behavior of ruthenium chloride solutions toward chloride and acetate form of the anion exchange resins, Permutit ES, and found that some of the ruthenium m s retained on the resin, and some passed through into the effluent. They offered no explanation for their observations. Berman and IlfcBryde (2) also observed the same behavior of chlororuthena.ce solution toward the anion exchange resin, Amberlite IRA 400. These authors explained this observation by assuming that the tetravalent ruthenium chloride complex was strongly adsorbed to the resin, but appeared to be partially reduced by it so that there was a comparatively large loss of ruthenium t o the effluent, as the chlororuthenate(II1) complex has a Ion- affinity for the resin. Attempts t o reduce the tetravalent ruthenium on the exchanger n.ith hydroxylamine nere unsuccessful. I n any case the authors’ explanation was not supported by data. As a solution of chlororuthenate does not pass through the cation exchange
resin quantitatively, one must not exclude the possibility of the existence of partially hydrolyzed cationic species of ruthenium in the solution. Support for this suggestion is the fact that prior evaporation in the presence of hydrochloric acid and sodium chloride allow the quantitative recovery of ruthenium in the effluent. Assuming that the chlororuthenate complex is principally in the tetravalent state the following reaction may indicate the character of the hydrolysis in dilute hydrochloric acid solution upon prolonged standing. [RuClC]-*
+ 6H10 S [Ru(H10)6]+4
+ 6C1-
In any case one cannot doubt the existence of a variety of dissolved partially hydrolyzed species of ruthenium. Treatment with hydrochloric acid and metal chloride may convert these species back t o anionic complexes. The hydrolysis of ruthenium may also explain the very small loss of ruthenium to the resin Then the buttonsample solution was passed through the
column, as ruthenium in this solution had little time to undergo hydrolysis. LITERATURE CITED
(1) Beaniish, F. E., Talanta 5 , 1 (1960). ( 2 ) Berman, S.,McBryde, IT. -4. E., Can. J . Chenz. 36, 845 (1958). ( 3 ) Blasius, E., Wachtel, V.,Z. A n d . Chem. 142, 341 (1954). ( 4 ) Gilchrist, R., J . Res. S a t l . Bur. Std. 12, 291 (1934).
(5) Kavanagh, J . AI.) Bearnish, F . E., hx.4~.CIIEM.32, 490 (1960). (6) Pluniiner, M. E. V., Beainish, F. E., Ibid., 31, 1141 (1959). ( 7 ) Rogers, W. J., Beamish, F. E., Russell, D. S., Ind. Eng. Chenz. 12, 561
(1940).
(8) Sandell, E. B., ”Colorimetric Metal hnalysis,” 3rd ed., Interscience, New
York, 1959.
(9) Sant, B. R., Beaniish, F. E., A x . 4 ~ . CHEY.33, 304 (1961). (10) Tertipis, G., Beamish, F. E., Zbid., 34, 108 (19G2).
(11) Thiers, R., Graydon, W.,Beamish, F. E., Zbid., 20, 831 (1945). (12 1 Kestland, -4. D., Beamish, F. E., Ibid., 26, 739 (1954).
RECEIVED for revierr February 2, 1962. Accepted March 26, 1962.
Analysis of Ruby and Sapphire Maser Crystals E. M. DODSON’ War Office, Woolwich, England
b These investigations, carried out in conjunction with the Royal Radar Establishment, Malvern, attempted to coordinate chemical composition and maser performance. Both ruby and sapphire (fused alumina doped with Cr, Fe, etc.) are difficult to break down quantitatively and require alkali fusion. The investigations have shown that most of the common crucible materials have serious disadvantages when used for the estimation of trace metals in maser crystals, particularly for iron. Some will not withstand alkali fusions (silica, porcelain) and others are severely corroded, yielding corrosion products which interfere with subsequent analysis (Pt, Au, Ag, Ni). Others, b y the formation o f solid solutions (Pt, Au) exchange trace metals between crucible and melt in an unpredictable manner, and carbon absorbs trace metals from the melt onto the surface o f the crucible. Provided precautions are taken to reduce surface oxidation, high purity zirconium appears entirely satisfactory, as it i s not corroded and does not form solid solutions with the trace metals investigated. Agreement between chemical and spectrographic results i s satisfactory, and recovery from synthetic standards i s theoretical. The pro966
ANALYTICAL CHEMISTRY
posed analytical methods described, fulfill the requirements for the estimation o f several elements, often a t trace level, on a single small sample weight, following a breakdown which does not interfere with the subsequent colorimetric analysis.
