(3) C. D. Smith, C. G. Schuetz, and R. S. Hodgson, lnd. Eng. Chem., Prod. Res. Dev., 5 , 153 (1966). (4) C. 2. Maehler and A. E. Greenberg, J. Sanit. Eng. Div. SAS, 969 (1968). ( 5 ) F. K. Kawahara and D. G. Ballinger, Ind. Eng. Chem., Prod, Res. Dev., 9, 553 (1970). (6) C. W . Brown, P. F. Lynch, M. Ahmadjian, Environ. Sci. Techno/.,8 , 969 (1974). (7) M. F. Spencer, F. W. Spencer, T. T. Jones, and J. S. Mattson, 1975
Pittsburgh Conference on Analytical Chemistry and Applied Spectros-
copy, March 3-7, 1975, Cleveland, Ohio, 1975, Abstract 407. (8) F. K. Kawahara, J. F. Santner, and E. C. Julian, Anal. Chem., 46, 266 (1974). (9) D. L. Duewer. T. F. Schalzki, and E. R. Kowalski, Anal. Chem.. 47, 1573 (1975). (10) H. A. Clark and P. C. Jurs, Anal. Chem., 47, 374 (1975). (11) "Tentative Method of Test for Preparation of Sample for Identification of Waterborne Oils," D 3326-74T, Book of ASTM Standards, Part 31, American Society for Testing and Materials, Philadelphia, Pa., 1975. (12) W. J. Potts, Jr., "Chemical Infrared Spectroscopy, Volume I: Techniques," John Wiley & Sons, New York, N.Y., 1963, Chap. 5, 6, and 7. (13) Ref. 12, pp 174-177. (14) Ref. 12, p 82.
"Recommended Practice for General Techniques of Infrared Quantitative Analysis," E168-67, 1968 Book of ASTM Standards, Part 30, American Society for Testing and Materials, Philadelphia, Pa., 1968. Ref. 12, pp 165-170. W. J. Dixon, "BMD, Biomedical Computer Programs," University of California Press, Berkeley, Calif., 1971, pp 214a-214t. 1181 , R . A. Fisher, Ann. Eugenics, 1, 179 (1936). , (19) E. D. Beitchman, J. Res. Nat. Bur. Std., Sect. A, 63, 189 (1959). (20) P. G. Campbell and J. R. Wright, lnd. fng. Chem., Prod. Res. Dev., 5 , 319 (1966). (21) F. K. Kawahara, "Recent Developments in the Identification of As-
phalts," Contributed Papers, Marine Pollution Monitoring Symposium and Workshop, National Bureau of Standards, Gaithersburg, Md., May 13-17, 1974.
RECEIVED for review November 3, 1975. Accepted January 5 , 1976. Mention of products or manufacturers is for identification only and does not imply endorsement by the U S . Environmental Protection Agency.
Determination of Fluorine by Titration with Triphenyltin Nitrate and by Steam Distillation under Vacuum George H. Cady Department of Chemistry, University of Washington, Seattle, Wash. 98 795
Fluorine is determined by titrating a solution containing Fwith an acidic 0.1 M ethanolic solution of Ph3SnN03. The titration is followed with a specific ion electrode for fluoride, and the inflection point in the titration graph (at a value of pF of about 5.9) is taken as the end point. A still made from polypropylene tubing is used for steam distillation of fluorine, mostly as hydrofluoric acid, under the dynamic vacuum of an aspirator. The still contains 1 to 2 ml of sulfuric acid and is held at 100 O C by steam. In the absence of interferences, 3 g of water vapor is a sufficient amount of carrier gas.
