identification of Chemical Compounds by Voltage Fluctuation in the Direct Current Arc
.
JAMES W. MELLICHAMP
S. Army Elecfronics Command,
lnsfifufe for Explorafory Research, U.
The effectiveness of a semiquantitative spectrochemical procedure can be extended in some cases to include recognition of simple chemical compounds, in addition to impurity analysis, by recording the voltage fluctuation of the d.c. arc during sample consumption. The recorded patterns differ sufficiently between compounds to permit identification with the use of reference patterns. A cored cathode i s used to increase arc stability so that the observed fluctuations are attributable primarily to effects originating from the anode. Reference patterns have been made of over 100 compounds to determine the extent and usefulness of this method. In general, compounds with boiling points between 1000" and 3500" C. are in-
Fort Monmoufh, N. J.
cluded. Patterns of mixtures or trace compounds are not, thus far, practical for interpretation.
cationic components and give little or no indication &s to the anionic components. By a systematic study of the voltage fluctuation of the d.c. arc during sample consumption, a method has been developed whereby, for many analyses, the chemical compound of the matrix material can be recognized by a comparison of its characteristic voltage fluctuation (CVF) pattern with that of a reference pattern. No modification of the semiquantitative procedure is necessary other than the incorporation of a strip recorder across the analytical gap. In a prior publication (S),a study was made of the voltage fluctuation patterns for different materials, including 67 elements in the elemental state, to determine the reasons for the observed fluctuations. This study has been continued
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spectrochemical procedures, such as that developed by Harvey ( I ) , are an important step in the identification and analysis of a wide variety of materials. More than 60 element.s are detected by d.c. arc excitation along with an estimation as to quantity, even with no special preparation for specific elements. While this may be sufficient for some purposes, further analysis by chemical or other methods is needed for greater accuracy; or, if identity of the original compound is required, x-ray diffraction can be utilized. Spectrochemical methods are generally limited to the analysis of metallic or EMIQUANTITATIVE
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SECONDS Figure 1. A composite of CVF patterns of Na halide compounds to show periodicity of responses based on boiling points and molecular weights
SECONDS Figure 2. A composite of CVF patterns of K halide compounds to show periodicity of responses based on boiling points and molecular weights
Voltage and current ore inverse potterns
Voltage and current are inverse potterns
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ANALYTICAL CHEMISTRY
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CVF patterns of silver and silver compounds
Patterns are characteristic of analytical conditions and properties of the compounds
and now includes a wider selection of materials. The recorded patterns are found to be a function of both the analytical conditions under which they are made (arc gap and current, electrode design and material, presence or absence of a buffer, etc.) and the properties of the sample material (volatility rate, density, ionization potential, molecular weight, decomposition products in the anode cup, and reactivity with the electrode material or surrounding atmosphere). Under standardized conditions, such as that required for a semiquantitative procedure, voltage fluctuation patterns differ between chemical compounds and are characteristic of the compound. Because a semiquantitative procedure is designed to cover as large a variety of materials as may be practical, a specific procedure is not the optimum for all materials. The procedure developed by Harvey (1) based on total sample consumption with no added buffer other than graphite powder is found to be suitable for inclusion of voltage fluctuation patterns. While not optimum for
any one material, it is satisfactory for compounds with intermediate boiling points between 1000° and 3500’ C. EXPERIMENTAL
