8. W. Smith and M. L. Parsons1 Arizona State University Tempe, 8.5281
Preparation of Standard Solutions Critically selected compounds
It is often necessary in chemical analysis, especially trace elemental analysis by flame spectrometric methods, molecular absorption techniques, etc., to prepare solutions containing known concentrations of given elements. In many cases it is difficult to make a choice as to what compound is best suited for these solutions. No complete. . . up-to-date tabulation of well-suited compounds has been found in the literature. General textbooks of analytical chemistry occasionally give lists of primary or secondary standards often used in acid-base and oxidation-reduction titrations, hut do not discuss the preparation of solutions for flame analysis. A few references are available (I, 2) which give details for preparing a great many chemical reagents, both organic and inorganic. One of these (1) contains a brief listing of how to prepare solutions for some 29 elements. It is felt that a more complete listing of good choices of compounds to be used for preparing solutions of the elements would he useful not only to the practicing analytical chemist hut also to those engaged in basic flame research. As a basis for making the selections of compounds suitable for use in preparing standard solutions of the elements, the following criteria were followed Stability I ) The compound shauld not he deliquescent, efflorescent, or
hygroscopic.
2) The compound should not undergo a change upon drying. 3) The compound should be easily weighable. 4) Bath the solution and the eampound from which it is pre-
pared should be stable purity 1) The compound should be available to a purity of 99.5% or
better. Ease of Preparation
The compound should be soluble in water, common acids or hases. 2) The compound should be easy to handle. 1)
Auoilnbility 1) The compound should be readily available from common
chemical suppliers.
2) The compound should not be unnecessarily costly. High Molecular Weight 1) The compound should have as high a molecular weight as possihle for accurate weighing. Toxicity 1) The compound should have as low a toxicity as possihle.
Of course, very few of the chemical compounds examined conformed to these criteria completely. Those few that do are the well known "Primary Standards." These have been noted in the accompanying tahle. When examining the possibilities for a given compound the criteria were considered in the order listed. Some of the choices are less than ideal, h u t it is felt that the compounds listed are the best possihle and that with careful preparation, solutions made from these will he of accurate concentration. Several sources were consulted in making the initial list of possihle com~ounds.The "Merck Index" (3) . . and the andho hook of Chemistry and Physics" (4) were first
'To whom all correspondence should he addressed.
searched, one element a t a time, and the likely candidates were listed with their properties. These lists generally included the common salts (sulfates, halides, nitrates, and carbonates), the oxides, and the elements themselves. Compounds containing the element as the anion and the cation were both considered, when possible. Where further details concerning the physical properties were required, more complete references were consulted (5-9). The criteria were then applied in detail to arrive a t a maximum of three choices. With the exception of hafnium and zirconium, samples of these compounds were then ohtained and examined in the laboratory. Solubility data were checked qualitatively, and compounds were checked for hygroscopicity, deliquescense, and efflorescense. In some cases, compounds which were noted as being slightly efflorescent or deliquescent in the literature were found to he suitable for accurate weighing. These were checked by placing a small sample (ahout 1 g) of the compound on a n analytical balance and recording its weight a t periodic intervals for up to 3 hr. Compounds whose weights changed by more than 2 mg (two parts in a thousand) were rejected. It should he noted that the humidity in our laboratory is probably lower than that in many parts of the United States. Once the final choices had been made, 1000 ppm solutions were prepared of all of the elements in the tahle except for europium, terbium, zirconium, thulium, rheuium, iridium, scandium, hafnium, rhodium, lutetium, osmium, and ruthenium. Solutions of these twelve elements were not prepared due to high cost and, in the case of osmium, extreme toxicity. Several chemical suppliers in the United States produce "standard" solutions of the elements for use in atomic ahsorption analysis. As possihle support for our final choices, data sheets showing the composition of these solutions were ohtained from five different companies. In only a few cases did our choices coincide with these. In general, the commercially availahle standards were prepared with emphasis on water solubility. Two firms used water soluble compounds in all solutions, many of which were highly hygroscopic or even liquid. This would indicate that these solutions are prepared by approximate weighing followed by standardization by atomic absorption spectrometry or other suitable techniques. In the authors' opinion, the preparation of standard solutions by accurate weighing of a compound of known composition is a more satisfactory approach. Whereas weighing may he done to a t least *0.01% precision and dilution to about 0.190,a standardization technique using, for instance, atomic absorption spectrometry will give a precision of only about +I%. The compounds chosen in this study are selected with emphasis on weighability and accuracy of composition. (In cases where matrix effects or pH are important or where water solubility is required less desirable compounds may he used as secondary standards and standardized against solutions prepared from more suitable compounds.) The tahle lists the final choices for 72 elements and includes all elements commonly determined by flame spectrometric methods. In some instances more than one compound is given. In these cases, the choice is a matter of availability, matrix or simply convenience. Tabulated Volume 50. Number 10. October 1973
/
679
Compounds for Standard Solutions Element Aluminum Antimony Arsenic Barium Beryllium Bismuth Boran Bromine Cadmium Calcium cerium cevum Chmmium Cobalt copper Dyspmsium Elbium Europium Fluorine Gadolinium Gallium Geimanium Gold Indium ISodirn Iridium 1mn Lanthanum Lead Lithium Lutetium Magnesium Manganese Mercury Molybdenum N~odymivm Nickel Niobium Osmium Palladium Phosphorus Plstinum Pofsssium
Camoound
FormuleWeizht 1.
