Determination of Aluminum in Chromic-Phosphoric Acid Solutions Determination of Aluminum in Aluminum Corrosion Products CORNELIUS G R O O T , R. M. PEEKEMA, and V.
H. TROUTNER
Fuel Technology Sub-Section, Coatings and Corrosion Unit, Hanford Atomic Products Operation, Richland, Wash.
I n the determination of aluminum in aluminum corrosion products, the corrosion products are stripped from the aluminum with 2% chromic-5qo phosphoric acid solution. This solution is analyzed colorimetrically for aluminum using aurintricarboxylate (aluminon). Interference caused by chromate ion and phosphoric acid is minimized by precipitating the aluminum as phosphate, thus removing the chromate ion and limiting the amount of phosphate, or by separating the chromate ion from the aluminum with anion exchange resin. The second method is more rapid and convenient. Interference caused by phosphoric acid is not serious if the 2% chromic-5q0 phosphoric acid solutions contain at least 170 p.p.m. of aluminum. This aluminum determination is accurate to within 3 ~ 3 % .
A
THOROUGH study of aluminum corrosion requires a knowledge of the amount of aluminum in the corrosion product film. This film can be dissolved without attack on the base metal in a 2% chromicd% phosphoric acid solution a t 90' to 120" C. The object of the work reported was to develop a method for determining the aluminum content of this solution, thereby determining the aluminum content of the corrosion product film. -4colorimetric method was chosen for the determination of aluminum in chromic-phosphoric acid stripping solutions. Most analytical methods for aluminum are either gravimetric or colorimetric, Gravimetric procedures require complete separation from nonvolatile elements and have difficulty in getting precipitates of knoam composition. In addition, so many elements are carried down by aluminum precipitates that several different precipitations may be required for complete separation. Colorimetric procedures are generally more rapid and often can be carried out in the presence of other elements. The colorimetric procedure selected for the determination of aluminum uses the red complex formed with aurintricarboxylate (aluminon). This method mas chosen because moderate amounts of trivalent chromium, which are always present in used stripping solution, do not interfere. Iron does not interfere when reduced to the divalent state with mercaptoacetic acid. The aluminon methods are ne11 described in the literature (1-5, 7-15). Because chromate ion absorbs appreciably a t the wave length selected for the procedure, a t least one separation must be made before the aluminum can be determined ( 5 ) . Two methods of separation have been developed. One method consists of precipitating the aluminum as the phosphate and filtering it away from the chromate and the bulk of the phosphate. The other uses anion exchange resin to effect a separation by replacing the chromate and some phosphate with chloride ion. Although the anion exchange separation is the most rapid and convenient, it has its drawbacks. It does not remove much phosphate and it requires analytical grade (aluminum-free) resin, which is difficult to obtain. For these reasons, both separation methods are described in this report.
Buffered aluminon reagent is made from 136 grams of sodium acetate (CH3COONa.3Hz0), 57 ml. of glacial acetic acid, and 0.160 gram of ammonium aurintricarboxylate. Make up to 1 liter with distilled water and store in a brown bottle. Let stand 3 days before use. Mercaptoacetic acid. Dilute 5.0 ml. of 95f % mercaptoacetic acid to about 100 ml. and titrate to a pH of 4.7 with ammonium hydroxide (about 10 ml. of 6 N ) . Make up to 250 ml. with distilled water and store in a brown bottle. Prepare fresh every 5 days. PROCEDURE
T o determine aluminum in 2% chromic-5% phosphoric acid stripping solution, perform a separation using either the precipitation procedure (A) or the anion exchange procedure ( B ) . A. Precipitation Procedure. Take an aliquot containing 2 to 40 mg. of aluminum from the stripping solution to be analyzed, and dilute a t least sixfold with distilled water. Neutralize with ammonium hydroxide to a p H of 5.0 to 5.5. Allow the precipitate to stand overnight. Filter through No. 40 Whatman filter paper (11 cm.) washing with water until the dichromate color is gone. Dissolve the precipitate into a 100-ml. volumetric flask, using four 5-ml. portions of hot 1 to 1 hydrochloric acid. One 5-ml. portion can be used to rinse out any precipitate adhering to the beaker. Rinse the filter paper well with distilled water.
