larger than 60 microns would be in error for this reason. ACKNOWLEDGMENT
The author acknowledges the helpful suggestions given to him by Jacob G. Rabatin. LITERATURE CITED
(1) Arnes, D. P., Irani, R. R., Callis, C. F., J . Phys. Chem. 63, 531 (1959). (2) Avgustinilc, A. I., Dzhansis, V. D., J . . l p p l . Chem. (U.S.S.R.) 24, 433-8 (1951). (3) Bachmann, D., Gerstenberg, H., Chem-1ng.-Tech. 29, 589-94 (1957). ( 4 ) Berg, R. H., Am. SOC.Testzng Materzals Spec. Tech. Publ. iVo. 234,
245 (1958).
(5) Bostock, K., J . Scz. I n s t r . 29, 209
(1952). (6) Coutte, R. H., Crowther, E bI., Trans. Faraday SOC.2 1 , 374 (1925). (7) Gaudin, A. hI., Schuhmaiin, R., Schlechten, A . W., J . Phys. Chem. 46, 903 (1942). (8) Hayakawa, T., Tomotsu, T., Takagi,
Table
111.
Comparison of Sedimeter with Sieves
Mesh Screen 1000 500 400 325 230
Wt. yo Greater than Stated Size Sieve Sedimeter 18.0 19 .c 14.7 15.9 8.0 10.8 4.5 5.7
Size, p 22 25 37 44 62
R . , Makishima, S., KBgy6 Kagaku Zasshi 60, 1249-52 (1957). (9) Irani, R. R., ANAL.CHEM.32, 1162 (1960). (10) Jackson, C. E., Saeger, C. M., J . Research Natl. Bur. Standards, R. P. 757, 14, 59 (1935). (11) Jacobsen, A. E., Sullivan, W. F., IND.ENG. CHEM., ANAL.ED. 19, 855 (1947). (12) Lincoln, K. A , Rev. Sci. Instr. 31, 537-9 i 1960). (13) Oden, S.; Intern. Mitt. Boden 5 , 257-311 (1915). (14) Oden, S.,PTOC. Roy. SOC.Edinburgh
0
):1(
0
36, 219 (1916). Orr, C., Jr., DallaValle, J. M.,
Fine Particle Measurement,” Chap.
2, Macmillan, Kew York, 1959. (16) Rabatin, J. G., Card, C. S., ANAL. CHEJI.31, 1689 (1959). (17) Rabatin, J. G., Gale, R. H., Ihid., 2 8 ,
1314 (1956). (18) Stairmand, C. J., Symposium on Particle Size Analysis, Supplement to Trans. rnst. Chem. Engrs. 25, 128, (1947).
RECEIVEDfor review October 27, 1960. Accepted January 18, 1961.
A Simple and Rapid Method for Fluoride Ion Determination SIR: I n the course of studying the mechanism of chemically etching singlecrystal silicon (4, 5 ) in solutions of nitric and hydrofluoric acids, a n interesting effect was observed which may prove useful in developing a simple and rapid method for fluoride ion determination. When a n n-type silicoii electrode (single-crystal, transistor-grade Si) is made the anode of a n electrolytic cell, shielded from room light and in a solution which does not chemically etch silicon, the current is limited to a f m microamperes per square centimeter of anode area. Brattain and Garrett ( I ) and Flynn ( 2 ) have found that the anodic dissolution of semiconductors requires positive electron holes, but only a relatively small number are available in the surface region of n-type silicon in a nonetching solution. In a n etching solution, such as HF and H S O s acid mixtures, the over-all reaction a t the surface of the silicon includes the formation of excess electron holes (4) and these can increase the anodic limiting current by several orders of magnitude. The number of excess electron holes produced is directly proportional to the chemical etch rate and the etch rate is directly proportional to the amount of HF in nitric acid (up to about
10 neight % HF). Therefore the anodic limiting current of a n n-type silicon electrode in HF and HSOs solutions should be directly proportional to the amount of HF in the chemical etch. This has been verified experimentally (5) The effect just described suggests that the experiment may be useful in developing a general method for fluoride ion determination. The technique involves adding the sample, preferably condensed in volume, to a known volume of concentrated (70%) nitric acid. The mixture chemically etches silicon, and the anodic limiting current density of the electrode should be
APIEZON W WAX
_ _ _ _ ~ F-
I N CONC. H N O ~
Figure 1. Experimental arrangement for fluoride ion determination
proportional to the amount of fluoride ion in the original sample. The experimental arrangement is simple, as illustrated in Figure 1. An ordinary 1.56-volt dry cell is a n adequate power source, as indicated by anode potential-current density curves for n-type silicon in HNO,-HF solutions ( 5 ) . The cathode material should be insoluble in fluoride and H N 0 3mixtures; platinum meets this requirement. Current through the cell is determined by the anodic limiting current density at the silicon electrode. It is measured nith a standard ammeter. The shape of the single-crystal silicon electrode is not important. The bar form is usually the easiest to obtain. The resistivity of the n-type silicon should be about 1 ohm-em. or less to avoid a n excessive I R drop in the electrode. .In ohmic contact is made to one end by abradinq the surface with No. 600 mesh S i c , depositing electroless nickel ( S ) , and soft-soldering it to a copper wire. To protect the solder joint and copper wire from fumes of nitric acid, a region above and below the contact is coated with a suitable masking material, such as Apiezon W wax dissolved in toluene, which can be painted on. The exposed surface area of the silicon electrode must be known with reasonable accuracy, since the total limiting current is dependent upon surface VOL. 33, NO. 7,JUNE 1961
959
area. A convenient surface area is 1.0 sq. cm. The silicon exposed to solution must be chemically polished before use to ensure that any damaged surface material is removed from the single crystal. Silicon may be chemically polished in 50 ml. of concentrated HNO, plus 30 ml. of 49% HF plus 30 ml. of glacial acetic acid.
