Recommended inorganic chemicals for calibration

Inorganic Chemicals for

John R. Moody, Robert R. Greenberg, Kemeth W. Pial, and Theodore C. Rains Inorganic Analytical Research Division Center for Analytical Chemistry National Institute of Standards and

Technology (NET) (Formerly National Bureau of Standards) Gaithersburg. MD 20899

All analytical techniques depend on the use of calibration chemicals to re- late analyte concentration to an instrumental parameter. A fundamental component ip the preparation of calibration solutions is the weighing of a pure chemical or metal before preparing a solution standard. The analyst must be assured that the purity, stoichiometry, and assay of the chemical are known. These terms have different meanings, and each has an important influence.

This REPORT is intended to assist the analyst in the selection and use of chemical standards for instrumental calibration. Purity, stoichiometry, and preparation of solutions for different purposes are discussed, and a critical evaluation of the best materials available for each element is presented for use in preparing solutions or calibration standards. Information on the

I - chemical form, source, purity, drying, and appropriate precautions is given in Table I. In some cases, multiple sources or chemical forms are available. Cer- tain radioactive elements, the trans- uranic elements, and the noble gases are not considered.

There is a subtle difference between the two twesof calibration solutions-

more accurate than -0.1%. Accurate assays of elements in compounds are not easily obtained. The second method is to determine the assay of one or more elements and infer the concentration of the last element by subtraction from 100%. The uncertainty in the concentration of the last element is increased because of the uncertainty

those foiassay standards and those for matrix matching-commonly used in the laboratory. Assay solutions can be less expensive to prepare than matrix- matched calibration solutions; the latter often require the highest available purity for the matrix component in order to avoid uncertainty in the analyte concentration. Because high-purity materials may be expensive, the analyst must balance the cost against at- tainable accuracy. Occasionally, there is no way to prepare a calibration solution with the required accuracy. By re- viewing the principles discussed in this REPORT, analysts can better evaluate the accuracy of their calibration solutions.

from each component element. Sub- traction from 100% also does not account for species that are not determined (e.g., HzO, Con, and Sod. Therefore, the actual assay of an element in a high-purity compound can be lower than that calculated by subtract-

r----r ing all other constituents from 100%.

For metals, the easiest route is to use a spectroscopic or other multielement technique to determine the impurities in the metal. The results are expressed as “total metallic impurities” or “total impurities” and usually are given as an upper limit (e.g., <10 pglg). When these impurity levels are subtracted from an assumed 100% (for the compound ormetal),a so-called metalpuri- t y is derived. For example, Fe metal with <10 pg/g impurities is equivalent

Interpretation of purity claims There are two ways to establish the elemental assay for a compound. The first way is to obtain the assay of each element directly by a suitable method. Usually these assays are not much

This article not subject to US. copyright Published 1988 American Chemical Society

ANALYTICAL CHEMISTRY, VOL. 60, NO. 21. NOVEMBER 1, 1988 - 1203A

to 99.999% Fe. This is usually short- ened for very high purity metals by ex- pressing the number of 9’s of purity. (The Fe metal example would he five 9’s pure.) However, most analytical techniques do not measure elements such as C, H, 0, and N, which can exist as water, hydrocarbons, or gases dissolved in the sample. A “pure” Fe rod that measured 10 Sg/g metallic impurities could have 20 or even 50 pgfg oxygen present. Hence the estimation of the assay by subtraction of the measured impurities from 100% can provide inaccurate results.

When comparing sources of high- purity materials, the analyst must be aware of information that is being transmitted by the specification of the materialas wellas informationthat has been left out. With pure metals, for example, the dissolved gases are rarely determined, and therefore it is difficult to know just how much to allow for uncertainty in standard solutions that are made from pure metals. Generally, manufacturers’ claims are reasonable, and one can count on a six 9’s metal being better than a five 9’s metal with respect to the impurities measured. But this is not always true, because the uncertainty in the impurity measure- ment is relatively large. Finally, the analyst should be aware that vendors typically sell products made by another company. Thus, even if it were possible to endorse a particular vendor, there would be no assurance that the same material would he available in the future. For these reasons, all commercial identification in this survey is avoided.

Physical form of high-purity metals For most elements, high-purity metals are the best source for assay standard solutions of high accuracy. The use of a pure element instead of a compound eliminates stoichiometry as a factor in calculating the concentration of the standard solution. In addition, drying procedures usually are not required for metals prior to weighing, whereas they are essential for most compounds. Sol- uble transition metal compounds generally exist as hydrates and cannot be used for preparing assay standard solutions because a sufficiently accurate hydrate form cannot be obtained. Hence, the use of the corresponding metal is essential if a high degree of accuracy is required. The only uncertainties for metals are associated with the dissolved gases, which for some metals could change with time.

