Spectrophotometric Determination of Aluminum in Nickel-, Iron-, and Copper-Base Alloys SIR: I n this work, aluminum at the 0.005 to 1% level is rapidly and accurately determined in a wide variety of alloys, including nickel- and iron-base materials by combining a sodium hydroxide separation of aluminum with a n extraction of its yellow 8-hydroxyquinoline complex. Masking agents used with the caustic separation prevent serious coprecipitation of aluminum. Variation of sample size, and/or dilution allows the determination of between 0.0005 and 5% aluminum. A spectrophotometric method is ideally suited for the determination of less than 0.1% of aluminum. The most frequently used spectrophotometric methods make use of a colloidal suspension or lake of aluminum with a dye substance. This method requires close control of pH and time, in addition to preliminary separations (11). A more useful method would be one which is not based on a lake formation. Such a method is the chloroform extraction of aluminum 8-hydroxyquinolate. One advantage of this procedure is that it produces a stable yellow solution. However, many metals interfere with this chromogenic system and extensive separations are necessary. Of the elements which are likely t o interfere, iron(III), iron(II), nickel, and copper all form complexes with 8-hydroxyquinoline. Other chromogenic reagents form true solutions with aluminum but the 8-hydroxyquinoline system was chosen for evaluation because it has been extensively studied, and the effect of masking agents is known. Extraction and electrolysis with the mercury cathode are most useful preliminary separation methods for aluminum (2). Precipitation methods are very seldom considered for the separation of trace amounts of aluminum because of the difficult separation from the many metals which are more basic than aluminum. Thus,
Table 1. Guide for the Selection of Sample Size and Aliquot
Sample wt.,
grams
1608
Aliquot from
per aliquot
250-ml. pg.
flask
ANALYTICAL CHEMISTRY
full advantage of the amphoteric nature of aluminum has not been taken because of serious coprecipitation of aluminum on metal hydroxides--e.g., iron and nickel prevent complete recovery of aluminum (6). The procedure consists of two steps after dissolution of the sample and preparation of a calibration curve. First, sodium aluminate is separated from interference by a sodium hydroxide precipitation of hydrous metal oxides in the presence of boric acid and sodium cyanide. The precipitate and supernatant liquid are diluted to volume and an aliquot of the sample is taken after filtration or centrifugation. A complete filtration is not necessary as only a portion of the supernatant liquid is needed for complete analysis. Second, the yellow 8-hydroxyquinolinealuminum complex is extracted into chloroform, its absorbance measured spectrophotometrically, and the results are compared with a calibration curve.
terference is encountered with chromium (VI). Therefore, it is possible to avoid this interference by either of two methods: first by fuming the sample with perchloric acid to oxidize ohromium(II1) to chromium(VI), or second by boiling the sample with sodium hydroxide to ensure complete precipitation of chromium(II1). SEPARATION.Add about 0.4 gram of iron as iron(II1) nitrate, if the weighed portion of the sample is known to contain less than 0.1 gram of iron. Dissolve a mixture of 40 grams of sodium hydroxide, 6 grams of boric acid, and 10 grams of sodium cyanide with 100 ml. of water in a 250-ml. beaker made of Teflon (du Pont). Platinum containers may also be used. Stir the mixture until dissolution is complete. The temperature of the caustic solution should be 70' f 10' C. before the precipitation. It is not necessary to heat the sodium hydroxide solution if the sodium hydroxide is dissolved a few minutes before the precipitation because the solution is sufficiently warmed by the dissolution process. Cautiously pour the sample into the hot caustic solution while stirring. If a filterable EXPERIMENTAL amount of iron(II1) hydroxide is not observed, a solution containing 0.4 Reagents. STANDARD ALUMINUM gram of iron(II1) is added. Transfer the caustic solution of the sample to a SOLUTION, 25 gg./ml. Dissolve 250-ml. volumetric flask, cool, dilute to 0.0500 gram of super-pure aluminum volume, and mix well. Place boro(99.995%), British Chemical Standard silicate-glass beakers beneath funnels 198D, in hydrochloric acid and dilute fitted with Whatman No. 52 filter paper. to 2 liters. Filter a portion of the sodium hySTANDARD ALUMINUM SOLUTION, 2.5 droxide solution and discard the first pg./ml. Transfer 10 ml. of the above 15ml. fraction. Substitute a dry solution to a 100-ml. volumetric flask beaker made of Teflon (du Pont) for the and dilute to volume. borosilicate-glass beaker. BUFFER,p H 9. Mix 207 grams of Strong sodium hydroxide solutions ammonium chloride with 266 ml. of should not be in contact with glassware concentrated ammonium hydroxide and for long periods, as aluminum may be dilute to 2 liters. introduced into the sample (10). Pipet 8-HYDROXYQUINOLINE, 1%. Dissolve an appropriate aliquot (see Table I) 1 gram of 8-hydroxyquinoline, 8-quinofrom the clear filtrate and place in in a linol, HOCsH&, mol. wt. 145.16, in 150-ml. beaker. 