This is to be compared with the value in the absence of chloride, (at = 0), which gives AE = ( 2 R T I F )sinh-1 ___
(20onts,,)
=((LETIF) In ( 106al)
which for al = 0.01 mole liter-’ gives AE‘ = ( 2 R T / F ) X 41n 10
Hence aE’/aE = 4, so that there is about a fourfold drop in the e.m.f. change near the end point as a result of the addition of a hundredfold excess of chloride ions. However, the change over the range *l% each side of end point is, by Equation 13, of the order of 100 mv., even when this large excess of chloride is present, so that the end point is easily determined. Figure 1 shows actual titration curves for three
equal samples of iodide, to one of which no chloride has been added, to another a hundredfold excess of chloride, and to the third an equimolar amount of chloride. The erroneous result in the third case is readily apparent. These curves were obtained with a Muller ( 2 ) capillary electrode system, the actual electrodes being silver wires very lightly coated with silver iodide by anodic deposition from potassium iodide solution. The potentials were measured every 0.2 ml. near the end point by means of a potentiometer with a vacuum-tube voltmeter as null indicator. For routine determinations where actual voltages are unimportant, the simple battery-operated titrator described by Garman and Droz ( 1 ) is very satisfactory. Magnefic stirring is employed throughout the titration. The three types of titration show very
different visual appearances: In the absence of chloride, the silver iodide formed remains colloidally dispersed until it precipitates sharply a t the equivalence point; with a large excess of chloride the silver iodide remains colloidally dispersed throughout; and when only a small amount of chloride is present, the precipitate coagulates well before the equivalence point. LITERATURE CITED
(1) Garman, R. L., Droz, M. E., IND. ENG.CHEM.. ANAL. ED. 11, 398 (1939). (2) Muller, A., 2. physik Chem. 135, 102 (1928). (3) Seidell, A,, “Solubilities of Inorganic and Metal-Organic Compounds,” 3rd ed., p. 40, McGraw-Hill, New York, 1953.
RECEIVEDfor review May 14, 1956. Accepted July 8, 1957.
Flame Spectrophotometric Determination of Microgram Quantities of Magnesium H. STRUNK, and S. L. ADAMS Research Department, Joseph E. Seagram & Sons, Inc., louisville, Ky.
LOUIS MANNA, D.
F A rapid procedure is described for the flame spectrophotometric determination of micro quantities ( 1 to 6 y ) of magnesium. It is precise and requires no preliminary separations. The radiant power of magnesium was greatly enhanced by aspirating from an 80% acetone solution. The inhibitory action of aluminum was circumvented b y the use of a multipleion radiation buffer consisting of 7 5 0 y of calcium per ml., 25 y of aluminum per mi., and 2M acetic acid. The accuracy of the method was verified b y analyzing National Bureau of Standards samples of limestone and magnesite.
0
few papers dealing with the flame photometric determination of magnesium, only Dippel (6), Ikeda, (Q), and Knutson ( I S ) described procedures that can be applied to its microdetermination. Close, Smith, and Watson ( 1 ) determined the magnesium oxide content of Portland cement, limestone, and cement mortar after the separation of silica, iron, and alumina. In determining the magnesium content of glass, Roy (19) used synthetic standards corresponding to the known composition of the glass to be analyzed in order to overcome radiation interference. F THE
Kuemmel and Karl (14) similarly determined the magnesium content of cast iron. Pro and Xathers (18) used a radiation buffer of dextrose and phosphate in determining the magnesium content of wines. This study describes a rapid procedure for the determination of small concentrations (1 to 6 y per ml.) of magnesium. Although the method was developed principally to determine micro quantities of magnesium in beverage alcohol, it is applicable t o complex solutions containing many inorganic components without preliminary separations. APPARATUS AND CHEMICALS
Emission measurements were made with a Beckman Model DU spectrophotometer equipped with a Model 4300 photomultiplier accessory and a Model 9200 flame photometry attachment. A model 4020 atomizer-burner utilizing an oxyhydrogen mixture was the source of excitation. Analytical grade chemicals were used throughout the investigation. Stock solutions of aluminum, copper, iron, and magnesium ions were prepared by reaction of each metal with a minimum of concentrated nitric acid and then diluting to volume with double distilled
water. Solutions of lead, cobalt, lithium, manganese, nickel, potassium, and sodium ions were prepared from their nitrates and a solution of calcium ions rvas prepared by reacting calcium carbonate with a minimum quantity of concentrated nitric acid. The solutions were stored in polyethylene containers. EXPERIMENTAL PROCEDURE
The 371- and 383-mp maxima in the magnesium oxide bands have been used for the determination of magnesium. However, unresolved radiation from the iron lines a t 373.0, 373.6, and 386.0 mp caused positive errors in the magnesium determinations. For this reason the much weaker atomic line a t 285.2 mb was used for the determination of this element. The following Beckman DU spectrophotometer settings were used throughout the investigation: Wavelength, mp Selector Sensitivity on photomultiplier battery box, 60 volts per dynode Resistor, megohms Slit width, mm. Hydrogen, lb./sq. inch Oxygen, lb./sq. inch
285.2 0.1
Full 22 0.025 1.25 12
The wave-length setting must be exVOL. 2 9 , NO. 12, DECEMBER 1957
* 1885
act to obtain the niaxinium energy response. The procedure used t o minimize instrumental errors has been reported (16).