T
HE RAPID DEJ-ELOPMCKT of solid state masers for low noise microwive amplification ( 2 , 19) has been accompanied by increasingly stringent requirements in some of the chemical aspects of the maser crystals used. T o eludicate these chemical aspects a conibination of techniques is desirable, so that not only may the paramagnetic nature of the crystal be found by physical means, but also that, chemically, the concentration of the desired paramagnetic ion and the nature of any impurities may be determined. The former problem is most suited for study by microwave spectrometer techniques (18), and this paper is concerned with chemical techniques designed to clarify the latter. Most of the requirements for successful three-level maser action (e.g., suitable energy levels, paramagnetic ion content, chemical stability, etc.) are satisfied in synthetic ruby (Cr+3 in
fused alumina, -41203), which has been widely used as a maser material, and “white” sapphire (Fef3 in fused alumina A1208-blue sapphire contains titanium as well as Fe) . Analytical techniques are necessary to solve some of the problems u-hich are common t o most maser development and associated resonance studies. The most important of these are: The determination of concentration of the required paramagnetic ion. This usually lies in the range about 0.01 to 0.1% by rreight. I n most maser crystals this concentration determines the absorption line width of the sample, and hence the band iyidth of the maser for a given cavity or traveling wave structure. I n ruby it may also severely influence the spin-lattice relaxation time (18) and hence alter the pump power requirements. I n addition, it is desirable to know the paramagnetic ion concentration in the single crystal to establish controlled methods of crystal groivth. For the latter, variations of the flame fusion process are widely used or the crystal is grown by the Thermal Syndicate method from the m p o r phase Present address, Kational Chemical Laboratory, D.S.I.R., Teddington, England.
and the nominal concentrations quoted usually represent the stoichiometric composition of the mixture before fusion. Under furnace conditions the oxides of both chromium and iron are volatile and this causes uncertainty in the respective concentrations in the solid boule (see Table I). The detection of other paramagnetic impurities. These give unwanted transitions which may occur near the required transitions. I n simple cases this may lead merely t o wastage of pump power, but in experiments on cross-relaxation and masers using this effect it may be particularly harmful. Very low impurity levels may be encountered, requiring trace analytical techniques. The homogeneity of the sample. I n masers n-orking a t wavelengths of 3 cm. and above, crystals of a few centimeters in size may be required and i t is desirable t h a t the concentration be uniform, since localized high concentrations not only lead to regions of short relaxation time, thereby increasing the necessary pump power, not also may act as highly absorbing centers in an otherwise active material. .4t shorter wavelengths where t h e sample sizes may be only a few millimeters, the problem becomes more
Table 1. Nominal and Actual Chromium and Iron Concentrations in Ruby and Sapphire
Sample No. 1
2
3 4
5 6 7 8 9 10
Spectrographic % 70 Found, yo O.1Cr 0 . 0 3 C r 0.032Cr 0 . 5 C r 0.013Cr 0.012Cr 0.02 Cr Trace Cr 0 . 0 1 (trace) O.1Cr 0.026Cr 0,022 0 . 1 Cr Trace Cr 0.01 0 . 1 Cr 0.055Cr 0.052 1 . 0 C r 0 . 5 5 C r 0.56 0.1 Fe 0 . 0 2 F e 0 . 2 F e 0.018Fe 0.014Fe (polarographic) 0 . 3 F e 0.022Fe ...
acute, particularly in the case of maser oscillators (17, 18). These considerations suggest that the analytical techniques used should possess the following features. Since analysis will, in general, be performed on a piece of crystal cut from a boule, adjacent t o t h a t used in the maser, the technique should be suitable for the determination of several elements on a single small sample %-eight (maximum
Platinum
Complete fusion possible
Gold (thick-walled, high purity material)
Complete fusion possible. Less corrosion than with Pt
Silver (thick-walled, high purity material)
Complete fusion possible Less retention of trace metals by crucible. Results in agreement with those of Shell (1.4, 15)
Zirconium (pure element)
I
The need for accurate chemical and Spectrographic analysis is shown in Table I which gives the relation between nominal paramagnetic ion concentrations (taken from the stoichiometric composition before fusion) and the true concentrations in the boule. For both ruby and sapphire the matrix is the same-fused alumina. I n ruby the color is due t o chromium and in white sapphire, t o iron; these two
Crucible Materials for Crystal Breakdown
Advantages Complete fusion possible Complete fusion possible
Specpure nickel (3). Spec- Complete fusion possible pure iron
Zirconium (commercial grade)
Chemical Found,
netic constituent should be possible without inhibiting subsequent estimations of other trace impurities in the same sample. A high sensitivity method is necessary, involving the minimum number of simple processes. Since several constituents may be required a t trace level, special precautions are necessary to avoid contamination from reagents, containing vessels, or, particularly, air-borne dust. Chemical methods have been developed along these lines and are in good agreement with spectrographic results (see Tables VI and VIII) CHEMICAL ANALYSIS
Table II.
