Radiometric Determination of Inorganic Fluoride E. I. ONSTOTT and W. P. ELLIS' University of California, Los Alamos Scientific Laboratory, Los Alamos,
europium has a suitable half life of 12.4years ( 4 ) . However, this lanthanon is SO expensive that it was decided that the titration would be more useful if a much cheaper juxtalanthanon, samarium, was used as the main precipitant, with europium as the carrier-tracer.
Sodium fluoride and sodium hexafluosilicate are titrated with samarium ion containing europium carrier-tracer. The end point is determined by measuring the excess of titrant by a radiometric procedure. Under optimum conditions, 20 to 30 mg. of fluoride can be determined with an error of less than l7~.Dilution of the fluoride bv a factor of 5 results in an error generally less than 376. Errors in the determination of sodiuni hexafluosilicate are greater than with sodium fluoride. Colloid flocculation is usually incomplete. The colloid remaining in suspension leads to low results. Acetic acid helps considerably in flocculating the colloid, but many other flocculents cause coprecipitation and high results. The titration can also be done on a micro scale. Fluoride in the amount of 38 y is determined with generally less than 29'~ error.
EXPERIMENTAL
Chemicals. Sodium fluoride, Baker and Adamson, was used for most of the determinations. It was heated a t 500" to 600" C. for several hours immediately before weighing. Pure sodium fluoride was prepared by the procedure given by Reynolds and Hill ( E ) , and was used for most of the microtitrations. Sodium hexafluosilicate, Baker's analyzed, was dried at 110" C. for about 15 hours prior to use. Samarium oxide, containing 1.5% europium oxide, was obtained from the Soci6t6 de Produits Chimique des Terres Rares, Paris, France Stock solutions were made by dissolving the freshly ignited oxide (ignited a t 950' C. for about 5 hours) in dilute nitric acid and adding the tracer and acetic acid as required. The amount of europium added in the tracer was insignificant. Tracer. Pure europium oxide from Johnson, Pvlatthey, and Co., New York, was subjected to neutron irradiation in the Los Alamos water boiler. The specific activity was about 25 mc. per gram. Equipment. An International clinical centrifuge was used. For microwork, a Misco air-driven centrifuge (Microchemical Specialties Co., Berkeley, Calif.) was used. Centrifuge tubes of about 0.5-ml. capacity were made of capillary tubing. Counting was done with a scintillation anticoincidence counter made by Group CMR-7 of this laboratory. The samples were surrounded by about 4 inches of lead during counting. Time of
T
FIE fluoride method of Willsrd and Kinter ( 7 )has been used n i t h a great deal of success. Iierently Popov and Knudson ( 5 ) reported a gravimetric method for fluoride in which lanthanum ion Tr-as used as the precipitant. A radiometric-volumetric method is presented here which employs samarium ion as the precipitant and europium carrier-tracer to detect the end point A y-emitting tracer is ideal for tracer IT ork, because solutions can he handled and counted directly in test tubes. High counting efficiency (of the order of 57,) Tr-ith a modern scintillation anticoincidence counter is attained. Counting errors can be reduced to less than 1%. A radiometric procedure (3)offers some advantage over other instrumental procedures. The specific activity of the tracer can be changed between extreme limits, so that microdeterminations can be carried out with accuracy determined primarily by the microanalytical equipment and the chemistry of the titration. Amperometric and conductometric titrations and similar methods which depend on properties of an ion or ions in solution are disadvantageous because the property being measured to detect the end point usually cannot be varied between wide limits, These methods require auxiliary equipment in the solution; hence, there is a physical limitation to the amount of solution employed. Smaller volumes could probably be used with a radiometric procedure. The radiometric procedure also has disadvantages. It is limited to methods where there is an actual physical separation of the unknoxn from the solution phase, such as a precipitation reaction. For counting, the solution phase must be well shielded from the separated phase, as has been done by Langer (b), or an aliquot of the solution may be removed and counted apart from the vicinity of the titration ( 3 ) . The counting of an aliquot removed from solution was the procedure used in this work. Every point on the titration curve, however, requires a separate sample, unless the aliquot is returned each time. Hence, for a good end point, three or four times as much unknoir-n is required as with other procedures. I n a routine titration, two points probably would suffice to determine the
E
9 m'
'0
.
