Qua Iitat ive a nd Qua nti ta tive An a lysis of Organic Compounds Use of Low-Voltage Mass Spectrometry CHARLES J. VARSEL, FRANCIS A. MORRELL, FRANK E. RESNIK, and
W. ALLAN POWELL
Philip Morris, Inc., and University o f Richmond, Richmond, Va.
b A method which permits qualitative and quantitative analysis of multicomponent mixtures by low-voltage mass spectrometry has been developed. It is rapid and shows precision and accuracy comparable to the normal method of mass spectrometric analysis. It eliminates the necessity of subtrading components from the spectrum of a multicomponent mixture. The use of simultaneous equations is generally eliminated in quantitative analysis. Qualitatively, the method can be used to supplement the information obtained from a normal mass spectrum. It provides a rapid means of obtaining the molecular weight and ionization potential of an organic compound.
SEC SUPPRESSOR
0
ANALYTICAL CHEMISTRY
'10 I;
-4
&
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TO DRONE (
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TO 101 CDLLECTOI'
Figure 1. Diagram of modified ionizing-voltage circuit and preamplifier
EVERAL
182
'
PREAMPLIFIER - -v
S
recent publications have described the advantages of low ionizing-voltage techniques for quantitatively analyzing organic compounds. These methods, however, were used primarily for the analysis of a certain class of compounds or a number of compounds in a homologous series. These analyses included inorganic gases ( l a ) , monodeuterated paraffins (2.2, 14), and unsaturates in petroleum hydrocarbons (6,8). This laboratory was interested in extending the use of low-voltage mass spectrometry to permit the rapid analysis of complex mixtures containing various types of organic compoundsi.e., aldehydes, ketones, paraffins, olefins, aromatics, etc. Such application is more involved than a simple analysis for a single compound type because ionization potentials for the different compound types overlap to some extent and, because of the overlap in ionization potentials, it is not always possible to avoid certain fragmentations of molecules. Generally, the lowest potential necessary to ionize all compounds present in a mixture was selected for an analysis. On occasions, several different voltages were used to obtain the various compound types in a mixture. The selection of an ionizing voltage was dependent upon the components present in the mixture. With proper calibration data,
131"
mmfd. Micromicrofarad
all components of a mixture could be determined quantitatively. One major advantage of the technique developed here was that it eliminated the necessity of using simultaneous equations and all calculations could be handled on a desk calculator or slide rule. The rapidity with which an analysis could be performed was also a distinct advantage. The major disadvantages of the method were caused by interferences from fragment ions (in some cases) and the low sensitivities encountered with certain molecule-ion types. INSTRUMENT MODIFICATIONS
A
Consolidated Electrodynamics Corp. Model 21-103C mass spectrometer was modified to permit operation with 3 to 20 e.v. potentials on the ionizing electrons. The modifications are shown Figure 1. The ionizing voltage of the Model 21-103C mass- spectrometer normally can be varied between 20 and 100 volts. To obtain ionizing voltages lower than 20 volts it was necessary t o add additional resistance to the ionizing-voltage network. The original potentiometer was replaced with a 10-turn, fine-control,
in
300,000-ohm potentiometer which permits redistribution of the ionizing voltage so that a low value of 3 volts can be applied to the anode. A widescale, low-range (0 to 20 volts) voltmeter was installed to permit accurate and reproducible settings of the ionizing voltage. These modifications were connected to a five-pole trvo-position switch to permit operation a t either low or high (70 volts) ionizing voltage without interrupting the normal operating schedule for the instrument. At low ionizing voltages it is necessary to lower the repeller voltages to a potential below that of the anode. The reduction in repeller voltages was accomplished by connecting two 8000ohm potentiometers across a %volt mercury battery supply through the appropriate contacts of the five-pole switch. This permits selection of the repeller voltages from the movable contacts of the potentiometers (for lowvoltage studies) or from the existing repeller-voltage supply. For low-voltage operation of the mass spectrometer, the inner and outer repellers are set a t 1.1and 1.0volts, respectively. Initially, ionizing currents of 50 and
35 pa. were employed for low-voltage studies, but because of "warping" effects on the filament, this current was reduced to 10 pa. when low voltages were employed. The use of an ionizing current of 10 pa. caused a considerable reduction in instrument sensitivity. This, however, was compensated for by increasing the sensitivity of the preamplifier. To do this, the 5 X 1Olo ohm resistor was moved from the high-sensitivity position to the low-sensitivity position and a 5 x 10" ohm resistor was placed in the high-sensitivity position. The 62-ppfd. capacitor normally used in the low-damping position of the preamplifier was removed and the 18pMfd. capacitor from the high-damping position was used to replace it. A 2ppfd. capacitor was used in the highdamping position to replace the Wppfd. capacitor normally used in this position.
