further as they were known to be aminoguanidine derivatives (9) (11). Some attempts were made to suppress the displace-
factory. Such conditions, for example, gave better yields of the triazole from S-tert-butylisothiourea (IO) (VII; R = SC(CH&; R' = H) and N-phenylthiourea (VII; R = SH; R' = Ph). ACKNOWLEDGMENT
We thank Dr. C. H. L. Kennard for X-ray measurements. !L!
!y!
ment of hydrogen sulphide or mercaptan in those tests where the competing reaction ( A ) appeared significantly great and the modified test conditions given above proved most satis(9) G. Pellizzari and A. Gaiter, Gazz. Chim. Ztul., 44, 78 (1914).
RECEIVED for review February 24, 1969. Accepted April 21, 1969. Research supported by Rural Credits Development Fund through a grant to R. G. D. (IO) G. Kresze and U. Uhlich, Chem. Ber., 92, 1048 (1959).
Direct Determination of Fluoride in Phosphate Rock Samples Using the Specific Ion Electrode C. R. Edmond The Australian Mineral Development Laboratories, Adelaide, South Australia
IN A RECENT paper, Singer and Armstrong ( I ) described the application of the fluoride ion activity electrode to the determination of fluoride in bone samples and suggested that a similar method could be used for the analysis of rock phosphates. At these laboratories an investigation into possible applications of the electrode method in fluoride analysis has been undertaken, the preliminary results of which have been reviewed elsewhere (2). A simple, routine method for the determination of fluoride in natural phosphate samples is highly desirable and the method of Singer and Armstrong for fluoride in bone samples offered promising possibilities. This method, however, depends on the dissolution of the sample in cold mineral acid, which is suitable for relatively pure forms of natural phosphate such as calcined bone, but may not result in a complete attack with samples having a more complex mineralogical composition. Moreover, the total amount of fluoride must be present in a completely ionized form in the final sample solution, because the electrode responds only to fluoride ions and not to fluoride complexes. For this reason the interference of those elements which form strong complexes with fluoride (e.g., Si, Al, Fe) must be eliminated before total fluoride can be measured in a solution containing all these ions. Although the bone samples analyzed by Singer and Armstrong would not be expected to present difficulties due to fluoride complexing ions, it was by no means certain that their method would apply generally to all grades of natural phosphate. The present report sets out the results of attempts to use the fluoride ion activity electrode for the determination of fluoride in a variety of phosphate rock samples. EXPERIMENTAL
Apparatus. An Orion Model 94-09 fluoride ion electrode and a Beckman Type R.L.B. calomel reference electrode were used in conjunction with a Jones Model Z Electrometer (1) L. Singer and W. D. Armstrong, ANAL.CHEM., 40, 613 (1968). (2) C. R. Edmond, Bull. Aust. Min.Deo. Lab. No. 7, 1-14(1969).
(N. L. Jones Scientific Instruments, Melbourne, Australia) for the measurement of solution potential. Reagents. Standard fluoride solutions were prepared as required for a stock solution (100 ppm F), made by dissolving 0.221 g NaF (previously dried at 120 "C) in 1 1. of water. Total Ionic Strength Adjustment Buffer (TISAB, Orion) was prepared by dissolving 58 g sodium chloride and 0.3 g sodium citrate in approximately 500 ml of water, adding 57 ml acetic acid, adjusting the pH to 5.0-5.5 with 5M NaOH and diluting to 1 1. M Sodium Citrate Buffer was prepared by dissolving 294 g sodium citrate dihydrate in 1 1. of water and adjusting the pH to 6 with 5M hydrochloric acid. Procedure. About 50 mg of phosphate rock sample, accurately weighed on a microbalance, were transferred to a small polythene beaker and dissolved by stirring with 1 ml of 5M hydrochloric acid. The solution was made to 100 ml in a volumetric flask and transferred without delay to a plastic container. To ensure that the ionic strength and pH of the solution remained constant, 10 ml of sample solution were mixed with 10 ml of buffer (either TISAB or M sodium citrate) in a closed plastic container. Standards containing 5, 15, 20, and 40 ppm F, respectively, were diluted in plastic containers with an equal volume of buffer to give working standards of 2.5, 7.5, 10, and 20 ppm F. The electrodes were placed into the solution which was swirled until a constant millivoltage reading was obtained. Readings obtained with the standard solutions were plotted on semilogarithmic paper, the fluoride concentrations on the logarithmic axis, and millivolts on the linear axis. The calibration curve was linear over the concentration range 2.5 to 20 ppm fluoride. The concentration of fluoride in each sample solution was read off the calibration curve and the percentage of fluoride in the original sample calculated. RESULTS AND DISCUSSION Measurements Made in the Presence of TISAB. The results obtained with a series of standard phosphate rock samples are shown in Table I, together with other relevant information about the sample composition and values for fluoride obtained using other methods. The results obtained with the electrode in TISAB buffered solution (comparable VOL. 41, NO. 10,AUGUST 1969
1327
Table I. Comparison of Results for Fluoride in Phosphate Rock Samples Determined with the Ion Activity Electrode F,
Sample
Acid insoluble matter (or SiOd, Z pzo5, Z ND 38.1 ND 38.5 ND 37.8
Origin P-1 Ocean Is. P-2 Christmas Is. P-3 Nauru Is. P-6 Christmas Is. dust 0.4 P-7 Morocco 3.1 P-8 Florida 9.2 NBS 120a Florida 4 . 3 (SiOz) Check Sample 15c Florida 7.2 ND Not determined. a F by distillation and titration with Th(NO&. * Certificate value. Association of Florida Phosphate Chemists.
