Extraction of Magnesium with 8-QuinoIinoI STANLEY J. JANKOWSKII and HENRY FREISER2 Department o f Chemistry, University of Pittsburgh, Pitfsburgh, Pa.
b The 8-quinolinol extraction of microgram quantities of magnesium into mixed organic solvents such as chloroform with butyl Cellosolve, isopentyl alcohol, or ethanolamine was studied. The effect of quaternary ammonium salts on the course of the extraction was also investigated. The variation of extraction of 8-quinolinol between water and butyl Cellosolve-chloroform with pH showed that the ionization constants at 25.0' C. were 8.0 y lod and 2.4 X 10-lo, respectively. The nature of the magnesium 8-quinolinate complex appears to change if quaternary ammonium salts are present during the extraction, the wave length of maximum absorption shifting from 3 8 0 to 3 8 8 mp and the molar absorptivity increasing from 6.12 X lo3 to 7.08 X lo3. From a study of the variation of magnesium extraction with tetrabutylammonium iodide and 8-quinolinol concentrations, it was concluded that the extracted complex Is (CdH&N+, Mg(CeHaNO)a-. Microgram quantities of magnesium can be determined in the presence of milligram quantities of calcium, barium, and strontium with the aid of tartrate as a masking agent.
A
8-quinolinol has long been known as a precipitant for magnesium (IO), only recently has it been applicable to the solvent extraction of this metal. Luke and Campbell (7) successfully extracted magnesium by using butyl Cellosolve as a supplementary solvent to chloroform as well as increasing the 8-quinolinol concentration to 3%. More recently, Umland and Hoffman (9) extracted magnesium with 8-quinolinol in the presence of n-butylamine. It seems reasonable to attribute the magnesium extractability to the formation of an ion pair: butyltris-8-quinolinolatomagammonium, nesium. The work reported here was undertaken to investigate systematically the extraction of magnesium with 8-quinolinol, with a view to learning more about the role played by the supplementary solvent and ascertaining the possibility of anionic magnesium chelate formation. LTHOUOH
EXPERIMENTAL
A Beckman DU quartz spectrophotometer with a photomultiplier attachment was used for Apparatus.
776
0
ANALYTICAL CHEMISTRY
all spectrophotometric measurementa in the ultraviolet and visible regions. The cella were the 1-cm. rectangular type made of silica and were matched a t various wave lengths. A uniform spectral slit width of 0.5 mp was employed in all measurements. A Beckman Model G pH meter with external glass and saturated calomel electrodes was regularly standardized with Beckman buffer solutions. A rotary shaker (S),small enough to fit into a battery jar constant temperature bath 12 inches in inside diameter, was used. The temperature was controlled a t 25' f 0.1' C. Reagents. The water used was passed through a column of Dowex 50 cation exchange resin to remove cations and was stored in borosilicate bottles. 8 Quinolinol. Technical grade 8quinolinol, obtained from Lemke and Co., Inc., was distilled under vacuum. It was then recrystallized from an ethyl alcohol-water mixture and dried in vacuo in a desiccator to 've a melting point range of 72.5-3.5' Butyl Cellosolve. Technical grade butyl Cellosolve (monobutyl ether of ethylene glycol), obtained from Union Carbide Chemicals Co., was fractionated through a 4-f00t, glass helix-packed column. The fraction boiling a t 166-7' C. (730 mm. of Hg) was collected and stored in the dark, Chloroform. Fisher certified reagent grade chloroform was used without further purification. Quaternary Ammonium Salts. All quaternary ammonium salts, Ewtman reagent grade, were used without further purification after qualitative tests showed that not more than 1 p.p.m. of any interfering metal ion was present. Isopentyl Alcohol. Fisher certified reagent isopentyl alcohol waa used without further purification after qualitative tests did not reveal the presence of any interfering ions. 2-Aminoethanol. Eastman reagent grade 2-aminoethanol was used without purification after preliminary extraction blanks showed no signs of interfering substances. Magnesium Solution. A sample of Baker's reagent grade magnesium nitrate was dissolved in water to give a solution which was near 0.1M. The solution was standardized by gravimetric determination of the magnesium as the 8-quinolinolate ( 4 ) and was 0.095911.1. Aliquots were used as required. Effect of Butyl Cellosolve on Final Phase Volumes. The final volumes, after equilibration, of tho organic and aqueous phases, whenecci butyl Cello-
-
8.
