Determination of Magnesium Oxide in Finely Divided Magnesium Metal T. A. Hiller and V. A. Stenger Analytical Laboratories, 574 Building, The Dow Chemical Company, Midland, Mich. 48640
It has long been known that magnesium metal and its common commercial alloys are inert toward aqueous solutions of chromic acid. However, the acid will dissolve magnesium oxide. One of the authors utilized this difference in behavior a number of years ago to determine magnesium oxide in magnesium millings or turnings, titrating the free acid remaining after treatment of the metal sample with a standard chromic acid solution. The volumetric determination of chromic acid had been studied by Kolthoff and Vogelenzang in 1921 and good results had been obtained in the presence of barium chloride. Recently a more convenient method based on the same reactivity difference has been developed, in which dissolved magnesium in the chromic acid solution is determined by atomic absorption. This made it possible to show that a “blank” on the purest metal available was actually due to magnesium in the solution and not to adsorption of chromic acid on the metal. Good agreement is obtained between these two methods, two other methods (chelometric and eudiometric), and also with direct determinations of oxygen by neutron activation. The new work also led to improved conditions for leaching, avoiding filtration and saving lime in the titration procedure.
plied the chromic acid treatment procedure successfully to this problem and developed two modifications, in one of which active magnesium is determined by a chelometric titration after removing oxide with chromic acid and dissolving the remaining metal. In the other, magnesium from the dissolved oxide is determined directly by atomic absorption measurement. As the original method was not published, it seems appropriate t o describe both that and the modifications a t this time. The titration of chromic acid as a dibasic acid (K2 = 3.2 x was studied long ago by Kolthoff and Vogelenzang (3).They arrived a t the best results by adding barium chloride shortly before the end point, to precipitate barium chromate and thus, in effect, to change the titration from that of a weak acid to a strong acid. This permitted the observation of a fairly sharp end point instead of a rather vague one a t about p H 10. Phenolphthalein was used as indicator and data accurate to within 0.25% were obtained. If the barium salt were added too early in the titration, some dichromate ion would apparently be occluded with the precipitate and the acidity found would be low. Because the transition from dichromate to chromate is slow, a five-minUte waiting period was recommended to assure that the end point has actually been reached.
In the early years o f the magnesium alloy industry, problems arose from inclusions of magnesium oxide in metal castings. Oxide inclusions affect the bulk properties of the metal, causing embrittlement and loss of tensile strength. The senior author (VAS) was asked in 1943 to devise a laboratory method for analyzing magnesium samples for their oxide content. I t was known a t the time that chromic acid solutions would dissolve magnesium oxide without significantly attacking the metal, and chromates were being used in conjunction with nitric or hydrofluoric acids as corrosion-proofing treatments for magnesium alloy products. Based upon this solubility difference, a titrimetric method was developed in which any loss of acidity upon exposure of a standard chromic acid solution to a sample of the finely divided metal was attributed to reaction of the acid with oxide to form magnesium chromate. Use of the method in connection with foundry studies led to improved practices of fluxing, gas blanketing, and gating which largely eliminated oxide inclusion problems. More recently it has become desirable to know the oxide content, primarily superficial, of magnesium powders used in the preparation of flares. Military specifications ( I , 2) call for the determination of active magnesium in such powders, wherein a sample is dissolved with dilute sulfuric acid and the evolved hydrogen is measured in a eudiometer. In the absence of other metals, this procedure can be used to furnish a measure of the oxide content by difference, but the precision is not great and a direct oxide determination would be preferred. The junior author (TAH) ap-
EXPERIMENTAL
(1) U.S. Military Specification MIL P-14067 B (MU): Powders, Metal; 10
March 1967. (2) Military Standard MIL-STD-1234,Pyrotechnics; Method 41 2.1.
