LITERATURE CITED (1) H. A. Mottola, Crit. Rev. Anal. Chem., 4, 229 (1975). (2) H. U. Bergmeyer and A. Hagen, fresenius' Anal. Chem., 261, 333 (1972). (3) H. H. Weetall, Anal. Chem., 46, 602A (1974). (4) 0. G. Guilbault in "Enzyme Engineering", Vol. 2, E. K. Pye and L. B. Wingard, Jr., Ed., Plenum Publishing Corp., New York, N.Y., 1974, pp 377383. (5) E. W. Chlapowski and H. A. Mottola. Anal. Chim. Acta, 76, 319 (1975). (6) H. Hall, B. E. Simpson, and H. A. Mottola, Anal. Biochem., 45, 453 (1972). (7) MPlAppl. Notes, 2, 23 (1967).
N. V. Rao and V. V. S. Eswara Dun, Mikrochim. Acta, 1970, 512. C. Paal and L. Friederici, Ber.. 64, 1766 (1931); 65, 19 (1932). F. Feigl, "Chemistry of Specific, Selective, and Sensitive Reactions", Academic Press, Inc., New York, N.Y., 1949, p 144. P. Hayword and D. M. Yost, J. Am. Chem. SOC., 71,915 (1949). P. N. Rylander, "Catalytic Hydrogenation over Platinum Metals", Academic Press, New York, N.Y.. 1967.
RECEIVEDfor review J u n e 19, 1975. Accepted October 16, 1975. This work was supported by the National Science Foundation.
Micro, Ultramicro, and Trace Determination of Fluorine Wolfgang J. Kirsten Department of Chemistry, Agricultural College of Sweden, 5-75007 Uppsala 7, Sweden
A method for ultramicro, micro, and trace determination of fluorine In organic and inorganic compounds is described. The samples are decomposed in a quartz tube with tungsten trioxide and phosphoric acid. The liberated fluorine and the other combustion products are hydrogenated in the same operation in the same tube and absorbed in water and measured spectrophotometrically. Down to 0.2 pg of fluorine can be determined in samples of up to 1 g of inorganic or organic material.
A method for t h e micro- and ultramicrodetermination of fluorine in organic and inorganic material, which is almost free from common interferences, was recently described ( I ) . T h e method has now been modified for the destruction of large samples of both organic and inorganic material. Tungsten trioxide is used to expel the fluorine from the samples. The spectrophotometric measuring method has been refined. Down to 0.2 pg of fluorine can be determined in samples up t o 1 g of organic material and even more of inorganic material. EXPERIMENTAL Apparatus.
The apparatus is similar to that earlier described
( I ) except that the combustion tube has a further chamber which
makes it possible to pyrolize large samples in a nitrogen flow, and that a wider combustion tube is used. A further stopcock has been introduced, which makes it possible to fill the whole combustion tube with nitrogen in order to avoid all risks in the combustion of volatile organic liquids. Reagents. Tungsten trioxide: Heat tungsten trioxide powder or tungstic acid to 600 OC in a flow of oxygen for about 1 hr. Let cool and keep in a well closed plastic vessel. Potassium hydroxide solution, 50%. Sodium hydroxide, 1 M. Phosphoric acid: Dilute 10 ml of concd orthophosphoric acid to 100 ml with water. Hydrochloric acid, 1 M. Complexan: Weigh out 241 mg of alizarin-3-methylamine-N,N-diacetic acid dihydrate, Merck, Darmstadt, into a 250-ml volumetric flask and dissolve it in the least possible amount of freshly prepared 1 M sodium hydroxide. Dilute to 50 ml and add 125 mg of sodium acetate, NaAc-SHzO. When all is dissolved, add slowly 1M hydrochloric acid until the solution just becomes red. Add 25 ml of acetone and make up to the mark with water. Lanthanum nitrate: Dissolve 271 mg of lanthanum nitrate, La(N03)yGHzO in water and dilute to 250 ml. Acetate buffer: Dissolve 105 g of sodium acetate, NaAc-SHzO in 100 ml of glacial acetic acid in a beaker with careful heating. Dilute with water to 500 ml. Cool to room temperature and make up to 500 ml. Sodium acetate solution: Dissolve 25 g of NaAc-SHzO in water and dilute to 500 ml. Nitrophenol: Dissolve 200 mg of p-nitrophenol in 100 ml of water. Absorbent: Mix 5 ml of nitrophenol solution with 95 ml of 84