Crucible Material Carbon Yickel commercial grade
Nominal,
0.02 gram, usually very much less). Determination of the major paramag-
Disadvantages Absorbs trace elements from melt Crucible corrosion by alkali melt introduces unknown quantities of impurities from the crucible into the melt. Cannot be used where Ni is a possible crystal contaminant Crucibles are not readily available, but otherwise are satisfactory, for low temperature KOH fusion Corrosion of crucible by alkali melt can yield colloidal Pt compounds which can absorb trace metals from the melt very strongly (Tables I11 and IV). Pt crucible can retain trace metals from the melt in solid solution, and release these later to a subsequent melt in a completely random fashion Corrosion increased with use yielding increasing amounts of colloidal gold corrosion products which absorbed trace metals from melt, though less than for platinum. Exchange of trace metals between crucible and melt was as unreliable as for Pt. Especially regarding the long term retention of Fe. See Table V Considerable corrosion of the crucible, giving interference with this particular analysis due to the difficulty of removing colloidal Ag without losing trace metals
Remarks Not satisfactory Not satisfactory
satisfactory Not satisfactory due to: corrosion of crucible; exexchange of trace metals between crucible and melt is unpredictable; and creeping of the melt r o t entirely satisfactory
Less retention of trace metals but not entirely satisfactory
Contains too much Fe (O.lyoto 700 p,p.m.) and Cr (150 p.p.m.) Little or no corrosion. No h'eeds care in use to prevent surface Satisfactory providing cruexchange of trace metals cibles are used under specioxidation between crucible and melt. fied conditions. (Tables Satisfactory recovery on VI, VII, and VIII) synthetic standards
VOL. 34, NO. 8, JULY 1962
967
~~~
Table 111.
Retention of Trace Metals Using Platinum Crucibles
(Iron retention; potassium hydroxide fusion) Fe Recovery 1st Fusion pg. Fe State of Added, (Repeated H2S04 Boils Repeat Fusion, pg. Fe Crucible pg. after 1st Fusion) 1 2 3 New smooth, 10 1. 6 . 8 2. 3 0 3. -pg. 10 0.5 Xone uncorroded 5 1. 3 . 5 2. 0 . 5 3. 0.5 !Sonea 0.2 Used, not 10 1. 7 . 0 2. 0 . 8 3. 1.5 None 0.3-0.5 muchcor5 1. 4 . 0 2. 0 . 2 3. 0.6 ... ... roded 10 1. 5.0 2. 1 . 0 3. 3.4 1.5b 0.5* Old, much 10 1. 5.4= 2. 0 . 5 3. 2.0 Sone 1.5b corroded 10 1. 4.0° 2 . - 3. 2.5 Sone 2.3b 10 1. 5 . 2 a 2. - 3. Sone (trace) 5.0h 3.0 a Some loss of Fe on colloidal Pt ppt, formed by corrosion. * Indicates release of extra Fe retained from previous fusions. ~
Retention of Trace Metals Using Platinum Crucibles (Chromium retention; potassium hydroxide fusions) Cr Added, Cr Recovered, pg. Crucible State wg. Fusion 1 Fusion 2 Fusion 3 ... New smooth 10 10.0 ... 5 4.0 0 5 10 9.8 Trace Tr&ce Used, not much corroded 20 17.6 Trace Trace 18.5 0.5 Trace Old, much corroded 1.0 10 1. 2-3a Probably 15 2. Xoneinsolu- 1-2 tion0 a Heavy or complete loss of Cr absorbed onto colloidal Pt PPt. formed corrosion of crucible by KOH melt. Table IV.
Table V.
Retention of Iron in Gold Crucibles
(Potassium hydroxide fusions) Fe rldded, Fe Recovery, Mg. fig. Fusion 1 Fusion 2 5 4 0.5 5 4.5 Trace
Crucible State Fusion 3 Nem Trace After 4 fusions Negligible 10 6-7 2 Nonea After 8 fusions Long KOH fusion (6-7 hr.) a t 400" to 450' C. gave 2.0 pg. Fe extracted from crucible (long-term retention).