100 ml. of chloroform. Store the soluCOLORDEVELOPMENT.Adjust the tion in a dark bottle. Prepare this pH of the aliquot to between 9.5 and solution fresh daily. 10 with hydrochloric acid. Add 10 ml. All chemicals were reagent grade. of pH 9 buffer, and transfer the solution Procedure. DISSOLUTION. Acto a 125ml. separatory funnel fitted curately weigh the sample into a with a stopcock plug made of Teflon. 150-ml. beaker. Select a sample Dilute to about 100 ml. with water. weight so t h a t the aliquot will contain Add 10 =t 1 ml. of 1% 8-hydroxy10-100 gg. of aluminum (see Table I). quinoline in chloroform and equilibrate The procedure is scaled to a dilution of the phases for 1 minute. Allow the 250 ml. Dissolve the sample-e.g., phases to separate and draw off the 20 ml. of hydrochloric acid and 5 ml. of lower layer into a 50-ml. volumetric nitric acid are normally a satisfactory flask. To ensure a complete transfer, solvent. Boil off the oxides of nitrogen add two 5 m l . portions of chloroform and dilute to about 40 to 50 ml. with and add these rinsings to the flask. water. If chromium is present, add Be careful not to transfer any solid 5 ml. of perchloric acid, and fume. matter which may cling to the interface More than 0.1 gram of chromium(II1) with the chloroform as it may cause will cause low results because of the the blank to increase. Dilute the reaction between chromium(II1) hysample to volume with acetone and droxide and sodium hydroxide to form measure its absorbance at 390 mp in chromites. Precipitation of chromium 1-cm. cells against acetone. A reagent (111) hydroxide is complete on boiling blank as well as a standard are carried with sodium hydroxide solution. NOin-
through this procedure. The blank has
a value of 0.025 f 0.01. Calculate the per cent aluminum after reference to a calibration curve prepared in the same fashion. The molar absorptivity of the tris-(8-quinolino1o)-aluminum(II1)complex in chloroform-acetone at 390 mp is 6970 liter/mole cm. A calibration factor of 195 pg./absorbance was obtained with the Cary Model 14 spectrophotometer for 1-cm. cells and a final volume of 50 ml. Beer’s Law is followed to a t least 200 pg. DISCUSSION
The proposed separation scheme is similar to the system used for the determination of magnesium in aluminum-base alloys (I) wherein the dissolution, separation, and masking steps are telescoped into one step. In the present procedure, after dissolution of the sample, the sodium hydroxide separation step is performed in the presence of masking agents. The separation produces a solution of aluminum which can be used for the determination with &hydroxyquinoline, almost without interference. Separation and/or masking is required before the 8-hydroxyquinoline method can be used (6) because a t pH 9 it reacts with aluminum, antimony, bismuth, cadmium, cerium, cobalt, copper, lead, mercury, iron, nickel, tin, titanium, uranium, and zinc. For instance, more than 50 pg. of iron interferes and is usually removed by electrolysis over the mercury wthode along with other interferences : molybdenum, cobalt, copper, nickel, etc. Then titanium and vanadium are separated with cupferron. After a separation step (4), complete extraction of aluminum can be obtained by equilibrating 50 ml. of pH 8.9 buffer with a single 10-ml. portion of 1% &hydroxyquinQhe solution in chloroform. Extraction is complete in the pH range 4.5 to 6.5 (acetate buffer) and 8.0 to 11.5 (ammonia buffer). In the pH interval the 6.5 to 8.0 extraction is not complete as insoluble aluminum hydroxide forms. When aluminum was determined in thorium (8),some acetate was extracted musing ionization of the 8-hydroxyquinoline and producing an absorption band a t 370 mp. This interference is avoided by using the ammonia buffer system. A pH 9 ammonia buffer also eliminates any interference from zirconium, molybdenum(V1), vanadium (V), beryllium, magnesium, manganese, and the rare earths. A method has been reported for the spectrophotometric determination of aluminum in steel with 8-hydroxyquinoline (6, 7). This method does not require extensive separations for steels if the aluminum level is greater than 0.08% and if the portion of the sample used for analysis contains no more than 0.017 mg. of
Table 11.