In the preparation of assay standard solutions from metals, the physical form of the metal must he taken into account. High-purity metals are available in several forms, including powder, sponge, sheet, wire, rod, shot, and drop. Those forms with a high ratio of surface area to volume are generally

L

least suitable for the preparation of assay standard solutions. Powder samples are worst in this respect and can he difficult to dissolve quantitatively because of passivation or poor wetting of the small particles. Sponge presents similar problems after partial dissolution and breakup of the sample. For- mation of surface oxide layers on the metal is a surface-controlled process and consequently occurs to the greatest extent with powder and sponge samples.

Sheet, wire, and rod samples must be cut to the desired size before dissolv- ing. Sheet metal, wire, or bolt cutters are required for most metals, although certain soft metals (e.g., In, TI, Au) can be cut with a knife. After cutting and before weighing, the sample should be cleaned (pickled) with a small portion of the acid (diluted if necessary) used to dissolve the sample; this removes any contamination from the cutter and also removes surface oxide. Inspection with a microscope may be useful in de- tecting surface films. In some cases, such as TI, a thick layer of surface oxide forms after extended storage in air and must he scraped off before pickling and weighing. Some of the more active metals (e.g., rare earths) are available in sealed ampoules in an inert atmo- sphere, which prevents the formation of surface oxide. However, weighing samples that come in ampoules can he difficult.

Purity vs. stoichiometry of compounds Most compounds are nonstoichiometric to some degree. They are sold with the same type of information on impurities that was discussed for pure metals. Purity and stoichiometry are not related, however. The purity claims usually do not take into account ab- sorbed COz, Hz0, or nonstoichiometry (e.g., KHSOa in K2S04). For example, high-purity NaCl could have <10 cg/g of impurities, yet still have an assay for Na that is in error by 4.5%. The vendors are usually explicit hut selective with their analytical information. Of- ten, the selection of a poor compound can be traced to the analyst’s incorrect interpretation of the available analytical information. Purity and stoichiometry are distinctly different aspects of chemical compounds.

Compared with high-purity metals, compounds that are assayed for one or more constituents may he better sources for assay solutions in some circumstances. For example, the alkali metals require special conditions so that they are handled without the formation of oxide layers. However, compounds generally are not as pure as metals and usually are less desirable if a pure metal is available. Both oxides and carbonates me available in fairly high degrees of purity, but both suffer from varying degrees of nonstoichiometry-for example, traces of Ca(OH)z, CaO, or CaHC03 in CaC03. Usually the stoichiometry is assumed, but the analyst should not prepare solutions from compounds that have not been proven stoichiometric (for the given lot).

Different compounds may be a per- cent or more in error from stoichiometric composition as received. Carbon- ates can be ignited under COz to elimi- nate some of the uncertainty. Some oxides (e.g., those of the rare earth elements) may also need ignition in 0 2 to achieve a more stoichiometric composition. Thermogravimetric analysis can provide information about ignition temperatures for achieving stable weighing forms ( I ) . However, there is no assurance, especially with oxides, that stoichiometric compounds result.

Other salts, such as the sulfates, may actually be mixtures (e.g., sulfate and bisulfate). The solution to these nonstoichiometry problems is less clear. For instance, recrystallization of the compound may he required. In these cases, an ion chromatograph can be a valuable tool in searching for anionic impurities that are indicators of nonstoichiometry.

Compounds as a class are less desirable than metals because the problem of stoichiometry is extremely difficult to address. Furthermore, the magnitude of the problem is typically 10

1204A ANALYTICAL CHEMISTRY, VOL. 80. NO. 21, NOVEMBER 1. 1988

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times more severe than the problem of dissolved gases in metals.

Assay wlutlons As implied, these solutions are intended for use in elemental assays, at per- cent or trace levels. The uncertainty in the calibration solution must be negligible when compared with that of the entire analysis. For example, assuming an uncertainty of 0.1% for the analytical technique and allowing a factor of 10 less uncertainty for the standard, i t is apparent that a metal with a purity of 99.99% (*O.Ol% uncertainty assumed) would he adequate. Because few analytical techniques have uncertainties below 0.1%, a metal of this purity is sufficient for most analytical work.