X
E
-
0
I
z 0
+ 103 1
0 v)
z >
c
E c
::
50
-
1
metric composition ( I , 6) with fluoride ion. Some of the lanthanons also have isotopes which have excellent characteristics as tracers. Of the lanthanons which are high energy y-emitters,
* Present address, University of
N. M.
I
I
I
I
I
Figure 1. Titration of 20 ml. of 0.04296M sodium fluoride containing 0.05iM citric acid with 0.07528M rare earth nitrate in 1M acetic acid
Chicago, Chicago, Ill.
Line represents ideal extrapolation
393
ANALYTICAL CHEMISTRY
394 Table I.
Determination of Sodium Fluoride
(Titrant, 0.1004M rare earth nitrate, pH about 2, samples not heated before centrifugation) Fluoride Centrifuge, Taken, Found, Error, Time, Hr. Molarity mg. mg. % 28.1 2 0.093 28.3 -0.6 26.7 2 0,093 28.3 -5.7 26.6 3'/1 0.093 28.3 -6.0 27.5 3 0.093 28.3 -3.0
Heating of the samples prior to centrifugation was tried aB a method of flocculating the colloid. The procedure was similar to that used for samples without heating. A 20-ml. sample wa8 taken, except for sodium fluoride which was O.O993M, when a 15ml. sample was taken. Titrant was added to each of the four Sam les to exceed the end point, and the samples were stirred. Wit{ the stirring rod still in each sample, they were heated in a water bath for 15 minutes a t 60' to 80" C., stirred again, and allowed to cool to room temperature. The samples were centrifuged and aliquots taken as before.
counting was 5 or 10 minutes and the total count was sufficiently high to reduce errors to less than 1%. A Gilmont microburet was used for delivery of the titrant in the microtitrations. The 0.1-ml. delivery tube was replaced with one of a larger diameter so that the buret could be more easily filled. This tube was made in the laboratory glass shop. Necessity for the control of pH in fluoride determinations has been stressed by Popov and Knudson ( 5 ) . I n this work the p H was kept between 1.8 and 3.5. Acetic acid was used to control the pH and also to serve as a colloid flocculent. Measurements of pH were made with a Model G Beckman instrument. RESULTS AND DISCUSSION
Titration of Sodium Fluoride. Although the microdetermination was done first, the results on the conventional titration are presented first to show the difficulties likely to be encountered. Popov and Knudson ( 5 )found that colloidal lanthanum fluoride is usually formed when precipitating fluoride ion with lanthanum ion. They brought down the colloid by using an acetic acid flocculent and by heating the solutions. The same difficulty was encountered in this work. Fifteen milliliters of neutral sodium fluoride solution was added to each of four 40-ml. centrifuge tubes, followed by 10 ml. of 1M acetic acid. Increasing amounts of titrant were added to the fluorid? aliquots, so that the end point was exceeded in each case. Each solution was stirred with a stirring rod, which was removed without washing. The samples were placed in the centrifuge and revolved a t about 3000 r.p.m. to separate the fluoride precipitate. A 10-ml. aliquot of the supernatant liquid was removed, placed in a counting test tube, and counted. The activity was corrected for dilution by the titrant. A plot was made of activity in the supernatant solution us. volume of titrant, and the end point was determined from the extrapolated line. Table I shows the results of some titrations which were done without heating. The results are all low, showing that extended centrifugation does not remove all of the precipitated rare earth fluoride. Other flocculents were tried in an effort to effect coagulation and separation of the precipitate rapidly at room temperature. The colloid should have a positive charge on the excess rare earth ion side of the end point; hence, the anions of citric acid, I-malic acid, and d-tartaric acid were tried. Citric acid flocculated the precipitate nicely but also caused coprecipitation of rare earth citrate, as shown in Figure 1. The line represents the ideal extrapolation as calculated from the tracer activity in the titrant. The points are all low past the end point, and reflect the decrease in activity in the solution caused by the removal of rare earth ion by coprecipitation. Because malic acid complesed the rare earth ion and prevented precipitation with some samples, it was ineffective as a flocculent. Tartaric acid did not completely flocculate the colloid. W70rk on finding a flocculent was discontinued because of the coprecipitation problem. However, for a practical method, the coprecipitation may not be objectionable. Figure 1 shows that an extrapolation in the immediate vicinity of the end point (which is within 0.5 ml.) should give a result close to the stoichiometric end point. On the other hand, an extrapolation from the data taken far from the end point gives a result which is much in error.
