poses, many substanccs may be removed by volatization. For example, fluoride and cyanide can be volatized in the presence of a strong acid. Silicon may be volatized as fluoride. Chromium may be removed as chroniyl chloride from a strongly acidic medium. Arsenic, germanium, selenium, tin, etc., may he removcd as their halides. ACKNOWLEDGMENT
Thc author is grateful to A. J. Barnnrd, Jr., and TI. F l a s c h b for helpful discussions concerning this work and to Francis J. Warmuth for performing some of the qualitative trsts.
LITERATURE CITED
(1) Bode, H., 2.anal. Chem. 26, 143, 182 (1954); 27,90, 144 (1955). (2) Cheng, K. L., ANAL.CHEM.26, 1038 ( 1954). (3) Ibid., 27, 1594 (1955). (4) Ibid., 28, 1738 (1956). (5) Ibid., 30, 243 (1968). (6) Zbid., p. 1027. ( 7 ) Ibid., p. 1941. (8) Cheng, K. L., Talantu 2 , 61 (1969). (9) Ibid., p. 266. (10) Ibid., 3, 81 (1959). (11) Cheng, K. L., Bray, R. H., ANAL. CHEM.25, 655 (1953). (12) Cheng, K . L., Bray, R. H., Melsted, S. W . ,Zbid., 2 7 , 24 1955). (13) Flaachka, H., hemisl Anal@ 44, 2 (1955). (14) hlajumdar, A. K., Chowdhury, J. B. R., Anal. Chim. Acta 15, 105 (1956).
LI
\ - - - - ,
H., Freiser, H., “Solvent Rxtrnction in Analytical Chemietry,” pp. 169, 162, Wiley, New York,
(15) Morrison, G.
1957. (16) Pfibil, R., Jenik, J., Collection Czechos~ov. Communa. 19, 470 (1954). (17) Schwarzenbach, “Complexometric Titrations,” p. 16, Interscience, New York, 1957. (18) Sediveo, V., Collection Czechoslcv. Chem. Communs. 16, 398 (1951). (19) Scdivec, V., Vasak, V., Ibid., 15, 260 (1950). (20) Vasak, V., Sedivec, V., Ibid., 15, 1076 (1950).
ch.
d.,
RECEIVEDfor review July 13, 1959. Resubmitted February 15, 1961. Accepted March 10, 1961. Presented a t the 12th Annual Summer Sympo~iiim on Analytical Chemistry, Complex Reattione in Analytical Chemistry, University of Illinois, Urbana, Ill., June 10-12. 1959.
The Aluminum-Alizarin Complex as a Measure of Friedel-Crafts Catalysts in Paraffin Hydrocarbon Systems HARVEY POBINER Analytical Research Division, Esso Research and Engineering Co., linden, N. 1.
b A rapid and sensitive spectrophotometric measurement of the aluminumalizarin complex can be used to determine the aluminum halide content of paraffin hydrocarbon systems. The method i s valuable in Friedel-Crafts reactions to measure aluminum halide consumption. Sampling procedures were established to circumvent the problem of hydrolysis, sludging, and equipment corrosion which are inherent in handling aluminum halide.