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
sodium acetate solution. Color reagent: Mix 50 ml of complexan, 50 ml of acetate buffer, 50 ml of lanthanum nitrate, and 250 ml of acetone. The solution is stable for at least 2 weeks. Prepare and keep all reagents in well closed plastic vessels. A d j u s t m e n t of A p p a r a t u s . Set up the apparatus as shown in Figures 1 and 2 and as described in ( I ) . Adjust the gas flow rates for micro and ultramicrodeterminations as described in ( I ) and for trace determinations with large samples to 75 ml of oxygen, 20 ml of nitrogen, and 250 ml of hydrogen per minute. Capillaries A4 and A5 are experimentally adjusted so that only a few ml/min of oxygen pass through A4 when stopcock H2 is opened, but that the total flow of oxygen does not decrease very much when H2 is closed. C o m b u s t i o n M e t h o d s . Ultramicro and Microdeterminations. Carry out the combustions as described in ( I ) . Add 5 r l of 10% phosphoric acid to all samples. Mix ash-containing and inorganic samples with an equal volume of tungsten trioxide and add 5 11 of the phosphoric acid. No nitrogen is used in the combustion. Trace Determinations, Solid Inorganic Samples, or Samples with Very Low Contents of Organic Material. Push back furnace N1 from the combustion tube and remove spoon S.Weigh the finely ground sample into a small, glass stoppered specimen tube. Add an equal volume of tungsten trioxide. If the sample is alkalinesuch as lime, magnesia, sodium carbonate, or hydroxides etc.-add at least twice the weight of tungsten trioxide which is necessary for the neutralization of the sample. Mix thoroughly. Pour the mixture into a big quartz boat. If the sample is small or adheres to the walls of the specimen tube, wash the latter with a few small portions of tungsten trioxide and add the washings to the boat. Add 5 1 1 of phosphoric acid to the upstream end of the boat and introduce the boat to a distance of about 3 cm from furnace N2 through the opening at A5. Close A5 and turn stopcock H2 so that oxygen passes into the combustion tube through A5, and then turn H1 so that hydrogen passes into the hydrogenation chamber. Attach the absorption vessel and draw furnace N1 across the tube with the sample. After completed combustion, detach the absorption tube and turn stopcock H1 so that the hydrogen passes out through its ball joint. Trace Determination, Samples Containing Considerable Amounts of Organic Material. Prepare the samples and introduce them into the apparatus as described above for inorganic substances. Close A5 and turn stopcock H2 so that nitrogen passes into the combustion tube, and then H1 so that hydrogen passes into the hydrogenation chamber. Attach the absorption vessel and draw furnace N1 over the tube somewhat upstream from the boat and let is slowly move to furnace N2. When no more gases are developed from the sample, turn stopcock H2 so that oxygen passes over the boat and the sample is completely burned. Detach the absorption vessel and turn stopcock H1 so that the hydrogen passes out through its ball joint. Trace Determinations, Oils and Liquids Which Can Giue Explosiue Vapors. Use spoon S. Push back furnace N1 from the tube. Turn stopcock H2 so that nitrogen passes into the tube and close stopcock H3. The whole tube will now be filled with nitrogen.