Indeed, to achieve the 0.01% level of uncertainty in the preparation of a solution, we would have to employ gravimetric dilutions. Volumetrically prepared solutions usually have an uncertainty not better than 0.1%. In general, when metals are available, i t is easy to prepare a standard solution gravimet- rically with much greater accuracy than is needed. Metals and some chemicals (such as Na2C08) are available a t much higher levels of purity than are usually required. Except for cost, there is no reason not to use the purest materials. However, there is little benefit in preparing a standard solution that is several orders of magnitude more accurate than needed. If calibration solutions are a factor of 10 more accurate than the method for which they are to he used, the uncertainty from the calibration chemical will be negligible.

Matrix matching Some analytical methods require that the analyte standard he prepared in a sample matrix form that simulates the sample insofar as major constituents are concerned. This can complicate matters considerably, because a trace amount of analyte can result from impurities in the major constituents. For example, trying to prepare a 1 pg/g by weight Cr standard in a 10% by weight Fe solution could he extremely difficult because Cr contamination in the Fe matrix could easily exceed 1 pg/g by weight. Here, the purity of the matrix determines the achievable accuracy in the analyte element. Because this is a general problem with matrix-matching solutions, materials (oxides, carhon- ates, or metals) are identified in the table that are particularly suited as matrix-matching materials. Some of these may not be exactly stoichiometric and thus are not recommended for use as assay materials for analyte calibrations.

For matrix matching, purity (as measured by analyte impurity level) is the single most important factor. Whereas

some uncertainty in the concentration of the matrix elements can he tolerat- ed, a significant impurity level for an analyte element is unacceptable. One can predict the necessary purity of the matrix element from the quality of analyte element data that is required. For example, preparation of a 10 pglg V standard in a 10,000 pg/g Fe matrix with an uncertainty of f0 .1 pg/g V re- quires an Fe matrix containing <0.1 pg V per 10,000 pg of Fe. This corresponds to 1 part V in 100,000 parts of Fe or five 9’s pure Fe (with respect to V). Because of this effect, in certain cases the available purities of the matrix element do not suffice for the intended use. In other cases, a material can be both pure and stoichiometric and thus can function either as a source for assay (analyte) or matrix- matching solutions. This guide should enable the user to predict the likeli- hood of success in preparing a particular matrix-matching solution.

Mid analyte solutions The main focus of this REPORT is the preparation of primary calibration solutions. There are, however, multielement techniques that require suitable multielement standards for calibration. Without going into the me- chanics of how such solutions are prepared, it is useful to note the additional requirements for these solutions. If an extreme ratio of two analytes is required in the standard (e.g., 105-loS), then the requirement for purity of the major analyte becomes as stringent as that discussed previously for matrix- matching purposes.

Even where the analyte ratio is 1:1,

analyte impurities from other constituents in the mixture must be negligible. In certain circumstances, this can become difficult or nearly impossible to achieve.

Dissolution of calibration materials Although the CRC, Lange, Meites, or other handbooks may be consulted for advice on how to dissolve a particular material, sometimes the suggested methods may not work. Many ultra high purity samples are more difficult to dissolve than their less pure forms. The analyst should experiment with a small portion of the material first to see if it will dissolve by the usual methods. If it does not, some other method must be tried, such as a mixture of acids or perhaps with the addition of a drop of HF or H2S04. Each of the authors has had different experiences in this re- gard, and it is difficult to offer broad advice because the same material may behave differently when obtained from different sources.

In general, most metals are readily dissolved in 1:l HN03 or 1:l HCI, or a mixture of the two acids with heating if necessary. In certain cases, more dilute acids must he used to avoid loss of sample from overly rapid dissolution. Con- centrated acids, notably HN03, can present problems with some metals (e.&, Cr) that are passivated and do not dissolve. High-purity metals are particularly susceptible to passivation compared with normal metals and al- loys. Frequently, dissolution com- mences suddenly after depassivation, following a period of apparent inert- ness. Passivation is less frequent in dilute acids, particularly HCI. In certain applications the choice of the acid for dissolution may be dictated hy the analytical technique being used.

All metals should be regarded with suspicion in their “as received” condi- tion. Metals should be etched to remove surface impurities and oxide layers. The etching solution may he the same as the dissolution mixture hut somewhat more dilute. The metal should he etched until an oxide-free surface is obtained, then the sample should he rinsed copiously with distilled water. Without this rinse, one might he left with chloride or nitrate salts on the metal surface. The more reactive metals may need to be rinsed with a solvent to remove the water and quickly placed in a vacuum desiccator to finish drying. The use of a vacuum desiccator greatly reduces the risk of such an oxide film forming during the drying step. When dry, some etched metals may take on a matte finish, which in some cases could be mistaken for an oxide layer. The metal should be weighed promptly when dry.

