Microdetermination of Fluorine Elimination of Effect of Chloride WALLACE D. ARMSTRONG, Laboratory of Physiological Chemistry and School of Dentistry, University of Minnesota, Minneapolis, Minn.
W
equal ionic concentration of all ILLARD and Winter (3) The fluorine content of biological and s o l u t i o n s ; (5) the use of a have made the most valusome other materials is so low as to rethorium nitrate solution in 48 per able contribution to fluorine quire the use of a micromethod for its accent alcohol; (6) various comanalysis in recent years. They curate determination. Based on the titrabinations of these procedures. proposed the s e p a r a t i o n of tion of fluorine in aqueous solution with The results obtained on the fluorine from substances which d i s t i l l a t io n and titration, in interfere with its determinathorium nitrate, such a procedure has been alcoholic solution, of 10 to 25 y tion by distillation of hydrodeveloped. The use of aqueous rather of f l u o r i n e , i n t r o d u c e d a s fluosilicic acid and the titration than the usually employed alcoholic solusodium fluoride, averaged 20 y of the evolved fluorine, in an tion gives better results because the equivain excess of the t h e o r e t i c a l alcoholic solution, with standard amount even though the alilence of the thorium nitrate solution for thorium nitrate solution. Bequots contained 2.5 y of fluorine cause t h e i n d i c a t o r which fluorine through the range 0.5 to 10 microand the Hoskins-Ferris buffer they employed combined with grams of fluorine is exact, a situation which was employed. T h a t t h e fluorine, being a lake of zircodoes not obtain when alcoholic solutions cause of the high results was nium and sodium alizarin sulare employed. In aqueous solution the due to some substance collected fonate, and was otherwise inconduring the d i s t i l l a t i o n was concentration of perchloric acid required venient, this writer ( I ) simplishown by experiments in which fied the procedure by using an to influence the results is greater than in no fluorine w a s d i s t i l l e d . a q u e o u s solution of sodium alcoholic solution. The amount of perTwenty-five gamma of fluorine alizarin sulfonate as the indichloric acid evolved is reduced by distilling were a d d e d d i r e c t l y t o t h e cator. This modified procedure hydrofluosjlicic acid in the presence of receiver contents and the results has become the most generally of titration were again 20 y in used fluorine analytical method sodium perchlorate. The fluorine content excess of the theoretical quanand has u n d e r g o n e further of some samples of dental enamel, dentine, tity. The interfering agent was m o d i f i c a t i o n , chiefly at the and inorganic phosphates has been deterfound to be perchloric acid colhands of Hoskins and Ferris (a), mined. A procedure for the removal of lected in t h e r e c e i v e r . The who present an adequate review chloride, when present in interfering titration of dilute solutions of of the literature. perchloric a c i d s h o w e d that There is need for a simple amounts, by the use of silver perchlorate is this acid behaved as fluorine in presented, p r o c e d u r e applicable to the proportion to its concentration. determination of v e r y s m a l l The a m o u n t of p e r c h l o r i c quantities of fluorine, since many acid volatilized varies with the temperature of distillation and naturally occurring substances contain little of this element volume of distillate. At 140" the equivalent of about 2 cc. and large samples must be employed. The manipulation of of 0.1 N acid is collected in 150 cc. of distillate. At 130" large samples is inconvenient and the quantity of extraneous the distillation blank is reduced to the equivalent of 8 y of substances thus introduced causes inaccurate results. The fluorine but the recovery of fluorine is only 70 per cent of the technic herewith presented permits the determination of theoretical quantity. While the distillation blank may be microgram quantities of fluorine with an accuracy fully as maintained reasonably constant under controlled conditions, great, if not greater than, that obtained with the macroits magnitude and variations prevent the accurate determinaprocedure or semi-microprocedure previously published. tion of small quantities of fluorine. It was not possible to obtain satisfactory results by direct adaptation of the previously described methods. The Other expedients which were tried in attempts to eliminate the effect of perchloric acid but which were unsuccessful were adaptation consisted in a reduction in volume of the alcoholic (1) reduction of the perchloric acid with hydrazine hydrate; solution for titration to 2 cc. and in the use of 0.0004 N instead of 0.01 or 0.001 N thorium nitrate solution. The results of (2) its removal by aeration of the acidified solution; (3) redistillation of the concentrated receiver contents from phostitration of 1.25 to 10.0 y of fluorine in 48 per cent alcohol phoric and sulfuric acids; (4) the use of the Hoskins-Ferris fell on a straight line when plotted, but the plotted data of buffer; and (5) the use of a distilling flask provided with a the titration of 0.5 to 1.25 y fell on another line (Figure 1). "spray trap." The smaller amounts of fluorine required more thorium nitrate than that which corresponds to the amounts predicted Principle of Method from the projection of the first-mentioned line. Attempts to It was found that the results of titration of 0.5 to 10 y correct the lack of equivalence of the thorium nitrate solution of fluorine in 