within the limits of experimental error, or 3~0.09volt. QUALITATIVE ANALYSIS
Knowledge of the molecular weight of a compound and an investigation of its mass spectrum can often lead to its identification. I n the same sense, the ionization potential of an unknown can aid in its identification. Knowledge of ionization potentials can be even more helpful when the normal spectra of two or more compounds overlap, making it difficult to determine the molecular weights of one or more of the compounds. Lowering the ionizing voltage eliminates fragment ions and leaves only the molecule ion. I n mixtures of compounds with closely related ionization potentials and lowest appearance potentials, each peak in a lom-voltage mass spectrum (providing the ionizing voltage is higher than the ionization potentials, but lower than the lowest appearance potential) represents the molecular weight of a compound which is present in the mixture. rln ionization potential alone will not be sufficient evidence for the identification of a compound, but this in addition to the information obtained from the 70volt mass spectrum with regard to functional groups, dissociation, and isotopic-abundance ratios will, in most cases, provide the necessary information.
DETERMINATION OF IONIZATION POTENTIALS
I n studies of ionization potentials, the method of plotting ionization curves was the same as that described by Dibeler et al. (2, 3). Table I presents the ionization potentials of the compounds studied. Argon was used as a calibrating gas and was introduced simultaneously with the compound whose ionization potential was being determined. Certain values are indicated as electron-impact values which Field and Franklin (4) consider the most correct. Some of these values are averages which were determined when several reported values differed only slightly. Photoionization values reported by Watanabe (16-18) are also indicated. The reproducibility of the measurem m t s recorded in Table I is generally
QUANTITATIVE ANALYSIS
Molecule-Ion Sensitivities and Correlation of Molecule-Ion Intensity with Concentration. The constancy of the proportionality factor is a basic requirement for successful quantitative analyses by mass spectrometry. At a n ionizing voltage of 70 volts, the proportionality (sensitivity) factor is
Table 1.
NO.
Electron Impact", E. V.
17
10.50
Hydrogen cyanide 2-Butene 1-Buten-3-yne Propene
27 56 52 42
13.73 9.23 9.87 9.81
Cyclopentene Cyclohexene
68 82
8.85
Furan
68
9.04
2-Methylfuran 2,3-Dimethylfuran 2-Butanone
82 96
8.31 8.01 9.58
Compound -4nimonia
Ion
Mass
72
9.09
Furfural 96 9.31 Methyl formate 60 10.92 Acetonitrile 41 12.42 2,3-Dithiabutane 94 8.53 Values considered most correct by Field and Franklin. * Reported by Watanabe.
constant over the pressure limits (1 t o 150 microns) employed in mass spectrometric analysis. To determine whether this factor is constant for low ionizing voltages, a number of compounds, representing the various types which were t o be included in future quantitative analyses, were studied to correlate molecular-ion intensities with partial pressures. Isoprene, 1-butene, ?a-butane, benzene, toluene, methyl chloride, methyl bromide, acetone, 2-butanone, methanol, furan, 2-methylfuran, acetonitrile, and carbon dioxide were determined. Plots of molecule-ion peak height (molecule-ion intensity) vs. pressure were constructed for the molecule ion of each compound over the pressure range of 0.25 to 20 microns. All plots produced straight lines, which gave evidence of constancy of the proportionality factor and showed that the moleculeion peak height varies directly with partial pressure. I n cases where many compounds are encountered in a mixture, it is virtually impossible to determine instrumentally the base-peak or molecule-ion sensitivity of each individual compound each day. Consequently, other methods must be relied upon to produce the molecule-ion or base-peak-ion sensitivities. The feasibility of calculating molecule-ion sensitivities a t low ionizing voltages from the sensitivity of a calibrating gas vias studied and the results were favorable. The method used was that outlined by Field and Hastings ( 5 ) . Toluene was chosen as the reference standard because of its large molecule-ion intensity. To illustrate the day-to-day variations in the molecule-ion sensitivity for
Ionization Potentials
Other Values, E.V. (10.52)" 10.15 f 0.Olb (13.86)" (9.29)" 9.90 rt 0.09 (9.80)" 9.73 f 0.01b 10.2 rt 0 . 2 (9.24 f 0.07)" 8.945 =ICO.Olb (9.0)" 8.89 f 0.01b
..