For most metals, the suggested mini- mum weight of a cleaned, dry piece is

1206A ANALYTICAL CHEMISTRY, VOL. 60, NO. 21. NOVEMBER 1. 1988

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-200 mg. With this size sample, the eye can detect significant amounts of surface salts or oxides for most metals. As larger samples are taken, the mass-to- surface-area ratio improves and surface oxides become less of a problem.

Preparation of solutions Once the salt or metal is cleaned, dried, and weighed, i t must he quantitatively transferred to the dissolution vessel or storage container. There are a number of ways to do this, hut whatever process is used must be quantitative and must not cause contamination. For all but the most concentrated solutions (-1 mg/g), Teflon, polypropylene, or poly- ethylene vessels should be used, depending on the reactivity of the dissolution mixture. The sample material is then dissolved by an appropriate method. The remaining steps largely depend on the method of solution preparation used (e.g., volumetric or gravimetric). The storage of calibration chemicals as dilute solutions (1 nele or less) is not . . _ I recommended.

With metals and carbonates a laree tivelv retained.

I ' i r D O 0

amount of gas will be formed, and"a mist that can carry concentrated sample out of the dissolution vessel could be produced. Hence some means of condensing or trapping the mist must be provided. This can be as simple as a Teflon lid on a beaker or a hollow poly- ethylene stopper loosely fitted in the mouth of a Teflon hottle. When the dissolution is complete, the various de- vices can he rinsed into the dissolution vessel so that the sample is quantita-

The total acid concentration is then adjusted, and the sample is diluted to the desired point. Volumetric procedures are familiar to everyone and are not discussed here. However, many analysts are not aware of just how inaccurate such dilutions are. A lab supply catalog should be consulted to determine the tolerances for various pipets and volumetric flasks. Volumetric errors can be quite large, and even an experienced analyst will have trouble

exceeding one partin a thousand precision-even when using calibrated volumetric ware (2). Equally revealing is an examination of the temperature coeffi- cient of volumetric solutions. It is not hard to induce an error of 0.1% just from poor temperature control.

It is for these reasons that gravimetric dilutions are so attractive. Whereas one might he able to reproduce a 1-L volumetric dilution to f0.3-0.5 mL, one can easily weigh 1 kg to fl mg reproducibility, or even better, depending on the halance used. Similar advantages exist for all gravimetric op- erations when compared with corresponding volumetric ones. Even if an inaccuracy of *0.25% were tolerable, which is often the case, such errors can propagate considerably when serial dilutions are used to prepare a dilute solution from a concentrated standard. Gravimetric dilutions are easy, require no special techniques, and can yield ex- cellent accuracy when a number of serial dilutions are made.

Because the results obtained are in micrograms of analyte per gram of solution, the concentration is indepen- dent of temperature. If a volumetric unit is desired, it is only necessary to determine the density, which is easy to do. The final advantage to those who prepare and are responsible for calibration solutions is that solution concen- trations can be monitored by changes in the storage weight of the bottle; a loss in weight signals an evaporative loss over a period of time. This gives analysts an extra quality check on their

Table 1. Recommended corn Group I

LI SRM 924 (LizCOo)

Assay based on carbonate lciulated from impuriti

Assay standard solutions Assays based on CI: Na come

Purity based on metallic impurities

I Z W A ANALYTICAL CHEMISTRY, VOL. 60. NO. 21. NOVEMBER 1, 1988

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ANALYTICAL CHEMISTRY, VOL. 60. NO, 21, NOVEMBER 1. 1988 * 1209A

calibration solutions. For the most careful and accurate

work, the analyst must correct sample weights for the buoyant effect of air. This vacuum correction is most significant for chemicals and objects with densities greatly different from those of the balance weights, which typically are 8.0 g/cm3. Some balances may employ weights that are not exactly of density 8.0 but have an adjusted apparent density of 8.0. In either case, follow the balance manufacturer's instruc- tions. Electronic balances may or may not have a factor for the balance weight correction, depending on how the balance has been calibrated. The buoyan- cy factor for the sample will still apply. The following equation can be used for these corrections, and the analyst should make the calculation at least once to see if a significant correction exists.

where w, is the weight in vacuum, in g; w. is the weight in air, in g; Do is the density of object being weighed, in g/cm3; D, is the density of balance weight (8.0 is usual), in g/cm3; and 0.0012 is the density of air, in g/mL at average temperature. pressure, and rel- ative humidity (for sea level, 45' lati- tude).