Table I.
Radiant Power of Magnesium in Various Solvents
Radiant Power, Scale Divisionsa Solvent Net Mg
+ solvent
Solvent Methanol
%
Ethyl alcohol
95 80 40 20 95 80 40 20 95
99.6 90.4 73.3 60.6 97.1 88.2 74.9 64.0 96.9
47.3 51.8 48.9 41,l 60.5 63.0 57.9 46.9 68.9 67.8 60.6 48.5 68.9
80 .~
86.9
A5 7.
73.3 58.2 59.2 86.4 64.7 51.6 38.9
62.5 46.7 20.5 47.5 41.5 35.2 32.1
Mg
RESULTS
Effect of Organic Solvents. Smit, $lkemande, and Verschure (20) first suggested that organic solvents such as acetone, butanol, and propanol might be beneficial in the flame determination of small amounts of sodium and potassium in blood serum. Since then other workers (3-6, 8, 11, 1.2, 15, 16) have demonstrated the enhancement of radiant power obtained by the use of various organic solvents. Different solvents were tested a t varying concentrations to determine their effect on the radiant pomer of 4 y of magnesium per ml. The data presented in Table I agree with the results reported by Ikeda (9) for 50% methanol solutions. Although %yomethanol produced the greatest net emission, this Concentration is not practicable. An 80% solvent concentration is more desirable because it contains sufficient water to facilitate the preparation of samples for analysis. Acetone gives the highest radiant poF-er a t 807, concentration. The data for acetone are unique. The net radiant powers of magnesium in 80 and 95% acetone are identical. However, the backgroundto-line ratio is most favorable for 95% acetone, and next best for 95% methanol, followed closely by 80% acetone. The calibration data for magnesium in 80% acetone are presented in Table 11. A plot of these data show that the curve is not linear. Self-absorption would be expected, inasmuch as the magnesium line a t 285.2 mp is a resonance line of low energy of excitation. Jefferson (11), Wilson and Krotinger (af),and Ikeda (10) have also reported nonlinear curves when working with magnesium at concentrations of 0 to 60 y per ml. and 0 to 400 y per ml. in 20% ethyl alcohol utilizing an oxyacetylene flame. Close, Smith, and Watson ( I ) , however, obtained a linear curve with 60 to 400 y of magnesium per ml. a t 371 mp with an oxyhydrogen flame. Effect of Anions. Ikeda (9) reported that nitric and sulfuric acids below 1.5M concentration and hydrochloric acid below 3 M concentration did not inhibit the radiant power of 5 y of magnesium per ml. in 50% methanol. Table I11 illustrates the effect of acids in 80% acetone containing 4 y of magnesium per ml. The inhibition caused by phosphoric acid was not unexpected, inasmuch as other workers (7, 16-18) have reported a similar effect on the radiant power of other metals. Kitric and acetic acids increased the 1886
ANALYTICAL CHEMISTRY
Propanol
2-Propanol
Acetone
40 20 95 80 40 20 100
Water Instrument adjusted to read 100 on transmittance scale with 6 80% methanol.
radiant power of magnesium. Of the two acids, acetic acid gave the more pronounced intensity. This is to be expected, because acetic acid is an organic solvent. The radiant power of magnesium decreased as the concentration of sulfuric and hydrochloric acid was increased. This inhibition indicates that a radiation buffer is necessary to overcome the influence of acid concentration on the radiant power of magnesium. Effect of Cations. The data shown in Table I V confirm the results reported by Ikeda (9) and Knutson (IS). Aluminum seriously depressed the radiant power of magnesium; all the other cations exerted a slight inhibitory action. Many cations were tried as radiation buffers to determine whether the strong inhibition of magnesium by aluminum could be overcome. Results showed that solutions containing 750 y of calcium per ml. were the most effective in re-establishing the radiant power of magnesium. Because 2M acetic acid caused a considerable enhancement of the radiant power of magnesium (Table 111), this concentration of acetic acid was added to the radiation buffer containing 750 y of calcium per ml. The efficacy of this multiple ion radiation buffer in overcoming the inhibitory action of aluminum on magnesium was then studied (Table V). In the solutions containing calcium as the radiation buffer, the presence of 2M acetic acid caused a reduction in the radiant power of magnesium. However, upon adding aluminum to the solution the radiant power of magnesium increased considerably. Therefore, a radiation buffer consisting of 750 y of calcium per ml., 25 y
46.9 34.5 15.8 12.4 39.1 27.4 15.4 13.7 28.2 20.4 14.3 15.5 28.0 21.2 10.8 11.7 38.7 38.9 23.2 16.4 6.8 y per ml. of Mg in
of aluminum per ml., and 2M acetic acid was adopted. An 80% acetone solution containing 4 y of magnesium per ml. and this radiation buffer exhibited a radiant power greater than 6 y of magnesium per ml. in 80% acetone without
Table
II.