solve was employed, were measured in the following manner. The extraction samples were prepared in exactly the same manner FAS if an extraction were to be carried out, but, instead of being pipetted into a glass bottle or a separatory funnel, the various components were pipetted into a buret graduated in 0.10 ml. The buret was then inverted repeatedly for one hour. The phases were allowed to separate completely and the volume of each was estimated to 0.02 ml. The operations were carried out at 25" f 1.5' C. The distribution ratio expressed as a concentration ratio of the chloroform to water phase was 9.36 (standard deviation 0.28). The same procedure for determining final phase volumes was employed when other oxygenated solvents such as isopentyl alcohol and ethanolamine were used in extraction studies. The isopentyl alcohol went almost entirely into the organic phase, whereas the ethanolamine remained almost entirely in the aqueous phase. Distribution of 8-Quinolinol in Chloroform-Butyl Cellosolve-Aqueous System at 25' C. A chloroform solution (20 ml.) of 8-quinolinol of known concentration was shaken in a 100-ml. glass-stoppered flask with 50 ml. of aqueous solution which was 0.1M in KCI and 2.5 ml. of butyl Cellosolve. The aqueous solution was adjusted to various p H values with 0.1M HCl or 0.1M NaOH. The mixture was shaken for 24 hours in a constant temperature bath. The final volume of each phase was determined on duplicate samples as described previously. After shaking, the aqueous phase was drawn off and its pH determined. This W&S taken to be the equilibrium pH. An ali uot of this was transferred to a 50-ml.Xeaker and its pH adjusted to a value between 1.20 and 1.90. The solution was quantitatively transferred to a suitable volumetric flask. A solution of 0.1M HCI was used for rinsing and diluting to volume. The absorbance of this solution was measured at 250.5 mp against 0.1M HCl aa a blank. The concentration of 8quinolinol waa found by reference to an analytical curve prepared in a similar manner. The molar absorptivity of 8-quinolinol was 4.51 X 104 liter per mole cm. (standard deviation of 0.04 X lo'). TO 1 Present address, Neville Chemical Co., Neville Island, Pittsburgh, Pa. * Present addreas, Department of Chemistry, University of Arizona, Tucson, Ark.
Table 1.
Distribution of 8-Quinolinol in Chloroform-Butyl Systems at 25" C.
Cellosolve-Aqueous
Initial 8-quinolinol concn. in chloroform, 0.005M [HOx] Oyg.,
Moles/Liter x 10' 39.0 59.7 163 352 762 426 444 444 449 452 452 452 452 452 449 443 413 356 610 289 279 197 193 87.3
PH 1.62 2.00 2.53 3.36 3.52" 3.95 4.49 4.52 4.90 6.50 6.90 7.00 7.10 8.00 9.69 10.28 10.85 11.40 11.63a 11.75 11.87 12.05 12.22 12.51 0
[HOx] A
U~OUE,
Molee/(Ziter x 10' 181 172 127 44.5 64.0 12.6 6.20
4.67 2.22 0.926 0.933 0.926 0.941 0.933 2.22 6.24 18.0 42.8 130 72.2 76.7 112 114 160
D 0.22 0.35 1.28 7.91 11.9 34.0 85.1 95.1 202 488 484 488 480 484 202 84.5 22.9 8.31 4.70 4.00 2.79 1.76 1.69 0.546
log D -0.87 -0.46 0.11 0.90 1.08 1.53 1.93 1.98 2.31 2.69 2.68 2.69 2.68 2.68 2.31 1.93 1.36 0.92 0.67 0.60 0.45 0.25 0.23 -0.26
Initial 8-quinolinol concn. in chloroform, 0.01M.
Table II. Absorbance of Magnesium 8-Quinolinolate Solutions
Extraction by butyl Cellosolve method 380 mp pH, 10.2 A,
Molar
Absorp tivity, Liters/ Magnesium Concn. Mole Molesfliter AbsorbCm. x 106 T/ml. ance x lo-' 0
3.40 6.80 10.2 13.6 17.0 20.4
0
n
0.82 1.64 2.46 3.28 4.10 4.92
0.208 0.415 0.630
6:l2 6.11 6.16 6.10 0.830 6.12 1.04 1.25 6.13 Av. 6.12 Std. dev. 0.02
Table 111. Extraction of Magnesium 8-Quinolinolate in Presence of Isopentyl Alcohol as a Function of pH
initial 8-quinolinol concn., 0.267M Initial magnesium concn., 6.0 X 10-SM Phase, volume ratio, 50 to 20 aqueous to organic PH Absorbance %E 9.67 1.020 93.6 9.97 1.028 94.4 10.05 1.0871 100 10.20 1.089 100 10.28 1.089 100 10.30 1 ,038 95.5 10.42 0.960 88.1
prepare this analytical curve the pH was varied from 0 to 2 and a butyl Cellosolve concentration was made four times m large as that expected in the aqueous phase after equilibration. The results are shown in Table I. Butyl Cellosolve in Extraction of Magnesium 8 Quinolinolate. GENERAL EXTRACTION PROCEDURE. The 8-quinolinol was initially in the chloroform phase. This phase was shaken with a previously prepared aqueous phase for a period of time which preliminary work showed to be sufficient t o achieve equilibration. The two phases were then allowed to separate completely. The organic phase was drawn off (through a fast filter paper, to remove any large droplets of water) into :i 50-ml. Erlenmeyer flask. The absorbance of this solution was measured against a blank prepared in an identical manner, except for the absence of metal ion. The pH of the aqueous phase, measured after separation of the phases, was taken as the equilibrium pH. Aliquots of the aqueous phase were then taken for further determination of the metal in question, to determine whether the absorbance of the organic phase is a true measure of the amount of metal extracted. The final volume of each phase was measured as described above on duplicate samples.
