Interaction of Fluoride Ions and Ground Glass R. C. SPECHTI Engineering and lndostrial Experiment Station, University
of Florida, Gainesville, Fla.
A study of loss of fluoride ions, when solutions of fluorides are kept or concentrated in containers of borosilicate or soft glass, showed that fluoride ions are either adsorbed on the surface of the glass or bound on the lattice. The interacted fluorine ions are partially released when the glass is treated with dilute acid solutions containing ferric ion, and are completely released by dilute sodium hydroxide solutions.
the thorium nitrate !vas standardized each day that titrations were made, and blank determinations xere made with each standardization and analysis. K h e n the repeated standardization of the thorium nitrate and the repeated blank determinations were made before titration of samples, it was possible to check the fluorine content within &0.02 p.p.m. Based on the results obtained in the determination of fluoride by the Negregian-hIaier modification for use with electrophotometric instruments, in which a precision of 0.02 p.p.m. can readily be obtained ( 2 ) , it is believed that the importance of repeated standardization depends more upon the adjustment of the standard solution to the temperature a t which subsequent analyses are made and the training of the eye t o the exact color end point than upon any exact change in the normality of the thorium nitrate solution.
T
H E determination of fluoride ions has become significant in medical, biological, and other research. An accurate method is necessary when samples contain only a fern parts per million of fluoride ions, as significant errors are introduced when small amounts of these ions are lost in the analysis. Therefore, this study was made in order t o determine the loss that may be encountered when solutions containing fluoride ions are kept or concentrated in glass vessels under controlled conditions, and the nature of that loss.
XIETHOD
A dilute solution of fluoride of known concentration v-as passed over samples of both ground soft glass and borosilicate glass, folloiving which the solutions were analyzed to determine the amount of fluoride remaining. When there was no longer any removal of fluoride ions from a fresh solution, the ground gla& was treated with ferric sulfate solution, and then with a dilute sodium hydroxide solution, to remove the adsorbed fluoride ions. .4 material balance is shown in Table I.
LOSS O F FLUORIDE I O Y S DURIYG EVAPOR4TION
I n the determination of the fluorine content of dilute solutions, a n evaporation may be necessary, either to decrease the volume and facilitate separation of fluorides, or to bring the concentration within the range of the colorimetric or volumetric method that is to be used. Reynolds and Hill ( 7 ) were probably the first t o report loss of small amounts of fluoride ions during the evaporation of slightly alkaline solutions. A solution alkaline to phenolphthalein was evaporated in glassware; it x a s noted that, when an original volume of 150 ml. was evaporated to two thirds of this quantity, there n-as a loss of 1%, if more than 1 mg. of fluoride was present. Relatively greater losses were found by McClure ( 5 ) in samples containing 10 to 50 y of fluoride, when 150-ml. solutions were evaporated to less than 10 ml. in borosilicate glass, the loss was between 1 and 24Cc, in porcelain 8 to 307,; in platinum the change varied between 7% loss and 4y0 gain. Rinck ( 8 ) , noting similar errors due to evaporation in borosilicate vessels, recommended the use of platinum dishes or Jena glassn-are and evaporations a t a pH maintained between 6 and 8. llatuszak and Brown (6) attributed losses t o sodium silicate dissolved during the evaporation. Fellenberg ( 3 ) reported that evaporation of water samples of p H less than 7 causes no loss of small amounts of fluoride, if the hardness content is low. Losses due to adsorption on the glassmire were noted when the hardness x a s greater than 14 grains per gallon of calcium carbonate. I n the analysis of over 100 samples of water containing 3.0 p.p.m. or less of fluorides, calculated as fluorine, the author has found no loss in fluoride ions, if the evaporation is carried out in the same flask in which the fluoride is steam-distilled as hydrofluosilicic acid. These results indicated that fluorides were not volatilized from alkaline solution and t h a t such reported losses may actually be due to mechanical loss or interaction, including adsorption, with the container used. The method of the American K a t e r Korks -issociation ( 1 ) was used for concentrating and separating the fluoride as hydrofluosilicic acid. When sufficient sample \vas available, 250 ml. was used for concentration and titration. T h e fluoride content of the original stock solution % a s checked each time it was used, Present address, T h e American Agricultural Chemical Co., Pierce, Fla.