Flgure 1. Layout of apparatus, compare ( 7 ) (A4, A5) Restriction capillaries. Quartz tube C2 is filled with copper oxide wire. (H2) Twoway stopcock. (H3) One-way stopcock. Furnace N1 can be made to travel along the combustion tube as usual in elementary analysis. Grooves are ground into the side of furnace N2 to accept the side tube of the combustion tube. Lengths of furnaces 120 mm, temperature 900 OC. (N3) Preheating furnace for three purification tubes, 850 OC. (N4) Hydrogenation furnace, 1050 OC, length 120 mm. The rod of spoon (R) must be longer than furnace N1, so that N1 can be drawn over the combustion tube upstream from the sample without overheating the magnet. Tube connections are Tygon tubing, stoppers are silicone rubber
Open stopper Q and introduce the sample and 5 pl of phosphoric acid into the spoon. Close Q. Turn stopcock H1 so that hydrogen passes into the hydrogenation chamber and open stopcock H3. Attach the absorption tube. Push the spoon forward with the magnet so that the sample is in front of furnace N2. Draw furnace N1 over the tube upstream from the sample and let it pass slowly over the boat. When no more gases are developed from the sample, turn H2 so that oxygen passes into the tube and burns the remainder of the sample. Detach the absorption tube and turn stopcock H1 so that the hydrogen passes out through its free arm. When the sample is very volatile, it is better to draw the spoon with the sample near to furnace N2 and to let it volatilize gradually through the heat of radiation from this furnace, before drawing furnace N l across the tube. A too rapid volatilization might otherwise be obtained. In ultramicrodeterminations of fluorine in fluorobenzoic acid, the fluorine is recovered in the absorption flask 12 min after the introduction of the sample-spoon into the furnace. The time can be shortened to 10 min by placing a 0.7-mm platinum wire, 80 mm long, into tube J. We usually use a combustion time of 15 min. When large organic samples, such as oils, are analyzed, a longer combustion time is needed. The method is, however, much less sensitive to incomplete combustion than most other methods of elementary analysis. Unless considerable amounts of soot pass into the absorption vessel, correct results are obtained. For 100-mg samples of paraffin oil, we use a total combustion time of 45 min. Also, inorganic substances might need a longer time for complete decomposition. For the reported trace determinations in inorganic samples, we have used a total decomposition time of 20 min. When the decomposition of a sample is ended too early, so that not all of the fluorine is recovered, high results are usually obtained in the next analysis. This means that the fluorine remains somewhere in the tube. It is therefore important not to use a too short decomposition time. When many large samples of inorganic and ash-containing materials are analyzed, it is advisable to place the combustion boat into a short quartz tube to protect the combustion tube. Spectrophotometric Measuring Methods. High Sensitivity Method for Neutral Samples. Absorb the combustion products in 1 ml of water in a 10-ml plastic measuring cylinder. Wash the inlet tube with a few drops of water. Add 3.000 ml of color reagent and dilute to the mark with water. Let stand in the dark for at least 90 min and read at 620 nm using the same reagent mixture in the reference cuvet. Procedure for Samples Whose Combustion Products Can Influence the pH of the Absorption Solution. Absorb in 1 ml of absorbent. In case the solution becomes yellow, add dropwise 1 M hydrochloric acid until the solution just becomes colorless. Agitate with the inlet tube and lift up the inlet tube, so that the solution inside also is neutralized. If the solution is colorless, add first drop-
!
1-775-125-65
300
!
2 1 5 4
Figure 2. Details of apparatus, compare ( 7 ) (H2) Two-way stopcock with an outlet groove (K) ground into its inner part with a dentist's drill. It lets the nitrogen escape when the oxygen flow is on. (H3) One-way stopcock with two outlet grooves K ground in the same manner. (L) Combustion-hydrogenation tube, inner diameter 12, outer 15 mm. (M) Side tubes, inner diameter 1, outer 6 mm, length 110 mm. (I) Quartz capillary, inner diameter 3, outer 8 mm. (T) Opaque quartz. (Ul) Capillary, inner diameter 2, length 15 mm. (U2) Orifice, 3-4 mm