Apparatus. The atomic absorption work was done with a Perkin-Elmer Model 303 instrument, using a magnesium hollow cathode tube as the radiation source. The eudiometer employed in the determination of active magnesium was modified somewhat from that described in the military specification (2) to allow for weighing the sample in a smail glass cup rather than a paper wad. The cup is designed to he attached with silicone grease t o a glass arm which can be rotated, after equilibrium conditions have been reached, to drop the cup and sample into sulfuric acid, liberating hydrogen. For activation analyses, 6- to 7-gram samples in 7-ml (2-dram) polyethylene vials were exposed to 14-meV neutrons generated in a Texas Nuclear Corporation Cockcraft-Walton type 150-kV positive-ion accelerator. Oxygen was thereby converted to lSN, which has a 7.4-second half-life. The irradiated samples were transferred quickly via a pneumatic tube to a scintillation counter. The apparatus and procedure were approximately the same as described for the analysis of cesium metal by Anders ( 4 ) . A Beckman Model IR-215 non-dispersive infrared analyzer with a 13.3-cm gas cell was used for the determination of carbon dioxide (in order to allow for correction of the oxygen data obtained by neutron activation analysis, as will be discussed later). The procedure was similar to that of Van Hall and Stenger ( 6 ) except that, for the analysis of magnesium, the sample was treated with acid in an aeration bottle rather than in a heated tube. Reagents. T h e solutions employed were prepared from ACS analytical reagent grade chemicals or from material of the highest purity available. The water used was purified by passing steam condensate through ion exchange resins in a mixed bed. Chromic acid solution, 5.00 grams CrOs per liter (approximately 0.10N as an acid). (3) I. M. Kolthoff and E. H. Vogelenzang, Red. Trav. Chim., Fays-Bas, 40, 681 (1921). (4) 0. U. Anders, in V . A. Stenger et a/.,Ed., “Analytical Methods for Impurities in Cesium Metal,” Tech. Doc. Rep. No. ML TDR 64-199, Wright-Patterson Air Force Base, Ohio, May 29, 1964, pp 17-30, (5) C. E. Van Hall and V. A. Stenger, Anal. Chem..39,503 (1967).
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
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Barium nitrate solution, 26.0 grams Ba(NO& per liter (approximately 0.1M). Sodium hydroxide solution, aqueous, O.lON, standardized against potassium hydrogen phthalate. Chlorophenol red sodium salt, 0.10 gram per 100 ml of water. Magnesium standard solutions. Weight 2 grams of pure magnesium accurately to within one milligram, dissolve it in 125 ml of 1:4 hydrochloric acid (added slowly) and dilute with water to one liter in a volumetric flask. One milliliter contains 2 milligrams of magnesium. For atomic absorption measurements dilute 50 ml to one liter and dilute 50 ml of that solution to 500 ml. The final solution contains 0.010 mg Mg per ml. EDTA solution, 40.0 grams ethylenedinitrilotetraacetic acid disodium salt dihydrate per liter (approximately 0.1M). Standardize against the standard magnesium solution, 2 mg per ml, as in the chelometric titration procedure. Buffer solution for pH about 10. Dissolve 65.5 grams of ammonium chloride in 300 ml of water, add 570 ml of concentrated ammonium hydroxide and dilute to one liter with water. Eriochrome Black T indicator solution. Dissolve 0.8 gram of the powder in a mixture of 40 ml of methanol and 60 ml of triethanol amine. Sample. The sample may be in the form of finely divided shavings, millings, filings, or powder. T o compare various samples of solid metal, they should be milled under the same conditions, preferably in an inert atmosphere. Powders should be of the same mesh size. Generally a one-gram sample is used; if more than one per cent MgO is present, the sample weight should be correspondingly decreased. Acidimetric Titration Procedure. Place the weighed sample in a 150-ml beaker, add exactly 25 ml of O.1N chromic acid