Figure 2. Titration of 15 ml. of 0.0993M sodium fluoride with 0.1061M rare earth nitrate in 1M acetic acid Line represents ideal extrapolation
Table I1 shows that reasonable results were obtained with the more concentrated fluoride solutions. Errors in the determinations were less than 1%,and deviations from the ideal titration plot were minor, as can be seen by comparing the line in Figure 2 to the data. With 0.01M sodium fluoride, a greater error was encountered in the determinations, even after heating the samples according t o the method just described. Table I11 gives the results, and Figure 3 shows in detail what causes low results. There is considerable deviation from ideality on the upper portion of the plot.
Table 11. Determination of Sodium Fluoride (Titrant, 0.1061M rare earth nitrate in 1M acetic acid, pH 2.33, samples heated before centrifugation) Fluoride PH qf Taken, Found, Error, Counting Sample' Molarity mg. mg. % 2.8 0.0558 21.09 21.18 +0.5 2.8 0.0555 21.09 21.23 +0.7 2.9 0.0872 21.74 21.68 -0.3 28.30 28.428 +0.4 2.5 0.0993 a Value for sample containing largest amount of titrant. Sample containing least amount of titrant would have pH aB much a8 0.2 unit higher. b Plotted in Figure 2.
395
V O L U M E 28, NO. 3, M A R C H 1 9 5 6 Table 111.
Determination of Sodium Fluoride
(Titrant, 0.0212M rare earth nitrate in 1 M acetic acid, pH 2.35, samples heated before centrifugation) Fluoride P H qf Error, Counting Taken, Found, Samplea Molarity mg. mg. 7% i1.0 4.26 0.0111 4.22 3.1 -2.3 4.11 0.0111 4.22 3.1 -2.0 4.14 4.22 0.0111 3.0 -2.0 4.14 4.22 3.0 0 0111 +1.2 4.26 4.21 0 . 0 1108 3.1 -2.3 4.11 4.21 0 . 0 1108 3.0 -2.0 4.13 4.21 0.01108 3.0 -2.3 4.11 4.21 0.01108 3.0 -2.1 4.12b 4.21 0.01108 2.9 -4.6 4.03 4.22 0 0111 3.3e -2.4 4.12 4.22 0.0111 3.4c 4.23 f0.3 4.22 0.0111 3.4c +0.3 4.22 4.23 0.0111 3.4c Av. 1.9 Value for sample containing the largest amount of titrant. Sample containing least amount of titrant would have p H as much as 0.2 unit higher. b Plotted,in Figure 3 C pH of titrant, 2.72.
The limiting factor in this particular method appears to be the colloid flocculation problem, rather than the solubility limitation which is present in other similar titrations. Determination of Sodium Hexafluosilicate. The method developed for sodium fluoride was tried on sodium hexafluouilicate in order to find if the titration could be performed on the distillate of a Willard-Winter ( 7 ) separation. Results of these titrations are given in Table IV; the procedure used was the same as that used previously for samples which were heated before centrifuging. 911 of the results are low, showing that separation of the colloid is incomplete.
Table IV. P H of Counting Sample"
Determination of Sodium Hexafluosilicate (Samples heated) Hexafliiosilicate Taken, Molarity
mg.
Found
Error,
mg.
%
Titrant, 0.2248.11 rare earth nitrate in 1 M acetic acid, pH 1.92 62,80 57.5 - 8.4 2 1 0.01669 Titrant, 0.0212.11 rare earth nitrate in 1 M acetic acid, pH 2.32 2.7 0.00213 8.00 7.60 - ?.! 2.7 0.00213 8 00 7.56 J.J 2.7 0 00213 8.00 7.74 - 3.2 2 6 0.00213 8.00 7.18 -10.2 a Value for sample containing largest amount of titrant. Sample containing least amount of titrant would hare pH as much as 0.2 unit higher,
ilcetic acid was added to the samples in an effort to facilitate flocculation of the colloid. Results of these tests are given in Table V. The same procedure was used as for other heated samples, except that the volume of sample was different, and acetic acid was added to the sodium hexafluosilicate. Acetic acid helps in the flocculation, but deviations from ideality are greater than with sodium fluoride. Figure 4 shows the effect of the acid in flocculating the colloid, especially near the end point. With the larger amount of acetic acid present, the amount of unseparated colloid is a t a minimum near the end point, but increases on either side. The shape of the plot (square
Table V.
