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Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 1, 1979
COMMUNICATIONS Phosphomolybdic Acid Test for Mercaptans The scope of the work involved looking at a test involving phosphomoiybdic acid as a quantitative method of measuring mercaptan levels. The test was found to b e suitable for mercaptans in solutions but not in the gaseous phase.
Introduction In a previously published paper (Knight, 1976) a colorimetric method for field determination of mercaptans levels in natural gas was developed. This method was based on N-ethylmaleimide. From all the reactions studied there, only one other reaction, involving phosphomolybdic acid in aqueous NaOH (Karr, 1954; Schobel, 1937), had shown a little promise apart from N-ethylmaleimide. This method has been reported in the literature to be a qualitative test for mercaptans. In our earlier work, the reaction was seen to produce a blue product with mercaptans. Although the absorbance of the solution was concentration dependent, the colored product was unstable. Further work on this reaction was promised (Knight, 1976) at a later date. The current work presents the results briefly of the experiments done in order to see whether or not the test is definitely a quantitative test for mercaptans and also if it can be developed on the same line as the N-ethylmaleimide test for mercaptan level measurements in natural gas. Experimental Details and Discussion The phosphomolybdic acid test is carried out by adding to an aqueous solution of sodium hydroxide a known, or unknown, amount of mercaptans, glacial acetic acid, and an aqueous solution of phosphomolybdic acid. The re-
Linearity of Concentration Dependence as a Function of Conditions
Table I. Phosphomolybdic Acid Test. series
reagents
A
a. 1.0 mL NaOHa b. 1.0 mL PMAb c. 0.05 mL GAA
B
a. 1.0 mL NaOH b. 0.05 mL GAA c. 1.0 mL PMA
C
a. 2.0 mL NaOH b. 0.1 mL GAA c. 2.0 mL PMA
D C
a. 2.0 mL NaOH b. 0.1 mL GAAd c. 2.0 mL P M A ~
action involves the conversion of the acid, H3PMo120,,30H20to molybdenum blue, MOO~,~.:H~O, the latter giving rise to a blue coloration in the solution. For quantitative measurement of the mercaptans, the optical density of the solution was determined at 8350 A spectrophotometrically. A series of quantitative tests was carried out with phosphomolybdic acid in an effort to determine the most effective concentrations of the three reagents involved in terms of color intensity and stability. In each series five or six solutions, each containing a known number of micromoles of mercaptan, were tested. All runs were done with tert-butyl mercaptan. The optical densities of the solutions were measured and plotted as a function of mercaptan concentration. For each particular set of reactant proportions, the range over which the color response was linearly dependent on concentration could be determined. Some representative data from the experiments of this type are given in Table 1. The last series listed in the table, series D, represents the conditions of those investigated which give the widest sensitivity range with linear response. Those data are plotted in Figure 1. A set of tests was run using the method and gas bubbling apparatus described earlier (Knight, 1976) utilizing natural gas containing known quantities of mercaptans. Two such sets of data are given in Table 11. In each case the gas was bubbled through an 8% aqueous NaOH solution, following which the glacial acetic acid and phospho-
calibration data
pmol of mercaptan
s o h OD at 8350 pi
0.04 0.08 0.12 0.18 0.20 0.04 0.08 0.12 0.16 0.20 0.04 0.08 0.12 0.16
0.200 0.315 0.333 0.550 0.495 0.130 0.235 0.352 0.495 0.550 0.070 0.146 0.205 0.255 0.270 0.378 0.724 1.048 1.354 1.540 >2
0.20
0.11 0.22 0.33 0.44 0.55 0.88
linear response range, pmol of mercaptan
nonlinear
comments variations in timing of GAA addition gives scatter in optical density values
0.04-0.18
maximum optical density value attained after approximately 5 min
0.04-0.1 5
as in series B
0-0.4
optical density vs. concn plot linear and extrapolates through origin, response linear, 0.0-0.1 Mmol
10% aqueous phosphomolybdic acid solution; GAA = glacial acetic acid and 8% aqueous sodium hydroxide solution. Glacial acetic acid and phosphomolybdic acid solutions addPMA = phosphomolybdic acid. Data plotted in Figure 1. ed simultaneously. 0019-7890/79/1218-0082$01,00/0
C 1979 American Chemical Society
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 1, 1979
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Table 11. Phosphomolybdic Acid T e s t - G a s Bubbling Experiments" test soln OD calcd pmol gas vol, m L a t 8350 A of mercaptan
mercaptan concn, ppm
Series A. Flow = 200 mL/min 0.072 0.114 0.234 0.310
200 400 800 1200
0.028 0.044 0.091 0.120
3.5 2.8 2.9 2.5
Series B. Flow = 100 mL/min 200 400 600 800
:
0.070 0.127 0.175 0.210
0.027 0.049 0.068 0.081
3.4 3.1 2.8 2.5
a Mercaptan-containing natural gas bubbled through 2.0 m L of 8% aqueous NaOH; 0.1 m L of glacial acetic acid and 2.0 m L of phosphomolybdic acid added. Optical density values determined spectrophotometrically.
01
02
03
04
05
06
U U X E S MERCAPTAN
Figure 1. Plot of solution optical density as a function of mercaptan concentration in series D, Table I.
molybdic acid were added. The volumes of the solutions involved here were the same as those in series D of Table I. As can be seen, the results in Table I1 indicate a serious problem when the mercaptan-containing gas is bubbled through the reagents as opposed to when mercaptancontaining solutions are added. When the gas is bubbled through the reagents there appears to be a significant drop in the response per unit amount of gas involved as the bubbling time is increased. Thus the parts per million of mercaptan calculated from the optical density measurements decrease in series A (Table 11) from 3.5 to 2.5 ppm when the volume of the gas bubbled is increased from 200 mL to 1200 mL. The total number of micromoles of mercaptan in 1200 mL of gas, however, is appreciably less than the amount required to consume all of the phosphomolybdic acid present. The concentration range was also well within the linear response region determined earlier. Similar results were obtained when the gas flow rate was varied. It appears that the mass transfer rate of mercaptans
from the gaseous phase into the reagent solution is very much dependent on the concentration of the mercaptan in the solution at the low concentrations considered here.
Conclusions The phosphomolybdic acid test is not suitable for development into a colorimetric method for field measurements of mercaptans in natural gas. However, the method can be used for quantitative determinations of low concentrations of mercaptan levels in solutions. L i t e r a t u r e Cited Karr, C., Anal. Chem., 26, 528 (1954). Knight, A . R., Verma, A,, Jnd. Eng. Chem. Prod. Res Dev., 15, 59 (1976). Schobel, A,, Ber., 708, 1186 (1937).
Department o f Chemistry and Chemical Engineering University of Saskatchewan Saskatoon, Saskatchewan, Canada
A r t h u r R. Knight
Research and Deuelopment Centre Saskatchewan Power Corporation Regina, Saskatchewan, Canada
A r u n Verma*
Received for review January 12, 1978 Accepted November 28, 1978