Radiant Power of Magnesium in Acetone
Mg Concn., y per M1.
Radiant Power, Scale Divisions
0
36.8 52.2 65.5 75.6 85.5 94.4 100.0
1
2 3
4 5 6
Table 111. Radiant Power of M a g nesium in Acetone Containing Acids
Acid HXOa
AcOH HC1 HZS04
HSP04
M 0.1 1.0 2.0 0.1 1.0
2.0 0.1 1.0 0.1 1.0 0.1 1.0 2.0
Radiant Power, Scale DivisionsAlgfacid Acid Net Mg 85.2 89.0 95.7 86.8 92.0 97.2 84.9 82.7 84.7 90.2 86.2 102.3 100.4
42.4 45.0 44.3 42.1 41.5 40.5 44.0 48.3 41.5 52.1 48.3 75.7 84.5
42.8 44.0 51.4 44.7 50.5 56.7 40.9 34.4 43.2 38.1 37.9 26.6 15.9
Instrument set t o read 100 on transmittance scale with 6 y per ml. of Mg in 80y0 acetone; 4 y of Mg per ml. gave a net radiant power of 45 scale divisions. 0
the radiation buffer. Increasing the aluminum ion concentration of the radiation buffer to 100 y per ml. decreased the radiant power of magnesium but only to the extent of one unit on the transmission scale.
Table IV. Effect of Cations on Radiant Power of Magnesium in 80% Acetone
Radiant Power, Scale Divisionsa Cation y per Mg + Net Added M1. cation Cation Mg A1 40.6 50 74.2 33.6 41.5 500 60.3 18.8 Ca + + 41.3 50 84.1 42.8 45.2 500 86.0 40.8 co 42.2 43.9 50 86.1 44.4 43.1 500 87.5 c u ++ 42.4 43.1 50 85.5 43.8 42.8 500 86.6 Fe 45.1 50 86.2 41.1 47.1 41 .O 500 88.1 K+ 43.5 50 86.2 42.7 43.3 40.1 500 83.4 Li + 44.5 40.5 50 85.0 50.6 500 88.9 38.3 Mn +r 44.6 50 83.9 39.3 46.4 37.5 500 83.9 Na 46.5 40.8 50 87.3 51.7 500 91.2 39.5 Ni + + 45.2 40.8 50 86.0 45.7 42.2 500 87.9 Pb + + 45.2 41.3 50 86.5 43.6 42.4 500 86.0 4 y of Mg per ml. gave a net radiant power of 45 scale divisions. +T+
by analyzing National Bureau of Standards argillaceous limestone 1A. The sample was prepared for analysis by digesting on a hot plate 360 mg. of sample with 1.5 ml. of perchloric acid and about 5 ml. of 48% hydrofluoric acid. The mixture was evaporated to dryness, dissolved in 6M hydrochloric acid, and diluted to 100 ml. in a volumetric flask. An aliquot of this solution was used to make an SO% acetone solution having a magnesium concentration between 2 and 4 y per ml.
