Chemical Microscopy of the Platinum Metals Reactions with 4-Bromoisoquinoline and 4-(p-Nitrobenzyl) pyridine HAROLD F. SCHAEFFER Wesfminsfer College, iWton, Mo.
b
By chemical microscopy certain platinum metals and gold can be detected or identified b y dilute acid solutions of 4-bromoisoquinoline and 4-(p-nitrobenzyl)pyridine. In a 0.033ml. test drop a solution of 4-bromoisoquinoline will readily detect or identify 0.5 pg. of platinum by separation of a characteristic microcrystalline derivative. Everi at this dilution (1 part in 75,000 parts of sample), an excess of cupric ion does not interfere. In the presence of cupric ion the reagent may also detect 1 1 pg. of gold or 4.5 pg. of iridium. A 3% solution of 4-(pnitrobenzyl)pyridine in 2N hydrochloric acid will detect or identify 1.7 pg. of platinum, 7.5pg. of palladium, or 1 1 pg. of gold. It can also detect 16 c,g. of osmium in a fresh acidified solution of potassium osmate. One advantage of 4(p-nitrobenzy1)pyridine reagent over pyridine i s the absence of offensive odor.
S
previous work (1) showed that unsubstituted isoquinoline is a suitable reagent for detecting certain precious metals, it was considered desirable to determine whether 4bromoisoquinoline mig;ht offer advanINCE
tages as a reagent for the platinum metals and gold by methods of chemical microscopy. Furthermore, since Whitmore and Schneider (2) in 1935 described the use of pyridine hydrochloride in the detection of certain platinum metals and gold, it was also decided to investigate 4-(p-nitrobenzyl)pyridine as a reagent for this group. EXPERIMENTAL
To serve as a reagent the 4-bromoisoquinoline was made up as a 0.4M solution of base in 1N hydrochloric acid. This solution remained stable for many months. Tests were carried out by adding a small drop of reagent solution to a drop of test solution on a microscope slide. Uniformly small drops of approximately 0.033 ml. each were obtained by using commercially available melting point capillaries as pipets. Except in the case of osmium, test solutions of the metals consisted of the specified concentrations of their chlorides in I N hydrochloric acid. Most observations through the microscope were made with the 16-mm. objective and a 1OX ocular. In tests using 4-(pnitrobenzyl)pyridine, the reagent consisted of a 3% solution (approximately 0.14M) of the base in 2N hydrochloric acid. The procedure was identical with that employed with 4bromoisoquinoline.
Figure 1. Precipitate obtained from 4-bromoisoquinoline reagent in presence of plantinic chloride Pt: 1 part in 10,000 Approximately 150X
RESULTS AND DISCUSSION
The 4-bromoisoquinoline reagent showed its maximum sensitivity in the test for platinum. In uncovered preparations numerous characteristic yellow crystals (Figure 1) separated very promptly when the platinum concentration was as low as 1 part in 50,000. A satisfactory test was obtained within 2 to 21/2 minutes when the platinum concentration was as low as 1 part in 75,000. Vnder the conditions of the test this was equivalent to the detection of approximately 0.5 pg. of platinum in a 33-mg. test drop. There n-ere no brilliant polarization colors when the crystals were viewed between crossed Nicol prisms. Extinction appeared to be oblique, a t approximately 30". The presence of cupric ion offered no interference. Even in a solution which was 0.12M in respect to cupric chloride, satisfactory results were obtained with a platinum concentration of 1part in 75,000. A solution of 0.5Q/, osmium tetroxide in 1N hydrochloric acid failed to yield any type of precipitate with the bromoisoquinoline reagent; however, solutions of potassium osmate in 1N hydrochloric acid readily yielded an offwhite precipitate. Under the micro-
Figure 2. Reaction between 4-bromoisoquinoline and iridium trichloride IR
1 part in 1000 150X
VOL. 36, NO. 1, JANUARY 1964
0
169
Figure 3.