-
EXTRACTION OF MAGNESIUM.The procedure followed for the extraction of magnesium was that of Luke and Campbell (7), except that a reagent blank was also prepared and used as the reference solution for measuring the absorbance attributable to the magnesium 8-qninolinolate extracted.
Measurcmcnt of the absorbance as a function of wave lcngth in a chloroform phase which was 6.80 x 1 0 - k h f in magnesium revealed an absorption maximum a t 380 mp. A series of chloroform phases whose magnesium concentration varied from 3.40 x to 2.04 X IO-'M after extraction was prepared and their absorbances were measured at 380 mp (Table 11). Other Oxygenated Solvents in Extraction of Magnesium. ISOPENTYL
ALCOHOL. Extraction as a Function of pH. T o 15 ml. of a 0.267M solution of 8-quinolinol in chloroform in a 125-ml. separatory funnel was added 5 mi. of isopentyl alcohol. This was shaken for 30 minutes with 50 ml. of a previously prepared aqueous phase, 6.0 X 10-'M in magnesium, whose was adjusted with ammonium droxide-ammonium nitrate mixtures. After separation, the organic phase waa dried with anhydrous sodium sulfate and its absorbance was measured relative to that of a blank prepared in an identical manner. The amount of magnesium extracted was determined by reference to a previously prepared analytical curve. The pH of the aqueous phase measured after extraction waa considered the equilibrium p H (Table 111). Analytical Procedure for Magnesium. The magnesium content of 50 ml. of solution wm varied. The p H of this solution was adjusted with buffer m above. This was shaken for 30 minutes with 15 ml. of a 0.267M solution of 8-quinolinol in chloroform and 5 ml. of isopentyl alcohol. The organic phase was filtered and dried with anhydrous sodium sulfate. The absorbance of this solution was measured at 380 mp relative to a blank identically prepared (Table IV). ETHANOLAMINE. General Extraction Procedure. To 25 ml. of buffered aqueous solution containing various amounts of magnesium was added 5 ml. of aqueous ethanolamine of various concentrations. This was shaken for
/$
Table IV. Extraction of Magnesium 8-Quinolinolate in Presence of lsopentyl Alcohol
Analytical curve 380 mp pH, 10.2 Phase volume ratio, 50 to 20 (aqueous to organic) Initial Magnemum Molar Conch. Absorptivi ty (Aqumua), (,Organic) Moles/Liter Liters/Moie X 106 Absorbance Cm. X lo-' 1.50 0.272 7.26 3.00 0.550 7.34 4.50 0.820 7.29 6.00 1.089 7.27 6.75 1.225 7.26 A,
0
...
0
Av. 7.28
Std. dev. 0.02
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777
2 minutes with 15 ml. of 0.207M 8quinolinol in chloroform solution. The organie phase was filtered. Tho absorbance of the organic phase was then measured relative to that of a blank identically prepared. The p H of the aqueous solution measured after se aration of the phases was considerel the equilibrium pH. Extraction as a Function of pH. The pH of an aqueous phase which was 6.00 X lO-&M in magnesium was adjusted and 5 ml. of 1 to 1 by volume water-ethanolamine solution was added. The extraction procedure was carried out and the amount of magnesium extracted was determined (Table V). The extraction procedure was repeated with 5 ml. of 3 to 1 by volume water4hanolamine solution and the amount of magnesium extracted was determined (Table V). Analytical Curve for Magnesium. The magneeium concentration was varied in an aqueous solution buffered at pH 10.3, to which 5 ml. of 1 to 3 by volume ethanolamine-water solution was added. The absorbance of the organic phase a t 380 mp was measured relative to that of a blank prepared in an identical manner. The molar absorptivity was 6.19 X 10' liter per mole cm. (standard deviation 0.01 X IO*). Quaternary Ammonium Salts in Extraction of Magnesium SQuinolinolate. GENERALPROCEDURE. To 25 ml. of a chloroform solution of 8quinolinol in a 125-1111. glass-stoppered separatory funnel was added the previously prepared aqueous phase consisting of a given amount of magnesium, the quaternary ammonium salt, complexing agents, where studied, and suitable base and/or acid for
Table V. Extraction of Magnesium 8-Quinolinolate in Presence of Ethanola m i n e a s a Function of pH Initial 8-quinolinolate concn., 0.207M Initial magnesium concn., 6.00 X 10-6 M Phase volume ratio, 25 to 15 aqueous to organic Absorbance PH 380 Mfi %E Initial Ethanolamine Concn ., 8.34 % by Volume 9.91 0.340 55 10.06 0.402 65 10.13 0.447 73 10.20 0.522 85 10.29 0.615 100 10.33 0.617 100 10.40 0.606 99 10.50 0.492 80 10.62 0.354 58 Initial Ethanolamine Concn., 4.17 % by Volume 9.72 0.570 92 9.90 0.800 97 9.97 Q.600 97 10.13 0.608 98 0.619 10.24 100 10.46 0.619 100 0.608 10.53 98 10.69 0.577 93
778
ANALYTICAL CHEMISTRY
