ANALYTICAL CHEMISTRY
1766 Total Phosphite. Dissolve a sample containing 0.002 to 0.003 mole of phosphite in 100 ml. of 95% ethyl alcohol. Keutralize immediately to a light pink with phenolphthalein indicator. Add 5.0 ml. of 0.1N hydrochloric acid, mix, and allow to sit for a t least 10 minutes. Add 40.0 ml. of O.IN sodium hydroxide, mix, and titrate with 0.LY hydrochloric acid until the solution is colorless. The net moles of caustic consumed are equivalent to the moles of phosphite present. The trialkyl phosphite content is calculated by subtracting the dialkyl phosphite from the total phosphite. Sample size, strength of solutions, and the amount of alcohol used as solvent may be regulated as desired. The authors prefer using aqueous acid and base with enough alcohol to keep a clear solution rather than alcoholic acid or base. RESULTS
The method is simple, rapid, and accurate. It is applicable to alkyl phosphites ranging from ethyl to octyl alkyl groups, with little interference from materials commonly found in commercial trialkyl phosphites. Table I shows typical results ob-
tained from synthetic mixtures of pure fractionated di- and tributyl and di- and triiso-octyl phosphites. ACKNOWLEDGMENT
The authors wish to express their‘thanks to Betty Bruns for assisting in compiling the analytical data. LITERATURE CITED (1) hrbuzov, A. E., J. Russ. Phys. Chem. SOC.46, 291-4 (1914). (2) Deal, V. Z., Wyld, G. E. A , , ANAL.CHEM.27, 47-55 (1955). J . Chem. Soc. 1953, 2224-34. (3) Landauer, S. R., Rydon, H. K., (4) NylBn, P., “Studien uber organische Phosphorusbindungen,” thesis, p. 130, Alniquist och Wiksells Boketryckeri, A.B., Uppsala, 1930; Ber. 57B, 1023-38 (1924); 59B, 1119-28 (1926). (5) Shell Development Co., “Organophosphorus Compounds for Department of the h’avy,” Tech. Rept. 8 (March l , 1950, to hIay 31, 1951).
RECEIVED for review May 19, 1936. -4ccepted July 10, 1956. Division of .4nalytical Chemistry, 129th Meeting, 9 C S , Dallas, Tex., April 1956.
Decomposition of Organic Fluorine Compounds by Wickbold Oxyhydrogen Flame Combustion Method P. B. SWEETSER Chemical Department,
E. 1. du Pont d e Nemours & Co.,
/nc.,Wilmington, Del.
The Wickbold oxyhydrogen combustion method is an excellent way of decomposing organic fluorine compounds for determination of fluorine. In this method the vaporized organic fluoride or its partial combustion products are passed through an oxyhydrogen flame which decomposes the sample to carbon dioxide and hydrogen fluoride. The hydrogen fluoride is absorbed in dilute sodium hydroxide solution and subsequently titrated with thorium nitrate. This method is rapid and eliminates the difficulty resulting from high salt content normally present after a Parr bomb-type decomposition.