Because of its very low solubility, triphenyltin fluoride has been used as the basis for gravimetric determination of fluorine (1-3). One of the well recognized problems of the method is coprecipitation of triphenyltin chloride etc. along with the fluoride. The specific ion electrode for fluoride ( 4 ) now offers an opportunity to titrate solutions of fluorides with solutions of triphenyltin compounds and to determine the end point potentiometrically by the well known method first reported by Lingane ( 5 ) .Such a titration gives a well defined end-point signal, because of the very low solubility of Ph3SnF; however, coprecipitation of other triphenyltin compounds causes error unless suitable precautions are taken. A procedure has been developed which substantially avoids coprecipitation. The method is accurate and has the advantage of being less subject to interferences than titration by thorium or lanthanum nitrates. I t has the disadvantage of being slow and complicated. In fluorine analysis, an operation used for many years was distillation of the element as SiF4 from hot dry sulfuric acid containing silica ( 6 ) .This method gave low results, because part of the fluorine failed to distill. Willard and Winter (7) introduced the use of steam distillation from sulfuric or perchloric acid a t about 135 O C and found it possible
to distill all of the fluorine. Many modifications of their procedure have been made, and many forms of apparatus have been described. It still is the case, however, that the distillation, as commonly used, consumes much time and gives such a large volume of solution that it frequently is desirable to evaporate most of the water before proceeding with the analysis. I t is also recognized that silica in gelatinous form retards the distillation and may prevent the complete distillation of fluorine. A procedure which involves removal of most of the silica before using steam distillation has been described by Grimaldi, Ingram, and Cuttitta (8).The distillation has been modified by Singer and Armstrong (9) who used a combination of nitrogen and water vapor as the carrier gas and by so doing reduced the volume of distillate to about 20 ml. Additional modifications of this procedure were made by Wade and Yamamura (10). I t has now been found desirable to use steam distillation under the dynamic vacuum of an aspirator. The method is rapid and requires only a few grams of water vapor as carrier gas. Problems caused by silica are greatly reduced by using apparatus of plastic (mostly polypropylene).
EXPERIMENTAL Titration. Preparation
of Triphenyltin Nitrate Reagent. A 3.003-g sample of a commercial grade of triphenyltin chloride was dissolved in 25 ml of warm 95% ethanol to which 0.5 ml of 6 M HN03 had been added. This was then mixed with 10 ml of water containing 1.455 g of AgN03. The mixture was filtered under vacuum to obtain a clear solution which gave no test for C1-. Nine ml of 5 M NH40H was added to the clear solution. This produced a precipitate of Ph3SnOH which then was removed by filtration and throughly washed with water. A small portion of the solid was found to dissolve completely in 1 ml of 95% ethanol plus a drop of 6 M HN03. Addition of a little C1- then gave no precipitate of AgC1. The solid Ph3SnOH was transferred to a 60-ml flask having an 8-mm 0.d. neck and was dried to constant weight under vacuum a t 70 to 90 O C . The weight obtained was 2.5151 g (yield = 88% of theoretical). Twenty ml of 95% ethanol followed by 1.64 ml of 6 M "03 (Theor. vol. = 1.14 ml) was added to the flask. When the vessel was warmed, all of the solid dissolved. The flask was then
ANALYTICAL CHEMISTRY, VOL. 48, NO. 4 , APRIL 1 9 7 6
655
filled with 95% ethanol, giving 49.1551 g of solution. (Concentration of Ph3SnNO3: 0.1187 mol/l.). The flask was closed with a serum bottle stopper through which portions of liquid could be removed into a syringe. Alternate Preparation. Rather than drying the PhsSnOH, a time-consuming procedure, the moist solid was dissolved in ethanol plus nitric acid. The concentration of the solution was then determined by titration of a known amount of sodium fluoride. Various methods other than the above have been used to prepare Ph3SnOH. None of the writer's attempts to prepare pure solid PhsSnNO3 have been successful. Apparatus. Titrations were carried out in 50- or 75-ml Pyrex glass or polyethylene beakers which had no pour spouts. The beakers could be closed by stoppers. Three electrodes were used: 1) a reference electrode, 2) an Orion Research specific ion electrode for fluoride, model 94-09, 3) a glass electrode for measuring pH. No special precautions were taken in the care of these electrodes except to avoid mechanical damage. The glass electrode was protected with a rubber sleeve. All three were normally present in the