0.1 M l d l l
1WOoom id11
Salvrnf
Comments
Hot, dil. HCI water
Al mefal KSbOC.H.0e .HHm AsrOa BaCOt Be metal Bi.0l HaBOa KRr ~~CdO CaCOa iNH.12CdNO.). Cslso, KzCrzOi Co mste.1 C" metal cuo DYsO~ E120. Eva08 NaP
dil. HCI dil. HCI HCI HNOs water water HNOs dil. HCI water wster wete, .~~~
HNOs dil. HNO, HCl, hot HCl, hot HCI, hot HCI, hot water - ~ . ~ HC1, hot HNOs, hot I rn NaOH. hot Aqua Regia, hot HF, fusion HCI. hot HCI. hot water
FImetal
HCI, hot HCI. hot
La202 Ph(NOa1. LixCOa LurO. MKO MnS0..H20 HgCls Moo2 Nd103 Ni metal
w&.
HCI HCI. hot HCI water water I rn NaOH HCI HNOs. hot HF, fusion HISO,. hat HNO.. hot water water water wster water HCI HNOI water HzSOA,hot water water HCI, hot HCl. hot HNOa, hot NaOH. Coned.
Ruthenium samarium Scandium Sdenium Silicon
"P ~~~
water water water HCI water HF. fusion HCI HCI, hot
Silver sodium strontium Sulfur Tantalum Tdurium Terbium Thallium Thulium Ti" Titanium Tungsten uranium
wste, ..
HCI, hot HCI HCI 1.1 H*SO.. water HNO, HNOs HCl. hot HCl. hot HCI, hot HCI HF. fuaian
Sn metal SnO Ti metal Na,w01.2H.O
~
u01
Vanadium Ytterbium yttrium zinc zirconium
Ptimsrystandard. 'These compounds conform verywol to the eriterieand approachprimaryatsndard qualify. c Hizhlv toxie~ v e r y highly toxic. T h e rare earth oxides. because t h w absorbCox and water vsoorfrom the air. should befreshl~isnited odor to weiehine. .. r mp = 29.6.C.The m d s l may be warmed and weighed aathc iquid. I Sodium fluoride solutions will etch glandahauld he freshly prepared. "Sodium Lungstate lore3 both water molecules at llOaC. After drying. F.W. = 293.68,lWO ppm = 1.5962 8, 0.1 m = 29.383 g. The water ianot rapidlyregained hut thecampound should be kept in adosiccator after drying and should bo wclghedquickly once it iaremoved. 1 Zirconium and hafnium compounds were not investigated infhe laboratory. ThefollowingmcthSods have been recommendedlordissolutian ofrirconium and hafnium (10). 1 . m gof the powdered metal is placed in a platinum dish with 6-10 ml of vator and 1-2 ml of HFi1:51 andcovered v l t h a platinum lid or s parstflned wafehglars. Once di8aolved. the fluorine may he removed by adding 1-2 ml ofsulfuric acid (cold1 and evaporating t o d e n s ~fumes or todryness ifrewired. K2SzOli-rplaead in a platinum or quartz crucible along Mfh thosample. After moltingtoa homogeneous molten mssr in a muffle A fusion method may also be used. A5-10fold ex-of furnace or burner the fusionpmdurf isdi~solvedin 2Nsulfuric acid. A third method avoids the use of platinum ware The sample of the metal is finely ground and plseed in a smsll heat-resistant beaker. Two to four g a m s of ammonium sulfate and 3-6 ml of sulfuricacid arc then added. A homogeneous melt is obtained on s hot plate and dissolved in 2Nsulfuric acid. 0
~2
~~
~~~~
.-
680
/
Journal of Chemical Education
.
-
with each compound is the formula weight, the weight required to prepare 1 1 of 10M) ppm solution, the weight required to prepare 1 1 of 0.1 M solution, suggested solvents and comments concerning toxicity and solution preparation. The solvent listed is used to bring the compound into solution using as small a volume as possible. The solution is then brought to volume with distilled or deionized water. All acids are concentrated unless otherwise specified and should be analytical grade. Note that for some compounds dilute acid is specified in preference to concentrated. Although the solvents in the table have been shown to he satisfactory, they are by no means inclusive and other acids or bases may he utilized in many cases. For accurate work, compounds should always he brought to constant weight by drying a t 110°C prior to weighing. Recently an excellent compilation has appeared which is complementary to this work (12).
Literature Cited 111 Rosin. J.. "Reagent Chsmieak and Standards."Srh 4..D. van Nostrand company, In