For convenience in filtration, it is likely that these steps could
I.o
I
I
I
I
! O' Figure 1.
REAGENTS A.
Hydrochloric acid, 1 to 1, is made by adding one volume of 37% hydrochloric acid to one volume of distilled water.
B.
1571
400 4!50 500 550 600 650 WAVE LENGTH (mp) Absorption by aluminum-aluminon complex
20 y AI in 50 ml. of pH 4.7 buffered 0.008% ammonium aurintricarboxylate W i t h 0.08% mercaptoacetic acid
ANALYTICAL CHEMISTRY
1572
be prepared directly from potassium aluminum sulfate in distilled water without the anion exchange separation.) Only about 0.6 meq. of acid can be tolerated in 50 ml; of colored complex, if the error arising from variations in p H is to be held to less than 1%. If more acid is required, it should be reduced with ammonium hydroxide until the sample contains only 0.6 meq. Other operations and measures may be used. However, it is extremely important that the operations in timing be duplicated exactly on each standard and unknovin. DISCUSSION
I
I
IO Figure 2. A. B.
I
I
1
20 30 40 50 TIME (Minuter)
60
Development of aluminum-aluminon complex 40 y AI in 50 ml. of colored complex 10 y AI in 50 ml. of colored complex
be carried out on a smaller scale than that given, but this has not been tested. More efficient use of the hydrochloric acid can be obtained by rinsing after each portion Iyith distilled water. Care must be taken not to add more than a total of 100 ml. of liquids. If preferred, a larger volumetric flask (250 ml.) can be used, thus allowing a freer use of rinse water.
B. Anion Exchange Procedure. From a sample containing a t least l i 0 p.p,m. of aluminum in stripping solution, take an aliquot containing 0.1 to 40 mg. of aluminum and pass through the anion exchange column into a 100-ml. volumetric flask. Flush the column with three 10-ml. washes of distilled water into the 100-ml. flask. Dilute to 100 ml. with distilled xater.
Choice of Wave Length. The absorption curve shown in Figure 1was obtained with a solution containing 20 y of aluminum in 50 ml. of buffered 0.008% ammonium aurintricarboxylate solution. As maximum absorption is obtained a t 530 mp, all the following measurements were made a t this wave length. The addition of mercaptoacetic acid, to eliminate iron interference, reduces the measured absorbance but does not significantly shift the point of maximum absorption, as shown in the lower curve of Figure 1. Effect of Time and Temperature. The color of the aluminumaluminon complex develops slowly, as shown in Figure 2, and time and temperature therefore have a large effect. It was easier for this work to let the color develop for 30 minutes a t room temperature than t o heat t o boiling for a specified length of time, as called for in other procedures. Beer's law is not obeyed when the complex is developed a t room temperature, but the calibration curve so obtained is reproducible. Strict adherence to a given set of operating conditions is most important. Standards must be run by exactly the same procedure as the unknown. pH Control. The most troublesome variation in technique is in the amount of acid added with the sample, because the color intensity of the complex may be strongly affected by the p H of the solution. Therefore, there should be relatively little variation in color intensity with pH, or little variation in p H with the amount of acid in the sample. It would be even better if these regions could be made to overlap.
0.8 0.7
0" z
: 0
For aliquots containing less than 10 mg. of aluminum, analytical grade resin must be used, if good results are to be obtained. A larger volumetric flask (250 ml.) may be needed, if total volume of sample and wash water exceeds 100 ml.
0.6 0.5
v)
0.4 0.:
;Ifter the separation, take an aliquot containing from 10 to 40 y of aluminum and transfer it to a 50-ml. glass-stoppered graduate. Add 25.0 ml. of buffered aluminon reagent, then add, as soon as possible, 2 ml. of 2% mercaptoacetic acid. Dilute to 50 ml. with
distilled water and mix. At 30.0 rt 0.5 minutes from the time when the aluminon reagent was added, read the absorbance of the solution in 5-cm. cuvettes (or cells) at 530 mp, using a similar solution of aluminon, mercaptoacetic acid, and water as a blank. Determine the amount of aluminum in the sample from a calibration curve prepared in the same way. (If precipitation separation is used, the standard aluminum solutions for the calibration curve can be prepared by carrying knolvn amounts of aluminum in stripping solution through the precipitation procedure. These solutions can then be used in making the daily calibration curve. If anion exchange separation is used, the standard aluminum solutions for the daily calibration curve can
0.2 0.1
Figure 3. A. B.