37.1 37.1 32.9 34.4 33.9
Z
Electrode method Ah03, Z 0.25 3.5 0.25 6.0 0.9 01.7 .9 1.1
Z
TISAB
Citrate
0.15 1.4 0.15 2.2 1.2 1 .o
2.9 1.2 2.95 1.05 3.78 3.34 3.80
1.1
ND
3.01 1.58 3.07 1.87 3.91 3.71 3.88 3.69
Fe&
1 .o
Other methods
..*
.*.
...
1.925 3.95a 3 . 655 3.92b 3 .71b
with the Singer and Armstrong method) are lower than those found by other methods. An examination of the results indicates that with samples containing less than 1% A1203, some 96y0 of the total fluoride (as shown by other methods) can be obtained. For the sample containing 1.7% A1203 recovery of fluoride falls to 91%, while with the sample having 6% A h 0 3 recovery is only 54% of the total fluoride present. On the basis of these results, it can be predicted that the values found for fluoride in samples P-1 and P-3 (both containing 0.15% Al2O3) will be reasonably accurate, but for sample P-2 (3.5% A1203)the result will again be low. With samples containing more than 1% A1203,either simple cold acid dissolution does not'liberate all the fluoride or, if it does, the resulting concentration of A13+is sufficiently high to complex some fluoride ion, thus preventing its measurement with the selective electrode. Measurements Made in the Presence of Citrate Buffer. Assuming that low results are due to the presence of AlF3 in the test solution, it seemed probable that some improvement could be expected by substituting a citrate buffer for the less complexing TISAB originally used. Such a buffer has been used successfully by McCann (3) to prevent aluminum interference in a method for determining fluoride in mineralized tissues. To test the effectiveness of a M sodium citrate solution buffered at pH 6 for complexing A13+,nine solutions (10-ml volume) were prepared, each containing 200 pg of fluoride, with 0, 25, 50, 100, 200, 500, 1000, 1250, and 1500 pg of aluminum, respectively. To each solution 10 ml of citrate buffer were added, the solutions mixed thoroughly, and millivoltage readings taken as described above. The results indicated that there is no interference by aluminum at least up to 500 pg (equivalent to 25 ppm in the final solution) and that even in the presence of 1000 pg of aluminum, the error in measurement of 200 pg of fluoride (10 ppm) is only minus 4%. The effect of phosphate in the system was not tested by measurements made on these synthetic solutions. Since Baumann ( 4 ) has shown that fluoride can be measured in strong phosphoric acid solution, even in the presence of
The electrode method for the determination of fluoride in bone described by Singer and Armstrong ( I ) would lead to low results when applied to phosphate rock samples containing A1203in excess of 1%. Samples with up to 6% A1203 have been analyzed successfully for fluoride by using a citrate buffer in place of the less complexing acetate buffer employed by the above authors. Measurements made with pure solutions indicate that there is a possibility of analyzing samples containing up to 20y0 AhO3 in the same way. The procedure is rapid and very suitable for the routine determination of fluoride in phosphate rocks which are sufficiently soluble in cold mineral acid. Experience indicated extremely good reproducibility of both the calibration curve and millivoltage readings for individual solutions. It is possible to read the expanded scale of the electrometer within i l mV which, in the present method, represents an error of approximately +0.12% at the 3% fluoride level. Further work is in progress on the application of the electrode method to the determination of fluoride in other samples, including silicate rocks and plant material.
(3) H. G. McCann, Archiaes Oral Biol., 13, 475 (1968). (4) Elizabeth W. Baumann, Anal. Chim. Acta, 42, 127 (1968).
RECEIVED for review February 17, 1969. Accepted April 14, 1969.
1328
ANALYTICAL CHEMISTRY
added aluminum, it seems unlikely that the ternary system Pod3-, A13+, F- would produce interference. Results with phosphate rock samples containing relatively high concentrations of aluminum support this view. The original phosphate rock sample solutions were analyzed again, after treatment with an equal volume of citrate buffer. Standards were prepared as previously, but using an equal volume of citrate buffer instead of TISAB. The calibration curve was again linear, with the same slope as the former curve, but with a slight shift toward more negative potentials. The results are shown in Table I and a comparison with those previously determined confirms that improved values for fluoride are obtained with samples containing more than 1% Al2O3. As was predicted, results for samples P-1 and P-3 remain largely unchanged, while that for sample P-2 is appreciably higher. CONCLUSIONS