Test for gold with 4-bromoisoquinoline reagent Au:
1 part in 2000
Figure 4. Platinum precipitated dine reagent
1 SOX
1 part in 3000 150X
Pt:
scope much of the precipitate appeared t o consist of very fine material which was not distinctly resolved. Although such material was also present a t various lower concentrations, the results did not appear sufficiently reproducible t o constitute the basis for a good microscopic test. Acid solutions of iridium trichloride promptly yielded a derivative consisting of small amber-colored crystals which gradually increased in size within several minutes (Figure 2). Good results were obtained with an iridium concentration of 1 part in 7500, which corresponds t o the detection of 4.5 pg. When viewed by polarized light, these crystals were not pleochroic. Furthermore, between crossed Nicols they retained an amber color. Extinction of isolated fine individual components was oblique. Moderate concentrations of cupric ion offered no serious interference. Even in the presence of 0.12X cupric ion iridium could be readily detected a t a concentration of 1 part in 4000. By allowing 2 to 2l/2 minutes for the appearance of crystals, the test could be obtained a t an iridium concentration of 1 part in 6000, while the cupric ion concentration was still 0.12M. When the reagent was applied t o dilute acid solutions of auric chloride, Table I.
Ru Rh
Pd
os Ir Pt Au
the resulting crystals were essentially thin needles of a deep yellow color (Figure 3). Between crossed Nicols these crystals exhibited various dispersion colors (not dichroic) ; extinction was parallel. Good tests were obtained at a concentration representing 1 part of gold in 3000. The presence of cupric ion appeared to have no deleterious effect on the test for gold, as the characteristic crystals were still obtained with a gold concentration of 1 part in 3000, in the presence of 0.12X cupric chloride. With a 0.5% solution of ruthenium trichloride in 1N hydrochloric acid the bromoisoquinoline reagent caused the formation of numerous yellow- to light amber-colored, rod-shaped crystals, as observed under a 4-mm. objective and 10 X ocular. When the test was applied t o a sample containing 1 part of ruthenium per 1000 very small crystals slowly separated in some instances. However, results were not reproducible, and the controlling factors were not ascertained. Although the reagent was not considered satisfactory for ruthenium, the presence of 1 part per 1000 of this metal did not interfere with tests
comparison of 4-Bromoisoquinoline and lsoquinoline Reagents for Precious Metals
Metal
4Bromoisoquinoline Detected, w Concn. ... n
Isoquinoline Detected, erg.
5
...
Concn. 1/6500
...
rl (I
1/1500 l/lOOO 11 1/3000 6.5 1/5000 a Precipitate may be formed, but is not composed of characteristic crystalline material, or is not reproducible. 170
4.5 0.5
ANALYTICAL CHEMISTRY
by 4-(p-nitrobenzyl)pyri-
1/7500 1/75,000
22 33
for gold or platinum a t the same concentration. Palladous chloride solutions failed to give any crystalline product with the reagent. Although a palladium concentration of 1 part per 1000 yielded an immediate precipitate, it was not composed of characteristic crystals; instead i t appeared to consist of fine amorphous material which was not resolved under a 4-mm. objective. From solutions of rhodium chloride, fine amorphous material separated gradually, not satisfactory for identification purposes. Both palladium and rhodium should be absent when testing for other members of the platinum group. Although solutions of platinum, iridium, and gold salts individually yield crystals representative of the respective metals, it is not generally feasible to detect the individuals in mixtures. Results obtained with the Cbromoisoquinoline reagent are summarized in Table I. Comparing the 4-bromoisoquinoline reagent with the unsubstituted base, it appears that one advantage of the former is its greater sensitivity toward platinum and iridium. Isoquinoline, on the other hand, can detect as little as 5 pg. of ruthenium, which yields no precipitate with the bromo derivative. Neither reagent detects rhodium, palladium, or osmium. In the presence of any of these three, isoquinoline causes no separation of a solid product, while the bromoisoquinoline yields only noncharacteristic amorphous material with rhodium and ruthenium. The 4-(pnitrobenzyl)pyridinereagent was most efficient in the detection of platinum. With dilute acid solutions of platinic chloride the reagent caused the separation of a yellow precipitate. As
Figure 6. Figure 5. One type of crystal form obtained in test for palladium by 4-(p-nitrobenzyl)pyridine reagent
Figure 7.
