helium, because the component of the diffusive motion in the direction of flow is negligible compared to the linear velocity of the carrier gas. For this reason also, the expulsion of air prior to desorption and transference of the sample into the helium stream is essential. Polarity and, to a greater extent, latent heat of vaporization are responsible for tailing effects in regular gas chromatography (4). Because there is no mass transfer or change of state in the empty tube experiments, the absence of tails and the identical nature of the distribution curves are to be expected. It was first anticipated that in the absence of complicating influences of mass transfer and change of state, the areas under the curves would be simply related to the thermal conductivity and the number of moles of the solutes involved. Khen the quantities of mass (Table I) are expressed as number of moles, they are all of the same order of magnitude The recorder actually plots the voltage unbalance in the bridge circuit produced by the change in resistance of the measuring hot wire. The out-of-balance voltage is given by the expression E = s p ( l ) ,where p is the change in mole fraction of the vapor and 7 is the sensitivity parameter of the thermal conductivity cell. Table I shows that so long as the molecular weights and the thermal
conductivities of the different compounds are not very different, calibration curves can be constructed by plotting the mole fraction of any arbitrary compound against height or the area of its peak. For compounds with small molecular weights and large thermal conductivities-as in the case of ammoniaE, and consequently the area, will be appreciably different. For the majority of the compounds, it was satisfactory to express the quantity in terms of volume for quantitative Calibration in this kind of wo;!;. For example, the areas obtained varied between 4 and 5 sq. cm. for the same volume ( 2 pl.) of different liquids, although the molecular weights, densities, and thermal conductivities were different. The total error from such an approximation rarely exceeds 20%, which would be acceptable for an air pollution index, alarm system, or certain survey studies. Figure 3 shows that a shorter emergence time and a deviation from Gaussian distribution occur with increasing flow rate. For most practical applications, a slow flow rate is preferred. Changes in temperature have little effect on distribution. Figures 1 and 2 show that the complexity of the sample did not interfere with the general dispersion pattern. Solutes of different molecular weights
and densities, when introduced together into the helium stream, produced one resultant distribution curve which may be used as a general index for the total amount of pollutants. When the total amount of the pollutants is such that further analysis is desirable, it can be done with the same sample (Figures 2 and 3). The method described can be used for rapid determination of total pollutants in air without losing the sample. Separate calibration curves are required, however, for the quantitative interpretation of the results of the flow tube experiments and of the chromatogram. The technique might possibly be used for experimentally studying the changes of thermal conductivity with change of mole fraction and also for determining the diffusion coefficient of gases. LITERATURE CITED
(1) Keulemans, ,4.I. M., “Gas Chromatography,” Reinhold, New York, 1957. ( 2 ) Tavlor, Geoffrey, Proc. Roy. SOC. ( L o d o n )A219, 186 (1953). (3) Ibid., A225, 473 (1954). (4) West, P. W., Sen, B., Gibson, N. A., -4N.4L.
CHEM. 30, 1390 (1958).
RECEIVEDfor review June 19, 1958. Accepted October 16, 1958. Investigation supported by Research Grant S-43, Study Section of Sanitary Engineering and Occu ational Health, Division of Research &rants, Public Health Service.
Combustion Method for Determination of Sulfur in Ferrous Alloys Modifications for Minimizing Errors J. W. F U L T O N and R. E. FRYXELL Transformer Division, General Electric Co., Piffsfield, Mass.
b
The origin of losses has been studied using radioactive sulfur-35 and found to b e primarily the result of adsorption of sulfur trioxide on the glassware which delivers combustion gases to the absorption vessel. Modifications in apparatus and procedure are described. These, together with substitution of alkali for iodate as titrant, result in significantly better recovery of sulfur.