Recommendedc~npoundsormetak Group I: The alkali metals Many different salts of these elements are available. Pure forms of the ele-

I 'r -1 ments are either not availahle or im- practical to handle in the general analytical laboratory. In trying to decide which salts to recommend, we have considered the tendency of the salts to deliquesce and the available purity of the salts as well as the best evidence of stoichiometric accuracy of the commercial salts. As a group, the carbonates are most

likely to be easily available a t a high level of purity. These compounds may not be stoichiometric, because they may contain traces of oxide or bicar- bonates. Spectrometric analysis of metallic impurities does not provide information about problems with stoichio-

metry. The best use that can be recommended for the carbonates is for matrix matching where the alkali metal is an analyte.

Chlorides are often the best materials from a stoichiometric point of view (all of the available NIST standard reference materials [SRMs] for alkali metals are chlorides). Unfortunately, these compounds are not necessarily stoichiometric, and conditions for drying the chlorides may vary from one production batch to another. Of all of the salts listed for Group I, only KC1 (SRM 918) has actually been assayed for both K and CI. With all of the other salts, the stoichiometry is assumed to be correct and the assays are based only on the chloride content. This is unde- sirable because the uncertainty in the assay for the alkali metal can only be estimated. With commercial materials, the same problem exists. Salts such as nitrates or sulfates should have similar and potentially larger uncertainties in the assay for the alkali metals.

Lithium may exhibit isotopic varia- tion for which the NIST reference material can be used as a quality control for the atomic weight of the element. Variance in isotopic composition is a special problem for the lighter elements such as Li and B.

&np I/: The alkalina earth metals Only two elements from this group, Be and Mg, are available as high-purity metals, and Be is only available at 4 9 . 9 % purity. AU of the remaining materials are carbonates, except mag-

~~

Purity based on metallic impuritles

Mg SRM 929 (Mg gluco- 100.1 * 0.4% Clinical standard, not best available nate) 5.403 * 0.022% Mg Dry 24 h over Mg(CIO&

Commercial Mg metai fi Assay anomalies have beer between different sampll metal

crosscheck Suggest use of multiple sources to

Use as is-no drying

Usermust determine stoichiometry Dry at 200 OC for 4 h in CO1

l p t e to establish stoichiometry Dry 1 hat 110°C

Commercial materials may be -1 % nonstoichiometric

Dry at 200 'C lor 4 h May need ignition to achiek

Stoichiometry not asswed stoichiometry

.... .̂ *nn 0- .^_ . L 1210A ANALYTICAL CHEMISTRY, VDL. Bo. NO. 21, NOVEMBER 1. 1988

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ACME CHA h *DE-FASTENERS AEPOPPOGrCTS 1 RFO . BA R J A I \ A I > T CAL hSTRUMENY BA RI)DFT CA..SISTEMS 3AR6SDLECOh lROS 60STOhGiAR.CEC hSTilUMthTS C E h l a l E C - COhl?O.E* CDhT-RP DELAIA.CC\I.E\SER DEAVA.nlDR0P.W DEAA(A.SlOW DEAVA.l.RBhE UEAVA.T.RBhEGMBH

DE-ROID nOPM GEAP I UE.TI I EhTERPF. SE F NCOR i.ECTROh CS GEMS SENSORS HEM BEAR hGS I UO AB IUO PUMP MAIRCMEM PROD.CTS M .LER - 0 I W A R T n MCRSC CONlROLS MORS& COh-ROLS r l D R A STEPnEh 8 COMPAlhV .TU T R A M \SlRbMENTS *EKA PIG WlGG hS CONNECTORS

Second row transltlon metals Element SourcefCompound

Y Y metal

Assay and matrix matching

Matrix matching 1's Assay

nu nu m e n

Rh Rhmetal

Pd Pdmetal

Ag Agmetal

SRM 748 (Ag metal)

Cd Cd metal

Assay and matrlx

Assay and matrix

Assay and matrix

Assay and matrix

Assay and matrix

Assay and matrix

Assay and matrix

matching

matching

matching

matching

matching

matching

matchir

Comments

Available in high purity but expensive

Oxide stoichiometry must be

Ignite 2 h at 700 OC

Dissolved gases not known-avoid

May contain Hi

Assay based on impurities

Dissolved gases not known Oxide stoichiometry not assured



Dissolved gases not known





determined by user

sponge

sponge

sponge

1212A ANALYTICAL CHEMISTRY, VOL. 60. NO. 21, NOVEMBER 1, 1988

‘S

Matrix matching

ASMY Sam as above

Majw impurity is Zr; beware of purity statements that do not include Zr

Assay and matrix

Assay and matrix

Assay and matrix

Assay and matrix

Assay and matrix

Assay and matrix

Assay and matrix

Assay and matrix

Assay w matrix matching Assay

Assay or matrix matching

matching

matching

matching

matching

matching

matching

matching

matching

No estimate of dissolved gases



NO estimate of stoichiometry

Easily volatilized on dissolution



Very pure but relatively expensive

No estimate of dissolved gases Not best material for the cost Dissolved gases (oxygen) are proba-