(9.?6) 9.54 rt 0.01b
...
(12 .h'6).
..
Ref.
Spectroscopic, E.V.
Ref.
... 9.24
...
0.7
...
(ioj (ioj
9.2
(ibj
9.06 f 0.01
(11)
...
... ... ...
... ii:&
...
... ...
...
VOL. 32, NO. 2, FEBRUARY 1960
ilj ...
183
Table
II.
individual compounds were calculated from the molecule-ion sensitivity of toluene according to the procedure of Field and Hastings ( 5 ) and compared with the actual sensitivities for the respective molecule ions as determined instrumentally. Table I11 shows the actual and calculated molecule-ion sensitivities for these four compounds. The largest individual deviation in the calculated sensitivities occurred in the case of methanol. I n one instance, the calculated sensitivity deviated by S.670 from the actual sensitivity as determined mass spectrometrically. Since variations in the calculated sensitivities a t 70 volts have been observed to deviate occasionally by as much as 7o/c, this deviation of 8.6% for low-voltage ionization is satisfactory. A second method confirmed the practicality of calculating molecule-ion sensitivities from the molecule-ion sensitivity of a reference gas. The molecule-ion sensitivity of toluene a t 10.0 volts was used as a reference to determine the sensitirity factors for benzene (9.5 volts), 2-niethylfuran (10.0 volts), acetone (11.0 volts), and methanol (12.0 volts). The individual factors were then multiplied by the toluene nioleculeion sensitivity a t 10 volts for each of the 10 days to obtain the molecule-ion sensitivities for each of the compounds. These sensitivities shoived about the same degree of reproducibility as the previously calculated sensitivities; however, in several instances large variations were observed. The largest variation n-as again observed in the case of nieth-
1-butene and n-butane and a sharp drop in molecule-ion sensitivity was noted. The sensitivity then increased again after the initial drop. This is normal for a newly treated filament and it illustrates the effect of filament conditioning upon the molecule-ion sensitivity. The molecule-ion sensitivities of four
any given compound, as well as the increase in molecule-ion sensitivity with a n increase in ionizing voltage, the molecule-ion sensitivities of toluene over a 10-dag period are presented in Table 11. The molecule-ion sensitivities a t ionizing voltages of 9.5, 10.0, 11.0, and 12.0 volts are shown. Prior to the third day the filament was treated with
Sensitivity of Molecule-Ion (Mass
92) from Toluene
Voltage Day
9.5 v.
1st
4t,h
60.4 56.3 41.0 51.9
85.6 83.0 58.6 72.8
129.8 123.2 94.1 115.7
167. 6 163.7 127.0 157.8
10th
54.2
78.1
122.9
165.2
10.0 v.
11.0 v.
12.0 v.
Sensitivities, Divisions/RTicron Pressure 2nd
3rda
a
Filament treated with 1-butene and n-butane. Molecule-Ion Sensitivities in Divisions per Micron of Pressure
Table 111.
Benzene, 9 5 T’olts
Actual
Computed
40.2 36.3 25.4 33.5 32.4 32.3
38.9 36.3 26.4 33.5 33.1 32.9 34.9 34.6 35.1 34.9
3413 35.6 34.8 34.1
2-Met hylfuran, 10.0 Volts Actual Computed
Acetone, Methanol, .o Volts 12.0 Volts Actual Computed Actual Computed 11
15.4 17.1 12.3 16.3 15.9 16. 1 16.9 17.2 17.3 17.1
‘72.8 70.6 49.8 62.0 61.2 63.4 65.8 64.9 66.1 G6.4
76.9 ‘70.6 53.0 66.7 65.7 66.5 63.4 70.0 70.2 71.1
Table IV.
18.0 17.1 13.1 15.1 15.8 16.2 16.9 16.9 16.9 li.1
10.3 10.5 7.8 10.7 10.0 10.6 10.5 11.6 11.3 11.6
10.7 10.5 8.1 10.1 9.i 9.9 10.6 10.5 10.5 10.6
Quantitative Analyses of Five-Component Mixture
Composition Low Voltage
Compound Toluene Benzene 2-Methylfuran Acetone Methanol
Actual blend, mole yo 13.4 16.0 15.9 19.4 35.3
13.7 16.4
+2.3
33.i
Compound Toluene 2-Methylfuran Benzene 2-Butanone Furan Acetone 1-Butene 1,3-Butadiene Methyl chloride Acetaldehyde Propene Acetonitrile Propyne Methanol Ethane
184
92 100
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82 ... 100
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I
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ANALYTICAL CHEMISTRY
-6.0 +3.8 S5.0 +5.i -5.4 5.2
12.6 16.6 16.7
+1.9 +4.1 -4.5 3.1
XV.