T
measurement of the aluminum-alizarin complex provides an accurate and sensitive determination of aluminum halide eatalyst in Friedel-Crafts reactions. This paper describes the analytical method and includes a description of the special sample handling procedure developed for this analysis. The method is rapid, accurate, and reproducible. Thc use of alizarin, as a water-soluble sulfonate for the determination of aluminum, was first described by Atack ( 1 ) . Textbooks of colorimetric analyses, such as those by Mellan (6) and Wrlcher (8),cite references pertaining to the analysis of aluminum by alizarin. More recently, Barton (a) presrntcd an ultraviolet photomctric dctcrmination of the aluminum in phosphate rock by alizarin complexing. Contrary to many spectrophotometric dctcrminations of metals in organic systems, the alizarin method for aluHE SPECTROPHOTOMETRIC
790
ANALYTICAL CHEMISTRY
minum, as described herein, docs not involve destruction of the organic components. These organic components are saturated paraffin hydrocarbons and do not interfere with alizarin complexing in methanolic solution. This results in considerable eaving of time. The alizarin method furnishes the required rapid analytical data concerning catalyst consumption to pilot and process units which use aluminum halide catalysts. The alizarin method presents additional advantages in sensitivity and specificity. It offers high sensitivity at low catalyst levels (0.1% and less). This sensitivity could not be matched by an available volumetric method for aluminum (7). Although a potentiometric titration method for bromide (S,4) offered this sensitivity of detection of the catslyst, it could not distinguish between the bromide of AlBra and that of HBr. Three unusual characteristics of a n AIBrpHBr-hydrocarbon system were encountered in this work. First, this system must be protected from air and water t o avoid phase separation due to the formation of sludge and resultant loss of aluminum. Second, the system is highly corrosive to metal equipment and is likely to contain many potentially interfering elements. Third, and last, the ratio of AIBrs to HBr was subject to wide experimental variations which could interfere with the aluminum-
alizarin complexing and the stability of the complex. The AlBr~-I-IBr-hydrocarbon sample is taken into methanol without exposure to air. This is accomplished by handling the sample in a dry box or by eampling directly from the unit into methanol. Alizarin, 1,Mihydroxyanthraquinone, is addcd to form the aluminum-alizarin complex. RH
+ AlX, + 3CHaOH RH + Al(0CHi)s + 3 HX +
Al(0CHa)a
(1)
+ 3 &H
a
/
135
OF.
Alizarin A1/3
/\
? ?
Absorption max. 508 mp
A buffer, ammonium hydroxide plus thioglycolic acid, is added to prevent interference from the excess halogen acid in the sample. The eamplealizarin-buffer solution is heated at 135’ F. to develop the red color of
the aluminum-alizarin complex. The absorbance is measured spectrophotometrically a t the absorption maximum, 508 mp. A prrviously established calibration curve is rised to define aluminum content and results are reported as the aluminum halide. The formation of aluminum methylate (Equation 1) circumvents some of the sampling problems inherent in handling aluminum halide. These sampling problems are the rapid hydrolysis of aluminum bromide in the air and the formation of sludge during even relatively brief storage in hydrocarbon. The AIBrJ-HBr-hydrocarbon system is no longer labile to moisture in air when taken into methanol. The complexing may then be accomplished under normal atmospheric conditions. The sample handling, the buffer, the color development conditions, and the elapsed time between color development and measurement are all critical and will be discussed herein. APPARATUS
Spectrophotometer, recording, suitable for a differential absorption measurement at 505 to 520 mp, such as the Applied Physics (Cary) Model 11 equipped with a pair of matched, 1-cm. quartz cells. Dry box, such as the Fisher Isolator Lab, equippcd with a nitrogen purge system and a moisture monitor sensitive to water levels of 10 to 100 p.p.m. The dry box is used in the calibration and is routinely purged to 25 to 50 p.p.m, of HzO before using. A t these levels, aluminum halide shows no visible air hydrolysis. Constant tcmperature bath, capable of maintaining 135’ to 140’ F. A convenient control consists of the Fisher thermoregulator (No. 15-179) and an immersion heater (No. 11-463-5). A Guardsman temperature controller, Model JP (West Instrument Co., Chicago, Ill.) may be substituted for the thermoregulator. REAGENT
Alizarin (1,2 - dihydroxyanthraquione), Matheson Coleman & Bell, No. B201. Aluminum bromide, anhydrous, Fisher certified reagent. Thioglycolic a z d (mercaptoacetic acid, HSCH&OOH), Fisher No. A-319. Cvclohexane, Matheson Coleman & .Belly No. 2825.’ Methanol, Baker’s analyzed reagent grade, No. 9070. SOLUTIONS
Alizarin Solution. Dissolve 1.OOO gram of alizarin powder in absolute methanol and make up t o 2 liters. Into another 2-liter flask add 16 ml. of concentrated NHIOH, cool, and then add 64 ml. of glacial acetic acid. Dilute to the 2-liter mark with the alizarin-methanol solution. Mix well. Discard the excess alizarin-methanol
Calibration spectra are shown in Figure 1. Read absorbance within 1 hour or less. With the Cary Model 11, the reagent blank is also used to establish the 10 line from 450 to 575 mp. Plot a calibration curve. Anhydrous AlCla (Fisher certified, 99.7% pure) can also be used ae a standard. SAMPLING AND ANALYSIS WIVELENGTH.