solution being titrated. and electrodes 2 and 3 could be interchanged and used along with 1 connected to a Leeds and Northrup 7400-A2 series pH meter. This meter gave stable readings within the pH range 0 to 14 with the smallest scale divisions being 0.1 pH unit. For measuring fluoride ion activities, the meter was set to M NaF read 7.0 (on the pH scale) when the electrodes were in a t room temperature (21 "C). The meter was then used to measure values of pF ( 5 ) during a titration (pF = -log(F-)). A meter reading of 6.0 corresponded to pF = 4.0 while a reading of 4.0 corresponded to pF = 6.0. All values of pF and pH given in this article are direct meter readings, no correction being made for ionic strength or for the presence of ethanol. The meter was set to read p H or pF in water solutions but was then used, without change, in solutions containing ethanol. This means that the values reported for solutions containing ethanol are only meter readings and are not true measures of the activities of fluoride or hydrogen ions. A 1-ml disposable plastic syringe graduated with 0.01-ml divisions and equipped with a 25-gauge (0.513-mm 0.d.) needle was used in place of a buret. During the course of a titration, the volume of solution removed from the syringe was read from time to time with the help of a lens. This volume was only approximate, however, and was not used to establish the amount of solution added at the end point. This amount was determined by weighing the syringe at the beginning of the titration and a t the end point. T o avoid error caused by electrical charge, the syringe was screened within a folded piece of aluminum foil when being weighed. Procedure. The sample to be titrated contains 1 to 2 mg of fluoride in about 3.0 g of solution. I t is held in a beaker of 50- to 75-ml capacity without a pour spout and placed in contact with the assembly of three electrodes. One ml of 95% ethanol is added. Dilute nitric acid (about 0.3 M) is added drop by drop until the indicated pH falls to 2.0. Four drops of 1 M AgN03 is added. This gives a precipitate of AgCl with chloride ion coming from the reference electrode. If a copious precipitate forms due to C1-, Br-, I-, or other anion in the solution. enough silver nitrate is added to complete the precipitation of this ion. The volume of solution at this point should be about 5 ml. The specific ion electrode for fluoride is now used, and readings of pF and volume of standard PhsSnNO3 solution are taken, the added increments of solution being about 0.1 ml. The syringe is weighed at the start of the titration. As increments are added, their size is decreased after pF has risen to about 4.3. When pF is above 4.7 the additions are only one to two drops. Between additions, the beaker is shaken back and forth until the meter reading becomes about constant. The reading is taken while shaking is in progress. When the meter reading for p F has risen to 4.9 to 5.0, the titration is stopped and the meter reading for pH is observed. The value is likely to be about 1.5 to 1.6. Solid sodium carbonate is now added in tiny increments, with shaking of the beaker, until the pH rises to about 2.0. The beaker is then removed from the electrodes and closed with a single hole rubber stopper which carries a 40-cm length of 6-mm i.d. glass tubing to serve as an air-cooled reflux condenser. The beaker is then heated by steam for 5 min while any liquid which evaporates is returned by reflux. This prevents loss of fluorine as SiF4 or H F vapor. The beaker is then cooled to room temperature and shaken in a manner which splashes the liquid against the stopper to wash back hydrofluoric acid which may have distilled. The stopper is then removed and wiped across the lip of the beaker to remove liquid. No wash-water is used. The beaker is returned to the electrode assembly, and titration is continued by adding Ph3SnNO3 solution 656
ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
.OtOl
slip fitting
-
Polypropylene tublng 3 2 m m od 3mm wall 3 2 c m length. The vessel IS locketed and m a y be heated by steam from bottom 10 top
CondenseP
length
Figure 1. Apparatus for steam distillation of fluorine under vacuum
Table I. Titration of N a F with Acidic PhsSnNO3 in Ethanol
NaF, mg
Moles PhgSnNO3/ Moles NaF at end-pt
Final vol. % ethanol
1 2 3 4 5 6 7 8 9
3.878 3.890 3.886 3.880 1.893 7.737 3.876 3.932 3.866
1.011 1.004 0.998 0.999 1.004 1.003 1.005 1.008 1.000
12 20 28 36 24 26 28 28 28
10
3.888
1.068
28
11 12
3.891 3.842
1.007 1.005
28 28
13 14
3.861 3.865
1.016 0.997
28 28
15 16 17
3.873 3.873 3.911
0.993 1.005 1.002
28 28 28
18
3.870
1.001
28
19 20 21 22 23 24 25 26 27
3.871 3.892 3.862 3.893 3.862 1.841 7.784 3.872 3.858
1.016 1.025 1.019 1.011 1.007 1.023 1.015 1.007 0.999
28 12 28 36 44 24 26 28 28
Run
Other special conditions