T
combustion method termination of sulfur admittedly entails losses not well understood (3. 5 ) . HE
for the dein metals which are For many
years, the empirical 93% recovery factor has been generally accepted and used for correcting values obtained by potassium iodate titration (ASTM E 30-56). However, in the past few years, it has been increasingly evident that the actual per cent recovery is intolerably variable between laboratories and even between operators. In private communications from other laboratories, the authors have learned of recoveries ranging from about 70% to almost 100%. Although results obtained with induction heating are a t hand from only a few laboratories, it would appear that resistance heating
leads to more serious and variable errors. These errors are commonly considered toarise from one or more of the folloTing : Insufficient temperature of the burning sample to expel all of the sulfur. Loss of sulfur oxides in the delivery system between the burning sample and the absorption vessel. This may occur by either adsorption by fine metal oxide particles which are transferred to cooler regions of the system, or absorption by moisture in the oxygen which may condense on walls of the tubing. To minimize the former, various types of filters or plugs are used to retain oxides in the hot region of the VOL. 31, NO. 3, MARCH 1959
401
furnace. Also, the oxygen stream is usually dried before passing into the furnace. An inherent gas phase equilibrium between sulfur dioxide and sulfur trioxide. Any sulfur trioxide so formed is, of course, not titrated by potassium iodate. The extent of sulfur trioxide formation has been thought to be influenced by the moisture content of the oxygen. The present report is a summary of studies in this laboratory which were aimed toward the accurate determination of sulfur in ferrous alloys a t the level of 0.005% and lower. Because of its convenience, induction heating has been used. Both Lindberg and Leco bench-model vertical tube furnaces have been used with equal success. Except where noted, commercial mullite crucibles and covers were used, prefired in air a t 1200' C. A few isolated experiments were conducted which indicated that the following variables are not sources of error: Various fluxes were substituted for the usual tin. Although these made visible differences in combustion characteristics, no improvement in either precision or per cent recovery was detected. While copper, manganese, silicon, and vanadium pentoxide all produced good burning characteristics, lead, zinc, and tantalum did not. Reverse flow of oxygen (top to bottom) did not affect sulfur recovery from standard samples. Various types of bubblers were used for gas dispersion in the absorption flask. An extra-coarse fritted glass gas dispersion tube was found most convenient and reliable in operation. Samples were prepared as millings, drillings, or nibblings. If the sample burned properly, the preparation had no effect on the results. However, some drillings are voluminous and do not pack tightly into the crucible. This kind of sample was difficult to ignite, although input power control of the induction furnace minimized the difficulty. Moisture content of the inlet oxygen stream was allowed to vary from a dew point of -62' to $25' C. without any apparent difference in sulfur recovery. This was checked with steels a t the level of 0.014 and 0.0005% sulfur. ORIGIN OF LOSSES AND PER CENT RECOVERY USING POTASSIUM IODATE TITRATION
Procedure. Conventional starch iodide-potassium iodate titration was used, with oxygen flow rate of 1200 ml. per minute. Granulated tin flux was added t o the sample and combustion performed in an induction furnace. Theoretical titer of iodate was 1 ml. = 0.0001 gram of sulfur. Results and Discussion. A tracer experiment was carried out to determine qualitatively where losses occurred in the analytical system. Radioactive sulfur-35 in the form of ferrous
402
ANALYTICAL CHEMISTRY
sulfide was mechanically mixed with low-sulfur powdered iron. The distribution of sulfur in the system could then be determined after combustion of such a mixture took place. The absorbing solution, rinses of delivery train components, and samples of crucible and slag were worked up and the tracer sulfur was separated by barium sulfate precipitation for radioassay. If combustion occurred a t a satisfactorily high temperature (1550' C.), the amount of sulfur remaining in the slag was of the order of 1% of the total, with no detectable amount in the ceramic crucible. If, however, the combustion temperature was obviously low, a much larger amount of sulfur was retained in the slag. On the other hand, most of the sulfur losses were found to result from adsorption on the glassware between the crucible and the absorption vessel. If a glasswool plug was placed in the combustion tube above the crucible, most of the loss occurred at this place. I n the absence of the glass wool, essentially the same amount of sulfur was retained in the delivery train, but farther removed from the sample. On the basis of this observation, all further work was done without the glass-wool plug. With a variable transformer to limit combustion temperature (d), as many as three consecutive 1-gram samples could be burned in the same crucible without failure of the crucible. An experiment was therefore possible to determine the minimum temperature required to remove essentially all of the sulfur from the sample. NBS 125a (0.01301, sulfur) was burned in a new crucible while