Siloxane Interference in Karl Fischer Reagent Titrations SIR: ;1recent review of methods for analysis of silicone resins ( 5 ) included comments on interference by siloxanes when silanols are being titrated with Karl Fiecher reagent (KFR). KO supporting evidence was presented or cited, and the impression was given that all siloxanes interfere. One would conclude t h a t deterininatiori of water in siloxane materials yields high results also. Several investigators ( 1 , 3, 4) have titratcd silanols in methanol with K F R and have shonm or assumed that no hydrolysis or alcoholysis of siloxane bonds occurs, as indicated by the equations: =SiOSi= =SiOSi=
+ H20
+ CHIOH
B(&iOH)
0.90r
(1)
+
=SiOCH3
+ &iOH
(2)
No appreciable reaction of K F R with hesamethyldiailoxane in methanol !vas rtyorted bl- Giliiian and Miller (3) or by Grubb ( 4 ) . From the sharp break in reagent addition os. time curve obtained with siloxane polymers, Damm and Sol1 ( 2 ) concluded that water and silanol could be differentiated and t h a t any hydrolysis of siloxane must be slight. We present additional data which show that only few siloxane materials do interfere seriously with K F R titrations in methanol. EXPERIMENTAL
Apparatus and Procedure. The trace water present in 40 nil. of 1 : l methanol-pyridine or 3: 1 (2-ethyl-lhexano1)-pyridine was titrated with K F R using the continuous recording apparatus and biani1)erometric end point as previously described (6). Five-gram fluid samples and 0.1 to 0.2 gram of cyclic dimethylsiloxane materials were introduced, and K F R was added manually to maintain the same end point for 1 hour. The two fluids were spiked with 0.005 gram of pure hesainethylcyclotrisiloxane and the titration us. time measurement was repeated with both alcohol-pyridine diluents. The siloxane materials were analyzed for hexamethylcyclotrisiloxane by gas chromatography using the F & 11 720 dual colunin programmed temperature gaq chromatograph with a 1-mv. recorder and attenuation of 4. Bridge current of 150 ma., injertion and detector block temperatures of 325' C., and a temperature program of 50 to 350' C. a t 15" C. per minute were employed. Twenty-microliter samples were injected on a 2-foot, l/r-inch diameter SS column containing 1.5y0dimethylpolysiloxane gum on 120- to 140-mesh Chromosorb G. Hexamethylcyclotrisiloxane content (weight %) was less than 0.017, in the hexamethyldisiloxane, 0.0170 in octamethylcyclotetrasilosane, 0.037,
TIME. MIN.
Figure 1. Consumption of KFR by various siloxanes in 40 ml. of 1:l methanol-pyridine 1. 0.1 gram of hexamethylcyclotrisiloxane 2. 5 grams of hexamethyldisiloxane plus 0.005 gram hexamethylcyclotrisiloxane
3. 5 grams of 100-cs. Dow Corning 200 fluid plus 0.005 gram of hexamethyicyclotrisiloxane 4. 5 grams of hexomethyldisiloxane 5. 5 grams of 100-cs. Dow Corning 200 fluid 6. 0.2 gram of octamethylcyclotetrasiloxane 7. 0.2 gram of decamethylcyclopentasiloxane
in decamethylcyclopentasiloxane, and 0.06% in 100-cs. trimethylsilyl end blocked dimethylpolysiloxane. RESULTS A N D DISCUSSION
Of the siloxane materials tested, only hexamethylcyclotrisiloxane demonstrated appreciable reactivity toward the 1: 1 methanol-pyridine solvent as qualitatively shown in Figure 1. The methanol-pyridine mixture was used for better solubility, and no decrease in interference was observed by lon-ering the methanol content in the diluent. Other cyclic and linear dimethylpolysiloxanes showed no reaction, and stable end points 17ere found for 1 hour or longer. The initial consumption is attributed to reaction with traces of rvater (and possibly silanol, if present). The instability or reactivity of the siloxane bond in the cyclic trimer structure is related to the strained six-membered ring. D a t a presented by Damm, Golitz and Koll (1) show that the relative reactivity of this cyclic trimer is about 400 times greater than that of the cyclic tetramer species in dilute HI-methanol medium. These conditions are more acidic than found in the K F R system, b u t a similar trend is probable. Other cyclic trimer structures-Le., (RR'
