1 .00 0.015
2 72 2 72 2 72 2.72 2.68 2.66 2.73 2.81 2,376 2.70 2.78 2.67 2.69 0.06
C
1
10
W A-7 6
Ta,
Sb,
0 39 0 40 0 38
0 58 0 59 0 58 0.59 0.60 0.0P 0.57 0.66 0.53" 0.60 0.60 0.57 0.54 0.09
c;o
0.40 0.41 0.39 0.38 0.42 0.41 0.33 0.41 0.41 0.39 0.016
CY /O
Ta,