the temperature was monitored with the variable transformer and measured with an optical pyrometer, sighting directly into the burning sample. (This is the only experiment herein reported in which crucible covers were not used. Although combustion characteristics are probably somewhat different without covers, this technique was used for temperature measurements because the tracer experiment had shown that the sample slag retained a variable amount of sulfur depending on temperature.) After the crucible had cooled, a 1gram charge of a special low sulfur steel (0.0005~0sulfur) was added and burned a t full temperature. If this second titration was abnormally high, it was taken as evidence that sulfur from the previous sample had remained in the crucible. With this technique, it was determined that a temperature of 1550' C. must be attained, a t least briefly, for essentially complete sulfur removal from the sample. Having established the minimum necessary temperature, this experiment was repeated with several higher sulfur samples, up to 0.078% (NBS 20e). From these
data, it was concluded that a maximum of 2% of the total sulfur was retained in the crucible and slag if proper operating temperature were attained. With a reasonable amount of experience, operators can readily recognize if a combustion is satisfactory, on the basis of both visual observation and the amount of plate current drawn by a burning sample. (Although satisfactory combustions usually drew 400 ma. or more, no current conditions are specified because of the possible differences between induction furnaces.) Consequently, for all the experiments reported below, it is assumed that only a negligible sulfur loss has resulted from retention by the sample slag. COMPARISON OF POTASSIUM IODATE AND SODIUM HYDROXIDE TITRATIONS
Procedures. POTASSIUMIODATE. As described above. SODIUMHYDROXIDE. Operating procedure was the same as described for the iodate method with the following differences: 150 nil. of 0.6% hydrogen peroxide was used as the absorbing solution; titration with carbonate-free sodium hydroxide (1 ml. = 0,0001 gram of sulfur) using bromocresol purple indicator; and flushing of the fritted-glass bubbler as described later in the standard procedure. This flushing is required, as the peroxide produces considerable sulfur trioxide which is trapped at this point.
Results and Discussion. The iodate titration method in this laboratory has shown an average 85% recovery based on NBS standard samples covering the range of 0.010 to 0.272% of sulfur. These data appear in Table I. Such evidence coupled with the above observation that some sulfur is absorbed by glassware suggests that considerable sulfur trioxide is formed. If this is correct, the question arises whether some sulfur trioxide gets as far as the absorption vessel but is not titrated by iodate. To check this possibility, a careful comparison of potassium iodate and sodium hydroxide was made. Using a steel of 0.01570 sulfur for the comparison, no significant difference could be detected. However, additional sulfur was recovered by titrating rinsings of the delivery system with sodium hydroxide. On the other hand, no additional sulfur is found with an iodate titration of rinsings. The problem therefore resolved itself into one of finding a way to prevent retention of sulfur trioxide by the glassware between the crucible and the absorbing solution. The two variables introduced were : Length of delivery system. The conventional delivery tube is approxi-
mately 90 cm. of 7-mm. tubing from the top of the combustion tube to the bubbler. This was compared with a longer one (170 cm.) and a shorter one (45 cm.). The latter was connected to a specially constructed combustion tube (quartz) which was constricted to 7 mm. immediately above the crucible. The 45-em. tube is the shortest which could be constructed around the guard of the induction coil, Oxygen flow rate, which was allowed t o vary from 1 to 5 liters per minute. Exploratory experiments with a steel containing 0.015y0sulfur indicated that an increase in the oxygen flow rate significantly increased the recovery of sulfur but the length of delivery tubing was less important. Although flow rates of 3 liters per minute and higher appeared to yield identical results, the data given below with the sodium hydroxide titration were all obtained with a flow rate of 5 liters per minute and the shortest train mentioned above. Table I summarizes the data obtained on a series of NBS standard samples. Each value is the average of a t least five determinations. Onehalf gram of granulated tin was used as flux with 1 gram of sample, unless noted. KOattempt was made to evaluate and deduct a blank. For the sodium hydroxide titrations, three sets of values are given: 1. As-titrated values, which include flushings of the fritted bubbler. 2. Total values, which also include a rinse of the delivery and combustion tubes. The rinse was made with the solution from the absorption vessel after a series of like samples was analyzed. The solution was titrated back to the initial end point and the equivalent amount of sulfur was divided equally among the samples in the series. Conveniently, a series of samples consists of five although occasionally sufficient fogging occurs in the absorber (due t o tin oxide) so that fewer should be run. 3. As-titrated values corrected with a loss factor, 1.09. This factor provides the best adjustment of as-titrated values to the certified values of the standards over the entire range.
