Volumetric Determination of Concentrations of Sulfuryl Fluoride in Air STANLEY G. HEUSER Pest Infestation laboratory, Agriculfural Research Council, Slough, Buckinghamshire, England
b The reaction of the fumigant, sulfuryl fluoride, with alkalies is discussed, and two volumetric methods of determining its concentration in air are described. Sampling from atmospheres containing increased carbon dioxide from insect respiration is discussed, and corrections required under these conditions are considered. The additive relationship of thermal conductivity meter readings obtained with mixtures of the two gases is demonstrated, and difficulties in interpretation of such readings are indicated.
S
fluoride (SOZFZ, b.p. -55.2' C.) is used in the United States for the fumigation of buildings for termite control (1, 4). Its penetration into commodities and toxicity to certain stages of insects infesting stored products have been favorably compared with methyl bromide (8), the most m i versally used fumigant a t the present time. Recently Meikle and Stewart (9) have shown residues in various materials, including foodstuffs, after fumigation with sulfuryl fluoride, to be at a very low level. h a preliminary to investigations a t this laboratory of the value of sulfuryl fluoride as a fumigant for the disinfestation of stored products, a chemical method for determination of the concentration of the vapor in air was required. ULFURYL
In cold dilute alkali-e.g., 0.1N NaOH S02Fz 2NaOH + NaS03F NaF
+
+
+ Hz0
(2)
In hot, strongly acid conditions HCl HzO + HzSO~ HF NaCl (3)
NaSOIF
+
+
+
-+
Application of Equation 2 to Assay of Gas Samples. Samples of sulfuryl fluoride vapor in air were taken in 200-ml. or 1-liter all-glass flasks containing a known volume of standard NaOH, evacuated to 5 cm. of Hg absolute, and then allowed to stand for 48 hours. Direct titration of e x c w NaOH with 0.1N or 0.05N HCl after absorption and reaction of SOzFz according to Equaition 2, using acid-range indicators such as methyl orange or bromophenol blue to avoid interference from carbonate, gave indistinct end points. Change in pH, when plotted vertically against addition of acid during titration, produced curves with low slope and indistinct points of inflection a t equivalence, similar to those shown in Figure 1, when continued below p€I 8.0. IO
II
19
METHOD A. Titration of excess alkali in an aliquot of the absorbent reagent with 0.05N HCl, using thymolphthalein as indicator, after precipitation of carbonate with neutral barium or strontium chloride solution, gives a sharp end point a t pH 9.2. Strontium chloride solution (2 ml. of 20% S r C k 6HzO) is preferred to the barium salt m giving a slightly less soluble carbonate. This produces more complete precipitation. Additionally, lessened re-entry of carbonate into solution as the p H falls momentarily below 9.0 during titration minimizes running of the end point. The difference in titration from that of a reagent blank of the same volume ( A - B ml.) is due to removal of free alkali by the separate reactions of SOsFl and COz absorbed from the gas sample (Figure 1). Since COz is always present in the sampled atmosphere, often a t a higher than normal level where insect respiration is a factor, the amount absorbed by the reagent requires calculation. This is achieved by direct titration to pH 8.3 of excess alkali in another aliquot of the
1 -ML.
E
SULFURYL FLUORIDE CARBON DIOXIDE
-+
(11
Blonk
EXPERIMENTAL
The reaction of sulfuryl fluoride with alkaline solutions often given in textbooks SOrFt 4NaOH -.c NaZSOd 2NaR
+
+ + 2Hz0
(1)
did not take place in the cold, no sulfate being detected in the reaction mixture. Instead, on absorption of SOzFz in cold dilute NaOH (down to 0.05N) a quantitative reaction, completed in about 24 hours, with half the alkali required by Equation 1, was observed. Addition of barium chloride solution to the still alkaline reaction mixture gave a crystalline precipitate of barium fluoride which dissolved when slightly acidified with HC1 and warmed gently. On further acidification and boiling, barium sulfate ww precipitated and the glass tube was etched. 1476
ANALYTICAL CHEMISTRY
I
I
\
4 -
i I 15
IO ML.
Figure 1.