4PPAR4TUS
The apparatus consisted of a piece of borosilicate glass tubing, 33 mm. in inside diameter and TOO mm. long, to one end of xhich was fused a borosilicate glass stopcock. Separate samples of soft and borosilicate glass were ground t o pass 80-mesh and be retained on 100-mesh screens. The ground soft glass was made from &pint acid reagent bottles and the ground borosilicate glass was furnished by a manufacturer of this type of glass. The ground samples were digested in hot dilute sulfuric acid to remove any iron that might be present and then washed thoroughly with distilled v a t e r and dried a t 100" c. During the washing operation, any fine suspended particles were decanted. The dried samples were placed in separate borosilicate columns and supported on a bed of borosilicate glass wool which had previously been put in place in each column. I n one column 202 grams of ground soft glass nere placed, and in another 142 grams of ground borosilicate glass. The ground glasses filled about one third the total height of the columns, the soft glass occupying proportionally more space than the borosilicate glass. The remainder of the space was used as a reservoir for the solutions.
Table I.
%laterial Balance of Fluoride Adsorption and Release F, 1 0 - 1 Rlg. Soft glass
Adsorbed a n d interacted Unaccounted for gain Total Released by ferric s u l f a t e solution Released by sodium hydroxide solution Total
123 3 126 27 99 128
Borosilicate glass 28 1 29
27
2 29
Assuming that the average diameter of the particles is equal to the opening of a 100-mesh screen-Le., 147 microns-and that the particles are spherical, it can be shown that the specific sur6 face is equal to - ( 4 ) square meters per gram, where p is density Pd
IO15
ANALYTICAL CHEMISTRY
1016 (assumed to be 2.23 grams per cc ) and d is average diameter in microns. From these assumptions, the specific surface of the ground glass was calculated to be 0.0183 square meter per gram. A standard stock solution was prepared by diluting approximately 0.04S sodium fluoride solution in 0 . 1 s sulfuric acid and adjusting the solution t o contain approximately 3.0 p.p.m. (3.0 my. per liter) of fluoride, calculated as fluorine. The stock solution was analyzed before each addition to the apparatus, to ensure that there had been no loss in flrioiine content. PROCEDURE
The columns, with the glass in place, weie filled v i t h the stock solution and throughout the test the level of the solutions was maintained above that of the glasses. The stock solution was allowed to flow through the columns a t the rate of approximately 2 ml. per hour, the rate being adjusted by means of the stopcocks on the apparatus. The tests were performed a t room temperature, which vaiied between 25" and 32" C. The p H of the stock solution varied between 1.15 and 1.20, and there was no change in pH as the solution passed through the column. Samples of around 300 to 400 ml. xere collected and analyzed. The stock through the columns until fluoride solution was allowed to f l o ~ ions were no longer removed b) the ground glass (Table 11).
Table 11. Interaction of Fluoride Ions and Glass Particles F, Mg./Liter I n orig. In soln. sample Soft Glasi Particles ~
Sample. 311.
F Renioyed from Soln. (Calcd.). 10-1 AIg 13 . 3
456 450 427 334 433 388 ,380
13.2 12.8
DI S C U S S I O S
An attempt %vasmade to keep the rate of flow of the solutions through the columns a t approximatel>- 2.0 nil. per hour; however, this was not always accomplished. .I record of time of contact of the solutions with the ground glass \vas kept and it ~ v n snoted that with an increase in flow rttte there n-;is a decreascb in the rate of interaction (see footnote. Table 11). From the data obtained it was calculated that. on 511 equal weight basis, thr ground borosilicate giuss had interacted with a masiniuin amoiint of fliioride ions in approximately threr triiths the time required by the ground soft glass. The relative umount of solution requiretl for saturation was in appi,oximatel). the same ratio. .I comparison of the results given i u Tables I1 and I11 reveals thitt practically all of the fluoride ions were removed from thix borosilicate glass by the ferric sulfate solution, whereas approximately the same quantity of solution removed only 225; of that interacted with the soft glass. The remaining i87c of the fluoride ions on the soft glass was removed by treatment \vith dilute sodium hydroxide solution (see Table IV). The unaccounted for gain of fluorine shown in Tahlc I is attributed t o an error in trchniquee of handling and measuring s:tmples.
9.5
11.6 8 1 10 6 4 8 4.2
RIB :qiin . -
5.7 2 3 5 2 '2.9 2 6 2 0 2.4 3.0 1.3'L 1.0" 1 .3 0 4" 1 3 1 4 1.2
325 300 394 385 320 411 403 375
Total
turned to the column. The amount of fluoride ions removed by the ferric sulfate solution is shop-n in Table 111. The ground glasses which had been returned to the columns were then treated with a 0.05'V sodium hydroside solution in the same manner as with the ferric sulfate solution. Samples were collected and analyzed (Table IV).
440 476 515 347 510 480 310 402 456 300 400 __ 11,253
lr, 1 8 0 0
0 0 __ 123.3 Borosilicate Glass Particles 14 7 h
0 143 193 188 0 171 0 184 0 243 0 399 0 425 0 454 0 330 0 300 0 285 __ 18 Total 3.315 a Faster r a t e througli column. b Interacted in prpliminary test made before starting this series
7
::
~
1 0
Table 111. Sample,
311.