Determinations made by the iodate titration a t the usual flow rate of 1200 ml. per minute are given for comparison. Attempts to use a higher flow rate in conjunction with an iodate titration were unsuccessful. Unstable end points were presumably the result of side reactions induced by the relatively hot combustion gases. These data indicate: Recovery of sulfur by the iodate method averages 85% over the entire range studied. Recovery by the sodium hydroxide method a t an oxygen flow rate of 5000 ml. per minute is considerably better, averaging 92%. The actual advantage
Table I. Determination of Per Cent of Sulfur in Steel Sodium Hydroxide Method Potassium KBS Certified Iodate 1. As3. As-Titrated, Corrected Sample Value titrated 2. Total Value Std dev. Method 72d 0 011 0 0089 0 0106 0 0004 0 0097 0 0098 125a 0 013 0 0129 0 0003 0 0108 0 0118 0 0125 (0 0109) (0 0119) 0 000G 32e 0 021 0 0181 0 0197 0 0209 0 0192 0 0207 0 0006 0 0195 15e 0 022 0 0176 0 0190 0 0302 0 0006 19e 0 030 0 0262 0 0277 0 0284 (0 0250) (0 0270) 73a 0 031 0 0288 0 0304 0 0314 0 0006 19d 0 041 0 0349 0 0382 0 0391 0 0416 0 0009 8h 0 050 0 0424 0 0459 0 04i6 0 0500 0 0009 (0 0439) (0 0463) 0 0774 0 0007 20e 0 078 0 0703 0 0710 0 0724 0 0787 0 0026 6ea 0 079 0 0684 0 0722 0 0746 0 2787 0 0027 12gb 0 272 0 246 0 252 0 255 High-carbon cast iron which does not burn satisfactorily in the procedure tlescribeci, b 0.500 gram was mixed with 0.500 gram of low-sulfur steel for this dsta.
of the higher flow rate is typified by the comparatively lower values in parentheses (Table I) which were obtained a t a flow rate of 1200 ml. per minute. The amount of sulfur found in the rinse of the delivery system appears to increase with the sulfur content of the sample. The lack of a strictly quantitative relationship is probably the result of some variability in the oxygen flow rate or, more important, in the temperature of the delivery system. -4t the higher flow rate, the delivery tube gets too hot to touch, and variations in this temperature would probably affect adsomtion reactions of sulfur trioxide. Even the total values in Table I fall appreciably short of 100% recovery. Qualitative evidence obtained in the tracer experiments described suggests that some adsorbed sulfur trioxide resists removal by a water rinse. Additional possible losses are retention of a maximum of 2% by the crucible and slag, and escape of sulfur-bearing gases from the absorbing solution. Experiments with three absorbers in series resulted in no additional sulfur recovery. However, there is still the possibility of aerosol formation. The as-titrated values times a loss factor 1.09 shorn excellent precision and accuracy over the entire range of 0.010 to 0.272%. STUDY OF BLANK
Procedures. As described above. Results and Discussion. The data given in Table I demonstrate a difference between the two methods of titration. However, blanks contributed by the crucible and the tin flux have not been deducted. It is necessary to evaluate these factors before comparing the two titrations for samples containing less than 0.01 yo sulfur. All of this blank study utilized the special low sulfur steel referred to earlier. Because essentially complete expulsion
of the sulfur from the sample imposed no problem, it was assumed that the crucible which led to the lowest titers made the lowest blank contribution. Crucibles made from various materials by several vendors were tried. Only those of zircon and mullite had the required physical characteristics. Hand-fabricated Vycor crucibles gave the lowest titers, thus indicating a blank contribution by mullite or zircon. Vycor crucibles, however, arc not available in quantities for regular use. The blank in mullite and zircon can be slightly reduced by heating to 1500" C. in air. However, the advantage in heating a t this temperature rather than a t 1200" C. is negligible. Thus, for a routine decontamination, prefiring in air a t 1200" C. is acceptable. Attempts to reduce the blank of the mullite crucible by firing in argon in a carbonarc furnace a t 1700" C. and in a gasfired furnace a t lGOOo C. also failed. The results obtained with the lowsulfur steel averaged 0.000S70 sulfur using the iodate titration and 0.0016% sulfur using the alkali titration. For the former, a dilute iodate solution was used, 1 ml. = 0.00005 gram of sulfur. For the latter, the standard sodium hydroxide solution was used; oxygen flow rate was 5 liters per minute; and titration included a rinse of the entire delivery system. Each charge included 1 gram of sample and 0.5 gram of tin. These data were obtained with commercial mullite crucibles prefired in air a t 1200" C. Zircon crucibles from four vendors resulted in significantly higher results even if prefired a t 1800" C. Apparently, a fundamental difference exists between materials with respect to the lowest attainable blank. Such a difference undoubtedly contributes to the discrepancy in per cent recovery between laboratories, especially as the crucible blank is a certain absolute amount of sulfur which percentagewise VOL. 31,
