Recoveries of Fluoride by Microdiff usion: Heat and Diffusion Time SIR: Recently interest has increased in the use of microdiffusion techniques for isolating small amounts of fluoride prior to analysis. The basic difference in technique between investigators centers around the use of heat to effect complete microdiffusion recovery of the fluoride. Hall (4) has reported recovery data as a function of temperature showing that 60" C. for 24 hours is required t o obtain 99 to 100% recovery of 2 to 5 pg. of fluoride. Stegemann and Jung (7) used 50" C. and 16 hours for recovery of 1 to 20 pg. of fluoride although no recovery data are given. Recently Singer and Armstrong (6) have reported a microdiffusion method using 20-hour diffusions a t 55" C.; no data on range of fluoride or recovery are given. Rowley and Farrah ( 5 ) reported quantitative recoveries of 1 to 30 pg. of fluoride following diffusions a t 50' C. for 16 hours. Following the lead of Hall, Wharton (8) has shown that diffusions a t 60" C. for 24 hours recover all of the fluoride in a sample containing up to a t least 4 pg. of fluoride. Frere (S), however, reports that diffusions at room temperature for 24 hours are satisfactory. His data show that 93 to 95% of the fluoride in the 1- to 50pg. range is recovered under these diffusion conditions. It is evident that there is a lack of accord in diffusion conditions for quantitative recovery of fluoride. This communication emphasizes the requirement of heat to recover quantitatively small amounts of fluoride.
Table I. Microdiffusion of Fluoride (as NaF) at Room Temperature and at 60" C. (Static, 24 hours)
Sample
volume,*
Recovery, ml. Taken Found % Room 0.3 0.50 0.33 66.6 1.00 0.88 87.5 tempera2.00 1.90 95.0 ture 3.00 2.91 97.0 1.0 2.00 1.82 91.0 3.00 2.72 90.7 60" C. 0.3 0.50 0.50 100.0 1.00 1.00 100.0 2.00 2.01 100.5 3.00 3.00 100.0 1.0 0.50 0.54 108.0 1.00 0.99 99.0 1.50 1.47 98.0 2.00 1.97 98.5 3.00 2.99 99.7 Plus 0.5 ml. of concd. (62%) HC104 and 3 drops 0.1% wetting agent.
406
F-9 Pg.
ANALYTICAL CHEMISTRY
Effect of
Table 11. Microdiffusion of Fluoride (as NaF) as a Function of Diffusion Time under Static and Continuously Agitated Conditions (60"C., 0.3 ml.)
Diffusion time, F- taken, hours rg. 4 1.00 2.00 3.00 8 1.oo 2.00 3.00 16 0.50 1.00 2.00 3.00 24 0.50
i.oo
2.00 3.00
Static diffusions" F- Found, Recovery,' r g.
70
...
...
...
...
1.07, 0.88 1.95, 1.91 2.80, 2.83
97.5 96.5 93.8
1.02; '1.oo
ioi :o
...
2.02, 2.00 2.98, 2.95 0.50, 0.50 1.02. 1.00 2.01, 2.00 3.00, 3.01 I
...
100.5 98.8 100.0
101.0 100.5 100.2
Agitated diffusions" F- Found, Recovery, rg. 1.01, 0.83
1.85, 1.83 2.83, 2.83 0.95, 0.95 1.96, 1.96 2.88, 2.85 0 50 1.05, 0.98, 0.98
1.98, 2.00, 1.99 3.01, 3.00, 2.94 0.53 1.02, 0.99, 1.01 2.01, 2.01, 2.08 2.98, 3.02, 3.00
07 10
92.0 92.0 94.3 95.0 98.0
95.4 100.0 100.3
99.5 99.5 106.0 100.7 101.5 100.0
Over-all standard deviation is 3=0.030. Repeatability of the Beckinnn B spectrophotometer is 0.5% T or 0.006 A. at 0.4 A. (I), equivalent to 0.1 p g . of F-.
The results of static diffusions of fluoride from K'aF a t room temperature and a t 60" C. are reported in Table I. A comparison of static and continuously agitated diffusions, both a t 60" C., is reported in Table I1 as a part of the diffusion time study. The agitation was provided by mounting the heated desiccator on a platform rocking a t a rate of two cycles per minute. All diffusions and fluoride analyses were carried out as previously reported (8), except for diffusion variations under investigation. In addition, dental enamel samples which must be dissolved by the acid in the diffusion cell before complete diffusion of the fluoride can take place, were run by static diffusion at room temperature and 60" C. The average fluoride content found from triplicate analyses a t room temperature n-as 79.9 p.p.m. The actual fluoride content of 91.6 i: 3.3 p.p.m. was established from 17 determinations using either static or agitated diffusions at 60" C. (Yo difference in absolute fluoride content was found between static and agitated diffusions; the standard deviation for fluoride was decreased slightly for samples of dental enamel using continuous agitation.) The average fluoride recovered a t room temperature was 1.20 pg. from 15-mg. samples. Diffusions a t 60' C. averaged 1.38 pg. of fluoride from the same size sample. All diffusion results were compared to a standard curve prepared from undiffused fluoride (as NaF) with alkali (0.3 ml. of 1.3N NaOH) added as in the diffusion procedure. The addition
of wetting agent to the undiffused fluoride standards was erroneously reported (8) and is not recommended. Certain conclusions become apparent from this examination. For the quantitative recovery of fluoride, heating is required. For amounts of fluoride exceeding 5 to 10 pg., the residual, undiffused fluoride from room temperature diffusions represents only about 5% of the total fluoride and could be ignored. For less than 5 pg. of fluoride, it seems imperative that heat be applied, for the residual undiffused fluoride from room temperature diffusions is fairly constant (approximately 0.15 pg. in this