Sample no. 1
2 3 4
5 6 7 8
9 10 11 12 13 14 15 16 17
Evaluation of the Sodium Hydroxide Separation of Aluminum with 8-Hydroxyquinoline Wt. added, grams Aluminum, pg.
NaOH
H~BOI
...
... ...
40
... 40 40 40 40 40 40 40
40 40 40
40 40 40 40
... ... 3 3
3 3 3 3
3
3 3 3 3 3 6
NsCN
...
... ... ... ... ... ... ... *.. 3 3
3
3
3 3
10
titanium. Thus, some metallurgical systems could be analyzed without a separation step but the method would not be generally applicable to a variety of matrices. Another procedure requires about 3 hours (3) and uses the 8-hydroxyquinoline extraction after an amyl acetate extraction of iron(II1). Separation of aluminum from other metals by the precipitation of the latter with sodium hydroxide is a t best a dubious method due to the coprecipitation of aluminum with other metal hydroxides (13). Oelschliiger (IO) used the separation successfully but he carried standards through the procedure which compensated for any loss of aluminum by coprecipitation. Hill (5) showed that the caustic separation of aluminum from iron could be made if the loss of aluminum by coprecipitation with iron(II1) hydroxide was prevented. He accomplished this by introducing relatively large amounts of an element similar in chemical behavior to that of aluminum, but not reactive with the chromogenic reagent. Boric acid improved the separation. He applied this method t o iron-base materials successfully. Therefore, it is possible to use sodium hydroxide for the separation of aluminum from the other elements which are likely to be present in iron- and nickel-base alloys; that is, to precipitate chromium, copper, cobalt, iron, magnesium, manganese, nickel, the rare earths, titanium, niobium, ’ tantalum, and zirconium. In addition to aluminum, the elements which are not precipitated by sodium hydroxide are antimony, arsenic, tin, lead, zinc, gold, tungsten, molybdenum, chromium, vanadium, and beryllium. Thus, the only elements which should react with 8-hydroxyquinoline st pH 9 after a sodium hydroxide separation are aluminum, antimony, lead, and tin. Antimony, lead, and tin are likely to be present but a t such low levels that interference is unlikely because the
Fe(II1)
Ni(I1)
Added
... ... ... ...
... ... ... ... ... ... ... ...
50 50
0.3
0.6 1.0
1.0
... ..* ... ...