adjustment of pH. The two phases were shaken a t room temperature long enough for equilibration, After shaking, the two phases were allowed to separate complete1 and the organio phase was filteredl The absorbance of this solution was measured relative to that of a blank prcpared in an identical manner a t a sui,table wave length for the determination of the metal bein studied, The pH of the aqueous p ase was measured after separation of the two phases and was considered the equilibrium pH. Aliquota of the aqueous phase were analyzed for magnesium remaining to determine whether the absorbance of the organic phase provided a true measure of the amount of metal extracted. The final phase volumes were determined as described above on duplicate Ramples. The absorption spectrum for the extracted complexes determined by extracting the metal under optimal conditions and measuring the absorption of the organic phase against a blank. The resulting spectrum displayed an absorption maximum a t 388 mp. The concentration of the magnesium in the aqueous pha e waa vaned, other conditions being maintained a t values optimal for extraction, and the absorbance values of the organic phases were measured against a reagent blank a t 388 mp. Dependence on Quaternary Ammonium Ion Size and Concentration. Aqueous phases of 25-ml. volume, buffered a t pH 11.5 and 1.00 X lO-4M in magnesium, were prepared. Both the amounts and the nature of the quaternary ammonium Ralts were varied. The initial 8quinolinol concentration in the organic phase was 1.00 X 10-*M. The phases were shaken for 2 minutes and the amount of magnesium extracted was determined. Dependence of Extrartion on pH. Aqueous phwes of 25-ml. volume, whose pH valurs wcre adjusted with ammonium hydroxidc-ammonium nitrate mixtures, 5.20 X 10-3iCI in tetra-nbutylammoniuni iodide, and 1.00 X 10-4M were prepared. These were shaken for 2 minutes with a chloroform solution of 8-quinolinol whose initial concentration was 1.00 X 1O-*M and the amount of magnesium estracted was determined. The above procedure was repeated, except that the aqueous phase was also 0.1M in potassium sodium tartrate and that 5 nil. of 1 to 1 butyl Cellosolve with water was also added. The amount of magnesium estracted was determined. Dependence on Initial 8-Quinolinol Concentration. Aqueous phases of 25-ml. volume were prepared which were IM in ammonium hydroxide, 5.20 X 10-36!f in tetra-n-butylammonium iodide, and 1.00 X lO-4M in magnesium. These were shaken with chloroform containing various amounts of 8quinolinol for 2 minutes. The amount of magnesium extracted was determined. Extraction in Presence of Other Alkaline Earth Metals. Aqueous phases of 25-ml. volume, 0.6M in ammonium
a
hydroxide, 5.20 X lO+M in tetra-nbutylammonium iodide, 9.59 X lO-tM in magnesium, and containing various amounts of calcium, strontium, or barium nitrates were prepared. These were 0.1M in sodium citrate or potassium sodium tartrate. To each of these was added 5 ml. of 1 to 1 butyl Cellosolve with water and .then they were shaken for 1 minute with a 1.00 X 10-aM solution of 8quinolinol in chloroform. The amount of magnesium extracted was determined. DISCUSSION OF RESULTS
Reliability of Extraction Data. I n general, spectrophotometric methods were used for the determination of the concentrations of components under investigation. Since the absorbance cannot be read more closely than *0.002 and the absorbance in general averaged about 0.400, the concentration values were in error by about *0.5%, The volumes of the phases were read to *0.02 ml. and since the average volume was about 20 ml., the error in volume measurements for the extraction studies waa considered insignificant as compared to those of the other measurements that were made. The limit of error of the Beckman Model G p H meter is zk0.02 p H unit. However, in those solutions where the sodium ion concentration was 0.05M or higher, the average probable error was k0.05 pH unit above a p H of 11.5. Attainment of Equilibrium. Equilibrium was considered to have been achieved when the results coincided with those obtained a t 24 hours. These times have been indicated in the procedures. Variation of Distribution Ratio of 8-Quinolinol with pH. 8-Quinolinol in chloroform-water systems is not as well suited for theoretical study of the extraction behavior of metal chelates as in the acetylacetone-water systems used by Steinbach (8) and Krishen (6). There is added complexity from the pH dependence of 8-quinolinol solubility, whereas the solubility of acetylacetone was found to be constant in the pH range 0 to 7. Lacroix (6) has studied the distribution ratio of 8-quinolinol between water and chloroform as a function of pH. Although the distribution coefficient, K D , of the 8-quinolinol remains essentially constant a t 720, the stoichiometry of the extraction vanes very dramatically with experimental conditions. Evaluation of the data of Table I gives a value of 5.10 for ~ K Nand A 9.86 for ~ K o H . The addition of butyl Cellosolve to a water-chloroform system results in a similar variation in the distribution ratio of 8-quinolinol with pH, as may be seen in Table I. A value of 452 for the distribution coefficient, KD, of