Sensitivity of aluminum-aluminon complex 20 y AI in 50 ml. of colored complex 10 y A1 in 50 ml. of colored complex
V O L U M E 28, NO. 10, O C T O B E R 1 9 5 6
1573
The effect of p H on color intensity of the aluminum-aluminon complex is shown in Figure 3. The maximum falls a t pH 3.8. Other authors report the maximum absorbance to be a t pH 3.9 to 4.2 (IS, 14). The maximum a t pH 3.8 is rather sharp and lies close to the region of instability caused by precipitation of the free aurintricarboxylic acid. This precipitation can be prevented by making the solution 50y0in ethyl alcohol, but pH control is still necessary because of the varying amounts of acid in the aluminum samples. The p H is controlled most conveniently by a buffer, in which case the use of ethyl alcohol is not required. The addition of mercaptoacetic acid to eliminate iron interference (discussed under interferences) causes the optimum pH to be shifted from 3.8 to 4.4, as shown in Figure 4. At the p H of maximum color intensity, the change in color intensity with p H is zero, which makes this pH a very desirable operating condition. The ideal buffering system would keep the solution a t this optimum p H and not interfere with the analysis. Lacking this, a buffering system that comes as near as possible to this p H should be used. The buffering of a weak acid-salt system is a maximum when the hydrogen ion concentration is equal to the ionization constant of the acid. This means that an acid with an ionization constant of 10-4.4or pK of 4 . 4 is needed. Likely choices were: Acid Acetic Phthalic Formic Citrip Lactic Oxalic Tartaric
PK 4.75 2.90, 5.51 3.75 4.08 3.86 4.21 4.16
PH Figure 5 .
Acetate and phthalate buffers both worked well, allowing the formation of aluminum complex of greater intensity than did any of the other buffers listed above. Acetate buffer was selected because it is much more soluble than phthalate and therefore has a greater buffering capacity. The complex formed in formate buffer was about one fourth as intense as that formed in acetate.
0.8
I
0.7 -
I Figure 4. A. B.
C.
l--
I
i
I
I
1
I
Effect of mercaptoacetic acid on aluminon complex sensitivity
20 y A1 i n 50 ml. of colored complex 20 y AI in 50 ml. of colored complex with 0.08% mercaptoacetic acid Shift caused by addition of 0.08% mercaptoacetic acid
Variation in pH with acid added to 1 liter of 1 M acetate buffer
Citrate, lactate, oxalate, and tartrate buffers all complex the aluminum and prevent aluminon complex formation. The variation of pH with the amount of acid added to a l M acetic acid-acetate buffer is shown in Figure 5. The ordinate, dpH/dX, is the variation of pH with X, the equivalents of acid (or base) added to 1 liter of 1 M acetate buffer. For example, if 0.01 equivalent of hydrochloric acid (AX) is added to 1 liter of pH 4.75 buffer containing 0.5 mole of acetic acid and 0.5 mole of acetate, the p H is changed by 0.018 (ApH). Thus ApH/AX is 0.018/0.01 = 1.8 as compared to 1.76 for dpH’dX as shown in Figure 5 . Because the color intensity variation with pH is least a t pH 4.4 and pH variation with added acid is least a t pH 4.75, the determination should be carried out a t one of these two points or possibly between them. The relative error caused by varying amounts of acid in the aluminum samples is about the same a t both points and slightly higher in between. For convenience the point of minimum pH variation Jvith added acid was chosenthat is, a buffer equimolar in acetic acid and sodium acetate a t a pH of about 4.75, The error arising from adding acid to a sample of aluminum in 50 ml. of buffered aluminon solution (pH 4.75, 131 in total acetate) is shown in Figure 6. If the error due to pH variation is to be held to less than I%, a maximum of 0.6 meq. of strong acid can be tolerated in 50 ml. of buffered aluminon solution. Interferences. The most significant interferences are caused by chromate ion and phosphoric acid from the stripping solution. Trivalent chromium, from the reduction of the chromate, and iron contamination, from the aluminum sample or from the corroding media, are lesser causes of interference. Chromate ion interferes with the colorimetiic determination by its own color. The absorbance a t 530 mp of a solution of 0.5 ml. of 270 chromic acid in 50 ml. of buffered aluminon solution is about 0.460, as compared to absorbances ranging from 0.1 to 0.5 measured for the aluminum determination. Therefore,
4
ANALYTICAL CHEMISTRY
1574
2 -4
E
0
a
LL 0 v)
1.0
I-
z w
I
-I Figure 6.