wise 1 M sodium hydroxide until the solution is yellow, and then hydrochloric acid until it becomes just colorless. Then add 3.000 ml of color reagent and make up to the mark with water and measure as described above. Two pg of fluorine give an absorbance of 1.26 in our 10-cm cuvets. (Special 10-cm tube cuvets of black glass with a total volume of 10 ml were made for us by Hellma GmbH, Mullheim/Baden, Western Germany.) The absorbance in blank runs is about 0.035. Table I. Trace Determination of Fluorine in Inorganic Materials rime of
comContent of fluorine Substance
Calcium tablets/ ground bone Bone powder, food additive Slaked lime powder Raw phosphate fertilizer, G50/ 7 4-7 5 Raw phosphate fertilizer G50/ 74-75 Silica gela
Weight of sample, mg
12.72 8.27 10.33 7.09 8.04 8.38 6.55 46.5 36.1 45.8 698.7 1169.5 1238.6 943.8 1250.1 1501.0
Calcd
Found
bustion, min in 0 2
O.O5%d
0.079% 0.082% 0.079% O.O5%d 0.069% 0.072% 0.072% 0.071% ... 113 pgig 1 2 3 pgig 1 2 1 pglg ... 2.43% 2.56% 2.48%
30 30 30 30 30 30 30 32 35 40 20 20 20
3.39% 3.42% 3.53%
20 20 20
100.4 87.7 94.3 85.6 94.8 91.8
20 82 p g k 20 83 pg/g Sodium sili20 37 pglg cateb 20 39 w i g Tricalcium 20 27 pgig diphosphatec 20 27 pgig aSilica gel: Merck Germany 7731 for thinlayer chromatography. b Sodium silicate: BDH 2172020. c Tricalcium diphosphate: Merck Germany 2143. d Commercial declaration.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
85
Table 11. Trace Determination of Fluorine in Organic Materials
Content of fluorine,
Time of combustion, m in
Content of fluorine,
Time of combustion, min
Pg/g
Substance
Oxalic acid0 Oxalic acid,= 0.5 pg of fluorine added Sucrose b Sucrose,b 0.57 pg of fluorine added Dinon yl phthalatec Liquid paraffinc Diisoamyl ether Tri-n-butyl citrate Di-n-butyl ether
A m o u n t of sample
1.007 g 0.999 g
Calcd
Found
N,
0,
Substance
Greenfodder pills standardized for testing purposes Sewage sludge standardized for testing purposes Manure, standardized Hay, standar di ze d
*..
1.5 1.5
30 30
15 15
2.0
2.0 3.2
30 30
15 15
...
0.993 g 0.500 g
...
0.504 g 100 pl 9 0 pl 9 0 pl 9 5 1.11 5 0 p1 50 pl 9 0 1.11 7 5 p1 95 pl 9 5 pl
4.3 4.3 10.6 10.9 2.6 8.5 10.6 6.7 10.0 6.0 0.0
4.7 30 15 3.2 25 15 10.6 25 15 11.4 30 15 2.6 30 15 15 7.9 25 10.0 25 15 7.0 25 15 8.6 25 15 5.8 1 2 min with0.0 o u t furnace N 1 , 3 cm from furnace N2, then 5 min 1.5 cm from N2, then 5 min with N1, then 1 5 min with only
Hay, standardized, Gra’73d Hay, standardized, Gralf’7 4d Pine needles A Pine needles B Pine needles C
with1
only
Amount of sample
Calcd
Found
N2
0 2
261.8 mg 296.0 mg 364.0 mg
... ... ...
22 22 21
15 15 15
15 15 15
22.9 mg 14.6 mg 18.9 mg 6.35 mg
... ... ...
617 619 637 624
15 15 15 15
15 15 15 15
150.8 mg 151.4 mg
... ...
15.5 16.5
15 15
15 15
105.1 mg 173.2 mg 37.6 mg 29.9 mg 54.9 mg 79.8 mg 93.1 mg 100.0 mg 81.3 mg 97.8 mg 124.2 mg 154.2 mg 17.6 mg 11.6 mg 136.7 mg 137.6 mg
16
16.1 20.3 17.2 20.6 24.6 23.8 24.9 18.0 16.9 17.6 9.6 10.0 162.6 163.7 15.5 15.5
15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15
15 15 15 15 15 15 15 15 15 15 15 15 15 15 15 15
...
See below See below
’
0 2 .
Oxalic acid: Merck 6054667, Pro Analysi. b Sucrose: Mallinckrodt 8360. c Organic liquids: The liquids had been analyzed and found free from fluorine. Samples of a solution of p-fluorobenzoic acid in dinonyl phthalate were then weighed out and the reported volumes of the liquids were added and the mixture was analyzed. d Hays Gra’73 and Gralf’74 were kindly given to us by F. Kadijk, Bedrijf-
slaboratorium voor grund- en gewasonderzoek, MariendaalOosterbeek, The Netherlands. The substances had been analyzed in an international collaborative investigation. For Gra’73, 1 4 laboratories reported the following results: 0.3; 6; 1 2 (2x); 13; 20; 21; 22; 23 (3x); 24; 25; 32 ppm of fluorine in dry matter. For Gralf’74, 1 5 laboratories reported 11 (2x); 1 2 ; 13; 1 5 (2x); 1 6 ; 1 7 (4x); 18 (3x); 2 3 ppm.