A typical calibration curve relating the limiting anodic current density of the n-type silicon electrode to the fluoride ion concentration, added as KF.2Hz0, in nitric acid is shown in Figure 2. After each addition of KF. 2Hz0, the solution was stirred until the fluoride salt dissolved and then, with the solution unstirred, the cell current was measured. The current increases linearly with increased fluoride concentration up to about 3.5 gram-ions per liter of fluoride. The break in the curve a t about 0.7 gram-ion per liter is not understood a t present. The decrease in limiting current density above 3.5 gram-ions per liter of Fis caused by the precipitation of KzSiFs on the silicon electrode and this reduces the effective area of the electrode. Thus, the maximum potassium ion concentration that can be tolerated in solution is about 3.5 gram-ions per liter. Similar results are obtained when sodium or lithium ions are present, but
for fluoride ion determination as described. Their effect can probably be determined by the following procedure. The sample with the unknown amount of fluoride ion is added to a known amount of concentrated "0, and the limiting anodic current density for the n-type Si electrode is measured. Then a known amount of fluoride ionfor example, as KF.2HzO-is added and the new limiting current density is determined.
FLUORIDE ION CONCENTRATION I N G. - I O N S / L I T E R
Figure 2. Relation between amount of fluoride ion added (as KF.2Hz0) to concentrated HNOa and limiting anodic current density a t n-type silicon electrode (0.7 ohm-cm.) Solution unstirred, platinum cathode, cell voltage 1.56 valts; cell in dark
the corresponding fluosilicate forms a t a much higher cation concentration. Other cations, such as Al+, and Fe+3, which form stable fluoride complexes, may also interfere. Certain anions, such as the acetate ion, in F- - H N 0 3 solutions, reduce the chemical etch rate of silicon. Thus these anions interfere with the method
The increase in the limiting current density caused by the known amount of fluoride ion added should serve to calibrate the system regardless of the nature or amount of interfering anions present, if the linear relation holds between the limiting current and fluoride in concentration. LITERATUiiE CITED
(1) \ , Brattain. W. H.. Garrett. C. G. B..' Bell System. Tech. J. 34,129 (1955). (2) Flynn, J. B., J. Electrochem. SOC.105, 715. (,1-R-.-W-),. .
(3) . . sullivan, M. V., Eigler, J. H., Ibid.,
104, 2- -23 (1957). (4) Turne r, D. R., Zbid., 107, 810 (1960). (5) Zbid., in press.
D. R. TURNEB Bell Telephone Laboratories, Inc. Murray Hill, N. J.
RECEIVED for review February 13, 1961. .4ccepted March 20, 1961.
Resin-Sta bilized 4-Tri methyla mmoniurnbenzened iazonium Cation as a Reagent for Aromatic Amines and Hydroxy Compounds SIR: A continuation of a study of resin-stabilized diazonium cations shows that 4 - trimethylammoniumbenzenediazonium cation is superior to the 4-nitro-, the 2,5-dichloroJ and the 2-carboxy-4-nitrobenzenediazonium cations previeusly described as reagents (1). Whereas solid salts of diazonium cations are usually explosive, those stabilized on dry nuclear sulfonic ion exchange resin can be safely stored for periods of months or years and released into solution when needed by exchange with an inorganic cation. The characteristics sought in addition to storage stability include reactivity in the coupling reaction, the absence of undesirable side reactions, and ease of exchange onto and off the resin. Cations having polynuclear or conjugated ring systems produced dyes having more desirable hues (bathochromic effect) compared to those having a single aromatic ring, but the latter apparently are better able to penetrate the interstices of the resin structure. 960
ANALYTICAL CHEMISTRY
In the previous study, the 4-nitrobenzenediazonium cation was superior in every respect except that hydrolysis of excess cation in the neutral to alkaline solutions necessary for coupling with most aromatic hydroxy compounds produced yellow 4-nitrophenol which was especially noticeable in blanks. From structural considerations, the 4 trimethylammonium benzenediazonium cation should show similar coupling reactivity and stability without producing colored hydrolysis products. While the quaternary ammonium group aTould not be a chromophoric group like the nitro group, it would impart a desirable solubility a t any pH to any azo dye it formed. The parent amine, (4-aminopheny1)trimethylammonium ion, was used in a recent method for the determination of nitrite and nitrate ions ( 2 ) . Available as a result of this concurrent study, it proved superior to other aromatic amines due to the ease of diazotization, the reactivity of the resulting
diazonium cation in coupling quantitatively to form an azo dye, and its solubility in all forms a t the various p H values used. The list of aromatic amine and hydroxy compounds tested differs somewhat from that investigated previously, due to the interest shown by biochemists in the previous study (1). A sufficient number are repeated to provide a basis for comparison of reactivities and dye colors. Diazonium Special Reagents. cation-resin combination, 4-trimethylammoniumbenzenediazonium cation on Rohm & Haas Amberlite IR-120 (H) nuclear sulfonic cation exchange resin. (4-Aminophenyl) trimethylammonium chloride monohydrochloride ( 2 ) . Buffer solutions, pH 5 , 7 , 9, and 12 (1).
Aromatic compounds to be tested, purest grades available. Preparation of Diazonium CationResin Combination. Dissolve 11.15 grams of (4-aminopheny1)trimethyl-