bly low-oxides form a film at just a few ppm

(contmnwdonp. 1214AI

nesium gluconate (NIST SRM 929). The only advantage of this latter material is that it has a large gravimetric factor, but this is offset by a high uncertainty (f0.4%) in its assay. The comments on carbonate stoichiometry for Group I elements apply equally well for Group I1 elements. Unfortunately, there are no other salts of these elements that can he recommended. The transition metals Virtually all of the transition elements are available as pure metals, which sim- plifies the gewral recommendations on compound selection and dissolution. A few elements, notably Sc and Y, may be more readily available as the oxide. However, the stoichiometry of oxides is not sufficiently known and is of dubi- ous value for calibration purposes. In these cases, a titration or other assay may be necessary to determine the metal content of the compound.

We have also recommended a compound, (NH4)20sCle, for osmium because the metal itself is easily volatilized on dissolution. Again, little can be said of the stoichiometry of this compound unless an assay for osmium is performed. Mercury also forms volatile compounds, but most analysts appear to be familiar with these problems.

The lanlhanides andactinides

Uranium and thorium are both available from several commercial sources as well as in the form of Certified Ref- erence Materials from the New Bruns- wick Laboratory in Argonne, IL, and the Central Bureau for Nuclear Mea- surements in Geel, Belgium. The atomic weight of these elements may vary, depending on the source, and the oxides (such as U308) may have to be heated in oxygen to a high temperature to achieve certified compositions. Commercial oxide materials may not have known stoichiometries.

The rare earths are usually obtained as the oxides because the other salts tend to be less pure and are often deli- quescent. These oxides have the same properties as U308 and may need ignition in oxygen to an elevated temperature to achieve a reproducible composition. Again, there is no way to be sure of the metal assay of these compounds except by an actual assay (e.g., by titration).

The pure metals are available to a limited extent from commercial sources and are reactive toward oxygen and water; a dry hox may be necessary to handle the metals adequately. The Ames Laboratory (Ames, IA) supplies

the lanthanide metals (as a group) preweighed in vacuum-sealed ampoules, which effectively solves the weighing and handling problems for these metals. The metals from Ames Laboratory are of exceptional purity but are expensive for routine usage.

Group //I elements Except for boron, all of these elements are available in weighable forms as the metal. For boron, the use of boric acid is recommended. A suitable NIST reference material, SRM 951, is available that is certified both for the assay and for the isotopic composition of boron. NIST SRM 951 is to be used “as is” after exposure to room air of -35% rel- ative humidity for 30 min and should not be dried. An abundance of commercial sources exists for the other elements in this group.

Grwp IV elements These elements range in character from metals such as Ph and Sn to the nonmetal carbon. Carbon is available in high purity as graphite powders for use in electrodes for sample sparking. Because it is a nonmetal and is not determined by the usual spectroscopic techniques, we make no mention of recommendations for standardized solu-

ANALYTICAL CHEMISTRY, VOL. 60. NO. 21. NOVEMBER 1, 1988 1213A

Lanthanldes and acllnldes Element SourcelCompound pwny UW Comments

Assay High purity available but expensive La Lametal more than three 9 s La203 mwe than four 9 s Matrix matching Problems: drying, exact stolchlw

Ce metal more than three 9's Assay mew

High purity available but expensive know stolchiomet

'r metal

mcfe than three 9 s

more than three

re than four 9's

h o 2 ore than four

Assay and matrlx

issay and matrlx hing

NO estlmate of dissolved gases or

Exact composition of Thoz depen- matching atomic weight

- Element SwrceICompwnd Purlly

B SRM 951 (H3B09) 100.00 0.01

Ai Almetal

SRM 1257 (ai metal) more than four 9's Ga Gametal m e than five e's

uea

lsotoplc standanl, assay

Away and matrlx matching

Same as above Assay or malrlx matching

SRM 994 (Qa metal) more than five e's bolcplc stand&

In In metal A E ~ Y w mahlx matchino

Do not dry, open to r w m alr (-35% RH) fw 30 mln

No estimate of dissolved w e 6

NO Bstlmate Of dissolved gases EaSllY S u p e r ~ i e d

Available In llmlted quantkles

NO eStlmate Of d lE~lved gases Same as above

Pur& based on lmpwirias

1214A ANALYTICAL CHEMISTRY. VOL. 60, NO. 21, NOVEMBER 1. I888

Reductimetrlc standard tssay fw As not cer k! estimate of dlsso

tions. Carbonate salts might be a useful approach for solutions of known carbon contept, as might certain organic compounds such as formic acid, su- crose, or ethanol.

