Determination of Calcium and Magnesium in lake Waters By Means of a Rotating Silver Disk Electrode VlLLlERS W. M E L O C H E and R U B I N S H A P I R O ' Department o f Chemistry, University o f Wisconsin, M a d i s o n 6, W i s .
A spectrographic method was developed for the determination of calcium and magnesium in lake waters using a silver disk electrode. The range of concentration studied was 0.1 to 30 p.p.m. (0.00001 to 0.0030%). Rinsing the electrodes in concentrated hydrochloric acid provided an efficient method for removing oxide coat and residues from previous analyses. This made it possible to use the same electrodes repeatedly. The average standard deviation of a single measurement, based upon four observa-
tions on a given sample, was found to be dependent on the spectral line pair employed as well as upon the concentration. For the determination of calcium the average standard deviation ranged from 4 to 10%; for the determination of magnesium it ranged from 2 to 10%. No buffer was used; however, for the recovery tests indicated, the accuracy was adequately measured by the precision. The method was found to be rapid and sensitive for the determination of calcium and magnesium.
T
obtained from Goldsmith Bros. Smelting and Refining CO. This metal also showed traces of calcium and magnesium; however, as the spectrograph was employed at reduced optical speed, no correction was made for these trace impurities.
HE rotating disk technique is one of the more attractive
means for analyzing solutions by means of the spectrograph. The principal advantages, here, lie in the elimination of an ashing or evaporation procedure prior to the excitation of the sample on the electrodes. Since its development by Pierucci and Barbanti-Silva ( I 7 ) , the technique has been employed by a number of workers. Boyle et al. ( 3 ) determined alkali and alkaline earth metals in urine. Gnmbrill, Gassman, and O'Seill (8) and Pagliassotti and Porsche ( I $-16) determined elements in lubricating oils and cracking catalysts. All of these workers employed graphite as their electrode material. The present authors felt that the use of a metallic electrode system might be advantageous from the standpoint of sensitivity and absence of cyanogen interference. They have found that after 15 to 20 seconds of prespark, the metallic disk grows hot enough to evaporate most of the water solvent, leaving the periphery with a solid coat of solute. I n this respect the metallic disk technique is similar to the evaporation technique employed by Rollefson (7, 13) and Fred, Sachtrieb, and Tonipkins (7, I S ) in their respective silver and copper spark methods for analyzing solutions. Xachtrieb states that in general the copper spark method affords higher sensitivity than the graphite direct currrnt arc method, because the metal does not absorb the solution ns does'the porous graphite. To the writers' knowledge, only Blank and Sventitsky ( 2 ) have published vork in which rotating metallic electrodes were utilized. They employed copper disk electrodes in the determination of sulfur in sulfate solutions. It was felt that further investigations of the use of rotating metallic electrodes would be in order.
FABRICATION OF ELECTRODES
The silver was melted directly in a cylindrical graphite mold shown in Figure 1. Although the graphite was impure, none of its im urities was introduced into the melt. It was necessary to meft the silver directly in the mold in order to ensure solid castings. Molten silver has a tremendous affinity for oxygen (1, 9). and must be melted in an inert atmosphere. Presumably in this case the incandescent graphite (1000" C.) reacted with oxygen inside the mold to provide the necessary inert atmosphere.
AXIS-;
APPARATUS
The spectrographic equipment employed was manufactured by the ,4pplied Research Laboratories and consisted of a 1.5-meter Abney grating spectrograph, solution excitation attachment, silver disk assemblies and upper electrodes, spark-arc source unit, densitometer, developing machine, and infrared film dryer. Figure 1. Graphite Mold for Casting Silver Electrode Assemblies
PRELIMIN4RY WORK
Silver was found preferable to copper for use as electrode material. When copper disks were used as media for exciting solutions, it was extremely difficult to maintain a steady value of spark current; moreover, during a run the liquid sample was boiled out of the porcelain boat employed as a container. Silver, on the other hand, showed none of these effects.
Four sets of electrodes shown in Figure 2 were machined from cast silver. Each set consisted of an upper electrode and a lower disk electrode made in the form of a one-piece disk-shaft assembly. An upper electrode weighed approximately 15 grams with dimensions of l 7 / * X l / 4 inch. A lower electrode assembly weighed approximately 24 grams, and its specifications are shown in Figure 3. There was no reason to believe that the dimensions were critical except for the diameter of the shaft portion, which had to be 0.242 inch in order that the shaft would fit snugly in the bore of the solution excitation attachment.