0
VER 500 references have been listed by McKenna (8-4)
on methods of decomposition and analysis of fluorine compounds. However, none of the procedures given in the above reference are entirely satisfactory. The two main sources of error encountered in fluorine analysis are: (1) Difficulty in obtaining quantitative conversion of the organic fluorine to ionic fluoride. It is usually necessary to employ more drastic decomposition conditions than those required for other organic compounds, and even then some organic fluorides give low results due to incomplete decomposition. ( 2 ) The quantitative determination of fluoride in the decomposed sample. Volumetric methods based on formation of complexes and precipitates are complicated by the difficulties of end point detection, slow reactions, and general nonstoichiometric conditions caused by pH and salt effects. Gravimetric determinations generally are not suitable because of the solubility of metallic fluorides and the difficulty in filtering precipitates. Probably the most widely used method for fluorine analysis is a Parr bomb decomposition, followed by titration of the fluoride with thorium nitrate usiqg alizarin red as indicator. A Parr bomb method has proved satisfactory in this laboratory for the decomposition of most organic fluorides; only in the case of highly volatile compounds and compounds with high fluorine content has there been difficulty in the decomposition step. The nature of this decomposition, however, makes the final deter-
mination of fluoride difficult because of the large amount of salt residue from the Parr bomb fusion. This high salt content not only has a detrimental effect on the sharpness of the end point, but also changes the stoichiometry of the reaction. I n an effort to avoid these difficulties, a study has been made of the decomposition of organic fluorides by a Wickbold-type, oxyhydrogen flame combustion apparatus (5, 6). APPARATUS
The quartz flame combustion apparatus (Figure 1 ) is similar to that described by Wickbold (6) and consists of five main parts: the vaporization tube, A , the burner, B, the flame chamber, C, the condenser, I , and the absorption receiver, D. Absorption receiver and Reitmeyer attachment, E, are of borosilicate glass; all other parts are of quartz or Vycor (96% silica glass) glass. The vaporization tube is the section in which the sample is heated to give partial combustion or vaporization. Gaseous products then pass from the vaporization tube via a capillary tube into the burner head a t B. The oxygen and hydrogen are supplied to the burner from the two tubes, x and g, so that the capillary from the vaporization tube is surrounded by the hydrogen and oxygen tubes ahich end a t the burner head. The burner section is connected to the flame chamber, C, by a 19/38 joint a t B. The flame chamber is surrounded by a water jacket to prolong the life of the quartz tube and t o prevent possible melting of the quartz. The flame chamber tapers down a t the end to about 6 mm. and is connected to a spiral enclosed in a water condenser, which in turn is connected a t J to the absorption receiver with a 10/30 joint. The apparatus is mounted on a Transite panel. The flows of the flame oxygen, sweep oxygen, hydrogen, and exhaust lines are controlled by needle valves, mounted on the front of the panel, which are connected to the Brooks multitube Flowmizer flow meters mounted in a single four-unit mount. The apparatus is surrounded by a Lucite acrylic resin safety shield. A probe burner, 11/2 X 20 cm., P , is made of Vycor glass. The oxygen is supplied to the burner through the outer tube and the gas by the inner tube, both of which taper down a t the burner head. A platinum boat, 1 X 4 X 1 cm., was used in the vaporization of solid samples and high boiling liquids. A quartz pig similar to that used by Wickbold was used for the volatile liquid samples. The illuminator for the titrations is an H-3, 85-watt Westmghouse mercury lamp with a vapor lamp transformer. The lamp is housed in a mount that directs the ultraviolet light a t an angle
V O L U M E 2 8 , N O . 11, N O V E M B E R 1 9 5 6
1767
to the titrating vessel and screens out strong light from other portions of t,he n-orking area. REAGESTS
Thorium Nitrate Solution, 0.1N. Dissolve 14.0 grams of C.P. thorium nitrate tetrahydrate in water and dilute to 1liter. Standardize this solution with C.P. sodium fluoride which has been dried for 2 hours a t 110' C. Monochloroacetic Acid Buffer, 0.50M. Dissolve 23.6 grams of reagent grade monochloroacetic acid in 400 ml. of water, add l.OAr sodium hydroxide t o t'his solution until the p H is 3.0, cool, and dilute t o 500 nil. Store in a polyethylene bottle. This buffer solution should not be stored more than 1 or 2 weeks. Standard Sodium Fluoride Solution. Weigh out 2.2105 grams of C.P. sodium fluoride (dried for 2 hours a t 110" C.), dissolve in m t e r , and dilute to 1 liter. Store in a polyethylene bottle. This solution contains 1.00 mg. of fluoride per ml. Sodium Alizarin Sulfonate Indicator. Prepare a 0.05yo solution and make alkaline by adding 0 . 1 s sodium hydroxide dropwise until the color changes to a, deep red. PROCEDLRE