Table VI. Recovery of Chromium and Iron from Synthetic Standards
(Recoveries from single fusions, potassium hydroxide fusions) Crucible
Cr, fig. Fe, cLg. ReReNo. Added covered Added covered 1
5.0
4.9
5.0
5.0 2
10.0
3
20.0
10.0 10.1 20.0 19.9
10.0 10.0
5.0 5.0 10.1 10.0 10.1 9.9
have been widely used as paramagnetic ions. However, there may be Xi, Co. Mn, etc., also present as impurities, probably at very low levels, and these elements interfere with the maser prop968
ANALYTICAL CHEMISTRY
erties. For a successful analysis the requirements are : -4fusion reagent which gives complete and quantitative breakdown of the crystal and which yields a solution free from interference with the subsequent colorimetric determinations; a crucible material n hich is unaffected by the fusion conditions and which involves no exchange of trace elements between the fusion melt and the crucible; the estimation of all the required elements on aliquots from a single breakdown; and exclusion of air-borne dust, b y the use of, for example, a glove box. These conditions are fulfilled in the methods described later. Prior to about 1920, no very reliable analyses of ruby had been carried out; Fremy (1891) states that ruby cannot be analyzed, but that b y synthesis its color was shown to be due to Cr; Wohler (1899) and Doelter and Leitmeier (5) stated that the Cr content of ruby was
too small for analysis. O'Leary and Papish (9) fused 0.2 to 1.0-gram samples of finely powdered ruby iTith K H S 0 4 in platinum and obtained fair agreement for per cent Cr. Subsequently there have been several analyses of fused alumina or rubies, (1, 7-9), but none of these is particularly well suited to the special requirements of the present problem. EXPERIMENTAL
Crucible Materials. Many of the analytical figures for ruby and qapphire maser crystals n-hich are quoted in the literature have been obtained using various breakdon n materials contained in nickel, platinum, gold, or silver crucibles. The preliminary investigations are described in some detail in this paper, since they demonstrate the need for crystal breakdown under carefully controlled conditions. In the present 1% ork breakdon 11s n-ith sodium or potassium bisulfate, carbonate, bicarbonate, peroxide, carbonate-borate, carbonate-hydroxide mixtures, hydrofluoric-sulfuric, phosphoric-sulfuric, and phosphoric-hydrochloric acid mixtures have been investigated. Of these. a 1011 temperature (400" to 460" C.) fusion in a zirconium crucible with potassium hydroxide was most satisfactory. Other fusion mixtures either gal e incomplete breakdown unless temperatures of 800" to 900" C. were used, with consequent increase in corrosion of the crucible (13, 1 4 ) , or gave interference (phosphate), or increased solution of trace metals from the crucible (borate) (15). Crucible materials investigated have included silica, carbon, nickel, gold, silver, platinum, and zirconium. ill1 of these (except zirconium) present difficulties either because of corrosion (Si, S i , Pt, Ag), interference from crucible materials (?Ti, Pt, S g ) , or exchange and absorption of trace elements from the fusion melt (Pt, Au, C). Specpure Xi crucibles are satisfactory ( 3 ) but not readily available. Using a zirconium crucible under the conditions given later there is neither gain nor loss of trace elenients (particularly Fe) t o the crucible, corrosion is negligible, and any trace of zirconium compounds in the resulting solution does not interfere r i t h the subsequent analysis, providing rigorous precautions are taken against contaniination. Data are given in Table 11. Zirconium Crucibles. Precautions in use of zirconium crucibles. Surface oxidation. Cox (4) discusses the surface oxidation of zirconium and breakdown of the initial very thin protective film (showing interference colors) t o the thick white oxide layer, by grain boundary oxidation. I n the use of zirconium crucibles for alkali fusions. it
is necessary t o avoid the formation of t'his thick white oxidc layer, since this can adsorb trace metals. The most effective method appears to be as follolvs : Use a crucible of wide, shallo\v pattern (approximately 3 em. diameter, 0.5 em. maximum depth, capacity about 2 ml.) arid bed i t up to its brim in analytical reagent grade anhydrous sodium carbonate contained in, ideally, a platinum dish. Ceramic containers do not give even heat transfer and the melt may. therefore. cwep. Corer with a watch glass antl heat on the electric hot plate a t 400" to 450" C., Le., only sufficient heat to keep the KOH molten. With careful heat control and the restricted air sul)ply. no thick m-hite oxide film is forinetl and there is no creeping of the melt. Anhydrous analytical reagent grade sodium carbonate is used as a heat transfer medium since it is almost free from iron, does not decompose a t the temperature used, andwill not interfere if any adhering to the outside of the crucible reaches the fusion mclt. If the zirconium crucible is heated directly or a t a much higher temperature, the surface oxidation can be rapid and the thick layer of zirconium oxide formed can adsorb small amounts of trace metals from the melt. Gas heating invariably increased corrosion rates. Crucibles in current use have withstood n-rll ovrr 150 fusions 11-ithout difficulty. Any traces of zirconium dissolved in the course of the fusion do not' interfere with the subsequent, analysis. Agreement betn-een spectrographic and clieniical analyses for chromium is satisfactor\-, and consistent results are obtained for iron. Synthetic standards give satisfactory recoveries for bot'h Cr antl Fe. (Set, Tabltts IX; 9. nntl XI.) Analytical Procedure. Sample preparation. Crush the ruby or sapphire betwcrn pieces of hard nylon or Bakelite block t'o a coarse powder, or heat t h e crystal in a silica tube lined n-ith a platinum sleeve and drop into cold ivater or solid carbon dioxide to effect t h t initial fracturing. (The use of a tungsten carbide percussion mortw iiitrocluces contamination n-hich can lie IWJ. tlificult~to remove.) Sieve through finc cwiibric into an agate mortar : t i i d grind this portion to as fine n I)o\vder as possible. Repeatedly siew through cambric or fine nj-loii to produce :t sinall grain size, because the efficimc\- of fusion depends directly on thc grain size. Boil for 1 to 2 niinutr,h i n concentrated hydrochloric aritl. li!-(lu)fluoric acid, or aqua regiti. if nec'cw:ir>.. to remove mortar contamination (use Y a1)ectrographic check for co~it:itiiiti:itioiiI , centrifuge, wash ~v(,ll man!- tinios iii ion-free distilled water (until ('1- ion is negative), then wash n-ith ac~toiic.a n d d r j ~in an air oven. i i l l i'(l:igc,iitP slioultl be of high purity,
Table VII. Comparison of Chemical and Spectrographic Analyses for Chromium in Synthetic Ruby
4 Potassium hydroxide fusion in zirconium)
1
Suectroiraphic Cr, yo 0.032
2
0.012
3
0.55
4 5
0.25
Ruby No.