1 cc. of aqueous solution fell on a straight line for amounts of fluorine greater and less than 1.25 y by the when plotted (Figure 1) and also that, under this condition, following methods failed: (1) alteration of the amounts of the amount of perchloric acid collected in the distillate did not hydrochloric acid of various dilutions added in excess of that influence the results when the receiver contents were conrequired to adjust the color of the indicator to greenish yellow; centrated to a volume no less than 10 cc. The determina(2) use of nitricwid for adjustment of the acidity of the solution of 5 y or less of fluorine requires that the receiver contents tions for titration; (3) use of the monochloroacetic acid buffer be evaporated to at least 5 cc., in which case .the increased of Hoskins and Ferris; (4) the addition of 1 drop of 1 N concentration of perchloric acid in 1-cc. aliquots causes high sodium chloride to the solution t o produce approximately 384
SEPTEMBER 15, 1936
ANALYTICAL EDITION
results (Tables I and 11). The amount of perchloric acid collected in the distillate can be reduced, it was discovered, by as much as 50 per cent by the introduction of sufficient sodium perchlorate in the distilling flask. This salt, since it elevates the boiling point of the solution, reduces the concentration of acid per unit volume in the contents of the distilling flask and thus diminishes the quantity of acid volatilized. Hoskins and Ferris (8) reported that good results were obtained in the titration of alcoholic solutions only when buffered a t pH 3.5. It was found that the use of the buffer is unnecessary if sufficient care is used to adjust, with dilute hydrochloric acid, the acidity of the solutions before titration (Figure 1). However, because of the convenience gained by employing the buffer, its use has been adopted. While the p H of the monochloroacetic acid-sodium salt buffer is 3.5 in 48 per cent alcohol, its pH is 2.8 in water. For titration in aqueous solution a 2 M formic acid-sodium salt buffer was prepared and used for a short time. The p H of the undiluted buffer, by determination with the glass electrode, was 3.48 and when diluted 40 times was 3.68. One drop of this buffer was added to each 1-cc. aliquot, after preliminary adjustment of the acidity of the solutions to the transition interval of the indicator, but it was found that there was a tendency for amounts of fluorine below 2 y to require too much thorium nitrate. By the use of the Hoskins-Ferris buffer, even though the theoretical and determined pH is 2.8 in aqueous solution, exact equivalence of fluorine for thorium nitrate through the range 0.5 to 10 y of fluorine was observed. Thus the optimum p H for the titration of fluorine in 48 per cent alcoholic and in aqueous solutions vary, although the identical buffer is employed.
385
15
1.4
1 3
Y 1.2
Bs ::: E;
0.9
3 2 0.8 di
8 0.7 0
0
0.6
0.5 0.4 0.3
0.i
Special Reagents and Procedure Prepare a stock solution of sodium fluoride containing 1 mg. of fluorine per cc., and from this solution make others containing 1, 2, 5, and 10 y of fluorine per cc. The sodium fluoride used in the work described in this report was synthesized in this laborator and its purity checked by conversion to sodium sulfate. Adium perchlorate: Neutralize 60 per cent acid with strong sodium hydroxide and eva orate the solution until it boils at 140' C. Collect the crystars which form on chilling the solution and dry them at 110' C. Silver perchlorate: In the dark room add a slight excess of freshly pre ared silver oxide to 40.3 grams of 60 per cent acid. Filter anidilute the solution to 250 cc., giving a solution containing 1 gram of the salt per 5 cc. Store the product in a dark bottle. Buffer: Neutralize, with sodium hydroxide, 50 CC. of 4 M monochloroacetic acid to phenolphthalein, add 50 cc. of 4 M monochloroacetic acid, and dilute to 200 cc. Water: Ordinary distilled water apparently contains a trace of fluoride, which is removed by redistillation from alkali. Titration vessels: Cylindrical vials approximately 4.5 X 1.4 cm, are used. Those in which dentists receive gold foil are ideal and can be obtained in quantity. They should be selected for uniform size. The apparatus employed in the distillation of hydrofluosilicic acid is identical with that used by Willard and Winter (3). If the water employed in the distillation is freshly boiled and transferred to the dropping funnel while warm, the troublesome formation of bubbles in the capillary tube leading from the dropping funnel is prevented and bumping during distillation is reduced. Charge the 50 cc. distilling flask with the sample, 3 glass beads, 0.1 gram of acid-washed and ignited quartz, 10 cc. of water, 5 CC. of 60 per cent perchloric acid, and, if the sample contains 5 y or less of fluorine, 6 grams of sodium perchlorate. Heat the contents of the distilling flaslr to 140' and collect 150 cc. of distillate. This temperature of distillation and volume of distillate were
Total Fluorine Present Y
established by experiment as optimal for uantitative evolution
of fluorine and minimal distillation of percfioric acid. Maintain the receiver contents, during their collection, just alkaline to
phenolphthalein with dilute sodium hydroxide. Evaporate the receiver contents, while alkaline, to a small volume, cool the solution, and make it just acid to the indicator with hydrochloric
25 10 5 a
Fluorine Found Y
24 8 2 4 . 3 2 4 . 9 24 4 2 4 . 6 9 . 4 , i o . z ' 9 8 '10.2 '10 0
5.7,a5.4:a5:lb,a5.8,a4.85,
5.1, 4 . 8 , 5 . 1 2 1.90,2.06,2.02,1.86 Without NaC101. Not included in mean.