Figure 3. Titration of 20 ml. of 0.01108M sodium fluoride with 0.0212-Vf rare earth nitrate containing 1M acetic acid Solid l i n e represents ideal extrapolation; dashed l i n e gives experimental e n d p o i n t
However, the data follow a straight line which has a different slope from the ideal line. The amount of colloidal fluoride not separated from the solution appears to be directly proportional to the amount of excess rare earth ion present. Thus, a t the end point (the isoelectric point of the colloid) the experimental line extrapolates close to the ideal line and the error is not large, though appreciable. The hump in the plot of the data has significance and is approximately reproducible. Here a negatively charged colloid is formed, which also diminishes in quantity as the isoelectric point is approached. The amount of colloid left in solution as shown by Figure 3 is not detected when the amount of fluoride is 5 to 10 times greater. This is perhaps the reason that more concentrated fluoride gives better results. More dilute solutions were not investigated because of anticipated difficulties of colloid separation, which were actually shown experimentally in the microtitrations.
Determination of Sodium Hexafluosilicate
(Titrant, 0.1124M rare earth nitrate in 0.5M acetic acid, pH2.28, aceticacid added for flocculation) pH of Molarity of Hexafluosilicate Counting Bcetic Acid in Taken, Found, Error, Sample' Hexafluosilicate Molarity mg. mg. % 2.4 0 0.008345 31.40 30.0 -4.4 2.0 0.8 0,01391 31.40 30.3 -3.4b 0.01113 31.40 2.1 1.3 31.1 -1.1 2.1 1.3 0.01113 31.40 30.7 -2.4~ 2.1 2.0 0,008345 3 1 . 4 0 30.7 -2.4 a Value for sample containing largest amount of titrant. Sample oontaining least amount of titrant would hare pH as much as 0.2 unit higher. b Plotted as circles in Figure 4. c Plotted as squares in Figure 4.
Table VI.
hlicrodetermination of Sodium Fluoride
(Titrant, rare earth nitrate in excess nitric acid to give pH approximately 0.3 ) Rare Earth Fluoride Nitrate, Taken, Found, Error, hlolarity hlolarity Y Y % 0.1000 376 374 0.500 -0 5 0.1000 376 376 0 0.500 0.1000 376 367 -2.3 0.500 374 0.1000 376 i O . 8 0.500 0.1000 376 382 0,500 C1.5 288 i l 0 0.250 0.0500 285' 190 0.0500 190 0 0.250 0.0100 38.0 38.5 0.0202 +1.2 38.0 37 8 -0.5 0.0100 0.0202 38.0 38.6 f1.6b 0.0100 0,0202 Av. 0 94 a 300 pl. of fluoride sample taken. b Plotted in Figure 5.
ANALYTICAL CHEMISTRY
396 symbols) in Figure 4 is similar to that in Figure 3 for sodium fluoride. It appears that silica formed when the fluoride is precipitated by rare earth ion tends to stabilize the colloid. Acetic acid helps in reducing the stability of the colloid, but not sufficiently to allow an accurate determination of the end point. Microdetermination of Sodium Fluoride. Some difficulty was encountered in the initial work in flocculating the colloid. Acetic proved to be the acid, recommended by Popov and Knudson (6), best flocculent tried, although moderate concentrations of dichromate also helped. Pyrophosphate, ferro-, and ferricyanide, when tried as flocculents, coprecipitated as the rare earth salt on the excess rare earth side of the end point.
0
I
I I
2
3
-VOLUME
I
I
4
5
Table VII.
Microdetermination of Sodium Fluoride (Effect of cation impurities)
Impurity Fe--' Fe-+?