s i o ) where ~ R and R' are methyl, ethyl, phenyl, trifluoropropyl, and vinyl substituents-are also reactive in the KFRmethanol system. Interference by these materials is less than that found with hexamethylcyclotrisiloxane. The higher cyclic components again did not interfere. The use of 3: 1 (2-ethyl-1-hexano1)pyridine as sample diluent was effective in eliminating the interference from hexamethylcyclotrisiloxane. Essentially superimposed curves, similar to 4 and 5 in Figure 1, were obtained with each original fluid and their mixtures with 0.1% hexamethylcyclotrisiloxane. Even with addition of 0.1 gram of pure hexainethylcyclotrisiloxane to the high molecular weight alcohol-pyridine diluent, no consumption of reagent was observed for about 40 minutes after the initial water reaction. I n methanolpyridine, about 1 ml. of K F R would be consumed with this amount of hexamethylcyclotrisiloxane present. The following equilibria are important to consider when using K F R to determine water or silanol in siloxane materials llethoxylation: 4iOH Condensation : &iOH
+
CHsOH F? zSiOCH3 H20
+
+HOSk -SiOSi + H20
Cleavage of cyclic trimer siloxane : [(CH3),SiO]3 2 CH30H F? CH3 CH3 CH3
+
I
I
I
I
CHIO-Si-0-Si-0 CHI
CH3
1
+
-Si-OCH3
I
CHa
H20 (5) and the K F R reactions with water. Trimethylsilyl end blocked dimethylpolysiloxanes generally contain traces to appreciable amounts of water, silanol, hexamethylcyclotrisiloxane. As and the above equations indicate, these components are all reactive with the KFRmethanol system. Of the linear and cyclic siloxane components present, only hexamethglcyclotrisiloxane is sufficiently reactive t o cause interference when methanol solvent is used. Attempts t o determine silanol in the presence of hexaniethylcyclotrisiloxane and water using the conventional K F R titration in methanol diluent would certainly yield high results. Our experience has indicated that silanol results by KFR titration are generally low, however, due VOL. 38, NO. 4, APRIL 1966
0
647
to incomplete methoxylation and possibly some condensation. K a t e r in dimethylpolysilosanes may be determined using KFR and methanol if hexamethylcyclotrisiloxane and silanol are absent. The use of high molecular weight alcohols to eliminate or minimize silanol interference has been reported previously (6). Thus, water in dimethylpolysiloxanes can be measured in the presence Of both hexamethylcyclotrisiloxane and silanol with the same
modifications. The use of high molecular weight alcohol-pyridine diluents has the additional advantage of better sample dissolution because methanol is generally a poor solvent for most siloxane materials.
(3) Gilman, H., RIiller, L. S., J . Am. Chem. 737 2367 ( lgs1). ( 5 ) Grubb, (4) ~ W. ~J , , T., ~ ~Ibzd., l ~l l lH. s76, , A ,3408 l, "ldent,fica(1954). ~ tion and Analysis of Plastics," pp. 256-7, I7anK'ostrand, Princeton, N. J., 1965, L. (6) Smith, lt. c., Kellum, G. E., AN \ CHEM.38, 67 (1966).
~
,
ROBERTC. SMITH
LITERATURE CITED
( l ) D a m m ~ K., Golitz, D., N o h 1V.j Angew. Chem. 76, 273 (1964). ( 2 ) Damm, X., Ko11, W., Kollozd 2. 158, 97 (1958).