0.200 g NaN03 0.100 g NaZS04 0.005 g NaC1; 0.04 g AgNG 0.005 g NaC1; no AgN03 0.090 g acetic acid 0.039 g NaZCZ04; 0.11 g AgN03 0.031 g sod. citrate 0.029 g KC104; 0.031 g NaC103 0.009 g K2CrzO.i Pyrex beaker Pyrex beaker; 0.100 g silicic acid Pyrex beaker; 0.100 g 0.028 g KHiPO4
At 50 "C At 60 "C
one drop a t a time (with intervening shaking of the beaker) until the end point has been passed (probably at a p F reading of about 5.8 to 5.9). After each 1-drop addition (about 0.0036 g) in the region of the end point, the syringe is weighed. The end point is considered to be the point of maximum slope in the titration. This procedure involves little or no coprecipitation with Ph3SnF. One
may consider that the molar ratio, PhsSnNO3 to F-, in the titration is unity. In place of the syringe, it would be possible to use any other volumetric device capable of giving correct volumes to 0.004-ml accuracy. Steam Distillation under Vacuum. Apparatus. Figure 1 shows the apparatus. The 25-ml glass flask serves as an evaporator to produce steam. When the flask is in a bath a t about 40 O C , a 3-ml portion of water evaporates in about 15 to 20 min. The 3-mm Teflon spaghetti tubing may be replaced by polypropylene or high density polyethylene. The 32-mm polypropylene still is jacketed by glass tubing, which serves as a jacket for heating the still from bottom to top by steam. This jacket extends up to the tapered slip fitting and is capped with a piece of aluminum foil folded to keep steam in contact with the top of the still. The stirring bar is rotated by a magnetic stirrer placed below the still. Ice plus water held in a Dewar vessel is used to cool the condenser from bottom nearly t o the rubber stopper. All joints are tight so that no substantial leakage of air occurs. Under this condition, the distillate in the condenser contains essentially all of the fluorine which leaves the still. A protective trap for water is placed between the still and the aspirator. Because of its relatively low softening point, ordinary “Low density” polyethylene cannot be used for construction of a still to operate at 100 O C . Procedure. The sample for analysis is placed in the bottom of the still by adding the material through the spaghetti tubing. If the sample contains more than about 3 g of water, the solution is made alkaline and evaporated to about 1-ml volume. One to 2 ml of concentrated sulfuric acid is then added. If part of the sample has spattered onto the wall of‘the still, the tube is rotated or tilted to make sure that the whole sample has mixed with the acid. The apparatus is assembled as shown in Figure 1. The stirrer, aspirator, and steam are then turned on, and the distillation is continued until the desired quantity of water has evaporated from the flask.
RESULTS Titration. Table I presents experimental results for several titrations. Notes regarding these runs follow. 1) T h e PhsSnNOs used in these titrations was t h e 0.1187 M solution described in the text. I t was approximately 0.05 M in nitric acid. 2) All runs through No. 19 were made by the standard procedure, the solutions being heated for 5 min before completing t h e titrations. Runs 20 through 25 were like 1-6 except that the solutions were not heated. 3) Runs 26 and 27 were made in a covered vessel a t the temperatiires indicated until nearly all of the reagent had been added. The electrodes were not used in the hot solutions. The solutions were then cooled to room temperature and the titrations were completed using the electrodes. 4) All runs except 16-18 were made in a polyethylene beaker. 5 ) For each run, the total weight of solution before starting the addition of Ph3SnNOS was 5.0 g. 6) The end point for each titration was taken t o be the point of maximum rate of change of p F with volume, and the amount of PhsSnNOs was calculated from the weight of reagent added. For all of the above titrations except No. 22 and 23, the observed values of p F a t the end point fell within the range 5.7 t o 6.0, with the maximum rate of change for all runs except 17, 22, and 23 being within t h e range 0.21 t o 0.40 p F units per drop (0.0036 g) of titrating agent. The presence of silicic acid (No. 17) or a high proportion of ethanol (No. 22 and 23) gave a maximum slope less than 0.20 p F unit per drop. Most of the titrations in Table I required about 0.65 g of reagent. There was, of course, some uncertainty about judging the location of the end point. The writer feels that the range of his uncertainty may have been as much as 0.006 g of reagent. This means that for most of the runs in Table I, a true ratio, moles of Ph3SnNOs per mole of N a F of 1.000 might have been observed t o fall within the range 0.995 and 1.005. A datum within this range is considered t o represent 1 : l stoichiometry and t h e substantial absence of