0.3 0.5
1.0
1.0
1 .o
0.5
25 25
10 10 10 50 50
Found 50 51
25
24 11 11 12 48 4
0.5
1.0 0.5 1.0 0.3
50 50
47
50 25
53
0.5
50 50 50
48 48 48
1.0 1.0 1 .o
50
23 50
sensitivity of these complexes with 8hydroxyquinoline is only about 0.1 the sensitivity of the aluminum complex. Table I1 shows the data obtained in evaluation of our caustic separation of aluminum from iron and nickel. The method described in the procedure is used to evaluate the recovery of aluminum. For instance, in samples 1 through 4 we see that there is no interference when as much as 40 grams of sodium hydroxide is added to the sample contained in a 250-ml. flask. Samples 5 through 8 show that boric acid is effective in preventing any coprecipitation of aluminum with iron (111) hydroxide. When nickel is added to the system, aluminum is coprecipitated, as is illustrated with samples 9 and 10. The addition of cyanide prevents the precipitation of nickel with sodium hydroxide and eliminates this interference, as is illustrated by samples 11 and 15. The nickel cyanide complex does not react with the reagent (4).Mixtures of iron and nickel do not produce any interference as demonstrated with samples 13-17. A %minute equilibration period or multiple extractions are required to extract aluminum with 8-hydroxyquinoline ( 3 ) . However, in this work the extraction was 60% complete after 2 seconds and 97% complete in 10 seconds. A minute was sufficient equilibration period for complete extraction. A 1% solution of Ei-hydroxyquinoline in chloroform was used throughout this investigation. When I-, 5-, IO-, 1 5 , and 20-ml. portions of this solution were diluted to 50 ml. with acetone and their spectra recorded, there was no absorbance in the region from 380 to 450 mp. The absorbance began to increase rapidly at 380 mp; therefore, it would be difficult to increase the sensitivity of the method by making absorbance readings a t shorter waveVOL 38, NO. 1 1 , OCTOBER 1966
* 1609
lengths. After extraction, the absorbance due to the blank at 390 mp was measured as a function of 8hydroxyquinoline concentrations. No variation in absorbance was noted for 5 t o 15 ml. of the reagent. Similarly, the recovery of aluminum was constant when the volume of 8-hydroxyquinoline was varied from 8 to 11 ml.; therefore, it was not necessary to pipet the reagent. RESULTS
The usefulness of the proposed method was evaluated on the standard samples shown in Table 111. Accurate results are obtained for the range 0.005 _ _ ~
~
Determination of Aluminum in Various Standards with 8-Hydroxyquinoline after a Sodium Hydroxide Separation
Table 111.
Number CONi NBS 673 NBS 672 NBS 671 NBS 169 B 5967 B 5789 B 5871 NBS 162a NBS 349 BCS 310 NBS 1193 NBS 1194 NBS 1190
Description Nickelbase High-Purity nickel Nickel oxide 3 Nickel oxide 2
Waspaloy Nimonic alloy 90 (International Nickel Co.) High temperature alloys W 545 A 286
NBS 1174 NBS 55e BCS 326 NBS 1164 NBS 1166 BCS 327
Udimet 500 Copper-Base alloys Cartridge brass A Aluminum brass C Aluminum brass B Aluminum brass A Iron-Base ailoys White cast iron Cast iron Mild steel Low-alloy steel D Ingot iron Mild steel
INCO 74 INCO 73 NBS 1168 NBS l l l b NBS 1156 BCS 328
Maraging steel Maraging steel Low-Alloy steel Ni-Mo steel Maraging steel Mild steel
INCO 11 BCS 329
Maraging steel Mild steel
NBSc 1100 NRS 1120 NBS 1119 NBS 1118
Low-Alloy steel G Ni 26-Cr 15 steel Maraging steel Maraging steel Cr-Mo-AI steel Miscellaneous Refined silicon NBS 57 Zn-Die casting alloy NBS 94b Ti-base (4 A14 Mn) NBS 174 Permanent magnet alloy BCS 233 a Polarographic method ( I d ) . a Spectrophotometric method (3). NBS 1167 NBS 348 B 5877 INCO 75 NBS 106b
1610
c
to 5% aluminum in nickel-, iron-, and copper-base alloys as well as in other materials. The reported values listed in Table I11 are certified for the (NBS) National Bureau of Standards or (BCS) British Chemical Standards materials. The BCS standard 327 and 329 contain 0.03 and 0.018% antimony, a potential interference. Results were not high because of the presence of antimony. It is interesting to compare the results with the uncertified values for acid soluble aluminum-i.e., BCS 329 was reported to contain between 0.048 and 0.057%. The proposed method is designed for the determination of acid soluble aluminum. It can be modified and total