Fine particles are principally droplets of yellow liquid Original Au concentration 1 part in 2000
lS0X
Pd: 1 part in 1000 150X
seen under the microscope this included long needles, daggers, and thin plates which frequently resctmbled deformed square plates. Just which forms predominated in a given preparation appeared to depend upor1 room conditions (temperature and hu nidity) and the concentration range of the platinum (Figure 4). Occasionttlly a few of the aggregates included six-pointed stars, resembling snow flaker, in form. Good tests could be obtained on a drop containing 1.7 pg. of pl(ttinum-i.e., in a solution containing 1 part of the metal in 20,000. Between crossed Nicols the color of the crys;als varied from white to very faint ydlow, depending upon thickness. Extinction was parallel. The presence of appreciable amounts of cupric ion affected the sensitivity of the test. I n a solution containing 1 part of phtinum in 10,000
Test for gold with 4-(p-nitrobenzyl)pyridine
in the presence of 0.12M cupric chloride (8 parts of cupric ion per 1000), the test was positive but the crystals appeared somewhat smaller. Ruthenium interfered by preventing the separation of a crystalline product, or yielding a precipitate which was not characteristic. A sample having a platinum concentration of 1 part in 2000 gave a positive test in the presence of a rhodium concentration of 2 parts per 1000; however, this concentration of rhodium prevented a positive test in a solution in vihich the platinum represented only 1 part in 10,000. Addition of the reagent to a solution of palladous chloride resulted in a faint turbidity, caused by the separation of small crystals which developed into very thin plates or serrated blades (Figure 5 ) . In some instances the aggregates were in the form of very
Test for iridium by 4-(p-nitrobenzyl)pyridine Ir: 1 part in 1000 l50X
thin plates crossed a t right angles. Individually the smaller crystalline forms appeared practically colorless, but sufficiently large crystals or groups were yellow t o light tan in color. Good tests resulted with a palladium concentration as low as 1 part in 4500, corresponding to the detection of approximately 7.5 pg. Crystals appeared practically white between crossed Kicol prisms, and exhibited parallea extinction. At relatively high concentrationsLe., l%--auric chloride solutions promptly reacted to form a pale yellow precipitate, but dilute solutions produced an off-white turbidity. The 4-mm. objective revealed that the reaction first gave rise t o innumerable fine, oil-like globules, which ultimately developed into pale yellow crystalline forms. One type of basic structure consisted of very thin parallelopipeds, while some developed into jagged blades; others developed as branched aggregates (Figure 6). Satisfactory tests were obtained with gold concentration as low as 1 part in 4000, the equivalent of approximately 8 pg. per test drop. Appreciable concentrations of cupric ion to some extent interfered with the test for gold. TThen performed on a solution containing 1 part of gold in 2000, in the presence of 0.12M cupric chloride, an abundance of the yellow globules separated, but very few crystals developed. Appearance of thin crystah between crossed Kicols was white; extinction was oblique. Very interesting results followed addition of the reagent to solutions of iridium trichloride. Soon after mixing, the preparation turned turbid, and within 5 to 10 seconds the microscope revealed the appearance of characteristic red-amber crystals. In addition to very small individual prisms, there were crosses, daggers, and mosslike VOL. 36,
NO. 1 , JANUARY 1964
171
Table 11. Behavior of 4-(p-Nitrobenzyl)pyridine Reagent
Metal Ru Rh Pd
Detected,
os Ir
Pt Au
UfZ.
... ...
7.5
16 11 1.7
8
Concn.