8-quinolinol was obtained in this system. Dynscn ( 1 ) has reported a value of 458 for K D whwe the ionic strength in the aqueous phase was 0.1M and a value of 9.66 for pKoH in the same system. A log plot of t,he distribution data for acid media will give the value of PKOH when the value for the quantity log ( K D / D - 1) is zero. A value of 5.10 was obtained for PKNHof 8-quinolinol in a chloroform-butyl Cellosolve-water Rystem. A similar log plot of the distribution data for basic media will give the vnlue for pKon. A value of 9.62 was obtained for pKoA of 8-quinolinol in the same solvent system. The aqupous phases in this study were made 0.1M in potassium chloride to minimize the effect of variation in ionic strength upon the pK values of 8-quinolinol. The K D and pKoB values of 8-quinolinol which were obtained are in good agreement with those reported by Dyrssen. Thus, the addition of butyl Cellosolve does not affect the extraction behavior of 8-quinolinol between water and chloroform where the ionic strength in the aqueous phase is maintained constant at 0.1M. A plot of the distribution of the metal chelates as a function of p H for the 8-quinolinol system will give rise t o curves of lower slope than would be obtained if the 8-quinolinol concentration in the organic phase remained constant as the p H of the aqueous phase was vaned. The system is, however, suitable for the theoretical study of extraction behavior of the metal 8-quinolinolates at constant p H where the reagent concentration or some other variable is varied to determine the nature of the extracted species. The study of the extraction of the metal 8-quinolinolates as a function of p H has considerable analytical significance. The separation and determination of various metals as their 8-quinolinol chelates may be effected as a result of such studies, provided equilibrium conditions have been attained during such a study. There will be, however, a lesser dependence upon p H of the extraction behavior of the metals unless corrections for the variation in the reagent concentration can be applied. Use of Oxygenated Solvents in Extraction of Magnesium 8-Quinolinolate. The addition of small amounts of butyl Cellosolve t o a chloroform-water system has been found suitable for t h e extraction and determination of magnesium as its 8-quinoiinol chelate (7). The sensitivity of this method may be improved by measuring the absorbanye of the organic phase which contains the magnesium 8-quinolinolate relative t o that of a blank prcpared in a n identical manner rather than relative to that of ehloroform. The measuremenh were made
Table VI. Dependence on 8-Quinolinol Concentration in Extraction of Magnesium Using Quaternary Ammonium Salts
1.00 x lO-'M magnesium x lO-aM tetra-n-butylammonium iodide pH, 11.5 Phase, volume ratio, 25 to 25 aqueous to organic Precipitation at 7.5 x IO-'M and lower Initial SQuinolinol Concn., Moles/Liter Absorbance 6.20
x
108 10.00 8.00 6.00 4.00 2.50 1.80 1.60 1 .oo
(388 M r ) 0.650 0.650 0 . (340 0.620 0.555 0.455 0.428 0,202
%E 100 -.. 100 98.5 95.5 85.5 70 66 31
at 380 mp, which is the absorption maximum for solutions of this chelate, rather than a t 400 m r where careful reproduction of the slit width is required for reproducible results. The results for the absorbance measurements at 380 mp made after extraction of various amounts of magnesium are shown in Table 11. The addition of butyl Cellosolve to a chloroform-water system also aids in the extraction of calcium as its 8-quinolinolate. The extraction of calcium is not quantitative and larger amounts of butyl Cellosolve than that used for the extraction of magnesium are required. Magnesium 8-quinolinolate is quantitatively extracted into chloroform from aqueous solution when isopentyl alcohol is present in this system. The p H range (Table 111) of 100% extraction is 10.0 to 10.3. The absorbancc of the organic phase after extraction of the magnesium chelate (Table IV) may be used for the determination of magnesium. Magnesium 8-quinolinolate may also be quantitatively extracted into chloroform from aqueous solutions when small amounts of ethanolamine are added to this system. There is a greater dependence upon p H for the extraction of magnesium when the ethanolamine concentration is 8% by volume than when it is 4oJ, by volume, as is shown in Table V. Substitution of dioxane, ethylene glycol, methyl Cellosolve, pyridine, or methylamine for butyl Cellosolve in the extraction of magnesium did not increase the extractability of magnesium over that found when chloroform alone waa used. Othcr solvents such as methyl ethyl ketone, methyl n-propyl ketone, and
methyl isobutyl ketone were also substituted for butyl Cellosolve in the extraction of magnesium. It was not possible, however, to obtain equilibrium conditions when these solvents were used, even for the extraction of 8-quinolinol itself, whereas in the presence of butyl Cellosolve equilibrium conditions were attained in 2 minutes. I n those systems whcre equilibrium conditions were attained, the absorption
Table VII. Distribution of Magnesium as a Function of 8-Quinolinol Concentration
1.00 x lO-'M magnesium 5.20 X lO-aM tetra-n-butylammonium
iodide pH, 11.5 [HOxIa,' Moles/ Liter
log
x 101
[HOxjo
4.95 3.30 2.06 1.48 1.32 0.825 0 HOx
-2.306 -2.482 -2.687 -2.830 -2.881 -3.084
log D
D
65.6 1.817 21.2 1.326 5.90 0.770 2.34 0.349 1.94 0.286 0 . 4 5 -0.348
fi-quinolinol.