ANALYTICAL CHEMISTRY
Aluminum, yo RepM. Found 0.0007 i 0.0005 0.001 0,004 0,009 0.09 0.09 0.09 0.18 0.50 1.23 1.43
0.088, 0.091
0.21 0.39 2.83
0.22, 0.23 0 . 3 9 , 0.41 2.81, 2.81
0.008 1.46 2.14 2.80
0,009 1.47 2.14 2.82
(0.001) 0.002 0.005 0.005 0.015 0.020, 0.016", 0.016b 0.04 0.04 0.042 0.043 0.047 0.048, 0.046", 0.047a 0.05 0.056, 0.052", 0.052a 0.16 0.23 0.21 0.25 1.07
0.002, 0.003 0.002, 0.002 0.005, 0.005 0.005, 0.005 0.016, 0.017 0.018, 0.017
0.67 4.07 4.27 6.98
0.0008 0.001 0.005 0,009
0.081, 0.080 0.081, 0.081 0 . 1 6 , 0.16 0 . 4 8 , 0.48 1.19, 1.20 1.38, 1.38
0.039, 0.039 0.035, 0.036 0.036, 0.032 0,046, 0.046 0.047, 0.048 0.044, 0.043 0.051, 0.049 0.054, 0.054
*
0.15 0.006 0.23, 0.23 0.22, 0.21 0.26 f 0.01 1.03, 1.03 0.66 4.15 4.28 7.06
aluminum determined by adding a filtration step after the dissolution, and treating any residue with a fusion (.a). The NBS steel l l l b (Table 111) was analyzed by this procedure and the acid soluble aluminum was 0.033%. The values given in Table I11 for NBS l l l b represent total aluminum; similarly, all values given under * "Reported" are total aluminum values. There are no certified analyses for acid soluble aluminum; however, approximate values for the BCS 326 series are available. They indicate that the level of acid soluble aluminum accounts for between 64 and %yo of the total aluminum. The general trend toward low results can be attributed to the presence of some acid insoluble aluminum. It is interesting to contrast the certificate values for BCS standards 327, 328, and 329 and the independent results obtained by different procedures. I n general, the agreement is good. The results obtained by the proposed method are in good agreement with a polarographic procedure (18) and also with a multiple-separation spectrophotometric procedure (3). The precision and accuracy of the method was measured by analyzing a low-alloy steel (NBS 1167) and maraging steel (INCO 75) 18 and 16 times, respectively. The standard deviation for these samples is 0.15 f 0.006 and 0.26 f 0.01, see Table 111. The relative standard deviation and mean error were calculated for 1167 as 3.7 and 0.001, and INCO 75 gave values of 4.5 and 0.003 for the relative standard deviation and mean error. The average accuracy is 7y0. By using a 10-gram sample, it was possible to apply this method to the analysis of aluminum in high purity nickel. A single determination gave a value of 8 p.p.m. According to the solid mass spectrographic determinations, this type of nickel contains 7 f 5 p.p.m. The method was primarily intended for the determination of aluminum at the 0.05 or 0.1% level. The data in Table I11 show that there is not a n appreciable error when the method is extended t o the 1% or even 5% level by taking a small sample weight and using a smaller aliquot. The method can be applied to samples which would normally be analyzed by gravimetric or volumetric (8) methods. A magnesium-base sample (NBS 171) was analyzed by this method and the results were low by about 15%. A p parently, magnesium interferes with the sodium hydroxide separation procedure by causing some aluminum to be coprecip%ated. No provision has been made in the procedure to compensate for any interference which might occur due to magnesium. A single sample can be analyzed in
0.5 hour after dissolution of the sample. Twenty samples can be analyzed by an analyst in a single work day. ACKNOWLEDGMENT
(2) Burke, K . E., Davis, C. M., ANAL. CHEM.36,172 (1964). (3) Dagnall, R. hf., West, T. S., Young, P., Analysl90, 13 (1965). (4) Gentry, C. H. R., Sherrington, L. G., Ibzd., 71,432 (1946). (5) Hill, U. T., ANAL. CHEM.31, 429
The author t'hanks Thomas Ruppert for his assistance.