coprecipitation. With this in mind, the data of Table I may be taken t o indicate the following. 1) The presence of ethanol reduces coprecipitation (Compare runs 1-4, also runs 20-23). 2) Heating the solution just before the last few drops of the titration reduces coprecipitation (compare runs 1-6 with the corresponding runs in the series 20-25). 3) Carrying out the greater part of the titration a t about 60 OC (run 27) would have about the same beneficial effect as 5-min heating. This procedure is not recommended, because of possible thermal damage to the specific ion electrode. 4) Coprecipitation of chloride or oxalate is prevented by precipitation of these ions as silver salts (compare runs 9, 10, and 12). 5) Coprecipitation of other substances tested in Table I is not large but appears to occur to some extent for S042-or HS04-, citrate ion, and H2PO4- ion and possibly acetic acid. 6) The slightly low result for K2Cr20; might be caused by a trace of Cr3+ ion. 7) T h e beaker may be either of glass or polyethylene. 8) The amount of titrating agent required is proportional to the amount of fluoride being titrated (compare runs 3, 5, and 6; also compare runs 21, 24, and 25). The following additional observations were made. 1) When the proportion of ethanol is as high as 36 or 44% by volume, the value of p F a t the end point is lowered considerably and the rate of change of p F at the end point is reduced. 2) While silicic or boric acid does not prevent one from finding the correct amount of fluoride, either of these substances lowers the observed activity of F- and reduces the rate of change in p F per drop of titrant at the end point. Distillation under Vacuum. Data regarding several distillations are given in Table 11. Attention is called to the fact that four of the runs did not follow the usual procedure in all respects. In one run, the volume of H2S04 was 10 ml. In the last three runs, the acid employed was phosphoric, not sulfuric acid. The amount of fluorine which distilled was determined by titration with triphenyltin nitrate. When elements which form stable fluoride complexes were absent, a 3-g quantity of steam from the 25-ml evaporator flask usually was sufficient. A relatively large proportion of A13+ appeared to interfere enough so that about 6 g of steam was required. The data show that interference by silica (derived from NaaSiO3) was serious. T o obtain a substantially complete distillation with 9 g or less of steam, it appeared t h a t the ratio of fluorine t o active silica should be about 4/1 or more. Two types of interference were caused by boric acid. The distillation was retarded, and the distillate contained HBF4 which was difficult to titrate for its fluorine content. When the volume of sulfuric acid was increased from 1.5 t o 10 ml, the rate of distillation was retarded somewhat. A similar effect was found for phosphoric acid, and it was observed t h a t distillation of HF from phosphoric acid was slower than from sulfuric acid. Interference by silica was less serious for phosphoric acid than for sulfuric acid. Possibly this effect was caused by a reaction of silica with phosphoric acid.
DISCUSSION Titration. Explanation of Features of the Experimental Technique. Triphenyltin salts which are available commercially are the chloride and acetate. The materials obtained by the writer were not pure and were not suitable for direct use in the volumetric procedure. When an alcoholic solution of the chloride was used, a substantial amount of coprecipitation occurred, even in the presence of silver nitrate. Triphenyltin acetate is soluble in ethanol containing nitric acid. One might expect such a solution t o be satisfactory as the titrating agent, because the acetate would nearly all be present as acetic acid. When the author ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
657
Table 11. Steam Distillation of Fluorine Materials initially in still H20 from 25-ml flask, Fluoride in distillate, Fluoride recovery, g moles x 10-5 %
Other
1.6 1.1 1.1
9.202 95.72 9.347
None None None
1 1.5
1.1 1.6
9.288 9.252
&(S04)3 Alz(S04)s
15 30
1.5 1.5 1.5
9.216 9.290 9.262 9.194
Fez(S04)3 Crz(S04)s
1
1.1 1.6 1.8 1.3
NazSiOs
36 6.5 63 1.7
1 1
1.6 2.1
9.469 9.218
NaZSiOs Na2SiOs
4.6 9.2
1 1
2.1 3.7
95.35 96.09
NazSi03 NazSiOs
9.2 24
1
11.5
95.32
NazSiOs
96
1.5
Ti02
3 3 3 6 3 3 6 3 3 3 3 9 3 3 6 9 12 3 3 9 3 6 3 6 3 3 6 3 6
83
1.1
9.247
2" 10"
1.1 1.1
9.214 9.283
None None
2a
1.1
9.300
NazSiOs
9.2
9.25 96.0 9.14 9.29 9.25 9.00 9.21 9.22 9.24 9.20 8.79 9.15 8.49 8.43 8.63 8.74 8.82 95.4 95.0 95.5 71.7 74.6 8.40 8.88 9.01 7.18 8.39 9.00 9.21
100.5 100.3 97.8 99.4 99.6 97.3 99.6 100.0 99.5 99.3 95.6 99.5 89.7 91.4 93.6 94.8 95.7 100.0 98.9 99.4 75.2 78.3 90.8 96.0 97.8 77.3 90.3 96.8 99.0
The acid used for these runs was 85% HsP04. Sulfuric acid was absent.