Table VIII. Distribution of Magnesium as a Function of Tetra-n-butylammonium Iodide Concentration 0.01M 8-quinolinol 0.0001M Mg(ClO4)r pH, 11.5
IR4N +I ma
Moles/ Liter
X 103
3.90 2.91 1.91 0.91 0.45 0
log
[RdN+] -2.41 -2.54 -2.72 -3.04 -3.35
D 32.4 24.0 17.2 7.33 3.35
log D 1.51 1.38 1.23 0.87 0.52
R = tetra-n-butyl.
Table IX. Extraction of Magnesium Using Various Quaternary Ammonium Salts 0.01M fi-quinolinol 0.0001M Mg(C10dh pH, 11.5
Concn., Moles/ Liter
Quaterna
Ammonium %alt X IOa Tetramethylammonium bromide 200 Trimethyl henylammonium iogde 20 Tetra-n-propylammonium iodide 20 Tetra-n-butylammonium iodide
%E 3.8 39 61
40
84
60
86
0.52 1.0 2.0 3 0
77 88 94.5 96 97 1%
4 0
5.0
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Table X. Extraction of Magnesium Using Quaternary Ammonium Salts 1.00 X 10-*M Squinolinol 6.20 X 10-*M tetra-n-butylammonium iodide Phaae volume ratio, 25 to 23 aqueous to organic
A. Extraction as a Function of pH 1.00 X lO-'M Magnesium 9.70 0.010 1.5 9.92 0.036 5.5 10.25 0.205 32 10.40 0.285 44 10.72 0.498 77 10.84 0.565 87 11.03 0.642 99 11.38 0.650 100 11.51 0.650 100 11.58 0.650 100 11.62 0.650 100 11.72 0.590 91 11.81 0.510 79 11.88 0.380 59
B. Extraction as a Function of pH Using Tartrate 0.1N Sodium Potassium Tartrate 9.69 X 10-'M Magnesium 10.40 0.274 44 10.98 0.620 99 11.20 0.624 100 11.32 0.628 100 11.47 0.625 100 11.88 0,625 100
maxima for the extracted magnesium chelates ail appearrd at the same wave length (380 mu). Use of Quaternary Ammonium Salts in Extraction of Magnesium SQuinolinolate. An examination of the derivation of equations relating the various equilibria involved in the extraction of a metal ion by the combined
Table XI. Absorbance of Magnesium Tetra-n-Butylammonium 8-Quinolinolate Solutions after Extraction Initial Squinolinol Concn. 1.00 x 10-'M
Initial tetra-n-butylammonium iodide concn. 5.20 X IO-*M X , 388 mp pH, 11.3 Phaae volume ratio, 25 to 23 water to orqanic Molar Initial Absorp Magnesium tivity, Concn ., Litera/ Molea/Liter Mole X 10' Absorbance Cm. X 10-J 13.41 0.875 7.09 0.755 11.50 7.14 9.59 7.10 0.625 5.75 7.08 0.373 0.250 3.83 7.09 1.91 0.123 7.00 0 0 Av. 7.08 Std. dev. 0.03
processes of chelation and ion association shows that b log D/6 log [RdN+] gives the number of quaternary ammonium ions prr metal ion involved in complex formation. Thc 6 log D/6 log [HOx] gives the n u m h r of moles of 8-quinolinol per mole of metal involved in complex formation. The data in Tables VI to VI1 for the extraction of magnesium as a function of 8-quinolinol concentration show that 3 moles of 8-quinolinol per mole of magnesium are involved in complex formation. The data for the extraction of magnesium as a funrtion of tetra-nbutylammonium ion concentration (Table VIII) show that one mole of the quaternary ammonium ion pcr mole of metal ion is involved in complex formation. This shows that the extracted magnesium complex is a tetran-butylammonium, tris-8-quinolinolatomagnesium ion pair. The extraction of a similar complex has h e n reported by Umland and Hoffman (9). In the presence of n-butylamine, magnesium is reported to extract aa a butylammonium, tris8-quinolinolatomagnesium. It is difficult to imagine that the butylammonium ion exists in significant concentration at thc high pH value used. This complex gives an absorption peak a t 380 mp similar to that found by the extraction of magnesium in the presence of solvcnts studied in this work. The quatrrnary ammonium magnesium complex gives an absorption maximum at 388 mp. It was not possible to study the extraction of magnesium as a function of 8-quinolinol concentration in the presence of n-butylamine, because a precipitate was observed in the organic phase after the mixture was shaken for longer than 5 minutes. The effcct of the size of the quaternary ammonium ion on the extractability of thc magnesium complex is strong (Table IX). The cation must be relatively large and have nonpolar substituents to form an cxtractablc ion pair.