Silver, 99.95% pure, with a principal impurity of copper was 1
Present address, American C a n Co., Maywood, Ill.
347
ANALYTICAL CHEMISTRY
348 To remove oxide and traces of elements from previous analyses, the electrodes were dipped into concentrated hydrochloric acid after each run, then rinsed in distilled water and doubly distilled water, after which they were dried in an air jet. The whole procedure took approximately 1 minute. Each set of electrodes was used for the same number of sparkings throughout the course of this work. As shown in Figure 3, the diameter of a disk section was originally 0.495 inch. After being subjected t o approxim~tely400 sparkings, with an average duration of 48 seconds for each sparking, the diameter was found to be 0.488 inch. This amounted to a loss of less than 0.2 mm. Since the length of the 3-mm. spark gap could be approximated to no better than 0.5 mm., a loss of 0.2 mm. from the diameter of the 1orr.m elootrode did not appear significant. A change in symmetry of the tip of the upper electrode, however, did appear to affect the accuracy of the determins tions. EMULSION CALIBRATION
Eastman Spectrum Andvsis No. 1 emulsion was used exclusively, and cdihmted f i r the wave-length regions 2800 to 3200 A. and 3900 to 4050 A. The calibration was performed aceordins to the d i t filter method described hv Churchill (4,5 ) . 1 1 1 film w& developed at, 20" C. in D-19 sdution. PROCEDURE
Conditions. The disk was rotated a t 8. speed of approximately 5 r.p.m. employing a spark gap of 3 mm. The level of immersion was such that the lower edge of the disk was approximately 4 mm. helow the surface of the liquid at the start of a run. The slit was maintained a t n width of 60 microns. Exposure conditions werc 24 seconds of prespark, 24 seconds of exposure, and 30% grating aperture. Spark excitation was employed. The constants were 0.021 d d . of capacitance, 180 mh. of inductance, and 10 amperes of R.F. spark current (225 to 235 volt.8, input).
.150. I
Figure 3.
S p e d i c a t i o n s for Silver Disk Electrode Assembly
cumes for magnesium and calcium are shown in Figures 4 and 5. These curves represent log log plots of Im./Imn us. concentration and Ics/Ian us. concentration. Lake Water Analyses. Four observations were made on each sample. The procedure was replicated over three days. The average standard deviation of n single measurement based on four repetitive runs is shown in Table I under the heading Av. % s. Other data in Table I include the wave lengths of the various analysis and internd standard lines, emitation potentials as given by Moore (11, f b ) ,approximate wavelength difference between members of a line pair, concentration range, and index point. Recovery Tests. Results of the determinations of magnesium and calcium indicated that the weight ratio of calcium t o magnesium rrtnged generally from 3 to 1 to 5 to 1 in the samples studied. Since the synthetic standards had been prepared to contain calcium and magnesium present hy weight in the ratio 1 to 1, i t was deemed advisable to run a simple recovery test. This was necessary before it could he said that, here, the accuracy was adequately measured by the precision. Two solutions were prepared, each wntaining 1.00 p.p.m. of magnesium
rable I. Data for M g / M n a n d Ca/Mn Working Curves Ex.
Figure 2.
Silver Electrode S e t s
Preparation of Standards and Samples. A series of standard solutions WV&Bprepared oontaining equal weights of magnesium and calcium in the concentration mnge 0.100 to 30.0 p.p.m. Each solution contained 45 p.p.m. of manganese as the internal standard, and chloride ion provided a constant anion background. In addition, each solution contained 1.5 p.p.m. of sodium and 0.5 p,p.m, of potassium, as previous workers (10) had found that the lake waters in question contained these amounts of sodium and potassium on the average. The necessary reagents included calcium carbonate, 99.98% pure; magnesium, 99.9% pure; manganese, speetroscopic$ly pure; sodium chloride and potassium chloride, both spectroscopically pure. Calcium carbonate, magnesium, and manganese were brought into solution by dissolving each reagent in the required amount of hydrochloric acid. Douhly distilled water used in all dilutions. Water samples were procured from 24 northern Wisconsin and four Madison lakes. @.ch snniple was prepared to contain 45 p.p,m, of manganese as mternd standard. Bath standards and samples were stored in polyethylene bottles. Preparation of Working Curves. Each standard was run four times on each of three consecutive days. The over-all mean intensity ratio of the twelve observations was used as a point in the construction of the working curve. The various working
Total
Ex.