solution from a pipet, and cover with a watch glass. Stir magnetically for 15 minutes with a plastic-coated stirring bar, rinse the cover and bar with 25 ml of water, and add 0.5 mi of chlorophenol red indicator solution. Titrate with 0.1N sodium hydroxide to a change from orange to red-orange and add up to 10 ml more of the titrant. Introduce 30 ml of 0.1M barium nitrate solution; the mixture should be yellow unless too much sodium hydroxide was added. Continue the titration. slowly until the yellow or slightly greenish yellow suspension shows a pink cast. Upon overtitration by a drop or two, the solution becomes a more definite pink which can be confirmed by allowing the precipitate to settle. Run a blank in the same way. The difference between the sample and blank represents the amount of sodium hydroxide equivalent to chromic acid neutralized by the sample. One ml of 0.1N solution represents 2.016 mg of MgO. If the titration difference is more than about 5 ml, the determination should be repeated with a smaller sample. Chelometric Titration P r o c e d u r e for Active Magnesium. Place a one-gram sample (weighed to within 0.5 mg) in a 250-1111 beaker with a magnetic stirring bar. Add 50 ml of 0.10N chromic acid solution, cover the beaker, and stir for 15 minutes. While retaining most of the metal in the beaker, decant the solution through a Gooch crucible with a small moist asbestos pad, under suction. Rinse the metal, beaker, and crucible about six times with water, or until the final washing is colorless. Reserve the filtrate and washings if dissolved magnesium is to be determined by atomic absorption; otherwise they may be discarded. Transfer the filter pad to the beaker containing the remaining metal. Add 50 ml of water, cover the beaker, and add 1:4 hydrochloric acid in small portions to dissolve the magnesium. Boil the mixture for about a minute, then cool it, transfer to a 500-ml volumetric flask, and dilute to volume. Pipet a 50-ml aliquot into a 500-ml Erlenmeyer flask, add 250 ml of water, 20 ml of pH 10 buffer solution, and 0.5 ml of Eriochrome Black T indicator solution. Titrate with 0.10M EDTA solution to a color change from red to blue against a reflected light background. No red color should remain a t the end point. Calculate the magnesium content of the sample from the titration volume and the magnesium equivalent of the EDTA solution as found by standardization. Atomic Absorption P r o c e d u r e f o r Magnesium Oxide. Place a one-gram sample in a 150-1111 beaker and proceed as in the first paragraph of the preceding method. After obtaining the first filtrate and washings, treat the residual magnesium metal with another 50-ml portion of chromic acid solution and again filter and wash. Dilute each filtrate to 250 ml in a volumetric flask. Prepare a blank on chromic acid filtered through asbestos in the same way. Pipet a 5-ml aliquot of each sample filtrate (or enough to contain 25 to 100 micrograms of Mg) into a 100-ml volumetric flask. Dilute to volume and mix. Adjust the atomic absorption instru2020
ANALYTICAL CHEMISTRY, VOL. 46,
ment to obtain appropriate sensitivity for the 285.2-nm line of magnesium. Aspirate standards and sample solutions successively into the flame, obtaining the absorbance for each. The blank can be analyzed directly (without dilution) since it is very low in magnesium. Ordinarily no background corrections are necessary. Plot the absorbance us. concentration of the standards in micrograms per milliliter on linear graph paper. Calculate the concentration of magnesium in each sample solution and the corresponding percentages of magnesium oxide and magnesium (by difference) in the original sample, correcting for the blank if necessary. The sum of the magnesium oxide figures obtained in the two extractions is normally used as the content of the sample. However, additional treatments may be made to demonstrate whether additional oxide is dissolved or a constant level of attack upon the metal is reached.