6
Chemical Cr1 70 0.030 0.031 0.013 0.0139 0.57 0.54 0.26 0.022 0,020 0.010 0.0115 0.0119
0.025
0.022 0.01 0.01
7
0.052
0.053 0.051 0.053
0,053
and containers, etc., tested for absence of contamination. Avoid the use of filter papers by centrifuging, since filter paper often contains part per million quantities of metals or can adsorb trace metals. During experimental work take rigorous precautions to exclude air-borne dust and store apparatus and reagents in a dust-free atmosphere, otherwise it is very difficult to obtain steady and sufficiently low blank values. The use of a dry box as a dust-free environment is advantageous.
Table VIII.
Crystal breakdown. Reagents. Potassium hydroxide, analytical reagent quality, stored under dust-free conditions, Zr crucibles, pressed from pure element zirconium sheet; corrosion resistance is higher with increasing purity and the material is less brittle. Ion-free distilled water, distilled water run through a mixed-bed ion exchange column immediately before use and kept under dust-free conditions. This water is used for all reagents, etc., and throughout the procedure. Fusion procedure. Considerable care is needed in carrying out the fusions, as follons: The use of a thin scatter of the finely powdered sample and a wide shallow crucible prevent "clumping" of the denser sample at the bottom with resultant incomplete fusion. It is also adrkable to stir the melt toward the end with a Zr wire, keeping this wire in the crucible. Zirconium nithstands repeated alkali fusions extremely well, provided the precautions previously mentioned are taken to reduce surface oxidation. The liniited air supply and the exclusion of air from the under surface of the crucible by packing in carbonate reduce cxterior oxidation to a minimum and favor the formation of the thin protectiw film (showing interference colors) without its breakdown to the thick white oxide layer. (For more rapid fusion the crucible can be heated more strongly in a n argon atmosphere.) Tn o to three pellets of analytical reagent grade potassium hydroxide are fused in the crucible and allowed to
Iron Analyses of Synthetic Ruby and Sapphire
(Potassium hydroxide fusions in zirconium) Sample S o . 1. (Khite sapphire)
Fe, 5%
Cr, ?;
Sone
( ab)) 0.019 0.019;
'.02
(dl
:: !t! I o,
017 0,019 I ( e ) 0 0181 0.020:
2.
(Khite sapphire)
Sone
\;(c) 0 od"19' " 019 ' 0 022 ' ( d ) 0 019i
5 separate fusions on different sample weights from different p a r h of the same
4 separate fusions on dif-
ferent sample weights from different Darts of the same boule
0.021)
3. (White sapphire)
Sone
( a ) 0.023 0.022
i b ) 0.025
4. Ruby
5 . Ruby
0.02
0.023 (a) 0.014
0.01
( a ) 0.0043
0,015 ( b ) 0.016 0.015
( b ) 0.0046 6. R L h J
Trace
( < 0 001'; Cr)
( a ) 0 0039 ( b ) 0 0036
VOL. 3 4 , NO. 8 , JULY 1962
969
cool. An accurately weighed, finely powdered sample (approximately 0.02 gram or less) is scattered on the cooled surface and a few more pellets of KOH are added. The whole, covered with the watch glass, is heated for 2 t o 2l/2 hours a t 400' to 450' C. on an electric hot plate until fusion is complete. I n this way there is no creeping of the melt. Gas heating is avoided as it invariably increases corrosion rates.
Table IX.