(10) Ibid., p. 329; Ibid., 2124. ( 1 1 ) Pakalns, P., Anal. Chim. Acta 32, 57 (1965). (12) Rooney, H. R. C., Analyst 83, 54.6 (1958). (13) Strafford, N., Wyatt, P. F., Ibid., 68,319 (1943). KEITHE. BURKE
The International Nickel Co., Inc. Paul D. Merica Research Laboratory Sterling Forest, Suffern, N. Y. 10901
LITERATURE CITED
( 1 ) Burke, K . E., Anal. Chim. Acta 34,
Presented, 152nd hleeting, ACS, New York City, September 1966.
485 (1966).
Analysis of Methyl Octadecenoate and Octadecadienoate Isomers by Com bined Liquid-Solid and Capillary Gas-Liquid Chromatography SIR: Liquid-solid chromatography on silver nitrate-impregnated adsorbents is frequently used for the separation of unsaturated fatty acid methyl esters according to their geometry (cis and trans) or degree of unsaturation (monoene, diene, triene, etc.) (1,6,16). Capillary gas-liquid chromatograply is used extensively for the study of complex mixtures of isomeric esters, particularly the methyl octadecenoates (11, IS) and octadecadienoates (10, 12, 16). However, almost all reported analyses of fatty acid methyl esters on capillary columns have been confined to the separation of postional isomers of a given geometry or geometrical isomers at a given bond position. We have applied a combination of the above chromatographic techniques to the analysis of partially hydrogenated vegetable oils. The successful analysis of samples containing such a multiplicity of isomeric fatty acids has not been reported previously. I n the present work liquid-solid chromatography on silver nitrate-silica gel was used to separate monoenoic fatty
acid methyl esters into trans and cis isomer fractions and dienoic methyl esters into several distinct fractions depending upon geometry and bond position. These fractions mere then analyzed by infrared spectrometry, capillary gas chromatography, and oxidative cleavage to give a fairly complete picture of the fatty acid composition of partially hydrogenated fats and oils. A similar combination of nicthods was recently used by the authors to analyze a much less complex mixture of internal olefins (8). EXPERIMENTAL
Preliminary Treatment of Sample. Methyl esters of the fatty acids were prepared from the fat or oil by basecatalyzed alcoholysis according to procedures previously described ( 7 , 8). For oils containing appreciable dienoic fatty acids, the monoenoic and dienoic methyl esters were first separated by liquid-solid chromatography of their mercury derivatives (8). The unsaturated ester fractions thus isolated were then further separated by chromatography on silver nitrate-silica gel as described below. For samples of 21.5%
38.1%
13.3%
6.1%
-
-I
Sample 0.6309. Recovery- 100 %
ro
Eluant 60 140 PE1 Benzene
50150
"
'I
40160 'I " 30170 " '' 901 I O Benrene/Ether
O/lOO
"
'I
OJ-
IO
20
30
40
FRACTION NUMBER
50
60
I 70
Figure 1. Liquid-solid chromatography of Cis dienoates from hydrogenated corn oil
low or negligible diene content the methyl esters can be chromatographed directly on silver nitrate-silica gel. Liquid-Solid Chromatography. The technique used here was a modification of that reported by DeVries (5, 6). Silica gel impregnated with about 30'% silver nitrate was used in 39- x 2-cm. columns cont.aining 75 to 80 grams of adsorbent. The adsorbent was sometimes mixed with Celite 545 filter aid in 20: 1 or 30: 1 ratio to facilitate solvent flow. Sample sizes averaged about 1.0 gram for saturated-monoenoic methyl ester mixtures and 0.65 gram for dienoic methyl esters. The chromatography required 20 to 48 hours depending upon the complexity of the sample. Petroleum ether-benzene mixtures were used to elute the various components: 80:20 for the saturated esters, 70:30 for the trans monoenoic esters, and 55: 45 for the cis monoenoic esters. For dienoic ester mixtures, the elution was started with 60: 40 petroleum ether-benzene, followed by 50:50, 40:60, and 30:70 mixtures. The final dienoic ester fraction was eluted with 9O:lO benzeneethyl ether. The composition of these fractions will be discussed later. Ethyl et,her was used to strip the column of the non-ester components. The "valleys" 1
I
125
I20
TIME, MINUTES
Figure 2. Gas chromatographic separation of trans Cls monoenoate isomers VOL 38, NO. 1 1 , OCTOBER 1966
1611