“8”
Figure 1. Absorbance curves of aluminum-alizarin complex A. B. C.
0.362 mg. AIBr, 0.724 1.448
solution. This solution of alizarin should be prepared fresh every 2 weeks. When kept for longer periods, the standards will produce progressively weaker absorption intensities. Aluminum Bromide Master Solution. Dissolve about 2 grams of anhydrous AlBrs, weighed correctly to the 4th decimal, in 375 ml. of methanol. Dilute to 500 ml. with cgclohexane in a volumetric flask. It is necessary to use t h e nitrogen-purged dry box or an equivalent inert atmosphere in transferring AIBrs to a weighing vessel, and again when dissolving in methanol. Anhydrous AlBr3 (Fisher certified reagent) is 99 to 99.5% pure as determined by gravimetric aluminum analyses. This is sufficiently pure for its use as a primary standard in following the concentration of commercial grade aluminum halide catalysts in hydrocarbon samples. Buffer Solution. Add 360 ml. of concentrated ammonium hydroxide, reagent grade, to 600 ml. of methanol. Cool the solution in an ice bath and then add 40 ml. of thioglycolic acid. CALIBRATION
An initial master calibration curve of grams A1Br3 os. absorbance of the aluminum-alizarin complex is prepared. Thereafter, an aluminum standard, representing one data point on the calibration curve, is run with every series of samples, thus permitting a continuing check on reagents and technique. Pipet 20 ml. of the AIBrs master solution into a I-liter volumetric f l a k and dilute to the mark with methanol. Pipet the following aliquots from this solution into 50-ml. volumetric flasks: 0 (the reagent blank), 3, 5 , 10, 15, 20, and 25 ml. Pipet in 5 ml. of buffer solution and 15 ml. of alizarin solution. Allow the solutions in the volumetric flasks to stand 10 minutes in a constant temperature bath at 135’ to 140’ F. At room temperature, dilute to the 50ml. mark with methanol. Read the absorbance of the red-purple color us. the colored reagent blank a t 508 mp, the absorbance maximum, in 1-cm. cells in a recording spectrophotometer.
The sampling procedure must eliminate the air hydrolysis of AlBh and the accompanying losses via sludging in hydrocarbon systems. Two types of sample handling, depending on the physical state of the sample, were established. The sample handling procedures are for: aluminum halide primary standards and hydrocarbon sludge samples, and paraffin hydrocarbon samples which contain AlBr, and HBr. Pure aluminum halide standards are dissolved in methanol in the dry box as described. The dry box is also used for handling certain hydrocarbon/ catalyst sludge samples from R, reaction unit. A 250-ml. Erlenmeyer flask containing 25 ml. of methanol is weighed and placed in the dry box. About 1 to 3 grams of the sludge sample are transfrrred to the methanol in the flask in the dry box. The flask is then removed from the dry box and reweighed to establish sample weight. The solution is then analyzed as described under Calibration. Liquid hydrocarbon solutions containing A1Br3-H13r are transferred directly from the rewtion unit into methanol. A 250-ml. Erlenmeyer flask containing 25 ml. of mrthanol is weighed. Then, about 10 to 15 grams of the AlBr3-HBr-hydrocarbon sample are delivered into the flask via a dip-leg of the process unit. The flask is reweighed to establish sample weight. The mixture may then be exposed to air and analyzed as described under Calibration. This method is in routine use for conventional samples. When an amount of aluminum halide present in the sample aliquot is in exccbs of the calibration procedure, t h e absorption maximum shifts below 500 mp. I n that event, a smaller aliquot is selected for the procedure. As established for the Cary spectrophotometer Model 11, the absorbance curve must maximize within the range 506 to 508 mp and absorbance must not exceed 1.2. DISCUSSION
Sampling. The sludging characteristics of aluminum halide-hydrocarbon systems are well known and need no further elaboration. It suffices t h a t sampling directly into absolute methanol without exposure to air or moisture provides a stable VOL. 33, NO. 6, MAY 1961
791