I
I
-2 -3 - 4 - 5 ERROR (Per Cent)
Error caused by acid added to buffered aluminum-aluminon complex
Figure 7. Interference caused by phosphoric acid aluminum arid chromate ion must be separated to eliminate this interference. 0 Experimental points Curve calculnted from K 2 X 10-2 The extent to which phosphoric acid interferes with the determination was determined by spiking 30-y samples of aluminum with various known amounts of 5% phosphoric acid. Twentyfive milliliters of buffered aluminon reagent and 2 ml. of 2% mercaptoacetic acid were added to each sample. These samples Table I. Phosphoric Acid Complexing were then diluted with distilled water to 50 ml., and the absorbHaPOi-1" A I H I P O ~ ~ + K~ 5% HsPOa. 70 AI Found ance of each sample was measured on the spectrophotometer X10-6 Af Error MI. in 50 MI. X 10-4 .If X 10-7 M X 10-2 (Table I). Assuming that the interference caused by phosphoric 2.22 0 0.01 1.02 0 4.5 acid is caused by the fact that the HzPO,-~(which is the predom2.21 -0.6 2.04 1 4.6 0.02 -0.9 5.10 4 2.8 2.18 0.05 inant species) complexes the aluminum to form a complex of the 2.15 -3.1 10.2 7 3.1 0.10 2.10 -5.2 0.20 20.4 12 3.6 form A1H2P04++,the equilibrium constant for the complex can -17.6 51.0 39 2.4 1.83 0.50 The 1.47 -34.0 102 75 2.0 be calculated from K = (A~+++)(H~POI-~)/(AIH~PO,++). 1.00 204 116 1 9 1.06 -52.3 2.00 value calculated from the data which has the most significance 510 164 1.8 0.58 -73.7 5.00 1020 193 1 .5 0 29 8 7 . 0 10.0 (10 to 90% interference) gives K = 2 X 10-2. Using this value a As so little Hap04 is used to complex AI, HaPO4-1 is nearly equal t o for K, a curve can be calculated for the interference caused by total HaPo' concentration. phosphoric acid. The experimental and calculated interference b Difference between A1 found and AI taken (30 y per 50 ml. or 2.22 X 10-8 fif) is concentration of complex AlHnPO4++. curves are given in Figure 7 . The calculated curve of Figure 7 indicates that a maximum of 0.06 ml. of 5y0 phosphoric acid can be tolerated in 50 ml. of colored complex, if the phosphoric acid interference is to be held to less than 3%. The minimum aluminum to be taken in Trivalent chromium is always present in used stripping solu50 ml. of colored complex is 10 y. It follows that 50 ml. of colored tion. The experimental results plotted in Figure 8 show that complex must contain a t least 10 y of aluminum per 0.06 ml. 0.1 mg. of trivalent chromium present in 50 ml. of the aluminumof 553 phosphoric acid or about 170 parts of aluminum per aluminon complex solution will cause an error of 1.5%. When million parts of 5% phosphoric acid. trivalent chromium is added, the increase in the apparent abFerric iron interferes with the determination, but ferrous iron, sorbance of the aluminum-aluminon complex is much greater and particularly the ferrous-mercaptoacetic acid complex, does than would be predicted from the molar extinction coefficient of not ( 1 ) . When mercaptoacetic acid is used to reduce iron intertrivalent chromium (Figure 9). The error must therefore be ference, it also reduces the intensity of the aluminum-aluminon caused by a colored chromium-aluminon complex. complex. Sufficient mercaptoacetic acid to reduce the ironaurintricarboxylate complex intensity by a factor of 100 or more PRECIPITATION SEPARATION will decrease the aluminum complex intensity by a factor of Aluminum can be separated from 2% chromic-5% phosphoric about 2. Dilute aqueous solutions of mercaptoacetic acid deacid by precipitation of aluminum phosphate with ammonium compose and should be made up fresh about every 5 days. The decomposition of this reagent seems to be one cause of day to hydroxide under controlled conditions. Hillebrand, Lundell, Bright, and Hoffman (6) state that "the precipitation of alumiday variations in the calibration curve obtained from known num in the presence of large amounts of ammonium phosphate aluminum standards.