used a solution of this sort, he found the molar ratio of PhsSnAc to NaF a t the end point to be about 1.025, even when the steam heating step was included. Apparently some coprecipitation occurred. A commercial source for analytical grade triphenyltin hydroxide would help by making it easy to prepare solutions of PhsSnNOs of known concentrations. Nitric acid is present in both the titrating agent and the solution of fluoride, to prevent coprecipitation of PhsSnOH with PhsSnF. If one titrates a solution of NaF with an alcoholic solution of PhsSnCl (with no nitric acid added), the solution becomes acidic, the p H falling to about 3.0, because of hydrolysis to form PhsSnOH. When the titration with PhsSnNOs is nearly complete, the pH is raised to 2.0 by adding Na2COs. The increase in pH causes an increased change in p F per drop of titrant a t the end point. By having the same pH and % alcohol for all runs, the end points come a t about the same value of pF. These factors help one to decide when the end point has been reached. The observed value of pF at the end point is decreased by increasing the p H (because F- activity increases) and by increasing the proportion of ethanol in the solution. Examples of Titrations. Figure 2 gives examples of titrations made with a solution of PhsSnCl in 95% ethanol. In each case the initial volume of aqueous NaF solution was 5.0 ml. Before titration the following additions were made: 1)no additions; 2 ) 0.45 ml of 0.3 M "03 (gave a pH reading of 1.98 for the solution); 3) 0.45 ml of 0.3 M "03 and 0.3 ml of 1 M AgN03; 4) same as 3-the solution was heated by steam for 5 min. a t the point indicated. The difference between curves 1 and 2 shows the marked effect of hy658
Moles x
Substance
1 1 10
a
H20, ml
NaF, moles X 10-5
HzS04, ml
ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
I
"
"
"
I
I
3 h \i 4t Figure 2. Titration of 7.92 X Ph3SnCI in 95 % ethanol
mol of NaF with 0.0946 M
drolysis giving PhsSnOH. The difference between curves 2 and 3 shows that silver nitrate reduces coprecipitation of PhaSnCl. Curve 4 shows that the combined effects of acidity, silver ion, and heating when near the end point do not eliminate all coprecipitation of PhsSnC1. Even with an ethanol content of about 30% by volume, 1:l stoichiometry was not obtained. Titrations with the acidic solution of PhsSnNOs described in the section on experimental technique are shown in Figures 3 and 4. Each curve represents the final part of the titration. For each titration in Figure 3, the initial solution contained the NaF plus 0.64 ml of 0.3 M "03 plus 1 ml of 95% ethanol plus 0.15 ml of 1 M AgN03 plus enough water to give 5.0 g of solution. The titration was carried nearly to the end point. Solid NaZC03 was then added to give the desired pH. After the solution had been heated by steam for 5
t
'
\
I
\
"
'
I
c 1
I
'
'
~
"
"
'
i
5.0 -
PF
PF -
-
6.0
6.0
0.96
0.98
1.02 mole NoF
1.00
Moles O,SnNO,per
Figure 3. Titration of 9.23 X mol of NaF with Ph3SnN03 reagent described in the section on experimental technique. The influence of acidity is shown
min, the pH reading (at 21 "C) had the value indicated below. Each curve represents the titration from this point onward: (1) pH reading = 2.50; (2) pH reading = 1.94; (3) pH reading = 1.55. For each titration in Figure 4, the initial solution contained the NaF in 1.09 g water plus 0.15 ml of 1 M AgN03 plus the following additions: (1)0.50 ml of 0.3 M HNO,; ( 2 ) 0.64 ml of 0.3 M H N 0 3 plus 1.0 ml of 95% ethanol plus 2.3 g of water; (3) 0.81 ml of 0.3 M H N 0 3 plus 2.45 ml of 95% ethanol plus 6.0 g of water. Each titration was carried nearly to the end point. Solid Na2C03 was then added to give a pH reading close to 2.00. After the solution had been heated by steam for 5 min, it was cooled to room temperature and the titration was continued as shown in the figure. At the end point, each solution contained approximately 28% by volume ethanol. Volumes at the end point were: (1) 2.5 ml., (2) 5.9 ml.; (3) 11.2 ml. Figure 3 shows that increasing the acidity reduces the slope of the titration curve and increases the value of p F a t the inflection point. Figure 4 shows the influence of volume of the solution. From the curves, it is obvious that small volume