NO. 3, MARCH 1959
403
varies with the sulfur content of the sample. Because the high flow rate enhances the burning of steel samples, some samples can be burned without using tin flux, thereby permitting determination of the contribution of tin to the blank. One-gram samples of the low-sulfur steel were burned with and without the usual 0.5 gram of tin; the difference in results was taken as the tin contribution. Verification was made by burning 2 grams of tin and just enough of the low-sulfur steel, 0.3 gram, to maintain 1550" C. No direct method is available a t present for evaluating the blank of the crucible, as the presence of steel is necessary to induce heating. However, by using multiple weight burns of the low-sulfur steel (2- and 3-gram charges) and Variac control of power, an estimated value can be obtained for the steel. It is not an absolute value because an increase of slag mass may increase the ceramic blank. The difference between the estimate for the steel and the total yields an estimate for the ceramic blank. As a difference in crucibles and various tin fluxes was found, these two sources of blank should be determined for each new lot used. The following estimated values were obtained: low sulfur steel and commercial mullite crucibles, each 0.0005%; tin, 0.0003 to 0.0008%, depending on source. The accuracy of these values is believed t o be t o *0.0002%. The crucible and tin blanks refer to the analysis of 1-gram samples of steel. DETERMINATION O F SULFUR BELOW 0.01 % '
Procedures. As described above. Results and Discussion. After establishment of the optimum conditions for obtaining maximum recovery of sulfur, the fraction lost, and the contributions of the blank, this information was applied to the determination of less than O.Olyo of sulfur in steel. No reliable standard samples having less than 0.01% of sulfur are available. Mixtures of NBS 19e and the low-sulfur steel were prepared such that the equivalent of 0.0020, 0.0034, 0.0064, and 0.0094% of sulfur per gram was obtained. These mixtures were made individually for each determination and analyzed by both the standard iodate method and the alkali method using the modified procedure described later. Five samples a t each level were included in a series. The results, together with those similarly obtained with NBS 125a (0.013%) and the low-sulfur sample (0.0005~0)are plotted in Figure 1. As the sulfur content of the mixtures decreases, the influence of the blanks becomes increasingly important. Below 0.005% the total sulfur re404
ANALYTICAL CHEMISTRY
Figure 1. Comparison of sodium hydroxide and potassium iodate titrations in recovery of sulfur by combustion method
0 0
Total sulfur titrated, including rinsings for alkali titration Total sulfur litrated, blank corrected
covered by the alkali titration, rinsings, and blanks is obviously too high. A partial compensation of errors can be obtained by ignoring both a rinse of the delivery system and a blank correction, but better agreement with the known sulfur contents is obtained by subtracting a blank from total sulfur, as shown by the lower curve in Figure 1, sodium hydroxide titration. By applying a loss factor of 1.09 to this curve, a nearly perfect fit to the theoretical curve is obtained. For all results below 0.01% sulfur, a standard deviation of 0.000270 was observed. As sulfur trioxide is not titrated by potassium iodate, rinsing does not add to a total sulfur recovery and total sulfur is appreciably low a t the higher sulfur levels as shown in Figure 1. A t these higher levels, the constant blank does not compensate for the increasing loss of sulfur trioxide; the result is an average recovery of 85%. Percentagewise, both blanks and sulfur trioxide losses become very high a t extremely lorn sulfur levels, but apparently compensate to give results approaching 100% recovery. If blanks are subtracted from the iodate titration, the results are very low. INTERFERENCES I N ALKALI TITRATION METHOD
I n general, no interferences have been found. Although a large amount of carbon can interfere by leading to overtitration, it was shown with multiple absorbers that carbon dioxide is swept out of the system a t the p H of the bromocresol purple end point. In passing through the absorber, it does momentarily give an acid indication, but interference can be avoided by allowing the carbon dioxide to be swept out of the absorber before titrating the sulfuric acid. Other acidic constituents were considered and found not to interfere. Steels high in phosphorus were analyzed with excellent results. The blanks of the crucibles and tin have been evaluated but their composition is unknown.