Table V.
Mole 7@ found % dev.
+2.5
16.2 20.2
Method 2 Mole % found ye dev.
Method I
High Voltage Mole yo found 7*dev.
20.5
33.4
12.4 16.4 17.4 19.9 33.i
Method 3 Mole 70 found C,; dev.
-7.5
12.0 16.0 16.6 20.3 35.0
+2.5
+9.4 +2.6 -4.5 5.3
Peak Contributions and Relative Per Cent Abundances of Ions Mass Number 78 72 68 58 56 56 54 44 42 41
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anol, and in one instance the calculated sensitivity deviated by 16% from the actual sensitivity for the niolecule ion of methanol as determined instrumentally. This large variation in the molecule-ion sensitivity of niethanol is attributed mainly to the small ion intensity of the niolecule-ion beam. It may also be due in part to the error in setting and reading the voltmeter at a voltage different from that used for the standard. Applications of Low-Voltage Mass Spectrometry to Quantitative Analysis. ,211 effective method for quantitatis e analysis b y lon-voltage mass spectronietry should he capable of analyzing mixtures of compounds rapidly and nithout difficulty. It should have piecision and accuracy comparable t o the normal method of analysis. The most satisfactory means of drterniining the accuracy of anp analytical method is by comparison of analytical rcsults with the actual Composition of the Llcnd. This n a s done in one caw in this expeiimentation, but generally thp rcsultq n ere compared n ith those obtaiiid by the. normal methotl of mass spectrometric analysis. A X A L Y ~ IOSF FIVE-C'OMPOSEAT MIXTURE. A 5-coniponciit liquid mixture containing toluene, benzene. 2-meth) 1furan, acetone, and niethanol TT :is prrp:ircd. This mixture i t :is analyzed 10 timcs a t the normal high ionizing voltage ( i o volts) and 11 times a t Ion ionizing voltages (9.5, 10.0. 11.0, and 12.0 volts). The constituents 71 u e dettv mined quantitativdy at lon ionizing voltages by three methods: ( I ) by use of the ac+ual parent-ion sensitivities as determined ni:iqs spectromi3trically; (2) by use of the parent-ion sensitivities as dcteriiiiiied from the individual sensitivity factors given by the ratio of compound stmsitivity to toluene sensitivity, both at the same ionizing voltage; and (3) by use of the parent-ion sensitivities as determined from the individual scnsitivity factors given by the ratio of' compound sensitivity to toluene sensitivity a t 10.0 volts. The average results obtained by the three methods for coniputatioii of lowvoltage results are summarized in Table IV. The quantitative results for the individual constituents obtained by the normal high-voltage analyses were compared 11-ith the actual composition of the mixture. The accuracy of this method ranged from +1.5 to -4.5%, d(,pending upoii the compound being determined. The precision of the normal high-voltage method of analysis, expressed as per cent relative deviation, was in the range of :.tl to 3%. The precision of low-voltage method of analysis, expressed as per cent relative deviation, was in the range of & 1 to 401,.
Table VI.
Compound Toluene 2-llethylfuran Benzene 2-Butanone Furan Acetone 1-Butene l,%Butadiene Alethy1 chloride -4cetaldehyde Propene Acetonitrile Propyne Methanol Ethane
Quantitative Analysis of 15-Component Mixture
High voltage 4.8 6.5 7.4 4.8 7.0
6.5 6.5 8.1 2.8
8.1 8.5 5.4 8.6
7.2 8.2
Averages, Mole 70 70relative voltage deviation 5.0 1.6 0.7 6.7 Low
7.2
4.9 7.3 7.2 6.8 7.9 2 .-5 7.8 8.3 5.2 8.1
7.3 7.8
70
1.3
0.9 0.5
deviation +4.2 +3.1 -2.7 +2,1
+4.3
$10.8 +4.6 -2.5 -10.7 -3.7 -2.4 -3.7
2.1 0.9 0 3
26
1 5
1.3 !I , 2 1.5 1.5
-5.8 41.4
1.7
-4.9
Av.