Although silicon is readily available in very high grades of purity as the metal, i t must be dissolved in HF + HN03 and can be lost as SiF4 during dissolution. Thus, to prepare a solution of Si, many analysts resort to compounds of lesser purity and uncertain stoichiometry. Because of these problems, we do not have a specific recom- mendation for Si.

The remaining elements, Pb, Sn, and Ge, are all available as high-purity metals and should present no difficulty to the analyst. Lead exhibits natural vari- ations in its atomic weight because of the varying amounts of radiogenic lead.

NIST SRM 981 is a high-purity lead of defined atomic weight and should be used where solutions of the highest accuracy are needed.

G m p V elements Although nitrogen is a gas, i t is often determined by analysts as NO,, NH3, or NH:. Nitrogen gas, of course, is also available in cylinders a t high purity and is appropriate for calibrations using Nz gas. Freshly distilled nitric acid is a good high-purity source for NO;; however, other oxides of nitrogen may he formed in time, particularly if exposed to light. Nitrate salts do not make good weighing forms, whereas a simple acidimetric titration of HN03 (or even a density determination) will give an accurate measure of the NO; content. Pure NH&I can be prepared

ANALYTICAL CHEMISTRY, VOL.

from ultrapure HC1 and NH3. Al- though the solid material can be isolat- ed, it is best to prepare the solution of NH&l by mixing known amounts of HCI and NHaOH.

Phosphorus, another important nonmetal, is commonly determined as a phosphate. High-purity P205 is used at NIST as a source for high-purity H3P01. PzOa is also a good desiccant; it is hygroscopic and hence is difficult to weigh. Any PzOs used for calibration purposes should be taken from a sealed ampoule and used in its entirety to make a single batch of H3PO4. A number of phosphorous salts can be used, but all suffer from some uncertainty in their stoichiometry and their purities generally are less than for chlorides and nitrates. (NIST offers SRM 194, NH4H2P04, which is not certified for P

60, NO. 21, NOVEMBER 1. 1988 1215A

content although it is often used for this purpose.) Another possibility would he phosphoric acid, which has been titrated to the first two end points. If these agree well and other impurities are absent, the POY3 concentration can be calculated accurately.

As, Sb, and Bi are all available as high-purity metals that are recommended for use in preparing calibration solutions. SRM 8Sd, Asz03, is widely used along with other commercial sources. However, SRM 83d is pri- marily used as a reductimetric standard and is not assayed for As. The purity of the As metal is also better than that of the oxide.

Group VI elements In this group, Se and Te are available as high-purity metals that are recommended for use in preparing calibration solutions. Elemental oxygen and sulfur are both available in relatively high purity, but the elemental forms may be inconvenient to use for many

applications. Distilled water free of dissolved gases is a good source for known amounts of oxygen. For sulfur, a number of compounds can be used to provide calibration standards, but all carry some risk of nonstoichiometry. For salts such as KzSOa, an analysis must provide the necessary assurance that no bisulfate or other foreign anions are present.

Group VN elements The halogens are usually available in high purity as the element. Iodine, for example, is easily sublimed to provide pure material. However, there are usually drawbacks to the use of the pure element, such as converting the element to the proper oxidation state and also handling volatile samples (Fz, Clz, Brz, 12).

For each element, there are one or more salts, such as NaX or KX, that are good candidates for use as calibration chemicals. Inexact stoichiometry is usually a problem with these materials,

but it is fortunate that many of them are assayed on the basis of the halide. For example, NIST SRM 919 is certified based on the assay for chloride. A t the present time, these salts are the best materials to be recommended, particularly for methods such as ion chromatography. The potassium salts are usually the least hygroscopic except for the fluorides, where NaF is the least hygroscopic.