Spectrographic equipment consists of a Bausch and Lomb dual-grating spectrograph and excitation stand, a JarrellAsh Varisource, and accessories for photographic processing and densitometry. Additional equipment is a 10inch strip recorder (Bristol Dynamaster) with 0-100 volts full scale and a second scale of 1&15 amp. Voltage across the analytical gap is read to within 1 volt, the current to 0.05 amp. The analytical conditions are summarized in Table I. The specially designed cored cathodes, described previously (S), are cut from 0.18-inchdiameter graphite rods (Union Carbide Corp., Grade AGKSP), shaped and cored, and packed with a 1 to 2 mixture of BaCOa (Johnson Matthey and Co., Ltd., Grade 1) and graphite powder (Union Carbide Corp., Grade SP-1). A cored cathode is used to increase arc stability so that observed fluctuations are attributable primarily to effects originab ing from the anode. Patterns made
without the cored cathode have additional fluctuations that may obscure small differences between compounds. The BaCOa used in the core is of high purity (less than 3 p.p.m. impurities reported), however, a blank electrode should be checked for possible contamination. Table 1. Analytical Conditions for Identification of Chemical Compounds by CVF Patterns
Sample wt., mg. Anode design
10(+10 mg. graphite powder) Ultra Carbon Go., preformed lOlL, Grade T 1-2
Cathode design Sp&c’ally cored cathode Arc current, amp. 14(copper short, not constant current) Analytical gap, 5 (manually mainmm. tained) Impressed line
voltage, volts Time of burn
303
Total consumption of sample Electrode holders Not artificially cooled Strip recorder Bristol Dynamaster, full scale 0-100 volts VOL 38, NO. 10, SEPTEMBER 1966
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RESULTS AND DISCUSSION
current have a periodicity of responses bssed primarily on differences hi b o i i g
Over 100 reference CVF patterns covering a selection of chemical compounds have been recorded to determine the extent and usefulness of this method. All patterns are characterized by a drop in voltage as a function of the amount of sample material in the arc column. With no sample material, the voltage is maximum at about 58 volts in the free burning arc. With sample introduction, the voltage drops and reaches a minimum with the maximum distillation of sample material into the arc column. Upon sample consumption, the voltage returns to 58. The lowest voltage drop for compounds of the same element is approximately the same 84 determined by the ionization potential of that element. Because a constant-current source is not used, current fluctuations follow the same but inverse pattern a the voltage. The variety of CVF patterns is as great as the number of compounds tested. The simplest CVF patterns are represented by the alkali halides as shown in Figures 1 and 2. The figures are a composite to show that both voltage and
pointa and molecular weights. The voltage drops immediately to the lowest point and remainsthere until completion of sample consumption. For sodium compounds, when the voltage is minimum at 20 volts, current is maximum at 11.8 amp. ; for potsasium compounds, the same points are 19 volts at 12 amp. Figure 3 is the CVF patterns for Ag and Ag compounds. The patterns are repeatable and are sdliciently dserent to permit identification of the particular compounds. Several patterne--e.g., the nitrate and the cyanide-are similar overall but d 8 e r in the smoothness of burn as seen in the coarser fluctuations in the cyanide. The iodide diflers from the bromide and chloride because the iodide volatilizes partly 84 AgI before the remainder is reduced and volatilized a the metal, a seen in the pattern a two distinct burn periods. The lowest voltage for Ag compounds is around 31 volts. Compounds of other elements are similar to those of Ag in that recognizable d8erences are observed. Examples of how patterns of compounds with the same anion but dser-
ent cations ditrer are shown in Figure 4. Most carbonates decompose to the oxide in the anode cup with a resultant change in volatility rates and can be seen in the alkali patterns as a rise and fall in voltage. When two patterns are nearly coincident, such a with Ba and Ca carbonate, spectra in addition to the reference patterns are required for identification. The most complex compounds tested are those having two cations. It can be Seen in Figure 5 that the patterns of the titanates are similar. Many of the twocation compounds tested--e.g., BaCr04, K2Cr&, KMnOa, and KFe(CN)-have patterns that are quite distinctive and repeatable, while others--e.g., MgW04, and CaWO4-have erratic patterns with little or no repeatability. Some pattern features that indicate characteristics of a compound are: warm-up period to obtain boiliig point, down-slope (whether step or gradual), lowest voltage reached, stability during consumption period, deviations due to sample decomposition or other reactions, and consumption time. Mixtures of compounds have intermediate or super imposed patterns with little possibility,
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CVF pattern of carbonates