GENEE. KELLuhr
Analytical Department Dow Corning Corp. Midland, l l i c h .
Direct Continuous Quantitative Ultrasonic Nebulizer for Flame Photometry and Flame Absorption Spectrophotometry SIR: The use of ultrasonic nebulization for flame photometry and flame absorption spectrophotometry has been reported by several authors (1, 2 , 4). The main advantage of the method is the possibility of obtaining a finer degree of dispersion and a higher concentration of the aerosol. I n the experiments hitherto reported, focusing ultrasonic nebulizers, in which the liquid is dispersed from its surface in a relatively large vessel, have been used. The disadvantage of this procedure is that a relatively large volume of solution is needed, and it is very difficult to obtain a constant and reproducible rate of nebulization. The method could therefore be used only for special important experiments, in which the time and the troubles needed for the adjustments could be disregarded. I n order to make the method feasible in accurate routine analysis, it was desirable to find a n arrangement which allowed a continuously-fed liquid to nebulize instantaneously, and to introduce it quantitatively into the flame. Recently, a nebulizer in which the sample is directly dispersed from a solid vibrating surface ( 3 ) became available through Nivab, Electromedical Department, Birger Jarlsgatan 66, Stockholm, Sweden. The instrument was designed for use in respirators. It is reported to have a maximum nebulization capacity of 0.72 ml. of water per minute with 70% of the particles having a diameter of 0.8-1.0 micron (S). Table I. Increase of Emission with Ultrasonic Nebulizer Rate of
introduction,
Exp.
no. Metal 1 Na 2 Ca 3 4
648
Rlg RIg
Concn., ml./ Factor F mg./liter minute 5 100-5 5000-50 250-10
0.05 0.05 0.05 0.25
ANALYTICAL CHEMISTRY
2.8 4.6 3.5 (2.0)
-
suRF SOLUTION
VIBRATING
~
i l I 1
TRANSDUCER
Figure 1 . Nebulization chamber of ultrasonic nebulizer. Patents pending Solution f e d to the hypodermic needle with a motor-driven syringe
The solution is introduced into the nebulization chamber of this instrument through a vertical metal tube and allowed to drop upon the vibrating surface This method was not suitable for the present purpose, because it did not give a sufficiently continuous and quantitative nebulization. Lowering the metal inlet tube until its opening was situated tightly above the swinging surface, so that no dropping occurred, gave some improvement, but a part of the solution crept upward on the outside of the metal tube and came down again at irregular intervals, which caused an irregular nebulization. Different kinds of plastic tubing were then tried, because it was thought that these elastic tubes could be in direct contact with the swinging surface and thus provide for immediate nebulization of the liquid as it leaves the tube. However, these tubes were soon deformed through contact with the highly-energetic swinging surface. Finally the liquid was introduced from the side through a thin hypodermic needle of 0.5-mm. 0.d. (Figure 1). The needle has an angle of about 10' to 15" t o the vibrating surface, and its opening is situated about 0.3 mm. above the middle of this. This
arrangement gives a n immediate, quantitative, and continuous nebulization of volumes up to 0.3 ml. per minute. With larger volumes some creeping of liquid along the needle can occur. The arrangement was tested with a n Eppendorf flame photometer (Xetheler and Hinz, G.m.b.H., Hamburg, Germany) with a n acetylene-air burner, and the results were compared with those obtained with the conventional pneumatic nebulizer supplied with the instrument. With the same rate of introduction of solution to the flame and the same rates of fuel gas and air for both nebulizers the reading a t the instrument increased with the factors F given in Table I, experiments 1 to 3. Reading, u1tra;onic nebulizer F = - . Reading. pneumatic nebulizer I n experiment 4 the rate of introduction of solution was too high to be realized with the pneumatic nebulizer. The figure obtained with the low rate of introduction \Tas therefore multiplied by 5 in order to obtain a figure to compare. Obviously the emission is relatively loiver with the higher rate of introduction of solution, which is probably