Mole Impurity per Mole Fluoride
Fluoride Taken, Found,
0.2 0.2 0.2 0.23 0.23
Ni+Cr+'+ Cr-+-
Y
Y
190 190 190 190 190
172 168 2200 94 94
Error,
%
- 10 - 11 +- =505 -
50
smaller centrifuge. The line in Figure 5 is drawn through the data to give the end point rather than to depict the ideal titration line. Some deviation from the ideal line might be expected a t points relatively far removed from the end point. Extending the method to include the titration of 0.001M fluoride was not successful. In one titration in which 0.001M fluoride was titrated with 0.0025M pure europium, the plot went through the origin, showing that no europium fluoride was separated from solution. In a duplicate experiment, a rough end point 63% low was obtained. From the expected solubility of europium fluoride [obtained by comparison with gadolinium fluoride ( I ) ] , it was predicted that 0.003M fluoride could be determined with less than 2% error, From the plot in Figure 5, it could be expected that a fairly accurate determination could be made even with tenfold dilution. However, the limiting factor appears to be colloid flocculation and separation, not the solubility. Effect of Impurities on Microtitration. The effect of some common cation impurities was investigated in conjunction with the microprocedure. In Table VI1 are results obtained with iron, nickel, and chromium. Iron interfered by complexing part of the fluoride and giving a low result. The data for the titra-
I
6
7
OF TITRANT- rnl
Figure 4. Effect of acetic acid in the determination of sodium hexafluosilicate Circles represent titration of 12 ml. of 0.01391M sodium hexafluosilicate containing 0.8M acetic acid with 0.1124M rare earth nitrate containing 0.5M acetio acid. Squares represent titration of 15 ml. of 0.01113M d u m hexafluosilicate containing 1.3M acetic acid with same titrant. Line represents ideal extrapolation
In Table VI are data obtained on several titrations which were done according to the following procedure: Titrant was added in increasing amounts with a Gilmont m i c m buret to a separate centrifuge tube, followed by neutral sodium fluoride from a calibrated 200-pl. pipet, Flocculent, consisting of a p roximately 0.024M sodium acetate in 1M acetic acid, waa addefin the amount of 100 or 105 pl. The samples were stirred with a platinum wire, and centrifuged for 30 to 40 minutes at about 20,000 r.p.m. A 2 0 0 4 aliquot of the supernatant liquid from each sample was added to a counting tube, diluted to 10 ml., then counted 5 or 10 minutes. The activity was corrected for dilution by the titrant, then a plot was made of the activity in the supernatant solution us. volume of titrant added. The end point was determined from the extrapolated line. I t may be noted here that the flocculent could have been added as a solution in the titrant rather than separately. Also, the order of adding the reagents should not influence the end point. Heating of the samples was not necessary, because the centrifuge removed most of the precipitate. By comparing Figure 5 t o Figure 3, it is seen that more of the colloid is removed by the
IO
20
30
40
I
1
50
60
VOLUME OF TITRANT -,Ad,
Figure 5. Microtitration of 200 pl. of 0.01M sodium fluoride with 0.0202iM rare earth nitrate Line is actual extrapolation, not ideal line
V O L U M E 28, NO. 3, M A R C H 1 9 5 6
397
tions followed a straight line which was parallel to the line eypected if no iron were present. Hence, the amount of fluoride complexed by iron remained constant and independent of the amount of rare earth ion present. Kickel also interfered, but not in a predictable fashion. The data on nickel are given to show that it interferes, but they are not necessarily a guide for estimating its effect. The data for the titration deviate from a straight line and give a poor end point. Therefore, nickel should be eliminated prior to the determination of fluoride. Cliromium appears t o interfere in a predictable fashion. Data in Table VI1 show that about half of the fluoride is removed as a nontitratable species. The data show that for each mole of chromium present, there are about 2 moles of fluoride removed as a complex ion; hence, the interfering species can be depicted as difluochromium(II1) ion, which Wilson and Taube have described (8). According to their data, this ion should be formed in high yield under the conditions of the experiment described here. As with iron, the titration lines were parallel to the line expected if no chromium were present. Improvement of Procedure. I t should be possible to improve the accuracy of the titrations by measuring the excess titrant much closer to the end point-e.g., within 10 or 20y0 excess of t it rant,
As lanthanum fluoride is more insoluble than samarium fluoride ( I ) , perhaps more dilute fluoride could be titrated with lanthanum ion. I n order to ensure complete carriage of the europium tracer, the precipitation and flocculation should be rapid. Radioactive lanthanum-140 could be used as the tracer, but ita half life of 40 hours is too short to be practical for routine work. ACKNOW LEDGM E S T
The authors wish to thank Sue Krainock and Louis Geoffrion for technical assistance. and J. F. Suttle for the tracer. LITERATURE CITED
(1) Kury, J. W., Univ. of Calif., thesis, July 1953; U. S. Atomic
Energy Commission Document UCRL-2271.