Accuracy of the lom-voltage method is comparable to the accuracy of the normal high-voltage method of analysis, if all reference standards are analyzed daily. It is decreased somen hat by the use of sensitivity factors in the required calculations. Quantitative determination of toluene presented the greatest difficulty in the lon -voltagr analysis of the five-component mixture. The large deviation in the toluene determination cannot be fully explained, but adsorption in the instrument is undoubtedly a factor. Even though difficulty did arise in the determination of toluene, its usefulness as a calibrating standard n a s not affected because the reproducibility of its sensitivitj- !\as good. ANBLP~IS OF A FIFTEEK-COJIPOSEST MIXTURE. A synthetic mixture containing 16 compounds !\ as prepared as a gas sample. Because nine of the components ordinarily are liquids a t room temperature, these nine were vaporized in a vacuum system before being added to the mixture of gases. The mixture contained the following constituents: 1. Toluene
'7. 2-hlethylfuran
3. Benzene
4. 2-Butanone
5. 6. 7. 8.
Furan Acetone 1-Butene Methyl chloride
9. Aceta Idehyde 10. Propene 11. Propyne 12. Acetonitrile 13. Methanol 14. 1,3-Butadiene 15. Ethane
The spread of ionization potentials for the components of the mixture was between 8.82 e.v. (toluene) and 12.46 e.v. (acetonitrile). Some of the olefins, however, contribute fragment ions !\-hose appearance potentials are lower than the ionization potential of acetonitrile. For example, l-butene and propene contribute fragment ions at mass 41 (C3HjL) which have appearance potentials of 11.59 and 11.95 e.v., respectively. It is evident, therefore, that interference a t mass 41, the mole-
4.5
cule-ion peak for acetonitrile, was impossible to avoid. I n view of this fact, and because it is advantageow to use the highebt ionizing voltage feasible t o obtain maximum sensitivity aiid yet keep interferences to a minimum, a n ioiiizing voltnge of 12.0 1-olts n as selected for the analysis of thii inixture. This voltage exceeded the ionization potentials of all molecule ions except that of acetonitrile. Intcrfercnces u-ere studied by calibrating for each of the individual components of the mixture a t 12.0 volts. From these spectra, data TT ere obtained for each component, and, if fragment ions appraretl in addition t o the molecule ion, their abundance percentages relative t o the niolecule ion were calculated. These data are sunimarized in Table V. From this table, the compounds and their relative intensities at the various peaks can be observeti. Eleven analyses of the mixture nere carried out at low ionizing voltages (12.0 volts) and 11 a t the normal highvoltage conditions (70 volts). The results of these analyses are presented in Table VI. Acetonitrile could not be detcmuncd directly from its peak heights in the lowvoltage spectra of the mixture. Intrrferences from 1-butwe and propene and the lon sensitivity of the acetonitrile molecule ion caused serious errors in the quantitative determination of this coinpound. The acetonitrile n as determined in the ion-voltage anal) ses by attributing to it the pressure differrnce betn een that read from the micromanometer and the sun1 of the partial pressures of the remaining 14 components. The reproducibility of the acetonitrile concentration, \\-hen calculated in this manner. was poor, but the average concentration from the 11 analyses compared favorably !I ith the concentration obtained from the high-voltage analyses. The precision of the lom-voltage VOL. 32, NO. 2, FEBRUARY 1960
185
results for the analysis of this mixture, when expressed as the relative deviation, is within h3% for all compounds except acetonitrile. This is within the expected limits. Interferences from fragment ions and low sensitivities are probably the greatest hindrances to the method. These hindrances are to be expected, however, in dealing with a large number of components. The majority of organic compounds ionize a t potentials below 11 e.v., while, for the most part, appearance potentials of fragment ions are greater than 11 volts; therefore, in only relatively few cases will interference be serious. Aliphatic nitrogen compounds and first members of homologous series are the main compounds which will be subjected to interferences from fragment ions: the nitrogen compounds because of their low sensitivities and odd molecular weights, and the first members of homologous series because of their relatively high ionization potentials.
ACKNOWLEDGMENT
The authors express appreciation to Vernon H. Dibeler and F. L. Mohler for their helpful suggestions, to Thelma Heatwole for her valuable suggestions on the writing of this article, and to Edna K. Chavis and Ruth Wayner for obtaining the mass spectrometric data. They also thank Philip Morris, Inc., for granting permission to publish this work. LITERATURE CITED
(1) Cutler, J. A., J. Chem. Phys. 16, 136 (1948). (2) Dibeler, V. H., Reese, R. hI., J . Research Natl. Bur. Standards 54, 127 (1955).