Conen.
Range, P.P.M.'
Index, P.P.M.
9% s b
4.31 4.40
I.OO-IO.O
6.00
9 96
4.42 7 97
12.06
3.00-20.0
8.80
I.RO
Ca3933.7IIR Mn4034.5 I
3.14 3.06
9.26
3.06
0.100-1.00
0.540
5.56
C&3968.5IIR Mn4084.6 I
3.12
!:::
0.500-3.00
1.40
3.76
Ca3179.3 I1 Mn2889.6 I1
7.02 8.38
Line Pairs
Pot.
Ma2852.1 I-RMn2801.1 I
4.34 4.40
klg2790.8 I1 Mn2701.7 I1
3.06
Pot."
15.37
::;$:
Ax-.
c
3.0030.0
10.2
10.6
Ca3158.9 I1
7.02 10.0-30.0 20.7 5.08 NI" 2889.6 II 8.38 Fimt ionization potential: Mg, 7.64; C&, 6.11; Mn, 7.40. 100 : ( E - c i * % Q is oaleulsted from statiatieal formula. % B = -~ c " - 1 ooeffioient of variation, where E ia the mean of four determinations of eoncentration. and e i s an individual determination. ~
Table 11. Reoovery Tests on Solutions C o n t a i n i n g 1.00 P.P.M. of Magnesium a n d 6.00 P.P.M. of Calcium
1 2
1.00 0.972
0.0384 0.0395
1.02 1.00
0,0200 0.0307
5.82 5.78
0.392 0.330
V O L U M E 26, NO. 2, F E B R U A R Y 1 9 5 4
349
6.00 p.p.m. of calcium, 45 p.p.m. of manganese as internal standard, 1.5 p.p.m. of sodium, and 0.5 p.p.m. of potassium. Four observations were made on each solution. The values of concentration obtained are given in Table 11. Also shown are the precisions stated in terms of the standard deviation of the mean of four measurements, sq. Where i t appeared that there might be a significant difference between the true value of concentration, C, and the recovery value, c, the statistical t test (18) was employed. t=-
c-c S4
Where there are four measurements to a set, the critical value of t for the 5% level of significance is given by tables as 3.18. I n this particular investigation the experimental values of t did not exceed the critical value 3.18; hence, i t was assumed that the values C and c were not significantly different.
a t a ccnstant level of immersion in the liquid sample. However, an investigation was conducted to determine the effect on the accuracy when the level of immersion was varied to extremes on different runs. The study was made employing added manganese or silver from the matrix as the internal standard. Where manganese was employed, an extreme change in disk level resulted in a 20% change in intensity ratios of Mg/Mn line pain. Where silver was employed, an extreme change in disk level resulted in as much as a 100% change in intensity ratios of M~lg/~4g line pairs. Although this experiment indicated that different depths of immersion may net be used within a series of deterniinatiom, the actual depth is not important as long as the same value is approximated from run to run.
t0L 'O'O
DISCUSSION
Studies of precision were made in which analysis lines of magnesium were matched with lines of various internal standard elements. The investigation was conducted on the intensity ratior of the line pairs. Chosen as internal standards were silver from the matrix electrode, cadmium added to the solution, or manganese added to the solution. The precision obtained with manganese as the internal standard was far better than with the other two elements mentioned; hence, it was decided to employ manganese as the internal standard.
c
:
1.00
W
c E
CONCENTRATION. PPM.
Figure 5.
10.0
0
0
1
I
ki a
k c 5
r-
1.00
,100
1.00
10.0
CONCENTRATION, PPM
Figure 4.
Working Curves for Calcium A.
Ca 3933.7/Mn 4034.5
D.