homogrneous solution. Thcre is no sludge formntion from a methanol solution of paraffin hydrocarbons and aluniiiiuni methylate left standing and exposed to air and light for a t leaut 48 hours. If sludge or precipitate is seen, then the sampling technique is at fault and should be studied criticdy. Buffering. Color development of the aluminum-alizarin complex is p H sensitive. If acid halides or other acidic compounds are present in the aluminum halide in hydrocarbon samples, the solution must be buffered before complexing with alizarin. The buffer system of ammonium hydroxide and thioglycolic acid in methanol was demonstrated to maintain a pH of 8.5 in the final color development. Table I shows that the buffer system is effective in the determination of aluminum bromide in the presence of a large excess of halogen acid in a seriea of synthetic blends. Color Stability. The color of the aluminum - alizarin complex is developed at 135' to 140' F. for 10
Table 1.
Buffer If Strong Acids Are Present
Wt. Ratio HBr/AIBra
Wt. yo AIBrl Recovered With buffer No buffer 100
1/1
100
...
y1
2r/l 80/1 400/1
w
72 52
102 98
...
100
Table It.
Absorbance Vs. Elapsed Time 10 7 24
Mm. Hours Hours Absorbance (10.640 0.640 0.412 cm. cell) Position of absorption max.,
nu
508
508
495
Table 111. Possible Interferences in Aluminum-Alizarin Determination
Impurity Blend," yo
% AlBrr Rwovered by Al- Alizarin Complex
1 4 . 4 FeCI3.6 H 2 0 4 5 . 6 HnO 2 . 0 HgC12 17.8 MgCla.6 €110 8 . 3 MnC12.4 H 2 0 7 . 0 NiCl,. 6 H,O 3.8 Pb(N0dz 16.5 SIICI, 8 . 3 ZnC12 a
100 100 97 86 100 96 90 97 92 100
100 103 100
88 100
Each blend contains 2.070 AlBra.
792
ANALYTICAL CHEMISTRY
minutes. Longer times or higher temperatures should not be used since an insoluble aluminum-alizarin complex can form at the inore severe conditions. The complex solution should be examined for a precipitate before measuring absorption. The absorbance reading should be taken within 7 hours after' color development, preferably within l hour, since long standing fades the color and shifts the absorption maximum as shown in Table 11. There is no change in absorbance or location of the absorbance maximum for at least 7 hours. Interferences. Most metals that would be expected in commercial aluminum bromide and/or as corrosion products of process equipment do not interfere with the AlBr, determination (Table 111). In the preparation of the interference test blends, the insolubility of inorganic ealts in hydrocarbons was circumvented by adding aluminum halide and the impurity to methanol containing cyclohexane. This d u t i o n was then analyzed by the alizarin procedure. The thioglycolic acid used in the buffer solution here has been used to remove the interference of iron in another photometric test for aluminum (6). Its presence in the buffer solution does remove some of the iron interference. When undue amounts of an iron salt are present, the alizarin results for aluminum halide are somewhat low. Since interference data of synthetic blends do not necessarily duplicate actual sample conditions, material balance studies of aluminum halide catalysts in actual batch runs were considered more significant. In these studies a known amount of aluminum halide catalyst was added to a reaction unit. Analytical recovery of the aluminum halide was measured by the summation of all alizarin analyses from the unit, together with an alizarin analysis of an alcoholic wash of all parts of the unit. hlaterial balance studies of the aluminum halide catalyst ranged from 95 to 105y0 of theory. Sensitivity and Repeatability. The minimum detection limit is 10 fig. of aluminum. This was determined on synthetic blends of aluminum bromide, hydrocarbon, and alcohol in glass VCRsels. On the basis of a l e g r a m sample this means that 1 p.p.m. of aluminum or 10 p.p.m. of AISra may be detected a t a measurable absorbance of about 0.050 in a 1-cm. cell. The use of larger absorption cells to increase sensitivity to still lower limits is not practical with the usual spectrophotonieter brcause of the colored reagent blank. Thr use of 10-cm. cells in this differential rolorimetric procedure did not allow sufficient signal for slit control on the instniment.