Average Fluorine Found
Average Error
Y
%
24.60 9.92
-1.6 -0.8
4.96 1.96
-0.8 -2.0
TABLE11. DISTILLATION BLANK DETERMINATIONS Fluorine Added to Receiver
Fluorine Found
Average Fluorine Found
Y
Y
Y
%
24.5 9.67 5.05 2.00
-2.0 -3.3 +l.O 0.0
25
a
VOL. 8, NO. 5
INDUSTRIAL AND ENGINEERING CHEMISTRY
386
24.1,24.8,24.2, 23.9, 25.1, 2 5 . 2 10 9.60 9.75 6 5.5,d5.4,a5.5,a5.2,4.9 2 2.06, 1 . 9 4 Without NaCIO,. Not included in average.
Average Error
TABLE111. ANALYSISOF PHOSPHATES I
Fluorine Found Double 10 Y fluori?e distillation added to residue
Phosohate
Stngle distillation
%
%
“Cas(PO4)a” Ca HzPOSz NaLPOc Concd. HsPOd
0.631 0.0131 0.0000 0.0015
0.615 0.0134
..
..
Y
of 10 y of fluorine added t o the residue in the distilling flask after distillation of the fluorine originally present in the materials are further proof of the accuracy of the results. An indication of the completeness of the recovery of the fluorine contained in the substance is afforded by the determinations in which extra fluorine was added to the original enamel or dentine. The lower limit of quantity of fluorine for which the method is applicable has not been determined by experiment. Since the aliquots of the 2 y determinations (Table I) contained 0.4 y of fluorine, the lower limit seems to be in the vicinity of 0.4 y . The difficulties attendant on the concentration of the distillate t o 1 or 2 cc. have discouraged attempts to determine fluorine in a total amount of a fractional quantity of a microgram.
10.3 9.6
....
That the accuracy of the above analyses was not due to balancing errors is shown by the results in Table 11,in which determinations the fluorine was not distilled but added directly to the receiver contents. I n this manner all uncertainty as to the quantitative evolution and collection of fluorine was removed, and if any substance had been collected in the distillate which influenced the analyses the results would have departed markedly from the theoretical values. The most severe test of the effect of phosphate, which might be expected to interfere by causing high results, is the analysis for fluorine in inorganic phosphates. In Table I11 are shown the results of analysis of reagent grade materials. The surprisingly high fluorine content of the calcium phosphates was shown to be real by the results presented under the column “double distillation.” The receiver contents were concentrated, while alkaline, and the fluorine was redistilled as hydrofluosilicic acid in the usual manner. If phosphate were collected in the first distillate, the amount collected in the second distillate would have been greatly reduced and the apparent fluorine content of the materials by double distillation would have been correspondingly lower than by single distillation. The data in the last column were obtained on the distillation of and analysis for 10 y of fluorine added to the residue in the distilling flask after volatilization and collection of the fluorine originally present in the material. If any interfering substance, particularly phosphate, distilled in any event, it would also have been collected when the 10 y of fluorine were volatilized and the results of analysis of the receiver contents would have been very much higher than the theoretical quantity. The high fluorine content of the calcium phosphates is probably explained by the insoluble nature of the phosphates and of calcium fluoride. The “tricalcium phosphate” is a t least partially hydroxy-apatite and the fluorine contained in this material is probably combined as fluo-apatite. Being combined in insoluble states, the fluorine was thus not removed from the calcium phosphates by washing during their preparation. Further proof of the accuracy of the method is afforded by the results of analysis of unashed enamel (0.7 per cent protein) and dentine (22 per cent protein) of the same teeth, as shown in Tables IV and V. The sample weights varied from 45 to 70 mg. The results of the multiple distillation determinations one of which (Table IV) was a triple distillation, and the above considerations in regard t o the reduction of phosphate by successive distillations again show that phosphate, or any other material which interferes with the accuracy of the procedure, is not collected in the receiver. The recoveries