is desirable, because it helps greatly in deciding when the end pcint has been reached. The initial volume should be small and the use of wash-water should be avoided during the titration. When the slope of the curve is high, there is little doubt about the location of the inflection point. When the slope is low, the curve may be almost a straight line, making it hard to decide just where the inflection occurs. Interferences. Serious interference in this titration is caused by cations (examples are A13+ and Fe3+) which form stable complex ions with fluoride. Such cations must not be present in harmful amounts. The existence of this type of interference can be recognized by the shape of the titration curve. As the titration passes through the p F range 5.5 to 6.0, the slope of the curve is considerably smaller than one normally finds. A typical slope for strong interference may be only about one tenth of that usually observed. When either H2SiFs or the product from H3B03 H F is present, the slope is smaller than normal but still large enough to allow one to find the correct end point. These acids dissociate rapidly giving F- (or HF) which can be titrated. By contrast, HBF4 is difficult to titrate, because i t decomposes only slowly to give fluoride. If one carries the titration to p F = 5.0 and then heats the solution, fluoride is released and can later be titrated. By using several heat
+
7 . 0 " " ' ' ' i J 0.96 0.98 1.00 1.02 1.04 Moles hSnN03per mole NoF mol of NaF with the Ph3SnN03 Figure 4. Titration of 9.23 X reagent described in the section on experimental technique. The influence of volume is shown
treatments of this type, it is possible to titrate all of the fluoride in HBF4 or BF4-. Many salts of triphenyltin ion have low solubilities. The anions would interfere, if the pH were about 6. Most of these are anions of weak acids. At pH 2.0, such ions are converted to the acids themselves and do not interfere. The ions C1-, Br-, I-, and C2Od2- are precipitated as silver salts and, therefore, do not interfere in the titration. Some anions, for example HzP04- or S042-, if present in high concentration, may coprecipitate to a small extent with Ph3SnF (see Table I). Monochloroacetate ion interferes. Presumably the anions of some other rather strong carboxylic acids also can interfere. Permanganate ion is reduced by the solution and must not be present. Presumably, there are other strong oxidizing agents which would also be reduced. When K2Cr207 is used, little or no reduction occurs (See Table I). Comparison w i t h Other Methods. Some chemists now determine fluorine by using the specific ion electrode to measure the activity of F-. This is a rapid method, and good, if one is not interested in high accuracy. Since the instrument responds in proportion to log (F-), the method is less precise than linear methods such as titration or colorimetry. Probably the best known procedure is the Willard and Winter titration of fluoride with a solution of thorium nitrate in the presence of sodium alizarinsulfonate or other indicator (7, 11). This procedure has been improved by Lingane, who used the specific ion electrode for fluoride to establish the end point (5). Lingane also studied other reagents and concluded that lanthanum nitrate was superior to thorium nitrate for titration of fluoride. In the writer's opinion, neither La3+ nor Th4+ is truly an excellent reagent for determining fluorine. Both ThF4 and LaF3 are more soluble than they should be for good analytical methods. There are also many serious interferences. Cations, such as A13+ and Fe3+, which form stable complex ions with fluoride interfere. Anions, including oxalate, phosphate, sulfate, acetate, and other carboxylate ions interfere by forming insoluble salts, by coprecipitation or by formation of complex ions with La3+ or Th4+. A factor favorable for triphenyltin nitrate is the shape of the titration curve at the equivalence point. For ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
659