The use of tartrate was found to incream the pH range of 100% extraction for magnesium, as shown in Table X. The use of tartrate at higher p H to prevent hydrolysis has been reported ( 8 ) , in the extraction of copper and iron 8-quinolinolatrs. Separation and D e t e m i a t i o n of Magnesium. Magnesiuni mny be separated from calcium, strontium, or barium as its tetra-n-butylaminonium 8-quinolinolate complex. The absorbance of the organic phase after the extraction of the magnesium is proportional to the amount of magnesium present and can bc used for the dctermination of this metal (Tablc XI). Microgram quantities of mngncsium can be determined as the tetra-n-butylammonium 8-quinolinolate complex when present with equal quantities of calcium, strontium, or barium, as the amount of any of the latter that is extracted is negligible in the p H range over which magnesium is quantitatively extracted. The sensitivity of this method is higher than of those of Luke m d Campbell (7) and Umland and Hoffman (9). However, when the ratio of the amount of calcium, strontium, or barium is large compared to that of magnesium, small amounts of those metals are also extracted. The use of tartrate (Table XII) has been found effective for the elimination of interferences from miiligram quantities of calcium, strontium, or barium in t1:e srparation and determination of milligram quantities of magnesium as its tetra-n-butylammonium S-quinolinolate complrx. The use of citrate is not as effectivc as tartrate in the elimination of interferences from milligram quantities of calcium, strontium, or barium in the separation and determination of microgram quantities of magnesium. ACKNOWLEDGMENT
The financial assistance of thc U. S. Atomic Energy Commission iri this work is gratefully acknowledged.
Table XII. Extraction of Magnesium in Presence of Other Alkaline Earth Metals 5.20 X 1 O - J M tetra-n-but lammonium iodide 1.00 x 1 0 - 1 ~gquinolinoi' 9.59 X 10-6M magneaium Amount, Absorbance, %E Metal Added Mg. (388 Mu) PH (Magnesium) A. 0.1M Sodium Potassium Tartrate Barium 60 0.625 11.40 10c Strontium 60 0.625 11.35 100 Calcium 17 0.610 10.80 98 Calcium 5 0.632 i1.31 lcll B. 0.1M Sodium Citrate Barium 60 0.570 11.32 91 Strontium 60 0.550 11.33 88 Calcium 33 0.570 11.00 9: I _
780
e
ANA LYTIC.^^ CHEMISTRY
LITERATURE CITED
(1) Dyrsscn, D., Svensk Kern. Tidskr. 64, 213 (1952).
(2) Gentry, C. H. R., Sherrington, L. G., Analysl 75, 17 (1950). (3) HarriRon, G. C., Jr., Ph.D. thesis, University of Pittsburgh, 1956. (4) Kolthoff, I. M., Sandell, E. B., “Textbook of Quantitative Inorganic
balysis,” Macmillan, New York, 1948. (5) Krkhen, A., Ph.D. thesis, University of Pittsburgh, 1958. (6) Lscroix, S., Anal. Chim. Ada 1 , 260 (1947). (7) Luke, C. L., Campbell, M. E., ANAL. CHEM.26, 1778 (1954). (8) SFinbach, J. F., Ph.D. thesis, Universitv of Pittsbureh. -1953. --(9) Umfand, F., Hoffman, W., Anal. Chim. Acta 17, 234 (1957).
-
(10) Welch:?
It‘. J., “Or anic Analytical Reagents, Vol. I, ban Nostrand, Princeton, N. J., 1947.
RECEIVEDfor review May 3, 19Op. Accepted January 30, 1961. Taken in from thesis submitted by S. J. Janowski in partial fulfillment of the requirements for the Ph.L). degree, University of Pittsburgh, 1059.
yt
I
The Dichromate Oxidation of Glycols C. L. WHITMAN, GEORGE W. ROECKER, and CLAWELL F. McNERNEY Research and Development Department, U. S. Naval Propellant Plant, Indian Head,
b Oxidation with potassium dichromate has been the basis for a method for the determination of concentrations of 1% or less of diethylene or triethylene glycol in aqueous solutions not suitable for the use of refractive index as an assay method. By control of both acidity and reaction time the oxidation can be stopped after consumption of 16 and 20.5 equivalents of dichromate per mole of diethylene glycol and triethylene glycol, respectively. Reproducible results are obtained with recoveries of 99.5%. Data are also presented for the oxidation of propylene glycol.