Ca 3158.9/1Mn 2889.5
B . Ca 3968.5/Mn 4034.5 C. Ca 3179.3/Mn 2889.5
Inasmuch as the four sets of electrodes \yere used continually, the tips of the upper electrodes were altered from 120" points to practically flat sections. From the standpoint of time, it was not feasible to repoint an upper electrode after each run: hence, i t w a b desirable to learn what effect repointing the virtually flattened tips would have on the accuracy of the determination. This possible cause of determinate error m s studied by repointing the upper electrodes near the close of the second replication in the analyses of the lake water samples. I n other words, groups of four observations \\.?re made on a sample before and after the tips of the upper elrctrodes were renewed. T h r re-
Working Curves for Magnesium A . M g 2795.5/Mn 2701.7 13. Mg 2582.1/Mn 2801.1 C. M g 2790.8/Mn 2701.7
An analysis of variance as prescribed by Dixon and Massey (6) and Youden (18) was performed with respect to two possible variables. Stated as questions, these were: (1) Was there a significant difference between mean intensity ratios as obtained by the use of different sets of electrodes? (2) Was there a significant difference between mean intensity ratios obtained on consecutive days? The investigation was conducted on the line pairs, Mg 2852.1/Mn 2801.1 and hIg 2779.8/Mn 27'01.7. A solution containing either 3 p.p.m. of magnesium and 50 p.p.m. of manganese or 30 p.p.m. of magnesium and 50 p.p.m. of manganese was excited on each of the four sets of silver electrodes. The procedure was replicated over four consecutive days. A comparison between the variance of the electrode means and the variance of the residual indicated that differences in the electrode means were insignificant-that is, differences were due to randomness (chance). A comparison between the variance of the day means and the variance of the residual indicated that differences in the day means were liken ise insignificant. Throiighout this work an effort was made t o maintain the disk
Table 111. Effect of Repointing Silver Upper Electrodes Concn. after True Conen., Repainting, P.P.M." P.P.M. Effect upon Determination of Magnesium Weber 0.282 0.294 0.309 Starret 0.246 0.242 0.231 Lone Tree 0.393 0.396 0.412 Frank Bear 0.821 0.815 1.02 1 31 Lost Canoe 1.28 1.74 Rock 1 37 1.32 1.61 Muskellunge 1.59 1.52 1.84 Arbor Vitae 2 63 2.63 3.36 Mann 2 33 2.44 3.23 12 8 3lendota 3Ionona 14 6 Effect upon Determination of Calcium Weber 1,19 1.25 0 982 Starret 0.950 0.965 0 830 Lone Tree 1.42 1.52 1.33 Frank Bear 2.47 2.37 2.54 Lost Canoe 6.24 6.45 7.42 Rock 6.10 5.68 7.46 Sample
a
121uskellunge Arbor Vitae Mann Afendota hIonona Each value represents
6.00 5.47 11.6 10.8 12.2 12.4 25.4 27 4 mean of four determinations.
5.92 12.9 12.6 31.9 29.5
ANALYTICAL CHEMISTRY
350 sults appeared to be especially significant in the magnesium determinations. The important data obtained are listed in Table 111. I t is apparent from the data in Table I11 that a marked change in the tip of the upper electrode generally affects the accuracy of a series of determinations on a specific sample. It would then appear advisable to keep the upper electrodes a t the same shape for successive runs. Since i t is not feasible to machine after each sparking, the writers recommend that the upper electrodes be machined originally to have flat tips rather than 120” points. The data from Table I indicate that in general magnesium and calcium can be determined with the following precision, stated in terms c>f the per cent standard deviation of a single measurement: For magnesium: 5.70% in the range of 0.100 to 1.00 p.p.m.; 9.96% in the range of 1.00 to 10.0 p.p.m.; 1.8070 in the range of 3.00 to 20.0 p.p.m. For calcium: 5.56Y0 in the range of 0,100 to 1.00 p.p.m.; 3.7670in the rangeof 0.500 to3.00 p.p.m,; 10.6% in the range of 3.00 to 30.0 p.p.m.; 5.08% in the range of 10.0 to 30.0 p.p.m. Of course, a greater number of determinations for a given sample lessens the standard deviation. In this case, in which four determinations were made, the values of the standard deviation of the mean are half of the values given above. Whereas the sensitivity and reliability of the silver disk electrode method in the range of 0.100 to 1.00 p.p.m. are somewhat superior to those of the Beckman flame photometer a i t h photomultiplier attachment and oxyacetylene flame, the flame photometric method is capable of yielding a t least equivalent reliability in the higher ranges of concentration of the elements described. ACKNOWLEDGMENT
The work described was supported in part by the Research