Summary We have tried to elahorate on the important considerations for an analyst in preparing calibration solutions. Al- though we have recommended the best compound or source for an element, there are many elements for which there are no good choices for a weighable form of the element. The choices indicated in this REPORT represent our collective experience, hut different compounds may be preferred. We have indicated only those compounds or elements for-which primary standard cali-

Comments

No good direction on drying Stoichiometry not demonstrated

NEed to dry

Dry 24 h over Mg(CIO& Assay based on CI- Assay based on CI- Ignite at 500 OC for 4 h Assay for CI-

Need method to dry and demonstrate

Difficult to work with Convell to useful form

Very pure but dlfflcult to dissolve

Useful sources for I- or IO; In hig

stolchlometry

19 w l t y

itolchlometry

1216A ANALYTICAL CHEMISTRY, VOL. 80, NO. 21, NOVEMBER 1, 1988

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ANALYTICAL CHEMISTRY, VOL. 60, NO. 21, NOVEMBER 1, 1988 1217A

PAT PENDING

Now in two sizes, 23 ml and 45 ml. The speed and convenience of microwave heating can now be applied to the digestion of inorganic, organic, or biological materials in a Teflon Lined Bomb. The new Parr Microwave Digestion Bombs have been designed to combine the advantages of closed high-pressure and high temperature digestion with the requirements of microwave heating. Many samples can be dissolved or digested with less than one minute heating times. As with all Parr Digestion Vessels, careful design and test- ing effort have gone into the safety and sealing aspects of this unique vessel and operating environment.

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bration solutions can be prepared. The significant difference between

this article and earlier works (3-6) is that we have attempted to evaluate the suitability of a material for use in calibration. Earlier works have confused purity with suitability.

References (1) ,Duval, C. Inorganic Thermogravimet-

ne Analysis, 2nd ed.: Elsevier: Amster- dam, 1963.

(2) National Bureau of Standards. Techni- cal Report No. COM 73-10504,1959; Na- tional Technical Information Service,

Springfield, VA. (3) National Academv of Sciences. “Ret”

on the Conferenc; on Chemical Com- pounds of Certified High Purity”; De- partment of Commerce Office of Techni- cal Services: Washineton. DC. 1959 Doc- . , , ument PB166354. L.

(4) Degens, P. N., Jr. (Ed.). “IUPAC Draft Report on the Purity of Laboratory Chem- icals”; Amsterdam, 1965.

( 5 ) Michaelis, R. E. (Ed.). “Report on Available Standard Samples, Reference Samples, and High Purity Materials for Spectrochemical Analysis”; ASTM Data Series DS-2; 1963.

(6) Smith, B. W.; Parsons, M. L. J. Chem. Edue. 1973,50,679-81.

John R. Moody (top left) is a research chemist in the Center for Analytical Chemistry at the National Institute of Standards and Technology (NIST). He received a B.S. degree in chemistry from the University of Richmond in 1964 and a Ph.D. from the University of Maryland in 1970. He joined the analytical mass spectrometry group a t NIST in 1971. Moody’s current research interests are in the areas of sample preparation, chemical separations, purification, and ultratrace analysis.

Robert R. Greenberg (top right) is the deputy group leader of the Nuclear Methods Group in the Inorganic Analytical Research Division of NIST. He received a B.S. degree from Brooklyn College (New York) in 1971 and a Ph.D. from the University of Maryland in 1976. His research interests include investigations into improving the accuracy and precision of measurements made with instrumental and radiochemical neutron activation analysis and in the ultratrace determination of essential and toxic elements in biological materials.

Kenneth W. Pratt (lower left) is a research chemist in the Center forAnalytica1 Chemistry a t NIST. He received a B.S. degree in chemistry and a B.A. degree in Russian from Lafayette College in 1976 and a Ph.D. in analytical chemistry from Iowa State University in 1981. He spent I -% years as a staff engineer a t IBM Instruments, Inc. and was a National Research Council-National Bureau of Standards postdoctoral research associate before joining NIST in 1984. His research interests are in the areas of electroanalytical chemistry, sample dissolution, chemical instrumentation, separations, and history of science and technology.

Theodore C. Rains (lower right) is a senior research chemist in the Center for Analytical Chemistry a t NIST. He received a B.S. degree in chemistry from Eastern Kentucky University in 1950 and did graduate work a t the University of Tennessee (1957-62). Afterspendingmore than 13years a t the Oak Ridge Nation- a l Laboratory, he joined NIST in 1965, where he has served as group leader of atomic absorption and emission spectrometry. His research interests are in the areas of sample preparation, chemical separations, preconcentrations of trace elements, atomic absorption, and emission spectrometry.

1218 A ANALYTICAL CHEMISTRY, VOL. 60, NO. 21. NOVEMBER 1. 1988

Documents

Recommended inorganic chemicals for calibration