The calibration curve is a straight line. With shorter cuvets, up to 6 fig can be measured. Above 6 Mg, the calibration curve bends. Procedure for Larger Amounts of Fluorine. Absorb in 50-ml plastic volumetric flasks in 20 ml of water or 15 ml of water and 5 ml of absorbent. Then use the same methods for the color development as described above, but add 15.00 ml of color reagent. Ten r g of fluorine give an absorbance of 1.24 in ordinary 10-cm tube cuvets. The absorbance in blank runs is about 0.01. With shorter cuvets, up to 30 Fg can be measured. Above that, the calibration curve bends. Procedure for Unknown Amounts of Fluorine. Absorb in a 10-ml plastic measuring cylinder or in a 50-ml volumetric flask. Make up to volume with water. Transfer a small aliquot into another cylinder or flask and develop the color there. If the color is too weak or too strong for accurate measurement, repeat the color development with a more suitable aliquot. The suitability of the solution for measurement can usually be judged from its appearance just a few minutes after the mixing. The color is stable for at least 48 hr. Color solutions from analyses run at the end of the day can therefore be measured the next day.
precision as reported in t h e earlier paper ( I ) ,Table 11. For opal glass, t h e results obtained with t h e new method were 5.4% and 5.6% for calculated 5.7% of fluorine. T h a n k s t o t h e high sensitivity of t h e spectrophotometric measuring method, it is very seldom necessary t o analyze large samples.
a
RESULTS Determinations of fluorine in different materials a r e reported in Tables I a n d 11. Ultramicroanalyses of p-fluorobenzoic acid, sodium fluoride, potassium hexafluorotitanate, barium fluoride, a n d phosphate rock gave t h e same 86
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
DISCUSSION There is a considerable demand for trace determination of fluorine for environmental research purposes. Since t h e earlier described method ( I ) for ultramicrodetermination of fluorine worked extremely well without interferences, it appeared desirable to a d a p t i t to trace determination. I n this method, t h e fluorine was expelled from inorganic a n d ash-containing compounds with phosphoric acid or with a phosphoric acid-sodium phosphate flux. When large samples were analyzed, large amounts of t h e additives had t o be used, a n d t h e n t h e phosphoric acid interfered with t h e measuring method. Tungsten trioxide is a n acidic compound. It has been recommended by Belcher e t al. for t h e combustion of phosphorus compounds (2) a n d by Kissa ( 3 ) for t h e decomposition of carbonates in t h e CH-determination. We hoped t h a t i t would decompose fluorides a n d form volatile hexafluoride. We found t h a t it decomposed all
fluorides we encountered very well, a n d it does not interfere with the measuring method. Since we find it somewhat disagreeable t o add a sodium-containing compound t o the samples, we use it always now instead of the flux, though we have not encountered any troubles with the latter. We still add orthophosphoric acid, however, because it is liquid and volatile and might reach particles of t h e sample which are not in sufficiently good contact with tungsten trioxide. We add 5 pl of 10% acid instead of the earlier 2 pl of 25%, because it is easier t o wet the samples with the larger volume. T o avoid risks of explosion when large samples are burned which develop organic vapors, such samples are always pyrolized under nitrogen first, a n d t h e vapors are burned in t h e intermediate chamber. T h e orifice U2 is situated in the hot part of furnace N2, so t h a t it cannot be clogged by carbonaceous foam, which can be formed from some kinds of samples. Such foam is broken down by the heat of the furnace before it reaches t h e orifice. T h e method of introducing samples through Q into a completely nitrogen filled combustion-hydrogenation t u b e makes it possible t o analyze rather large samples of even rather low-boiling organic liquids. There is no possibility of any formation of explosive gas mixtures. We do not know whether the rigorous gas purification system used really is necessary in t h e determination of fluorine. We know t h a t it is necessary in the trace determination of sulfur ( I ) , and we use it because we intend t o use t h e same apparatus for this purpose also. T h e temperature of furnace N4 was 1050 "C in all analyses. We do not know whether this temperature is necessary or optimal or not. In any case, good results were obtained. T h e spectrophotometric method is essentially t h a t described by Belcher, Leonard, and West ( 4 ) .When we tried t o work with small volumes of absorption solution, it was difficult t o add the color reagents with a sufficient preci-