... 1/4500 1/2000 1/3000
1/20,000 1/4000
forms (Figure 7). Frequently some buff-colored amorphous material also appeared. The color of crystals between crossed Nicols was a reddish amber; small individual components exhibited parallel extinction. Detection of iridium wm reliable in samples containing 1 part of the metal in 3000, or approximately 11 pg. per test drop. At an iridium concentration of 1 part per 1000, the test was not reliable in the presence of 0.12M cupric chloride. Although a 0.5% solution of osmium tetroxide in IN hydrochloric acid failed to yield any crystalline precipitate with the nitrobenzyl pyridine reagent, interesting results were obtained with a
freshly prepared acid solution of potassium osmate. A 1% solution of the latter, containing about 5 parts of osmium per 1000, promptly yielded a copious white precipitate having the macro appearance of thin glistening flakes. When precipitates obtained in more dilute solutions were examined under a 16-mm. objective the material was seen to consist of very thin plates, many of which occurred in clusters resembling small blossoms. Individual components appeared as colorless lenticular or cigar-shaped forms; a few assumed shapes of double-ended chisels. In general, reliable identification was possible a t dilutions representing 1 part of osmium in 2000, or 16 pg. in a drop. Appreciably higher dilutions did not generally give positive tests within a reasonable time. The test appears adversely affected when room temperature rises above about 23' C. Between crossed Nicols the color was white, and extinction was parallel. A resume of results obtained with the 4(p-nitrobenzyl)pyridine is shown in Table 11. Under the conditions described above, unsubstituted pyridine was not a very satisfactory micro reagent for the
precious metal salts. The sole exception was in the reaction with auric chloride solution, which could be detected in a test drop containing 13 pg. of gold. Whitmore and Schneider (2) identified a larger number of precious metals by this reagent through the formation of microcrystalline derivatives; however, they were working in a higher concentration range (1 to 2% solutions of the metal salts), and they applied the reagent in the form of the pure solid-Le., pyridinium chloride. One unpleasant feature of the latter is its objectionable odor. Unfortunately, 4(p-nitrobenzyl)pyridine did not prove satisfactory for identifying specific platinum metals (or gold) in mixtures containing two or more members of the group. LITERATURE CITED
(1) Schaeffer, H. F., ANAL. CHEM., 31, 1111 (1959). ( 2 ) Whitmore, W. F., Schneider, Herman, Mikrochemie 17, 279-319 (1935). RECEIVED for review February 1, 1963. Accepted October 11, 1963. Based on a paper read before the Division of Analytical Chemistry, 139th Meeting, ACS, St. Louis, .hIo., March 1961. Work made possible, in part, by a grant from the
Research Corp.
Chelometric Determination of Aluminum KEITH E. BURKE and C. MANNING DAVIS Research Laborafory, The Infernafional Nickel Co., Inc., Bayonne, N. J.
b A rapid and accurate titrimetric method i s described for the determination of aluminum in a wide variety of alloys including nickel-, iron-, and copper-base systems. The mean absolute error is less than 0.02% for a multicomponent, high temperature alloy containing about 3% aluminum.
A
often one of the key elements used in the preparation of precipitation-hardened nickel- or ironbase alloys. Instrumental determinations are frequently impossible because of the lack of suitable standards. In the aluminum industry the gravimetric method most frequently used is that of Rg03 and difference. The ASTM gravimetric method (1) employs several separation schemes prior to precipitation as the 8-hydroxyquinolate or as the hydroxide. Gravimetric procedures are subject to errors of occlusion, tend to be long and tedious, and require an experienced analyst to obtain accurate 172
LuMINubi is
0
ANALYTICAL CHEMISTRY
results. There has been a need for a more rapid method, such as a volumetric method, for some time. This is evidenced by the existence of a ASTM task force (9) on this specific problem. Existing volumetric methods (15) are based on the titration of hydronium ions liberated in the reaction of aluminum and alkali citrate, the titration of hydroxide ion liberated when fluoride is added to a solution of aluminum containing tartrate, and the potentiometric titration of aluminum with sodium fluoride. In the volumetric determination of aluminum (10) in bauxite, iron ore, limestone, etc., fluoride is added to an aluminate solution, the complex fluoride is formed, and an equivalent amount of hydroxyl ion is liberated and subsequently titrated with the standard acid. This method tolerates more impurities than any other procedure; however, it would not be satisfactory for the determination of aluminum in alloys containing nickel, zirconium, titanium, and manganese. Sodium gluconate has been shown to be an
effective complexing agent in volumetric analysis (11). This method has been applied to steels, high temperature alloys, and nonferrous alloys (9). Aluminum is separated from its alloy constituents by the mercury cathode prior to the titration. This method is subject to possible errors in the presence of relatively large amounts of zirconium and titanium. A proposed ilSTPIl method (9) for aluminum in high alloy steels removes interfering elements with a mercury cathode separation followed by an extraction of the cupferrates and a sodium hydroxide separation. Aluminum is precipitated with 8hydroxyquinoline and subsequently titrated (3). P:ibil and Vesl6y (7') recently discussed the chelometric estimation of aluminum and iron using CDTA [ (l,%cyclohexylenedinitrilo)tetraacetic acid]. In the present work, it is proposed t o determine milligram quantities of aluminum in a wide variety of steels and expeiimental alloys by a titrimetric method using CDTX. The