(2) Langer, Alois, ANAL.CHEM.22, 1288 (1950). (3) Langer, Alois, J . Phys. Chem. 45, 639 (1941). (4) Lockett, E. E., Thomas, K. H., .Vucleonics 1 1 , S o . 3, 14 (1353) ( 5 ) Popov, A . I., Knudson, G. E., AKAL.CHEM.26, 292 (1954). (6)
Reynolds. D. S., Hill, W. L., IND.ENG.CHEM.,A N A L . ED. 1 1 , 21 (1939).
(7) Willard, H. H., Winter, 0. B., Ibid., 5, 7 (1933). (8) Wilson, -4. S., Taube, Henry, J . Am. Chem. SOC.74, 3509 (1952).
RECEIVED for review August 4, 1955. -4ccepted December 9, 1955. Work U. S. Atomic Energy Cornmission.
was done under auspices of
Spot Test for Diketones and Quinones Based on Catalytic Effect FRITZ FEIGL and CLAUD10 COSTA N E T 0 Laboratorio d a Produfio Mineral, Ministerio d a Agricultura, Rio d e Janeiro, Brazil
Translated by R A L P H E. OESPER University o f Cincinnati, Cincinnati, O h i o
The slow reaction between formaldehyde and 1,Zdinitrobenzene, which yields a violet alkali salt of the aci- form of o-nitrosonitrobenzene, is hastened by the addition of 1,Z-diketones and quinones. It is assumed that an intermediate catalysis is involved. Microgram quantities of the catalytically active compounds can be detected by drop reactions if certain simple conditions are maintained. New microtests for anthracene, phenanthrene, and inositol are made possible by the ready conversion of these compounds into anthraquinone, phenanthraquinone, and cyclic polyketones, respectively.
F
ORMALDEHYDE functions as a hydrogen donor in alka-
line solution and thus reduces 1,a-dinitrobenzene to the n c i - form of 1,2-nitrosonitrobenzene ( I ) , giving a violet alkali salt. However, the reaction
be due to the fact that formaldehyde in alkaline solution reduces them to 1,2-hydroxyketones as shown in Equation 2, and these products in turn reduce the 1,2-dinitrobenzene to the violet o-quinoidal alkali salt as shown in Equation 3. The diketone is thus regenerated and can react again according to Equation 2. Reactions 2 and 3, which occur again and again, proceed faster than Reaction 1. Addition of the partial Reactions 2 and 3 gives a net reaction xhich is identical with Reaction 1, in nhich the diketone does not appear, even though it is continuously consumed and regenerated. 2
2
-co -Lo
+ 2CHzO + 2 0 H -
-CHOH 1
-co
+
-CHOH --t
2
u=so*-
+ 2 H C 0 0 - + 3Hz0
(1)
proceeds slowly even with relatively large quantities of formaldehyde in sodium carbonate solution, and hence, has no analytical value. The redox reartion has been found to be rapid in the presence of 1,a-diketones and quinones, which act as catalysts. This effect is the basis of a sensitive and specific test, xhich may be used as a spot reaction, for these compounds. The catalytic hastening of Equation 1 by 1,a-diketones may
+ 2HC00-
(2)
+ 3HzO
(2)
+ 20H-=KO
-co
-
=SO>-
fi-so-
1
-CO
t 2
1
-co
There is no doubt as to the existence of Reagtion 3, because microgram quantities of acyloins and benzoins (compounds containing the group -CHOH-CO-) have been found to yield the violet color characteristic of organic hydrogen donors when they are warmed with an alkaline-alcohol solution of 1,2-dinitrobenzene ( 2 ) . No reports could be found in the literature about the reduction of diketones by formaldehyde as in Reaction 2. Attempts to convert benzil quantitatively into benzoin by warming an alcohol solution with a sodium carbonate solution of formaldehyde failed. However, the evaporation residue of such a reaction mixture contained benzoin, as revealed by the color reaction with l,2-dinitrobenzene and also by the production of hydrogen