RESULTS AND DISCUSSION The procedure for titrating excess chromic acid as given above was modified from that of Kolthoff and Vogelenzang (3)in two respects. Barium nitrate was substituted for barium chloride to avoid interference from magnesium metal, which is not removed prior to titration. Chloride ion is known to promote the action of chromic acid upon magnesium, whereas nitrate ion does not. This substitution should have no adverse effect on the titration. Chlorophenol red was used in place of phenolphthalein because it seemed more desirable to observe the end-point change a t a p H near 7 than a t about 9. The recommended conditions for treating magnesium with chromic acid differ from those originally employed in connection with the titration procedure. Originally two leachings were carried out in a medium porosity glass filter crucible, for ten-minute periods, with occasional stirring with a glass rod. A pretreatment with potassium chromate solution, 5 grams per liter, was given to remove chloride possibly present from fluxes. No pretreatment is needed when chloride is absent. In a test of the ability of 0.1N chromic acid to dissolve magnesium oxide under those conditions, with no free metal present, 93.5% of the oxide was found to have reacted during the first leaching and 3.5% more in the second. Under the conditions given in the current procedure, 99% or more was dissolved in one treatment. The improvement is probably due to the more effective stirring and the slight rise in temperature caused by the magnetic stirring apparatus. In these tests, 20-mg portions of pure magnesium oxide, previously ignited a t 800 "C, were employed. No standards of magnesium or its alloys with known oxide content were available a t the time of the original study or in the more recent work. However, several hundred samples from experimental and commercial castings were analyzed. These were generally alloys containing six percent aluminum and three percent zinc, equivalent to what is now designated as AZ63 alloy under the ASTM system. Results from 0.11 to 5.3% MgO were obtained. A t the lower end of the range the results for duplicate samples were usually reproducible within 0.03% and the castings were of excellent quality. Samples with more than 1% oxide, which were not numerous, showed poorer reproducibility and the metal quality was obviously also poor. No result below 0.11% was found. At the time, it was not known whether this apparent limit represented a true minimum oxide content, adsorption of chromic acid on the metal surface, or reduction of a little chromic acid by active magnesium or another metal. The observation of a green tint near the end point, perhaps from chromium(II1) hydroxide, offered some indication of the last possibility. The chelometric titration procedure has been utilized only for determining active magnesium in powders o r chips free from other metals. Table I presents a comparison of chelometric data with those of the eudiometer method. The
NO. 13, NOVEMBER 1974
The data cannot reveal whether any metal is dissolved with hydrogen evolution, for here the acid neutralized would correspond exactly with the metal and oxide in solution:
Table I. Active Magnesium in Samples of Magnesium Chips Active magnesium, % Sample
Eudiometer method
Chelometric method
RMC -305 -1 -2 -3 -4 -5
99.0 i 0.3 99.0 i 0.3 99.2 i 0.3 99.0 i 0.3 99.1 f 0.3
99.2 f 0.2 99.0 0.2 99.1 * 0.2 99.0 i 0.2 99.0 f 0.2
RMC-511-1 -2 -3 -4 -5
99.0 99.2 99.0 99.0 99.1
+ 0.3 i 0.3 i 0.3
0.3 i 0.3 &
*
99.0 99.1 99.0 99.0 99.2
+ H2Cr0,
MgO
+ H2Cr0,
-
-
MgCrO,
+ H,
(2)
MgCrO,
+ H,O
(3)
Also included in Table I1 are the results of total oxygen determinations by neutron activation analysis. These analyses were undertaken in an attempt to determine whether the chromic acid methods were revealing all the magnesium oxide in the samples. First indications were that the chemical results might be low. However, the realization that other forms of oxygen might also be present prompted the determination of carbonate by acid attack and infrared analysis. Correction for the oxygen equivalent of the
* 0.2 * 0.2 i
Mg
0.2
* 0.2 * 0.2
Table 11. Magnesium Oxide Content of Magnesium Chips Apparent magnesium oxide, %
Neutron activation analysis Acid-base titration
Sample
RMC -305 -1 -2 -3 -4
-5 RMC -511 -1 -2 -3 -4 -5 Mean of all s a m p l e s
Atomic absorption analysis
From total oxygen
Correction for CO2
hlgO
&et
0.66 i 0.02 0.85 i 0.01 0.99 0.06 0.81 i 0.02 0.88 i 0.03
0.65 1.02 0.95 0.91 0.80
i 0.03 i 0.05 i 0.05 i 0.05
* 0.04
0.91 1.16 1.31 1.16 1.28
0.20 0.18 0.18 0.15 0.15
0.71 0.98 1.13 1.01 1.13
0.97 0.91 0.88 0.91 0.74
0.84 0.88 0.81 0.83 0.65
t
0.04 0.04 0.04 0.04 0.03
1.16 1.01 0.98 1.11 1.08
0.29 0.27 0.22 0.21 0.20
0. a7
0.74 0.76 0.90 0.88
1.116
0.205
0.911
*
0.01 0.02 0.01 i 0.04 i 0.01 rt
i
*
0.860
i i
* rt
0.834