The absorption receiver is filled with 50 ml. of 0.1N sodium hydroxide and the flame ignited after first removing the burner, B , from the standard taper joint. I t is then possible, by applying a vacuum at E , to return the lighted burner to the flame chamber, C, so that the chamber is nearly filled by the oxyhydrogen flame. The various flov rates are important at this point and are adjusted to piedetermined values n i t h the aid of the flow meters. Although the exact flow rates r i l l vary to a certain extent with the apparatus, the following rates were found most suitable for the author's apparatus: a flow setting of 6.0 to 7 . 2 liters per minute for the flame hydrogen, y, 5.0 t o 6.0 liters per minute for the flame oxygen, 2, 1.6 t o 2.2 liters per minute for the sweep oxygen, G , and 4.0 to 4 5 liters per minute for the exhaust line, E . T h e sample is introduced into the vaporization tube through the removable joint a t F . Oxygen is passed over the sample from the sn-eep oxygen line, and the sample is heated m-ith a Fisher burner until vaporization or partial combustion takes place. The on>-gen sneeps the vaporized sample through the capillary tube and into the flame chamber. The temperature in this chamber IS aiound 2000" C. and all carbon-fluorine bonds are broken rrith the formation of carbon dioxide and hydrogen fluoride. The hydrogen fluoride is snept through the condenser coil into the receiver, v-here it is absorbed by the aqueous scrubbing solution. The continuous formation of water vapor from the reaction of the hydrogen ai$ oxygen assures complete washing of the flame chamber and condenser of adsorbed hydrogen fluoride. The fluoride solution is then removed from the absorption receivrr and diluted to 200 ml An aliquot of this fluoride solution containing 5 to 9 mg. of fluoride is then added t o a 210-ml. casserole along 11-ith 1 0 0 ml. of the alizaiin red indicator, 2.00 ml. of 0.5OJI monochloroacetic acid buffel, and enough Trater to make approximately 100 ml. The pH of the solution is adjusted to 3.4 xith 0.3iV hydrochloric acid or 0 3-X- sodium hydroxide and the final volume made approximately 103 ml. This solution is then titrated n-ith standard thorium nitrate until a slight color change from yellow t o pink appears under a white light. At this point the titration is continued in the dark n i t h the ultraviolet light source directed
down toward the casserole a t a 45' angle. A definite appearance of a faint pink color is now taken as the final end point. Liquid samples can be treated similarly. The sample is weighed in a quartz pig with a ground-glass stopper, which is removed as the sample is placed in the vaporization tube. The sample is gently heated until it has been completely vaporized into the flame chamber. Gaseous samples can be decomposed by connecting the gas sample tube directly to the vaporization tube and slon-ly sffeeping the gas through the vaporization tube into the burner. The combustion of gaseous samples requires a much slower rate of flow for the sweep oxygen. The most convenient procedure was t o allow the sample to diffuse into the burner by the slight negative pressure of the vaporization tube and combustion chamber. After 1 t o 2 minutes a slow stream (0.1 to 0.2 liter per minute) of sweep oxygen is allowed to flow through the gas sample tube for 2 to 3 minutes, after n-hich the sn-eep oxygen is increased to the normal value of 1.6 to 2.2 liters per minute until the gas is completely displaced from the sample tube. I n the decomposition of some solids and liquids, considerable amounts of tar may form during the vaporization process and collect in the capillary between the vaporization tube and the burner end. Large amounts of this tar can give low results in the final fluoride analysis, as well as be troublesome to clean out. An all-Yycor oxyhydrogen flame probe, P , provided a rapid and effective means for solving this problem. The usual procedure was to remove the boat from the vaporization tube after the vaporization and, with the oxyhydrogen flame still in the flame chamber, increase the vacuum slightly and direct the probe flame into the capillary tube through the joint at the end of the vaporization tube. The vacuum caused the probe flame to be drawn through the capillary tube, thus removing all the tar and carbon in less than 15 seconds. Care must be exerted a t this point to prevent melting of the capillary tube. DISCUSSION
The results of analyses of several fluorine compounds are given in Table I. I n most cases the results tend to be slightly on the low side; this is attributed t o a possible boron effect from the borosilicate glass absorption receiver. Addition of mannitol to the sample was found t o give a slight increase in acidity, thus indicating the presence of boron. Because this error was small and reproducible, a recovery factor of 1.006 was adopted for use on all known saniples. It is interesting that quantitative decompositions took place even with trifluoroinethane and tetrafluoromethane. These compounds have been notorious for their stubbornness in resisting decomposition. Freier and coworkers (1) recently reportid that, in the quartz tube combustion method, only about 3% of the theoretical fluorine content could be obtained in the deconiposition of tetrafluoromethane even in the presence of water vapoi The quantitative decomposition of tetrafluoromethane is more importaqt than it appears on the surface, for in many combustion methods it is thought that the cause of low results is the formxtion of tetrafluoromethane. K i t h the Wickhold method, com-
r
6
E
.