T
H E potassium dichromate oxidation of glycols was the basis of a n investigation t o establish the conditions for the determination of low concentrations of either diethylene or triethylene glycol in dilute aqueous solutions. The dichromate oxidation of these glycols was considered to have features desirable in a control method because no special apparatus or equipment is required and the analysis can be performed by laboratory technicians. The samples to be analyzed contained 1% or less of a particular glycol. The presence of other componcnts, such as aromatic amines and nitrate esters, would not permit assay by measurement of refractive index and, therefore, preliminary water and subsequent methylene chloride extractions were necessary t o ultimately recover the glycol as a n aqueous solution. Francis (3) used the dichromate oxidation for the determination of small amounts of diethylene and triethylene glycols in monoglycols. He stated that for the diethylene glycol reaction a n actual factor, 0.1381 gram of glycol per gram of dichromate, was used instead of the theoretical factor, 0.1082. Apparently, although i t is not stated, this approximates the consumption of
15.7 equivalents of dichromate per mole of diethylene glycol. Cardone and Compton (I), in studying the effects of sulfuric acid concentration on the oxidation of diethylene glycol, found that with 50% acid by volume at 100” C. the oxidation to carbon dioxide and water waa complete in 30 minutes with the consumption of 20 equivalents of dichromate per mole of the glycol. Also, Werner and Mitchell (4) assumed for the monoalkyl ethers of ethylene glycol t h a t the glycol portion goes t o carbon dioxide and water with consumption of 10 equivalents. Curme and Johnston (9) discussed the potassium dichromate oxidation of glycols and its application to control analyses. They state that this method is particularly useful for dilute aqueous solutions. EXPERIMENTAL
Prepare solutions by transferring an accurately weighed 0.7- to 0.8-gram sample of diethylene glycol (Fisher Reagent) and 1.0- to 1.1-gram sample of triethylene glycol (Fisher Reagent) to separate I-liter volumetric flasks and dilute to volume with water. Determine the amount of water present in the glycol by the Karl Fischer method and correct for this. Prepare standard 0.2N potassium dichromate solution from Bureau of Standards potassium dichromate. This solution is used both aa the oxidant and as a primary standard. Prepare a 0.1N thiosulfate solution and standardize in the customary manner. Dilute 2 volumes of 95% sulfuric acid with 1 volume of water for the sulfuric acid solution. DETERMINATION
OF
GLYCOLS
Transfer a measured portion of the respective samples sufficient to contain 0.01 to 0.02 gram of diethylene glycol, or 0.010 to 0.015 gram of triethylene glycol, to a 500-ml. iodine flask. Using a pipet, add 25 ml. of 0.2N potassium dichromate solution. Add sufficient 2
Md.
to I sulfuric acid to maintain the acidity at 30% for the diethylene glycol sample, or at 28% for the triethylene glycol sample. Stopper the flask loosely and heat on the steam bath for 2 hours. At the end of the heating period, remove the flask and cool it. Dilute with water to approximately 200 ml., add 15 ml. of 15% potassium iodide solution, stopper the flask, and allow to stand for 2 or 3 minutes. Using O.IN sodium thiosulfate, titrate the sample to the usual end point.
A blank, in which the sample is replaced by an equal amount of water, is prepared and run for each series of samples. The amount of the particular glycol can be calculated by using 0.006633 gram, the milliequivalent weight for diethylenc glycol, or 0.007325 gram, the milliequivalent weight for triethylene glycol. The standard solutions of diethylene and triethylene glycol were used to check yields. A convrnicnt and ticcurateJy measured aliquot of thc respective glycol solution was oxidized, and the recovc.ry was calculatvd as per cent of diethylene or triethylcne glycol. The recovery should fall in the range of 99.0 to 100%. RESULTS A N D DISCUSSION
The number of equivalents for the complcte oxidation of diethylene glycol to carbon dioxide and water is 20. Figure 1 shows the effcct of acid concentration on the extent of oxidation of diethylrne glycol for a 2-hour oxidation period. Howevrr, by suitable control 30% sulfuric of the conditions-i.e., acid for a 2-hour period of oxidationthe rcaction can he stopped at an empirical 16 equivalents. On the basis of 16 equivalents of potassium dichromate per mole of diethylcne glycol in a series of determinations, values of 99.8, 99.5, 99.5, 99.9, 99.7, 99.6 (average 99.7% f 0.1) were obtained. For the complcte oxidation of triVOL. 33, NO. 6, M A Y 1961
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