-
-
V O L U M E 28, NO. 10, O C T O B E R 1 9 5 6
1575
is always incomplete and may not take place a t all if very little aluminum is present.” However, by diluting the stripping solution about sixfold, and carrying out the precipitation a t p H 5.0 to 5.5, the amount of aluminum remaining in solution is held down to about 0.1 mg. Figure 10 shows aluminum recovery in the precipitation separation as a function of pH. To get
Table 11.
Elution of Aluminum from Anion Exchange Colunins
Sample Contents 1.000 nig. A I in 1-ml. stripping s o h ’ First 10-ml. wash Second 10-ml. wash Third IO-Inl. wash Fourth 10-ml. wash Fifth 10-ml. wash 10.00 mg. A1 in I-ml. striuping soln.1, First 10-ml. wash I Second 10-ml. wash Third 10-ml. wash Fourth 10-nil. wash Fifth 10-ml. wash
’
Aluminum Found, Mg.
ANION EXCHANGE SEPARATION
0.9925 0.0042 0,0015 0.0008 0 0003 125 0.0128 0.0027 0.0013 n , 0005
Table 111. Phosphoric Acid Transfer through Anion Exchange Resin % of Test Successive 2-ml. portions of stripping solution through same column
A.
R. Successive 5-ml. portions of stripping solution through same column C . Column washed with 10 nil. of 3.U HC1 between 5-ml. portions of stripping solution D. 6-ml. portions of stripping solution through different columns (1
Portion of Stripping Solution First 2 ml. Second 2 ml. Third 2 ml. Fourth 2 ml. Fifth 2 mi. First 5 ml. Second 5 ml. Third 5 ml. Fitst 5 ml. Second 5 ml. 5 ml.
5 ml. 5 ml. D a t a from first 5-ml. portions in tests R and C.
maximum recovery it is necessary to let the precipitate stand overnight; neither heating nor cooling does as much good as standing overnight. The phosphate precipitate filters more rapidly than the hydroxide, but still requires No. 40 Whatman filter paper. Precipitates formed at p H 6 not only have a greater aluminum loss but are also very difficult to filter, having a greater tendency to go through the filter paper. Chromate is completely separated from the aluminum by the precipitation and filtration. The phosphate, which is carried through the precipitation in an amount equimolar to the aluminum, is too small in amount to cause any significant error in the determination. Trivalent chromium and iron follow the alnminum through the procedure.
HaPo4 Transferred through Column 60.3 72.7 89 9 go 4 90.1
Anion exchange resin can be used to effect a more rapid and convenient separation. Dowex l-XS, 50-100-mesh anion exchange resin in the chloride form, was used in the column shown in Figure 11. The resin is discarded when used up (indicated by a distinct color change), as no means have been found for regenerating the spent resin. It can be seen from the data of Table I1 that one 10-ml. wash with distilled water effectively flushes all the aluminum from the column. To obtain a greater margin of safety, three 10-ml. washes are specified in the procedure. When chromic-phosphoric acid stripping solution is passed through the anion exchange column, the chromate ion is rapidly and completely removed, as evidenced by the clear, colorless effluent. That the phosphoric acid is removed slowly and incompletely was shown by passing successive portions of stripping solution through the exchange column. The percentage of phosphoric acid washed through the column was determined by titrat-
78.7 75.0 78.6 75 0 82 2
2.5
I
78 6 78.7” 75.0-
2.0
8
1.5
2
a m a
0
v)
2 1.0 0.5
O‘
4 0 0 450 S O 0 550 600 WAVE LENGTH (mp)
Figure 9.
INCREASE OF ABSORBANCE (Per Cent 1 Figure 8.
Error caused by trivalent chromium
6b
Absorption by trivalent chromium
A . CrCht B. C r + + + Both curves for 1000 p.p.m. chromium
1576
ANALYTICAL CHEMISTRY
Table IV.