L.
P
\ '
0
F i g u r e 1. Oxyhydrogen combustion apparatus A . Vaporization tube Burner B. C. Flame chamber D . Abaorption receiver E . Reitmeyer attachment F . Removable joint 0. Sweep oxygen inlet
H . 3-may stopcock
I. Spiral condenser J . 10/30joint f. Probe burner r. Burner oxygen inlet ? I , Burner hydrogen inlet
D
li
G
F
-
0 5 10 CM.
1768
ANALYTICAL CHEMISTRY
Table 1. Results of Analyses with Wickbold Apparatus % Fluorine
~~
Substance Teflon (tetrafluoroethylene resin)
Theory 76.0
Found 75.84 75.69 75.35
Sodium tritluoroacetate Fluorobeneene
41.9 19.77
Trifluoroacetylaoetone 1-Chloro-3-fluorobenzene
37.0 14.55
Perfluoro-(2-n-propyl)-pentamethyleneoxide
73.10
41.8 19.67 19.60 19.50 36.5 14.14 14.58 73.46 73.29
7.5 76
0 '
F
n-Heptafluorobutyric acid
62.18
Dichlorodifluoromethane
31.4
Trifluoromethane Tetrafluoromethane
81.4 86.4
60,83 61.88 31.1 31.4 81.3 86.0
85.8
plete combustion of tetrafluoromethane takes place, and, consequently, similar results on other samples which have been difficult to decompose should be expected. The data in Table I indicate that even the sodium salt of trifluoroacetic acid is quantitatively decomposed. I n this case the salt was first decomposed by the usual method, during which about two thirds of the actual fluorine was liberated. The boat was then removed from the apparatus and washed with water to remove the residual sodium fluoride, which was then combined with the solution from the absorption receiver and made up to volume. This procedure should m*ork for all salts that form nonvolatile soluble fluorides] and with possible modification should be of use with other fluoro-organic salts.
Some work has been done on the effect of sulfur on the determination of fluorine. Although no standard compounds which contained both sulfur and fluorine were available, results by the Wickbold method on unknown samples which contained substantial amounts of sulfur were 1 to 3% lower than the corresponding Parr bomb results. These low results are probably caused by a change in the stoichiometry of the thorium nitratefluoride reaction and are not due to the decomposition procedure. The Wickbold flame combustion method has several advantages over the more conventional methods. First, the method is rapid and gives excellent results with solid, liquid, or gaseous samples. The ease of analysis of gases makes the method ideal for elemental analysis of such samples. The actual combustion time for 0.1- to 0.2-gram samples is only 3 to 7 minutes, and the total time for an analysis is 15 to 20 minutes. One of the most important features of the method is that it eliminates the large amount of salt left by most decomposition methods. ACKYOWLEDGRIENT
The author .rvishes to acknowledge the development by E. S. Wilkins of this laboratory of the original procedure for determination of the alizarin red end point by the use of ultraviolet light from an H-3 mercury lamp. LITERATURE CITED
(1) Freier, H. E., S i p p o l d t , B. W.,Olson, P. B., Weiblen, D. G., - 4 S . 4 L . CHEM. 27, 14G (1955). ( 2 ) 1 l c K e n n a . F. E.. Sucteonics 8. 24 (June 1951).