Elimination of Effect of Sodium Chloride Any chloride in the sample is distilled as hydrogen chloride and in the preparation of the distillate for analysis is converted to sodium chloride. Hoskins and Ferris reported the permissible upper limit of sodium chloride to be slightly lower than 0.1 M in the titration of 57 y of fluorine in 50 cc. of alcoholic solution. In the present work the interfering concentration of sodium chloride in the titration of 10 to 0.5 y of fluorine in aqueous or alcoholic solution was determined to lie between 0.003 to 0.004 gram per 1 cc. Therefore, if the distillate is made to 10 cc., the sample may contain as much as 0.018 gram of chloride. The materials whose fluorine contents are reported above contained much less than the allowable quantity of chloride. However, many substances, especially biological materials, contain large amounts of this element. The accurate fluorine analysis of these materials requires the removal of the chloride from the distillate. The addition of an amount of sodium chloride equal to that contained in the unknown to the reference solution used in the titration does not correct the error in the determination of microgram quantities of fluorine because the recognition of the end point is difficult in the presence of a large excess of sodium chloride. Furthermore, the chloride content of the distillate must be determined if the correct amount of sodium chloride is to be added to the reference solution. Since silver fluoride is soluble, the possibility of removing chloride from the distillate as the silver salt followed by reTABLE IV. ANALYSISOF SOUND HUMAN DENTAL ENAMEL Fluorine Found Sinele d!etil&tion Sinile distillation with NaClOa One redistillation Two redistillations 10 y F added to residue 5 y, added to original enamel
F
~
TABLE
%
%
0.0163, 0,0154, 0.0164, 0,0160
0.0160
0.0157 0.0162 0.0153: 0.0155, 0.0158 0.0163
0.0160 0.0155 0.0163
Micrograms
Micrograms
1 0 . 0 , 10.6, 9 . 8
10.1
4.7,4.9
4.8
~~
v.
ANALYSISOF SOUND HUMAN DENTINE Fluorine Found
Single distillation One redistillation
a
Average Fluorine Found
10 y F added to residue 10 y F added to original dentine With NaC10a.
Average Fluorine Found
%
%
0,0200, 0.0208, 0,0204, 0 , 0207a 0,0206
0.0204
Microorams
0.0206
Micrograms
9.8
9.8
10.4
10.4
SEPTEMBER 15, 1936
ANALYTIC.AL EDITION
distillation of the fluorine in the filtrate was apparent. Silver nitrate cannot be employed as the precipitant, since the nitrate which replaces the chloride is volatilized in the second distillation and introduces as large an error in the titration of fluorine as does chloride. The use of silver oxide is not practical for the removal of large amounts of chloride, since silver chloride is precipitated only on the surface of the particles. Silver perchlorate was found t o be the ideal precipitant, since sodium perchlorate is formed in a n amount equivalent to the sodium chloride contained in the solution. It was shown above that the presence of sodium perchlorate in the distilling flask is advantageous since it reduces the amount of perchloric acid volatilized. The second distillates, on acidification t o phenolphthalein, were found, in many instances, t o possess a slight color which caused low results in the titration of the fluorine in the aliquots. No procedure was discovered whereby the production of the color could be prevented, nor has its source been entirely identified. However, the color can be removed with activated charcoal, thus permitting the accurate titration of the solution for fluorine. If the sample is soluble, dissolve it in 150 cc. of water and precipitate the chloride as described below. Perform the first distillation of solids as described above, omitting phenolphthalein
from the receiver contents. Maintain the receiver contents alkaline to phenolphthalein paper with 1 N sodium hydroxide, Since the greater part of the hydrogen chloride is evolved when the flask contents have boiled at 140' C. for 5 minutes, use 0.1 N sodium hydroxide during the last half of the distillation. Make the distillate acid with dilute perchloric acid and add, with constant stirring, silver perchlorate solution until the silver chloride coagulates. Continue the addition of silver perchlorate in 0.1-cc. increments until a drop of the solution produces a red color on filter paper which has been impregnated with 5 per cent potassium chromate. Make the solution alkaline to phenolphthalein paper and heat it to boiling to promote coagulation of the precipitate. After cooling the solution, filter it through retentive paper. All operations during the precipitation of the silver chloride should be carried out in subdued light. Evaporate the filtrate to about 10 cc. and filter off any silver oxide which may have formed during the concentration. Evaporate the filtrate to about 5 cc. and transfer it to the distilling
387
flask together with 4 to 6 grams of sodium perchlorate. Perform
the second distillation in the usual manner and maintain the receiver contents alkaline to phenolphthalein. Evaporate the distillate to a small volume, make it acid to the indicator, and dilute the solution to the optimal volume. Add 0.25 ram of activated charcoal per 10 cc. of solution and shake the !ask for about 3 minutes. Filter the solution, using dry apparatus, and titrate 1-cc. aliquots of the filtrate in the usual manner. TABLEVI. DETERMINATION OF 10 MICROGRAMS OF FLUORINE IN PRESENCE OF ONE G R A M SODIUM CHLORIDE Fluorine Found Undecolorized Decolorized Y
Y
9.06 9.3a 8.9 9.3 9.0
9.7: 10.1 9.8 10.2 9.7 9.7 9.8 10.3
.. ..