Aluminum Contamination from Resin
Resin Grade Commercial dnalyticala a
Stripping Solution, Ml.
Aluminum Taken
Aluminum Found, y
3 3
0 0
3 4
ANION EXCHANGE RESIN
0
Obtained from Biorad Laboratories, Berkeley, Calif.
ing potentiometrically with standard sodium hydroxide for the two end points. The results (Table 111)show that most of the phosphoric acid is passed through the column. Washing the column with hydrochloric acid between samples had no appreciable effect. As most of the phosphoric acid from the original stripping solution passes through the anion exchange column, and the final solution of colored complex must contain a t least 170 parts of aluminum per million parts of 570 phosphoric acid, it follows that the original stripping solution sample must con-
Table V.
Aluminum Recovery
Aluminum Takena, Mg.
Aluminum Found, Mg. 25.6 10.3
25.0 10.0
1.00 0.100 0.0220 0.0220 0.0100
%
Error +2.4 +3.0 +3.0 -2.0 -61.4
1.03
0.098 0.0085
-23.6
0.0168 0.0063
- 37
Amount of aluminum in sample passed through ion exchange column.
SINTERED GLASS CONNECTION
$l i trv
TO F I T FLASK
Figure 11. Anion exchange column
tain a t least 170 p.p.m. of aluminum for satisfactory analysis, if this method of separation is used. Commercial grade resin is frequently contaminated with aluminum. When 3-ml. portions of stripping solution were passed through both commercial and analytical grade resin, and the effluents were analyzed colorimetrically for aluminum, the results (Table IV) indicate that sufficient aluminum contamination is introduced by commercial grade resin to be significant when small quantities of aluminum are being determined. Analytical grade resin introduces no measurable aluminum contamination. ACCURACY OF METHOD
I
I
IO0
1
I I
Aluminum was determined in samples containing 1 ml. of stripping solution and from 0.1 to 25.0 mg. of aluminum according to the detailed procedure, using anion exchange separation. An error of 3% was obtained (Table I-). The sample passed through the ion exchange resin must contain at least 0.1 mg. of aluminum for satisfactory analysis. A similar degree of accuracy was observed using precipitation separation. LITERATURE CITED
(1) Chenery, E. AI., Analyst 73, 501 (1948). ASAL.ED.17, (2) Craft, C. H., Makepeace, G. R., IXD. ENG.CHEM., 206 (1945).
(3) Groot, C., Peekema, R. AI., “Determination of Aluminum
Content of Aluminum Corrosion Products,” Hanford Atomic Products Operation, HW-35199, April 25, 1955. (4) Groot, C., Troutner, V. H., “Improved Method for Determinittion of Aluminum Content of Aluminum Corrosion Products,” Hanford Atomic Products Operation, HW-38481, Nov. 14,
4.0
4.5
5.0
5.5
6.0
6.5
PH Figure 10. Recovery of 10 mg. of aluminum from 50 ml. of stripping solution A . 0.8% HiPOi B . 5.0% HsPO4
1955. (5) Hammett, L. P., Sottery, C. T., J . -4m.Chem. SOC.47, 142 (1925). (6) Hillebrand, W. F., Lundell, G. E. F., Bright, H. A., Hoffman, J. I., “Applied Inorganic Analysis.” 2nd ed., p. 501, Wiley, 1953. (7) Ibid., p. 511. ( 8 ) Rolfe, A. C., Russell, F. R., Wilkinson, S . T., J . A p p l . Chem. (London) 1 , 170 (1951). (9) Roller, P. S., Ibid., 55, 2437 (1933). (10) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” 1st ed., p. 116, 1944; 2nd ed., p. 146, Interscience, Kew York, 1950. (11) Schemer, J. -4.,Magerman, W. D., J . Research Natl. Bur. Standards 21, 105 (1938). (12) Short, H. G., Analyst 75, 420 (1950). (13) Smith, W. H., Sager, E. E., Siewers, I. J., - 4 v 1 ~ .CHEX 21, 1334 (1949). (14) Winter, 0. B., Thrun, W. E., Bird, 0. D., Ibicl., 51, 2721 (1929). (15) Yoe, J. H., Hill, IT. L., Ibid., 49, 2395 (1927).
RECEIVED for review February 27, 1956. Accepted June 8, 1956.