Chromium Analysis of Synthetic Ruby Comparison of Chemical and Spectrographic Analyses (Potassium hydroxide fusion in zirconium) Sample Chemical Spectrographic NO. Cr, % Cr, 70 1
2 3 4
5 6
7 8 9 10 11
0.91 0,60'1 0.59) 0.56' 0,541 0.26 0.18 0.053' 0.0531 0.03 0.022i 0.021, 0 026 0 013
: E{
0.89 0.58 0.55 0.25 0.15 0.052' 0.053: 0 029 0.02 0 025 0 012 0 01
0 012
12
Very faint, Less than 0.01 trace 0 0005 or less
After cooling, the crucible and melt are boiled in ion-free water twice, then for 1 minute in 10% H2SOI, and finally well washed, the acid solution and washings being added to the main solution. The whole is made just acid with analytical reagent grade H2S04 and any trace of cloudiness is removed b y centrifuging. Depending on the estimation required, this solution, S , is either evaporated or diluted and made up to a standard volume, between 5 and 50 ml. It is advisable to fuse 3 to 4 pellets of KOH in the same crucible and add this melt to the above solution to ensure the complete removal of the last traces of Fe from the crucible. Solution S . Aliquots are v,-ithdrawn for the subsequent colorimetric estimation of individual elements. It is advisable to complete the analysis in the minimum time after fusion to avoid dissolving trace metals from the glass containers if the solution is left to stand in them for any considerable time. The use of polyethylene, etc., is not recommended for the fusion solution because trace elements from a solution can be lost to the walls. All containers, etc., should be covered to exclude air-borne dust.
970
ANALYTICAL CHEMISTRY
COLORIMETRIC ESTIMATION OF CHROMIUM
Principle of Method. After fusion t h e chromium present is oxidized t o chromate and determined colorimetrically as the violet complex with diphenylcarbohydrazide (diphenylcarbazide) for chromium in the range 0 to 20 pg. Sample weights, final volumes, and Unicam cell sizes are adjusted to work within this range. Reagents. Ion-free distilled water, prepared as described. Sulfuric acid, analytical reagent grade concentrated acid (9801,) diluted to 5y0v./v. M ith ion-free water. Ammonium persulfate, analytical reagent quality, stored under dust-free conditions. Silver nitrate, analytical reagent quality, stored under dust-free conditions. Diphenylcarbohydrazide, analytical reagent quality (solid), stored under dust-free conditions. Use fresh solid reagent every 8 weeks to avoid deterioration on keeping. Diphenylcarbohydrazide, 0.25 gram of diphenylcarbohydrazide dissolved in absolute working solution, alcohol (Crfree) and diluted to 100 ml. with ionfree nater. Mahe up freshly for each estimation. Phosphoric acid, analytical reagent grade, 60%. Standard chromium solution. Dissolve 0.3740 gram of potassium chromate (analytical reagent grade in the ion-free mater and dilute to 1 liter. Dilute 10 ml. of this solution to 500 ml. Prepare freshly as required. 1 ml. = 2 pg. of chromium. Procedure. Sample anal3 sis. -4 suitable aliquot of solution S is used, the pH is adjusted to 1 with 5% sulfuric acid, and the chroniium is oxidized to chromate with 5 ml. of ammonium persulfate and 2 drops of silver nitrate, follon-ed by a long boil (15 minutes), or with potassium permanganate-sodium azide oxidation. Care must be taken to ensure complete removal of the oxidizing agent, control of the final salt concentration, and pH, because maladjustment of any of these will cause the color to fade. The volume is then made up to approximately 15 ml., the p H adjusted to 1, 5.0 mi. of the fresh diphenylcarbohydrazide added followed by water to 25 ml. The color is read within 10 minutes on the Unicam 600 at 5400 A. in a 1-cm. cell. -4blank is also run. If the ruby also contains iron in a quantity (2 0.1 Cr) to interfere with the Cr estimation, this interference can be prevented by the addition of 2 to 3 drops of phosphoric acid before the diphenylcarbohydrazide. This method gives satisfactory recovery of the synthetic standards and results are in agreement with chromium analysis figures obtained spectrographically by AIostyn (6). Low, or trace, chromium contents (e.g., in nhite sapphire) can be estimated by direct color matching, as for iron. Spectrographic estimation of chromium.
Table X.