Table V. Effect of Calcium, Aluminum, and Acetic Acid on Radiant Power of Magnesium in Acetone
Radiant Power’
++
+++
+
The radiation buffer was evaluated in 80% acetone solution containing 1 to 6 y of magnesium per ml. and a combination of 40 y of each of the following eleven cations: aluminum, calcium, cobalt, copper, iron, lithium, potassium, manganese, nickel, sodium, and lead (Table VI). Complete magnesium recoveries were obtained without the radiation buffer at the levels of 2, 3, and 4 y per ml., but the recovery was 0.2 y high a t the 1-7 level and low by 0.2 and 0.5 y for the samples containing 5 and 6 y, respectively. When the radiation buffer was employed in no instance did the results vary by more than 0.1 y from the true value The foregoing data demonstrate that a magnesium content of 1 to 6 y per ml. can be determined accurately by the application of the multiple ion radiation buffer. However, a concentration of magnesium between 2 and 4 y per ml. can be determined without the use of a radiation buffer provided the concentration of interfering cations is low. Analysis of Limestone. The validity of this procedure was determined
Radiation Buffer, y per M1. Ca A1 AcOH None . . . . . . 1000 . . . . . . 1000 . . . 2M 500 25 2M 750 25 2M 1000 25 2M 500 100 ZM 750 100 2M 1000 100 2M
“,‘,a:
ation buffer 75.7 81.5 74.5 89.7 91.7 9.5.2 .. 88.5 90.5 94.2 ~
Radiation buffer 38.2 44.2 46.9 39.4 42.5
Net Mg 37.5 37.3 27.6 50.3 49.2
45 R
49 X
42.0 45.3
48.5 48.9
39.4 i9:i
Table VII.
Sample Found Limestone 2.26 2.16 Magnesite 87.0 84.8
Given 2.19
85.9 i 1 . 1
85.69
...
The average of two separate analyses (Table VII) shows good agreement with the given values. No radiation buffer was used because the diluted sample had a small concentration of interfering cations. LITERATURE CITED
W. E., Katson, hl. T.. ANAL. CHERT. 25. 1922 (1953).’ Curtis, G. W., Knauer, H. E., Hunter, L. A., Am. SOC.Testing Materials, Tech. Publ. 116, 67 (1952). Dean, J. A., Lady, J. H., ANAL. CHEM.27, 1533 (1955). Dean, J. A., Lady, J. H., Ibid., 28, 1887 (1956). Dean, J. A,, Thompson, C., Ibid., 27, 42 (1955). Dippel, W. A., Ph.D. dissertation, Princeton University, October 1954. Dippel, W. A., Bricker, C. E., Furman, N. H., ANAL. CHEM. 26, 553 (1954). Fink, A., Mikrochim. Acta 1955,
(1) Close, P., Smith,
(4)
Analysis of Magnesite. About 40 mg. of National Bureau of Standards burned magnesite 104 was transferred into a 100-ml. volumetric flask and 5 ml. of concentrated nitric acid was added. After digestion on a hot plate for about 1 hour, the flask was
QO, % Average 2.21
filled t o the mark and an aliquot of this solution was used to prepare an 80% acetone solution having a magnesium ion concentration between 2 to 4 y per ml.
a Instrument adjusted t o read 90 on transmittance scale with 6 y magnesium per ml. in 80% acetone.
The results of two separate analyses (Table VII) indicate a good recovery. No radiation buffer was used because the diluted sample had a low concentration of interfering cations.
Analysis of Limestone and Magnesite
(5) (6) (7) (8)
__ _. 21 4
(9) Ikeda, S., Sci. Repts. Research Insts., Tohoku Univ. 7A, 575 (1955). (IO) Ikeda, S., Ibid., SA, 9 (1956). (11) Jefferson, J. H., Ph.D. dissertation, University of Wisconsin, August 14.56
(12) K&gi!&, G. R., Schaffert, R. R., J. Biol. Chem. 206, 807 (1954). (13) Knutson, K. E., Analyst 82, 211 11957). , \ - - -
Table VI. Effect of Cation Mixture’ on Radiant Power of Magnesium in Acetone
Magnesium, y per M1. Found without Found with radiation radiation Present buffer buffer 1 1.2 1.1 2 2.0 2.0 3 3.0 3.1 4 4.0 4.0 5 4.8 5.0 6 5.5 6.0 Final solutions contained Mg plus 40 y/mL of A1 + + + Fe + + + Ca Ni + + K +,Na +, Pb ++,I Mn +4, Co +%,Li c u ++. 0
++
+:
Kuemmel, D. F., Karl, H. L., ANAL. CHEW26, 386 (1954). Lady, J. H., Ph.D. dissertation, University of Tennessee, August 1955. Manna, L., Strunk, D. H., Adams, S. L., ANAL. CHEK 28, 1070 (1956). Parks, T. D., Johnson, H. O., Lykken, L., I N D . ENO. CHERT., ANAL.ED.20, 822 (1948). Pro, M.J., Mathers, A. P., J. Assoc. O ~ CAgr. . Chemists 37, 945 (1954). Roy, K.,ASAL.CHEM.28, 34 (1956). Smit, J., Alkcmande, C. T. J., Verschure. J. C. M.. Biochim. et Biophys. Acta 6, 508’(1951j. (21) Kilson, T. C., Krotinger, S . J., ASTM Bull. 189, 56 (1953). 1
RECEIVED for review January 22, 1957. Accepted August 17, 1957. VOL. 29, NO. 12, DECEMBER 1957
1887