Removal of Fluoride Ions from Glass Ferric Sulfate Solution F in Kecoi.ered Soln 3la./Liter
E Removed from Glass
(Calod.). 10-1 .\Ip
Soft, Glass 449 400 345 290 398 342 300
Total
2 , li 1.4 1 .o 1.0
11.7 3 .0 3.4 2 9
0.1
2.8 0.8 0 .3 __ 27,:
::>;
2624 448 400 345 590 412 343 300
Borosilicate Glass 3.1 1.1 1 1 0.4 0 . .54 0 . 2.i
13.9 4.4 3.8 L.2 2.4 0.8 0 5 __ '27.0
0 IIi
T o t a l 2j38 .
~
~
~~~
~
-
I,
8 3 0 0 0 0 0
Following the adsorption test the columns were drained; the glass was removed from each column, air dried, and then returned to the column. I n order to remove some of the fluoride ions as an iron complex ( 9 ) , a solution containing approximately 30 p . p m of ferric ion in a 0.05h' sulfuric acid solution was added to the columns, and removed through the stopcocks a t the rate of approximately 2 ml. per hour. Samples were collected in the same manner as in the adsorption tests and analyzed for fluorine content. When no more fluoride ions were removed from the glass by the acidified ferric sulfate solution, each column was drained; the ground glass was removed, air-dried, and then re-
Assuming that the fluoride ions were :idsorbed on the siiriace of the ground glasses, the amount adsorbed per square meter of surface was calculated as 1.07 mg. for the borosilicate glass and 3.33 mg. for the soft glass. Based on the assumption of geometriral similarity and an average particle diameter of 0.14i micron for the ground glasses, it can he calciilated that approximately 29 fluoride ions xould be adsorbed per square Angstrom of soft glass surface. I n the case of the borosilicate glass, there would be 9 per A . 2 . With an ionic radilis of 1.36 -1.for the fluoride ion with a charge of - 1, there would be five layers of the ions on the surface of the soft glass; on the borosilicate, 1.6 layers. There could not be a la)-er of fluoride ions to these depths because of the electrostatic charge that would he developed; consequently, the fluoride ions n-odd have to be adsorbed on the surface or bound on the lattice in some other manner. The calculation of the surface area is, of course, subject to considerable error. T h e exact nature of the interaction could possibly be determined if
1017
V O L U M E 2 8 , NO. 6, J U N E 1 9 5 6 the exact surfacbc aiea were known. I n any case, it is dernonstrated that there is an interaction of the fluoride ions and the ground glasses.
Table I\.. Removal of Fluoride Ions from Glass 13: Sodium €13 droxide Solution 111.
F in Recovered Soln.. 1Ig./Liter
son ~
20 1 4.2 .3 5 1 :3 0.2 0 0
Sample,
f Reinoved iron, Glass (caicCi.), 1 0 - 1 irg.
the plastic containers are used, this disparity is no longer encountered. QCKNOWLEDG31ENT
The author wishes to acknon-ledge the suggestions made b y J. H. Simons, the analysis of over 200 samples by IT. R. Smith, student assistant, the samples of ground borosilicate glass s u p plied by Corning Glass IVorks: and the sponrorship of the Lngineering arid Industrial Experiment Station.
Soft Glns-
Total
. .
1831
18.7 10.4 2 .0 0 4 0 .0 __
98.7
Boro-ilicate Glass
loo
Total
200 __
IiOO
0.5 0.0
L I l E R . A T U R E CITED
lis. li
2 0
0 0 __
1.0
Disparity in rcsnlts o h t a i n d in the determination of the flnorine content of w\-atrr stored in glass has resulted in a change to the rise of polyethj.lcnc containers in the author's laboratory for collection and storage of fluorine-containing n-aters. When
Kater Work3 .i.jsoc.. C'oiniiiittee Heport. J . &4n1. Water Works Assoc. 33. 1975-8 (1941). ( 2 ) .hi, Kater Works Assoc~..Subrominittee 8-11, C'onimittee Keport, J a n . 2, 1953, p. 13. (3) Fellenberg, T. van, Potterat. I[,,J l i t f . G'ehiefe Lebem771. H u g . 4 0 , 146-66 (1949). (4) Green. Henry. J . Fra7iklitz Inst. 192, 637-06 (1921,; 204, 713-29
(1)
.%in.
(1927). ( 5 ) SIcClure, F. J., ISD. ESG.( ' H E M , , .kx.u,. E D . 11, 1i1-3 (1939). (6) ALatuszak, 11.P., Brown. D . I