Tin and iron oxides sometimes cause fuming and cloudiness in the absorber, which, when excessive, may obscure the end point. This interference can be remedied by changing the absorber and cleaning the fritted glass dispersion tube. However, there is no evidence that these oxides in the absorber affect the results by altering the acidity. RECOMMENDED PROCEDURES
On the basis of the results obtained, the following two procedures are given as the most reliable for the determination of sulfur by the combustion method. Both include increased flow rate of oxygen and alkali titration, as well as a correction factor of 1-09. However, the two procedures differ in one respect. For the higher range of sulfur, rinsings and blank are ignored, as any error in their failure to compensate is negligible. For sulfur contents of less than O.Olyo,full corrections for rinsings and blank are recommended, as justified by Figure 1. Most of the work with the alkali titration has been with sodium hydroxide as titrant. Limited experience with THAM (tris(hydroxymethy1)aminomethane] as titrant indicates that it is equally reliable and is subject to the same correction factor. Others have pointed out certain advantages which this reagent has over sodium hydroxide (1, 4 ) . The following procedures are therefore given with a choice between these titrants left to the discretion of the operator. Standard Procedure. Install the modified combustion tube and short delivery tube described earlier. The rubber tubing connection which attaches the fritted glass bubbler is located beneath the absorbing solution. Any other rubber connections in the delivery system must be cooled by an air stream. Transfer 150 ml. of hydrogen peroxide (0.6%) to the cylinder of a 250-ml. gas washing bottle, which serves as a convenient absorption vessel with a constricted
neck to prevent spray loss of the solution. Adjust the oxygen flow to 5 liters per minute and allow 5 minutes to flush out carbon dioxide in the solution. Weigh a 1-gram sample and transfer to a prefired crucible. (These may be prefired in advance and stored in a desiccator.) Add tin to ensure good combustion. One-half gram is usually sufficient and many samples can be burned with no tin. Add 5 drops of bromocresol purple to the absorbing solution and adjust to colorless end point with carbonatefree alkali or acid. Flush the fritted glass dispersion tube by momentarily opening the gate of the furnace to interrupt oxygen flow and then closing. Readjust the end point. Repeat until end point is constant and note buret reading. Place the covered crucible in position and ignite the sample. Burn for 3 minutes, maintaining plate current as high as possible without overloading the induction furnace. Titrate with alkali (1 ml. = 0.0001 gram of sulfur) to colorless end point. Experiments
with samples as high as O.35y0 sulfur (NBS 133) indicate that no losses occur even if no titrant is added until the end of the combustion period. Repeat the flushing of the fritted glass dispersion tube and titrating until the end point is constant. Calculate per cent of sulfur with a correction factor of 1.09. Modifications for Low Sulfur Samples. Follow the standard procedure, using the same absorbing solution for a series of determinations. As many as 10 have been run by the authors before the absorbing solution became fogged and obscured the end point. Remove the modified apparatus and rinse with the solution from the absorption vessel. Titrate to the end point. Repeat the rinsing and titration until the end point is constant. Add a proportionate volume of alkali used for titrating the rinse to each of the individual titers. Calculate the per cent of sulfur with a correction factor of 1.09, after making suitable blank corrections for crucible and tin flux.
ACKNOWLEDGMENT
The authors are indebted to B. A. Thompson, General Electric General Engineering Laboratory, Schenectady, N. Y., for performing all of the radioassay work described. LITERATURE CITED
(1) Fossum, J. H., Markunas, P. C., Riddick, J. A., ANAL. CHEM.23, 491 (1951). (2) Fryxell R. E., Zbid., 30, 273 (1958). (3) Hale, H., Jr., Muehlberg, W. F., IND.ESG. CHEM.. ASAL. ED. 8. 317
6.