g.Za,b a
b
..
Fluorine Found Undecolorized Decolorized Y
9.2atb 8.8 9.4 9.0 9.2 9.3 9.0
.4verage 9.1 Av. error, % 9.0
Y
... ...
... ... ... ... ...
9.8 1.0
First distillation omitted. Blank. Fluorine added to distillate.
The results shown in Table VI were obtained when the determinations were begun in the presence of 1gram of sodium chloride. T w o determinations of the fluorine content of the pooled human dental enamel mixed with 1 gram of salt showed the enamel t o contain 0.0162 and 0.0155 per cent of fluorine. Titration of the undecolorized distillate indicated only 0.0151 and 0.0137 per cent of fluorine. The calculated amount of fluorine in the first sample, on a basis of 0.0160 per cent of fluorine as previously determined, was 8.4 y and 8.1 y in the second.
Literature Cited (1) Armatrong, W. D., J.Am. Chem. SOC., 55, 1741-2 (1933). (2) Hoskins, W. M., and Ferris, C. A., IND.ENQ.CHEX,,Anal. Ed., 8, 6-9 (1936). (3) Willard, J. H., and Winter, 0. B., Ibid., 5, 7-10 (1933). July 8, 1936. RECEIVED
A Sensitive Thermoregulator LYLE D. GOODHUE, Bureau of Entomology and Plant Quarantine, Department of Agriculture, Beltsville, Md.
T
HIS regulator is a modification of the type t h a t depends on the change in vapor pressure of a volatile liquid with change in temperature. It is constructed in such a way that no seals are required for the electrical leads and no contact points are present to corrode, and, finally, is made extremely sensitive by employing the principle of the differential manometer. Since the capacity of the regulator is large and the electrical circuit is broken with a rise in temperature, no relay is needed. It can be used directly on a 110-volt line for currents up to 2 amperes. The diagram shows the construction of the regulator. The tubes, A and A', are made large (10 mm.) compared with the heavy-walled tube (3 mm.) a t P where contact is made. The bulb, B , should not be less than 3 cm. in diameter. The total length of the apparatus is about 20 em. For temperatures between 15" and 40" C., the apparatus is completely filled with isopentane, and some inert gas, such as nitrogen, hydrogen, or butane, is introduced through a capillary until half the isopentane in the bulb is displaced. Mercury is poured into the arms until it stands about 10 cm. high. The bulb is gently warmed and pentane is then expelled until the two mercury columns meet a t P , when the regulator is cooled to approximately the temperature of operation. Further adjustment may be made by adding or removing small amounts of mercury. It is convenient to use a small plunger fitted through a small stopper in the end of one arm for the fine adjustment.
The current is conducted to the mercury through No. 18 Nichrome wire. A 1-microfarad condenser is connected across these leads to prevent arcking and the formation of colloidal mercury.
A
~
The sensitivity of the regulator depends on the angle a t which the two mercury columns meet at P. A range of 0.05' C. is sufficient to make and break the contact in the regulator in use in this laboratory. All regulators of this type are subject to fluctuations in barometric pressure. This could be eliminated by using ground-glass stoppers in tubes A and A', but it would complicate the apparatus and require sealed-in leads. The temperature of the bath being regulated does not usuallyvarymore than *0.2' C. from this cause and can easily be adjusted before beginning a n experiment. R E C ~ I V EJune D 17, 1936.