Committee of the Graduate School from funds supplied by the Wisconsin Alumni Research Foundation, LITERATURE CITED
Addicks, L., “Silver in Industry,” p. 173, Sew York, Reinhold Publishing Corp., 1940. Blank, 0. V., and Sventitsky, N. S.,Compt. rend. acad. sci. U.R.S.S., 44, 58 (1944). Boyle, A. J., Whitehead, T., Bird, E., Batchelor, T. If.,Iseri. L. T., Jacobson, S.D., and Meyers, G. E., J . Lab. Clin. M e d . , 34, 625 (1949). Churchill, J. R., IND.ENG.CHEX.,AXAL.ED., 16, 653 (1944). Churchill, J. R., “Modern Instrumental Analysis,” ed. by D. F. Bolta, p. 1, Ann Arbor, hIich., Edwards Bros., 1949. Dixon, W.J., and Massey, F. J., ”Introduction to Statistical Analysis,” p. 119, New York, McGraw-Hill Book Co., 1951. Fred, SI.,Kachtrieb, ?;. H., and Tompkins, F. S., J . O p t . SOC. Amer., 37, 279 (1947). Gambrill, C. AI., Gassman, A. G., and O’Neill, W. R., XSAL. CHEW,23, 1365 (1951). Latimer, W. >I., and Ilildebrmd, J. H., “Reference Book of Inorganic Chemistry,” p. 112, Kew York, RIacmillan Co.. 1940. Lohuis, D., hleloche, V. W., and Juday, C., Trans. Wiscoriain A c n d . Sci., 31, 285 (1938). Moorc, C. E., “A Multiplet Table of Astrophysical Interest,” Parts I and 11, Princeton, X.J . , Observatory, 1945. Rloore, C. E., Katl. Bur. Standards, Circ. 488 (1950, 1952). Nachtrieb, N. H., “Principles and Practice of Spectrochemical Analysis,” p. 264, Ken- York, LlcGraw-Hill Book Co., 1950. Pagliassotti, J. P., and Porsche, F. W., ANAL. CHEM.,23, 198 (1951). Ibid.,p. 1820. Ihid.,24, 1403 (1952). Pierucci, l I . , and Barbanti-Silva, L., .Vuovo cimcnto, 17, 275 (1940). Youden, W.J., “Statistical Methods for Chemists,” pp. 18, 84, Xew York, John Wiley & Sons, 1951. RECEIVED for review .4ugust 21, 1953. Accepted October 26, 1963.
Functional Group Analysis Characterization of Coal Hydrogenation Products R. A. GLENN and ELIZABETH D. OLLEMAN’ Coal Research Laboratory, Carnegie Institute
of
Technology, Pittsburgh 13, Pa.
P
REVIOUS studies in this series on the chemical nature of products from the hydrogenolysis of coal have described the resolution of the distillate oils by means of chromatography on alumina (10, 14)and/or silica gel ( 1 ) and by means of multistage molecular distillation ( 2 ) . The resultant fractions were investigated by means of such properties as molecular weight, elemental composition, spectral analysis, and in a few instances by acid and alkali solubility and by hydroxyl content as indicated by acetylation. This paper reports the progress made on the determination of the classes of oxygen and nitrogen compounds present in coal hydrogenation oils. The classes of compounds other than hydrocarbons that may be found in coal hydrogenation oils include primary, secondary, and tertiary alcohols; primary, secondary, and tertiary amines; and phenols ( 2 , 5-7, f4). Functional group analysis in combination with ultimate analysis and molecular weight constitute the basis of an analytical procedure for the quantitative, estimation of these compounds. The coal hydrogenation oil used in this study was accumulated from the hydrogenolysis of seventeen 200-gram batches of Pittsburgh Seam coal R-ith ddkins catalyst a t 375’ C. for 12 1
Present address, Verona Research Center, Koppers Co , Inc , Verona, Pa.
hours (4) using the procedure already described (6). The ultimate composition, average molecular weight, and average molecular formula are given in Table I for the neutral oil resulting from the alternate extraction of the distillate oil with acid and with both aqueous and alcoholic alkali (4). ANALYTICAL PROCEDURES
The various analytical procedures used for the determination of the different functional groups and classes of compounds, either individually and/or collectively, are, in general, applications of semimicrotechniques described in the literature, but which have been refined for application to coal hydrogenation products.
Sodium aminoethoxide titration (8) in anhydrous ethylenediamine determines quantitatively carboxylic acids and monohydric phenols, either hindered or unhindered, without interference from nitrogen compounds. The dihydric and the unhindered monohydric phenols are assumed to have been removed completely by the repeated extraction with both aqueous and alcoholic alkali. Carboxylic acids are not found in coal hydrogenation oils, so the observed value represents the hindered phenols present. Perchloric acid titration (9) in glacial acetic acid determines all basic nitrogen compounds-Le. all primary, secondary, and tertiary amines except those cydlic secondary amines of the