The rise and fall in voltage seen In some pattarns is c a d by decomposition of the carbonate in %e anode cup
1974
ANALYTICAL CHEMISTRY
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thus far, for interpretation. At what point trace or minor constituents interfere with the patterns has not been determined, but it is estimated to be approximately 1% for total impurities depending on how the impurities differ in properties from the major constituent. Compounds with boiling points less than 1000° C. have consumption periods of less than 20 seconds and requirements become critical to resolve small time differences. This is one limitation of CVF patterns for identification. When advantageous, however, consumption time can be increased by a decrease in arc current. Another limitation is the presence of water or other volatile components that cause fluctuations that are not repeatable. Compounds with boiling points between 1oOO’ and 3500’ C. generally give patterns which are s a t i s factory for interpretation. Above 3500’ C. the patterns are erratic because of uneven and incomplete distillation. Refractory compounds also have the tendency to condense near the tip of the cathode, thus adding to the fluctuations. Usually an arc-current increase is of no advantage with this type of material. Most patterns were made in duplicate, while some were repeated five or more times. The overall pattern characteristic of the compound varies slightly with each repeat burn because of variables not completely under control. As the an& lyst gains familiarity with the patterns, details are noticed that are of value for the improvement of both reproducibility and sensitivity of the d.c. arc. The d.c. arc with its inherent sensitivity has relatively poor reproducibility when compared with the condensed spark or other excitation sources. Factors causing deviations in CVF patterns are the same that limit the reproducibility of d.c. arc procedures in general, such as uneven distillation and sample ejection during consumption. Other factors causing differences in the CVF patterns are: inaccuracies in repeat weighings, inconsistancies in sample-loading techniques, moisture in the sample, arc instability, and inexact excitation parameters. Variations in dimensions and quality of preformed electrodes are not found; however, a change in design or graphite grade will give rise to a corresponding change in the pattern. Because it is imprac:ical to anticipate all compounds which may be encountered, CVF patterns are accumulated in the normal run of samples for future reference. Fluctuation patterns are recorded for both unknown and reference samples during routine semiquantitative analysis as a matter of standard practice. From the patterns, it is often possible to obtain an indication of what is to be expected prior to photographic plate development so that other reference standards can be added as needed. For most analysis, the spectrogram is
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Figure 5.
CVF patterns of titanates
Two-cation compounds are the mort complex tested
required for cation identification. Additional patterns are made to match with indicated compounds. Patterns have been used in this laboratory for phosphors, laser crystals, rare-earth compounds, etc. Patterns have not been attempted with other semiquantitative methods, such as that reported by Kroonen and Vader (6) in which LizCOa buffer is added to the sample material. Because the buffer is used for both arc stability and suppression of the matrix effect, dserences between properties and the resultant patterns are decreased. A constant-current source, such as that designed by Owen (d), has not been used and whether fluctuation patterns are accentuated or suppressed is not known. Identification of chemical compounds by CVF patterns is an indirect method based on reference patterns made under
standardized conditions. Ideally, reference patterns would be exchangeable between laboratories, and one set of patterns would serve all; however, this is prsctical only when analytical conditions BS well as methods of recording are the same. The use of reference patterns would be one step in obtaining standardization between laboratories. LITERATURE CITED
(1) Harvey, C. E,.: “Semi uantitative
Spectrochemlstry,
A p p d Research
Laborahria, Glendale, Calif., 1963. (2) Kroonen, J. K., Vader, D., “Line Interference in Emission Spectrographic Analysis,” Elsevier, New York, 1963. (3)Mellichamp, J. W.,ANAL. CHEY.37, 1211 (1965). (4) Owen, L. E., A p p l . Spectry. 12, 178 (1958). RECEIVEDfor review May 10, 1966. Accepted July 5, 1966. VOL 38, NO. 10, SEPTEMBER 1966
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