sion. T h e mixed reagent was therefore prepared. I t is added with a syringe with a precision adapter. I t was necessary t o use a somewhat lower concentration of acetone t o avoid precipitations in the mixture. Also the concentration of t h e buffer was increased, which was desirable because of the larger samples used. T h e mixed reagent is stable for several weeks. We keep it a t room temperature in the dark. Evaporation of acetone must, of course, be avoided. T o obtain a good spectrophotometric precision, we keep the water and all spectrophotometric reagents a t room temperature. We hold all long and narrow cuvets with wooden clamps and do not touch them with our fingers in order t o avoid the formation of schlieren in t h e solution, which would impair the spectrophotometric accuracy. With cuvets shorter t h a n 4 cm, this is not necessary, nor is it necessary with the common wide commercial 10-cm tube cuvets with a volume of 25 ml. T h e precision of the spectrophotometric method is remarkable in view of the fact t h a t the fluorine-free reagent solution used in the 10-cm reference cuvet has a light absorbance of 1.46 a t 620 nm. ACKNOWLEDGMENT T h e author is indebted t o Ylva K. S.Bjorkman and Lena M. Eriksson for skilled technical assistance. LITERATURE CITED (1) (2) (3) (4)
W. J. Kirsten and 2 . H. Shah, Anal. Chem., 47, 184 (1975). R. Belcher, J. Fildes, and A. J. Nutten, Anal. Chim. Acta, 13, 431 (1955). E. Kissa, Microchem. J., 1, 203 (1957). R. Belcher, "Submicro Methods of Organic Analysis", Elsevier, Amsterdam, 1966, p 62.
RECEIVEDfor review July 11, 1975. Accepted September 23, 1975.
Linear Graphical Kinetic Analysis of Mixtures Kenneth A. Connors School of Pharmacy, University of Wisconsin, Madison, Wis. 53706
A mixture of two reactants A and B, reacting by first-order or pseudo-flrst-order kinetics, can be analyzed for the Initial Cz) exp( kAt) vs. exp(kA concentrations by plotting (CF - k B ) f , where 2 is the common product. The slope of the The method can be exresulting straight line is equal to tended to three-component mixtures by including an extrapolation. It is applied to the analysis of mixtures of esters subjected to alkaline hydrolysis.
-
e.
If two reactants undergo a common reaction with differe n t rate constants, their mixtures may often be quantitatively analyzed by means of measurements of t h e total ext e n t of reaction as a function of time. For first-order or pseudo-first-order reactions, the most convenient way t o do this is with a conventional semilogarithmic first-order plot, which will be curved in the early stages of reaction, since both reactants are contributing t o t h e reaction; after the faster component has essentially completely reacted, however, the plot becomes linear, a n d its extrapolation t o zero
time yields t h e concentration of the slower reacting component in the mixture. Another way to treat the d a t a is by the "method of proportional equations", in which the extent of reaction is measured a t two times, and, with the aid of prior calibration measurements, two simultaneous equations are solved for the two initial concentrations. Mark and Rechnitz ( I ) have described these and related methods in detail. T h e advantages of the semilogarithmic extrapolation method include its simplicity a n d its independence of separate rate parameter determinations; in fact, the rate constants can be extracted from the same data used for the analytical determination (2). T h e weaknesses of this method are its reliance on an extrapolation, often a long one based on the poorest d a t a in the set, and its inapplicability when the relative rate is low and the ratio of slower to faster component is low. Moreover, it makes no use of the best d a t a in t h e set, those from the early part of the reaction. T h e method of proportional equations uses d a t a in this time range, b u t uses them inefficiently. An automated graphical extrapolation method, interesting in the present context ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
87