(1936). (4) Holler, -4. c., ~ A L CHEM. . 28, 1359 (1956). (5) Smith, T. B., Backhouse, A., Woodward, P., J . d p p l . Cheni. (London) 4, 75 (1954). RECEIVEDfor reviem- May 26, 1958. ilccepted October 6, 1958. Pittsburgh Conference on Bnalytical Chemistry and Applied Spectroscopy, March 1958.
Nonaqueous Determination of Acetylenic Hydrogen LUCIEN BARNES, Jr. Research laboratories, Air Reducfion
Co., Inc.,
b A combination of silver perchlorate, an amine titrant, and screened thymol blue indicator enables the direct determination of acetylenic hydrogen in nonaqueous solvents. The method is based upon the liberation of hydrogen ions when silver perchlorate reacts with monosubstituted acetylenic compounds. The acid liberated due to acetylide formation is titrated with a methanolic standard solution of tris(hydroxymethy1)aminomethane to the color change of a screened thymol blue indicator, permitting direct determinations of acetylenic hydrogen in waterimmiscible solvents and easily hydrolyzable esters. The determination may be performed in the presence of appreciable amounts of certain weak organic acids without prior neutralization. Strong acids and bases may be preneutralized so as not to interfere. Intermediate strength amines and acids interfere because of buffer action but there is no interference from halogens or aldehydes. Acetylide precipitation rarely occurs, which facilitates detection of the visual end point.
M
for the determination of acetylenic hydrogen have been reported (1, 4, 6). This nonaqueous procedure was developed for the many
Murray Hill, N. J.
mater-insoluble samples routinely received for analysis in these laboratories. Such samples range from pure acetylenic compounds to trace amounts (less than O.loJ,) contained in various waterimmiscible solvents and easily hydrolyzable esters. Because the strong bases normally employed as titrants readily saponify many esters, a weak amine, tris(hydroxymethy1)aminomethane (THAM), was used as the titrant Fyith a low pH indicator, thymol blue. Miocque and Gautier (5) published a silver nitrate procedure in which the complexing and basic properties of pyridine are used to excellent advantage. Alcoholic sodium hydroxide is used as titrant and thymolphthalein as indicator. The two methods approach the problem from opposite ends of the pH scale, and complement each other, as neither procedure is universally applicable. Silver perchlorate is appreciably soluble in many organic solvents and reacts with monosubstituted acetylenes in polar media according to R-CEC-H R-C=C--ilg.
+ 2dgC104 AgClOi
+
+ HClOi
(1)
ETHODS
The liberated perchloric acid may be quantitatively titrated with tris(hydroxymethy1)aminoniethane to the
low pH color change of a screened thymol blue indicator. REAGENTS
Silver Perchlorate, lilf. Dissolve 104 grams of anhydrous silver perchlorate (G. Frederick Smith Chemical Co.) in anhydrous methanol and dilute to 500 ml. Store in a polyethylene bottle. Xo explosions have been experienced with solutions prepared and stored in this manner. Barium Perchlorate, anhydrous (G. Frederick Smith Chemical Co.). Standard 0.1N Tris(hydroxymethy1)aminomethane (THAhI). With mechanical stirring, dissolve 12.15 grams of tris(hydroxymethy1)aniinomethane (Fisher Scientific Co.) in methanol and dilute to 1 liter. Filter any remaining insolubles. To standardize, dilute 40 ml. of this solution with 200 nil. of water and titrate \Tith aqueous acid to the methyl purple end point. Screened Thymol Blue Indicator Solution. Dissolve 0.100 gram of thymol blue and 0.025 gram of alphazurine in 100 ml. of methanol. Store in a brown bottle. Prepare fresh every week, as it decomposes on standing. Alphazurine is obtainable from the General Laboratory Supply Co., P. 0. Box 2607, Paterson, N. J., and is known as Alphazurine A or Patent Blue il, Catalog No. NB717. Standard 0.1N Perchloric Acid. Dissolve 8.5 ml. of 72% perchloric acid in VOL. 31, NO. 3, MARCH 1959
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