0.05 N
HYDROCHLORIC
20
ACID
Titration of excess Ba(0H)z after reaction with SOzFz and COz (Method
B) End point taken at pH 9.2
sample, without the addition of strontium chloride, using 3henoltetrachlorophthalein as indicator. The carbonate in solution is thus converted to bicarbonate. A moderately sharp end point is obtained when this titration is carried out at Oo C., with buret tip under the surface to avoid 10:s of COz. The amount of HC1 neutralized a t this equivalence point is e2actly half that required for complete decomposition of carbonate. 2HClf NanCOa-,CO2 Ha0 2NaC1
+
(11
Blank
121
co,
.alone
131
SO, f ,
141
SO7
olone
F , t CO,
+
(end point unsatisfactory in presence of SO2F2) HCl NazCOa-L NallCO, NaCl
+
+
pH 8.3
This titration volume, when subtracted from a similitr reagent blank (C - D ml.), is used to calculate the amounts of SOzFzand COI present. Let mg. of SOzFzpresent in aliquot = CY Let mg. of COz prese2t in aliquot = i3 A - B m l . = CY 3 B C - D ml. = CY 2 1 liter of 1 N NaOH = 51.0 grams of SOzFz (see Equation 2) 1liter of 1 N NaOH 22.0 grams of COz :. Mg. of S02F2 in aliquot = 2 (C - D) - (A - B ) ml. O.OJN X 2.55 Mg. of C02 in aliquot = 2 (A - B ) - 2(C - D) ml. 0.05N X 1.1
+ +
Accuracy = z t O . 1 ml. 0.05N = *2.5% a t 50 mg. per liter of SOzF2, taking 20ml. aliquots from 50 ml. in 1-liter flask.
METHODB. W h m 0.1N barium hydroxide (carbonate-free) is substituted for the sodium salt in Equation 2 a similar reaction occurs:
+ 2 Ba(0H)a
2 S02F2
+
Ba(S0sF)z IhF2 2H20 ( 4 ) -+
+
COz present in the atmosphere sampled is precipitated as barium carbonate As in together with the fluoride. Method A, direct titration of excess alkali with 0.1N HCI, using thymolphthalein as indicator to pH 9.2, gives the reduction in free alkali due to reaction of both SOzFz and COZ (Figure 1). Excess 0.1N HCl is t h m added, up to a standard total volum?. Barium fluoride redissolves and ?;he carbonate is decomposed with par tin1 evolution of GOa. An amount of acid equivalent to the original hydroxyl strength reacting with COzis thus neutralized. (Heating to remove all GO2 from solution a t this stage proved unsatisf tctory, resulting in partial decomposition of barium fluosulfonste.) The total amount of 0.1N HCl used is nott:d and the same addition i s made to the reagent blank. Bacli-titration with 0.02N Ba(OH)2 (carbonate-free) of excess acid remaining after decomposition of carbonate gives a pH curve of the form shown in Figure 2 with samples containing even small amounts of SC2F1. Compared
5 ML.
Figure 2.
15
IO OOZN
BARIUM
HYDROXIDE
Back-titration of excess acid after decomposition of carbonate (Method
B) End point taken at pH 5.0
with a reagent blank, the curve from pH 3.0 to 4.7 has a low slope due to buffering, precluding the use of indicators within this range to obtain the equivalence point for SO2Fa. At the same time indicators having color changes above p H 5.0 are sensitive to carbonic acid in solution (curves 2 and 4, Figure 2). The relatively short vertical part of the curve representing the equivalence point requires an indicator color change a t p H 4.7 to 5.0 for accurate titration. No single indicator meets this requirement, but two drops of a mixture of methyl red and bromocresol green gave the following color changes: pH 4.8 rose red pH 5.0 colorless pH 5.2 green giving a sharp end point using 0.02N Ba(OH)2. If determination of COS in the sampled atmosphere is not required, titration with HC1 is replaced by direct d d i tion of a standard amount of acid in excess by pipet, to sample and reagent blank. The amount of SOZFZpresent is then obtained directly by difference in titrations with 0.02N Ba(0H)z. A 50% excess of 0.1N Ba(OH)2 as reagent in the flask is desirable. 1 liter of 1N Ba(OH)2= 51 grams of SOnFa (see Equation 4) ;. Mg. of SOZF2 = ml. of 0.02N Ba(OH)2 (=a) X 1.02 Accuracy = A0.05 ml. = *0.5% at 10 mg. per liter with 1-liter flask. 1 liter of 1 N Ba(OH)2 I22 grams of COa
.:
mg. of absorbed C02 = [(blank titration-samde titration, ml. of 0.1N HCl) - 6/5 ml.] X 2.2 Methyl Red-Bromocresol Green Indicator. Dissolve 0.1 gram of bromocresol green in 1.44 ml.-of 0.1N NaOH and dilute to 50 ml. with water. Dissolve 0.1 gram of methyl red in 150 ml. of absolute alcohol and mix with bromocresol green solution. Absorption Correction in Sampling. I n either of the methods described, if the concentration of COZbeing measured is above about 1% (20 mg. per liter), the effect of its almost instantaneous absorption in the reagent has t o be taken into account. Consequent correction for the error in the calculated volume of sample transferred to the evacuated flask must be made, affecting both SOzFzand COz results. Let V be effective volume of sampling flask-Le., V = (vol. of flask - P where B is vol. of reagent) X B 7 the barometric pressure and p is the absolute pressure to which the flask was evacuated. If gaseous absorption is allowed to proceed to completion, true volume transferred, Vt=V+V-+V 100
-
+
V ($0)"
(5)
where 2 = true percentage by volume of COz in air. Since 5
m