Chromium Analysis of Synthetic Ruby Variation in chromium content across a single boule (Boule cut into 13 pieces, of 0.05 gram each, and each piece analyzed for chromium) SpectroSample Chemical graphic so. Cr, % Cr, 70 -4 0.0271 0.027 R
c
D E F G
H
0.025 0 025 0 027 0 024 0 031 0 038 0 037 ( 2 1
0.026 0 026 0 026 0 023 0 028 0 039 0 037
0 036
0.035
The spectrographic method finally adopted is a modification of the ironflux procedure originally developed a t the Chemical Inspectorate (6). Direct analysis of ruby using A1 as the reference element for Cr proved unsatisfactory owing to the very refractory nature of ruby, but using the iron-flux dilution method, and standards containing a n equivalent amount of A1203, the standard-sample comparison is then more valid. U'ith a sample weight of 0.01 to 0.02 gram. Cr may be determined in the concentration range of 0.01 to 0.2% with a precision of =t15%, and higher Cr concentrations by suitable dilution. COLORIMETRIC ESTIMATION OF IRON
Principle of Method. The iron is estimated as the red coniplex with 2,2'-bipyridine in a solution buffered to p H 4.5 to 5.0 with animonium acetate. I n spite of concentrated solutions and sniall volumes, the iron concentration in some of the samples is beloiv the range of the Unicam S.P. 600. Hence direct color matching against a series of standards is used. Range. For iron in the range 0.1 to 10 pg. Concentrations and final volumes are adjusted to work in one of the ranges 0.1- to 1.0-pg. standards differing by 0.1 pg. or 1.0- to 10.0-pg. standards differing by 1.0 pg. and color matching is effected in either 2-, or 4-ml. depressions in a white tile. Reagents. Standard iron solution. 1 nil. = 10 pg. Fe, is made up freshly as required, from Specpure Fe sponge. Small volumes are measured by a n Agla syringe. Color developing reagent. Hydroxylamine hydrochloride 2570 W./V.
Ammonium acetate 0.LY (9.63 grams per 250 ml.). 2,2’-Bipyridine solution. Dissolve 0.2 gram of 2,2’-bipyridine in 100 nil. of 0.02N hydrochloric acid. X i s 8 ml. of hydroxylamine hydrochloride solution, 50 ml. of 0.5N ammonium acetate, and 20 nil. of 2,2’-bipyridine solution. Dilute to 100 ml. 2% 2,2’-bipyridine in absolute alcohol for spot tests. Ion-free distilled water (as above). Procedure. Sample analysis. An aliquot of solution S from t h e fusion is adjusted t o near neutrality and concentrated to a small volume. An 0.5- to 2.0-ml. aliquot is withdrawn into t h e 4- or 2-ml. depression in t h e white tile, buffered to p H 4.5-5.0 with solid ammonium acetate. and 2 ml. of t’he 2,2’-bipyridine color reagent added. After stirring well with a Pt wire the color is matched in diffuse daylight against the Fe standards similarly prepared. It is adrisable t,o use tn-o different Fe concentrations on each sample (e.g., 0.5- and 1.5-ml. aliquots) to ensure that the total salt concentration of thc original solution has not been so high that full color development is inhibited. I n this way the iron content is rcproducible to 0.2 pg. of Fe. With rigorous precautions to exclude air-borne contamination, blank values are steady nncl n-ell below the range of iron being determined. The blank v a l i m rise sharply and are completely unpredictable if rstimations are carricd out in thc open laboratory. The red 2,2’-bipyridine color is stable for several hours. For iron in the rnngl. 0.002 to 0.1 pg. Adjust concentrations and final volumes to 0.01- to 0.1-pg. standards diffcring 1 ) ~ -0.01 pg., or 0.002- to 0.01,ug. standards diffrring by 0.002 ,ug. The colors lx~lon-0.1 pug. are often too fsint to bc ensily distinguishalile in the tile depressions, especidly in fluorescent light. For t h e lon-er ranges, spot’ test pctpw, on K h a t m a n S o . 41 filter paper (iron free! is soaked in 2% 2,2’-bipyri(dinein absolute slcohol, dried, and disks, 0.5 em. in diameter are punched out. ‘These arp placed on :t lightly grcased white tile to prerent the liquid spreading. and thc iron solution is added to them. ‘The color dewloped is rompared against :i series of standards similarly prepared. [f the iron solut’ionis buffered with solid ammonium acetate. the presence of this salt in thc spot t’ed paper prevents undesirable chroinntographic effects on drying out since it, keeps the paper inoist. For w r y lo^ iron contents the color intemity can be increased by using smaller dinmetor disks. *ill the rni~tllyolumes, especially for the color standards, are measured from :in Agla syringe. O t h e r Trace Impurities. These are usually present’ in very small amounts a n d their presence is first detected spect’rographically if possible. At these levels colorimetric methods of estimation are employed using aliquots from t h e fusion solution S (see
Table XI.
Iron Analysis of Synthetic Ruby and Sapphire
Nominal % ’ Fe Samples Taken Sam- from Different Chemical Ple Parts of 70Cr Fe No. Same Boule (Found) (Found) 1. (a) 0 . 1 (b) 0 . 1 (c) 0 . 1
None
0.020 0.020 0.018 0.021
Sone
0.0190 0.0195 0.016
(d) 0.1
(e) 0 . 1 2 . (a) 0 . 2 (b) 0 . 2 (cj 0.2 (d) 0 . 2
0.020
0.017
O.OlT5
(e) 0 . 2
(f) 0.2 3. (a) 0 . 3 (b) 0.3
0,0180
None
0.022 0.023
0.026
(c) 0 . 3
(ej 0 . 3
0,024
0,022 0,0155 5.