Table N. Repeatability of AIBr, Determination by Aluminum-Alizarin Complex
% AlBrr
Sample A 1.29 1.27 1.33 1.31 1.30 1.29 1.29 1.28 1.29 1.29 Av. 1.29
Sample B 0.094 0.094 0.097 0.094 0.093
Av. 0.094
%yoconfidence limit
2.9%of nmt. present in A 4.3y0 of amt. present in B
A series of repeatability runs for production samples indicates a 95% confidence limit of 2.9% of the AlBra present a t the 1% halide concentration. Another wries at the 0.1% level showed a 95y0 confidence limit of 4.3% of the AlBr, present. Repeatability data appear in Table IV. Accuracy. Aluminum halide analyses by the aluminum-alizarin complex agree with those obtained by a volumetric method for aluminum (7) and potentiometric method for bromide ion, similar to published methods (3, 4). The agreement is presented in the data of Table V. The volumetric procedure for aluminum is not sensitive a t aluminum levels of less than O.l'%, and consequently, is not suitable for aluminum bromide levels less than 1.0%. The potentiometric procedure for bromide is sensitive for 0.1% levels of A1Br8, but is not specific for the bromide of AIBr3 in n mixture of Alnr, and HBr. The alizarin procedure thus presents the advantage4 of serrsitivity and applicability to a wrricty of samples containing both Friedcl-Crafts catalysts and halogen acids.
Table V. Agreement of AluminumAlizarin with Other Methods
Wt. yo Aluminum Bromide Sample 1 3 3
4 5
Alizarin
1 1 4 8
1 2 4 4 94 0
Volumetric A1 1 2
1 4 4 2 8 2
95 9
Potentiometric
Bra
6
7
0 31 0 51
0.35 0.51
Samples 6, i did not contain HBr.
LITERATURE CITED
(4) Jones, H. B., Baum, H., Ibid., 27,
( 1 ) Atack, F. W.,J . SOC. Chott. Ind. Japan 34,936 (1915). (2) Barton, C . J., ANAL. CHEW 20,
99 (1955). (5) Luke, C. L., Braun, IC. C., Ibid., 24, 1120 (1952). (6) Mellan, I., “Organic Reagents in Inorganic Analysis,” pp. 227-50, Blakiaton, Philadelphia, Pa., 1941.
1068 (1948). (3) Blacdcl, W.J., Lcwip, \V. B.,Thomas, J. W.,Ibid., 24,50‘3 (1952).
(7) Snyder, L. J., IND.ENQ. CHEM., ANAL. ED. 17, 37 (1945). (8) Welcher
F. J v “Organic Analytical Reagcnta,” Vol. IV, p. 355, Van Nostrand, New York, 1948. RECEIVED for review October 18, 1980. Accepted January 3, 1961.
Ion Exchange Separation of Calcium and Strontium Application to Determination of Total Strontium in Bone MARVEN A. WADE and H. J. SElM Universify of Nevada, Reno, Nevada
b The difference in the stability constants of the EDTA complexes of strontium and calcium i s utilized to effect their separation b y ion exchange. The sample is dissolved in EDTA and, after the pH has been adjusted to 4.80, the solution is passed through a Dowex 50-X8 column. The remaining calcium is then eluted with EDTA at a pH of 5.30. After removal of alkali metal and ammonium ions with 0.75N HCI, the strontium is eluted with 3N HCI and determined flame photometrically at 460.7 mp using an oxygen-hydrogen flame. The method has been used for the routine determination of strontium in bone ash and is applicable to any type of sample. The procedure has been applied successfully to Ca-Sr ratios as great as 20,000 to 1 .