samples, intended for use in flares, consisted of rounded but irregular pellets generally less than 0.4 mm along their largest axis. They had been analyzed by emission spectrography with the following results: present at about 1 ppm, Cu, Fe, Mn, Ni, Ca; not detectable a t a sensitivity of about 1 ppm, Ag, Co, Cr, Pb, Sn, V, Zn, Zr. The indications are that the samples are quite similar in elemental magnesium content and that magnesium oxide by difference would not be expected to exceed 1%.The military specification for active magnesium is 98.0% minimum. Results of magnesium oxide determinations on the same samples are given in Table 11. The data from titrations and atomic absorption measurements represent the means of duplicate determinations (in some cases triplicates), the ranges of which are indicated by the plus or minus figures. It will be seen that, for at least half the samples, the results of the two methods differ by more than would be anticipated from their precisions. Nevertheless, the overall averages differ by less than 0.03%, the AA results being lower. This comparatively small bias argues against any significant loss of chromic acid either by adsorption on the magnesium metal or by a possible reduction process in which more chromic acid would be consumed than would correspond to the apparent magnesium oxide dissolved: 3Mg + 5H,drO, -+ 3MgCr0, + 2Cr(OH), + 2H,O (1)
evolved carbon dioxide yielded the data shown in the last column as net MgO. Generally the NAA results are considered to have a relative precision of &lo%. The apparent mean bias of +0.05% MgO for the NAA method over acidbase titration is within this precision range but is probably real. The presence of only 0.022% moisture would cause the NAA results to be high by 0.05% MgO; it would be difficult to determine or remove moisture from an active metal such as magnesium at this low level without affecting the oxide content. Consequently, we feel that chromic acid is effectively reacting with all the oxide in these samples. Additional justification for this view is found in the fact that magnesium has a higher expansion coefficient than magnesium oxide. When hot metal contracts around an oxide particle, it probably develops cracks which later expose the oxide as the metal is milled, turned, or ground. A series of samples turned on a lathe showed no differences due to cutting depths of 0.05 or 0.10 inch (0.127-0.254 cm) when analyzed by the titration method. A question remains as to the significance of the 0.11% MgO lower limit found in samples analyzed by the earlier titration procedure. The agreement between the methods on samples with higher oxide content indicates that the result is probably real. Analyses of very pure metal by the activation method would be necessary to decide the question.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
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INTERFERENCES Common alloying elements such as aluminum, zinc, and low concentrations (
Analytical chemists have studied adsorption of foreign ions on hydrous oxides from the viewpoint of contamination of precipitates as well as the removal of radioactive ell Present address, Department of Chemistry, University of Florida, Gainesville, Fla. 32611.
2022
ements from solution (1-4). In general, a dramatic increase in adsorption is observed with increasing p H and hydrogen ions are released into the solution. The adsorption and, in turn, the release of hydrogen ions is dependent on the surface area of hydrous oxide. In fact, it has been suggested (5-7) that adsorption of zinc ions be used as a measure of the surface area of oxides such as MnOp, ZrOn, and SiOp. Although the displacement of hydrogen ion from the surface is an indication of specific ion exchange adsorption, the number of hydrogen ions released per heavy metal ion adsorbed cannot be taken directly as a measure of the number of binding sites to which the heavy metal is attached. For example, four hydrogen ions are released from a silica sample for each copper(I1) ion adsorbed from an ammonium acetate solution ( 5 ) ,presumably because of the involvement of copper amine complexes in the adsorption process. (1) I. M. Kolthoff and B. Moskovitz, J. Pbys. Chem., 41, 629 (1937). (2) I. M. Kolthoff and L. B. Overholser. J. Pbys. Cbem., 43, 767, 909 (1939). (3) M. H. Kurbatov, G. B. Wood, and J. D. Kurbatov, J. Phys. Cham., 55, 1170(1951). (4) J. Korkisch, "Modern Methods for the Separation of Rarer Metal Ions," Pergamon Press, New York, N.Y.. 1969. (5) A. Kozawa. J. Electrochem. Soc.. 106, 552 (1959). (6) A. Kozawa, J. horg. Nucl. Chem., 21, 315 (1961). (7) A. Kozawa, S. C. Paterniti, and T. A. Reilly. in "Oxide-Electrolyte Interfaces," R. s. Alwitt, Ed., The Electrochemical Society Inc., Princeton, N.J., 1973, pp 72-90.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