6. 7. 8. 9.
(somirial 0.1 yo Cr) None Kone Kone (Vapor phase boule) A None (Vapor phase boule) B
eliminates the blank error. If the sample contains much Cr, the dimethylglyoxime color can be solvent extracted in chloroform (11, p. 471). COBALT(11, p. 277). A suitable aliquot of solution S is used and cobalt estimated as the nitroso R salt complex. The color is read at 415 mp or by direct color matching as for iron. The above methods give reproducible results at the levels required (usually 0 to 5 pg. of the metal) and the application of suitable standard colorimetric methods for other metals, t o aliquots of the fusion solution be extended if necessary.
0.023 0.016 0.020
0.039 0 . 0 4 0.054 0 , O L S None 0.0036 0.0038
None
0 0043 0 0046
ACKNOWLEDGMENT
The author thanks R. A. ILIostyn, Chemical Inspectorate, for supplying the spectrographic analysis and for his encouragement and interest; the Royal Radar Establishment, LIalvern, for most valuable cooperation throughout and J. J. Thomas (University of Keele) for details of geochemical analyses. Certain ruby samples were generously supplied by G. E. C. Hirst Research Centre and by the Thermal Syndicate Ltd., and a silver crucible was loaned for tests by Johnson Matthey Ltd. REFERENCES
Ilexander, A. E., J . Gemmol. 1, S o . 8, 4 (1948). 12) Bloembergen, IT., Phys. Rev. 104, 324 ( l Y J 0 ).
above). Since these methods are standard colorimetric procedures, they are given in outline only. M A X G A N E S E . A4 suitable aliquot Of solution S is oxidized with ammonium persulfate-silver nitrate, or with hydrogen peroxide and the resulting permanganate color matched visually against a series of standards prepared from analytical reagent grade potassium permanganate, covering the range 0 to 20 pg. of Mn. The color matching is effected either in small Sessler tubes (if there is sufficient solution) or in the depression of a white tile, as for iron. If sufficient l l n is present t o iiiterfere with the Cr estimation, Cr can be estimated by oxidation m-ith potassium permanganate instead of ammonium persulfate, the excess permanganate being just decolorized with dilute sodium azide, before the addition of diphenj-lcarbohydrazide. NICKEL (11, p. 4’71). -1 witable aliquot of the fusion solution S is used, and the nickel is estimated as the dimethylglyoxime complex. If the Xi concentration is belon- Unicam range, direct color matching against standards prepared from Specpure S i rod or powder (RIond Kickel Co. Ltd.) can be used, as in the procedure for iron (see above). The direct color matching
(3) Brookes, A. J., Mond Nickel Go:, Ltd.! Specpure .. rruribles . . in analvsis oi iused Coriinrl .-..-urns, private communication. ( A \ ,:ox, B., A.E.R.E. Rept. KO,R2874 (HMSO). ( 5 ) Handbuch der bheralchemie. Bund IV. Cornelius Doelter and Hans Leitmeier, eds., Dresden. Th. Steinkopff. (6) “I. P. Standards for Petroleum and pp. 523-33, March its Products. Part 1)’’ 1960. (I.P.122/55.) (7) Uendlina, N. G., Sovoselova, A. .I., Rvchkov. R. S..Zaoodskava Lab. 25, 1293-4 (1959). (8) Miller. C. C.. Chambers. R . A,, -Analyst’78, 694 (1953). ( 9 ) O’Leary, W. J., Papish, J., A rn . Xznerul. 16, 34-6 (1931). (10) Petretic. G. J., AXAL. CHEX 23 > ‘ 1’183 (1951). (11) Ssndell, E. B., ”Colorimetric Determination of Traces of Metals,” Vol. 3, Interscience, Xew Tork, 1960. (12) Scovil, H. E. I),,Feher, G., St?idel, H., P h y s . Ret,. 105, 7 G Z (1957), (13) Shell, H.F.. XSAL. CHEM. 22, 326 ,A,
\-,
(1950). (14) I b ~ d, 26, 591 (1954). (15) sinales, A. d , JT-ager, L. El., “Modern Methods of Geochemistry,” Inter-
science, Sew Tork, 1960. (16) Thorp, J. S Pace, J . H., Sampson, D. F., Brzt J d p p l . Physzcs 12, 705 (1961) (17) Thorp, J. S., Pace, J H., Sampson, D F J . Electron Control 10.138 (1961). (18) Thorp, J. S.,Pace, J. H., Sampson, D. F., Proc. Phys. Soc. ( L o n d o n ) , Vo1.
LXXVI, 697 (1960).
RECEIVEDfor review July 7 , 1961. aIccepted April 5, 1962. VOL. 34, NO. 8,
JULY 1962
971