has bccii applied by Roberts (4), who reported the strontium content of two different adult femurs to be 281 and 480 pg. per gram of ash. These methods of detcrmining strontium all require elaborate and expensive instrumentation Strontium can readily be determined in bone ash flame photometrically once i t is separated from the large amount of calrium present. I n bone ash, the calcium-strontium ratio is of the order of 4000 to 1. A method is described for the ion exvhange separation of calcium and strontium utilizing (rthylenedinitri1o)tctraacetic arid (EDTA). The Ca+l ions are complrxed with EDTA and the CaEDTA+ complex and Sr+l ions are separated using Dowex W X 8 resin. THEORETICAL
R
on thc metabolisin of strontium and its distribution in the body has been difficult because of t h r lack of an analytical method suitable for routine analysis Research in this direction is important because the radioactivr species, Sr”, is considered t o be the most dangerous fission product resulting from nuclear explosions and reactors. Strontium is present in only trace amounts in body tissues, and the main difficulty involved in its determination in samples such as milk and bone is separation from the large amount of calcium present. Neutron activation has been applied by Sowdrn and Stitch (7’) t o the determination of strontirini in bone. Thcse investigators found the concentration to be of the order of 100 pg. per gram of ashed tissue. Thurbcr el al. (B), using emission spcctroscopy, reported thc world-wide average of strontium in human bone to be 172 pg. per gram of ash. X-ray fluorescence ESEARCH
The reaction of a metal ion with EDTA is represented by Equation 1, where Y represents EDTA. M+a
+ HZY-1
=
MY-’+ 2 H C
the elution of the calcium through the resin. This technique of cation separation has bcen applied by Fritz and Umbreit (3) t o the separation of several binary mixtures, and by Davis (1) t o the separation of calcium and strontium when present in equal amounts. Farabee (9)utilized a similar procedure for the determination of radiostrontium in urine. APPARATUS
All flame photometric determinations were made with a Beckman Rlodel D U spectrophotometer equipped with a 9220 flame attachment and an osygenhydrogen burner. All p H mcasurements were made with a Beckman Model G pII meter. The ion rxchrtnge columns used were similar t o those described by Seim, Morris, and Frew (61, but were made from borosilicate glass 14 nim. in outside diametcr and held a column of resin 27.6 cm. in height. The reservoir had a 250-ml. capacity, and the column and reservoir were connectcd with a 19/22 standardtaper ground-glass joint.
(1)
This equation shows that the system is dependent on the H + concentration, and increasing the acidity of the solution will decrease the concentration of the metaI-EDTA complex. The concentration of the complex also depends on the stability constant; the larger the log K z , the lower the p H at which the metal-EDTA complex will exist. Since the logs Kz for the calcium and strontium complexes with EDTA are 10.59 and 8.63, .respectively (6),the calcium mill complex at a lower p H than the strontium. A second factor favoring the separation by ion exchange is that the resin affinity is greater for strontium than for calcium. Both of these factors favor the retention of the strontium and
REAGENTS
Ion Exchange Resin. Dowex 50X8, 50-100 mesh, analytical grade, hydrogen form (Bio-Rad Laboratories, 32nd and Griffin Ave., Richmond, Calif.), is thoroughly washed with water and 30 ml. arc transferred to a n ion exchange column. After elution with 250 ml. of 3 N HCl and 50 ml. of H20 t o remove impurities, 150 ml. of 10% NH,Cl are added to convert the resin to the ammonium form. After equilibration with 50 ml. of 2% EDTA“4, the column is ready for use. EDTA. The EDTA used in this study was supplied by Geigy Industrial Chemicals, Saw Mill Rd., Ardsley, N. Y. The EDTA is purified before use by dissolving in NHIOH, filtering, VOL. 33, NO. 6, M A Y 1961
793