(3) Dibeler, V. H., Reese, R. M., Mohler, F. L., J. Chem. Phys. 20, 761 (1952). (4) Field, F. H., Franklin, J. L., “Electron Impact Phenomena and Properties of Gaseous Ions,” Academic Press, Xew York, 1957. (5) Field, F. H., Hastings, S. H., ANAL. C H E ~28,1248 ~. (1956).
(6) Franklin, J. L., Field, F. H., J. Am. Chem. SOC.76, 1994 (1954). (7) Hissel, J., Bull. SOC. Toy. sci. Ligge 21, 457 (1952). (8) Lumpkin, H. E., ANAL. CHm. 30, 321 (1958). (9) Morrison, J. D., Nicholson, A. J. C., J . Chem. Phys. 20, 1021 (1952). (10) Price, W. C., Tutte, W. T., Proc. Roy. SOC. (London)A174, 207 (1940). (11) Price, W. C., Walsh, A. D., Ibid., A179,201(1941). (12) Stevenson, D. P., Wagner, C. C., J. Am. Chem. SOC.72,5612 (1950). (13) Taylor, D. D., U. S. Patent 2,373,151 (April 10, 1945). (14) Tjckner, A. W., Bryce, W. A,, Lossing, F. P., J . Am. Chem. SOC.73, 5001 (1951). (16) Watanabe, K., J . Chem. Phys. 22, 1564 (1954). (16) Ibid., 26, 542 (1957). (17) Watanabe, K., Marmo, F. F., Inn, E. C. Y., Phys. Rev. 91, 1155 (1953). (18) Watanabe, K., Mottl, J. R., J. Chem. Phys. 26, 1773 (1957).
RECEIVED for review March 23, 1969. Accepted September 14, 1959. Sixth Annual Meeting, ASTM Committee E-14 on Mass Spectrometry, New Orleans, La., June 1958.
Determination of Sodium Ions in Acidic Silica Sol Systems Solution Potent ia I Measurement Procedure THOMAS A. TAULLI Research Department, lnorgonic Chemicols Division, Monsonto Chemical Co., St. louis, Mo.
b A novel, rapid, and quantitative method for determining sodium ions in acidic silica sol systems uses an electrode assembly after dilution and o pH adjustment by ion exchange processing of the sample. The sensitivity of the electrode toward sodium ions is pH-dependent; however, at proper pH levels, electrode response is linear in aqueous solutions having a maximum sodium ion concentration of 100 p.p.rn. In the analysis of silica sols containing 12.1 to 333.8 p.p.m. of sodium ion, the average per cent deviation between the proposed method and other methods, including flame spectrophotometry, is 5.7, relative.
T
presence of even minute quantities of sodium salts is undesirable for some product uses, especially some end uses of silica sols, where small amounts of sodium ions can be detrimental (8, 1‘7). When there is a demand for a n essentially sodium saltfree product, usually a twofold problem HE
186
ANALYTICAL CHEMISTRY
exists: detection and removal. This problem is paramount in silica sols. Sodium salts are usually introduced into a silica sol system through the use of sodium-containing raw materialse.g., the reaction of aqueous sodium silicate with a mineral acid. After reaction, large quantities of sodium salts are removed to maintain sol stability (9). Several methods are reported for the preparation of acidic silica sols of relatively low sodium salt content (7, 10-11, 14, 16); other methods use ion exchange techniques (2, 5, I S , 15). These operations warrant analytical methods for monitoring the sodium ion level. Some analytical techniques for the quantitative determination of trace amounts of sodium ion, applicable in many systems, have been described ( I , 6), as well as numerous modifications and adaptations of the colorimetric method based on the color of uranyl ions. Recently, ion exchange techniques have been reported for quantitative separation of sodium from potas-
sium ion in silicate rock (4). Conventional methods are then used for the analysis of the separated alkalies. Even flame spectrophotometry does not lend itself readily for a direct sodium determination in silica sols, because the presence of colloidal silica will tend to clog the atomizer. hloreover, while all of these methods are possibly adaptable for a sol system, they are tedious and require exacting conditions for optimum accuracy. This paper describes the successful application of a simple electrode assembly for the determination of small amounts of sodium ion in acid silica sols, by potential measurements. The silica sol is diluted and an optimum pH level is obtained by a relatively rapid passage of the solution through the basic form of an anion exchange resin bed, A potential measurement of the effluent and subsequent calculation defines the sodium ion content of the silica sol. The average per cent deviation for this method, as compared t o a flame spectrophotometric procedure and