Ph3SnNO3, the inflection point appears to be the equivalence point. For La(N03)3, the equivalence point comes shortly after the inflection point ( 5 ) .This complicates the problem of selecting the end point, as one inspects a titration curve. The problem is enhanced by high acidity. In the opinion of the writer, a practical way to select the end point for a La(N03) titration is to choose the point a t which the slope of the curve starts to decrease. The principal advantage of titration with PhsSnNO3 over La(N03)3 or Th(N03)4 is that there are fewer interferences. The principal advantage of L a ( N 0 3 ) ~or Th(N03)4 over PhsSnNO3 is that less time is required, because coprecipitation is a less serious problem. The colorimetric method of Belcher and West (12) also has the advantage of being rapid and the disadvantage of suffering from more interferences than titration by triphenyltin nitrate. Steam Distillation under Vacuum. Many of the previous modifications in distillation procedure are the result of interference by silica either from the sample or from the glass still. Fluorine becomes bound in a form which does not distill. The writer is convinced that the glass still itself can be a serious interference, particularly in the old procedure of distilling from hot concentrated sulfuric acid, When the still is of polypropylene, the fluorine may distill largely as hydrofluoric acid. A little reactive silica can be present, because it is converted to a volatile material, probably SiF4, and distills with the fluorine. Samples which are high in silica must be treated to remove silica (8) before distilling the fluorine. Persons who use steam distillation under vacuum when dealing with minerals containing silica may find it advantageous to use either phosphoric acid or a mixture of phosphoric with sulfuric acid. They may also find it necessary to use substantially more than 3 ml of water to obtain complete distillation of the fluorine. Grimaldi et al. (8) have used mixtures of phosphoric and perchloric acids in their glass still. The writer has avoided perchloric acid, because his still was of organic matter.
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Interference by metals such as A13+ and Fe3+ is much less serious than by silica. This problem has been studied by Grimaldi ( 8 ) ,and by Wade and Yamamura (10) for distillation from phosphoric acid. An advantage of distillation of fluorine as hydrofluoric acid rather than silicon tetrafluoride is that the subsequent titration of fluoride is improved. The presence of silicon reduces the concentration of fluoride ion and has a tendency to obscure the end point when titrating. The obscuring effect is less serious for titration with triphenyltin nitrate than for the nitrate of lanthanum or thorium. As one titrates fluorosilicic acid or fluorosilicate ion, the solution increases markedly in acidity. An adjustment of p H shortly before the end point is reached is desirable. If the amount of fluorine collected in the condenser is so small that titration of the whole sample is desirable, a procedure should be followed which involves little or no washwater. The procedure of the writer was to pour the sample into a beaker titrate nearly to the end point. The resulting solution was then used to rinse the condenser, including the tapered slip fitting and connecting tubing. After continuing the titration nearly to the end point, the condenser system was rinsed again. The titration was then carried to completion.
LITERATURE CITED (1) N Allen and N. H. Furman, J. Am. Chem. SOC., 54, 4625 (1932). (2) H. Ballezo and H. Schiffner, Fresenius' 2. Anal. Chem, 152, 3 (1956). (3) J. Tscholakowa. Fresenius' 2. Anal. Cbem., 266, 288 (1973). (4) M. S.Frant and J W. Ross, Jr., Science, 154, 1553 (1966). (5) J. J. Lingane, Anal. Chem., 39, 881 (1967); ibid., 40, 935 (1968). (6) F. Wohier, Pogg. Ann., 48, 87 (1839); see also W. F. Hillebrand and G. D.F. Lundell, "Applied Inorganic Analysis", John Wiley and Sons, New York, 1929, p 599. (7) H. H. Willard and D.E. Winter, Anal. Cbem., 5 , 7 (1933). (8) F. S. Grimaldi, B. Ingram, and F. Cutlitla, Anal. Cbem,, 27, 918 (1955). (9) L. Singer and W. D.Armstrong, Anal. Cbem., 31, 105 (1959). (10) M. A. Wade and S . S. Yamamura, Anal. Chem., 37, 1276 (1965). (11) H. H. Willard and C. A. Horton, Anal. Chem., 22, 1190 (1950). (12) R. Belcher and T. W. West, Talanta, 8, 853 (1961); ibid., 8, 863 (1961).
RECEIVEDfor review July 14,1975. Accepted December 29, 1975.
