hotronic Colorimeter and Its Application to the Determination of Fluoride L. V. WILCOX,U. S. Department of Agriculture, Bureau of Plant Industry, Riverside, Calif. N CO”ECTION with the study of natural waters, a need was felt for a photoelectric colorimeter, and after Some Preliminary experimental work the apparatus here described was assembled. It uses as its light-measuring
I
elements two Weston photronic cells, which are photoelectric cells of the type that transforms light energy directly into e* electrical energy without the use Of an f* In the present assembly the cells are electrically opposed and the current develoned bv them is balanced bv means of a variable resistance. A sensitive galvanometer is used as the indicating instrument. The intensity of the light transmitted through a colored solution is measured in terms of the resistance that must be interposed to balance the photronic cells. The apparatus has been satisfactorily used for the determination of fluoride in natural waters by a modification of the acetylacetone method described b y Armstrong (1). I n earlier tests the thiocyanate reagent and method, essentially as described b y Foster (3), were used, but it has been found that the acetylacetone reagent is somewhat less susceptible to the effect of sulfate and chloride than the thiocyanate reagent. A
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DESCRIPTION OF APPARATUS A schematic diagram of the apparatus and circuit is shown in Figure 1.
dicated below, with a cell filled with the reference solution in each box. The reference solution in the cell in box B2is replaced by the unknown solution and the set is balanced by adjusting the variable resistance, VR. The change in this resistance from the original setting of 1000 ohms to some new value measures the difference in light transmission of the two solutions. For the present purpose this offersno advantage, but there may be applications where it would be desirable. Likewise, the use of allglass cells arranged in a horizontal position or Nessler cylinders in a vertical position might be indicated. Such modifications could be made without changing the electrical circuit. The general arrangement of the set is shown in Figure 2. I n the foreground are two glass cells of the type used. They are of metal, lacquered inside, with plain glass sides. To the left of the cells is an amber-colored glass, the resistance reading of which is frequently noted as a measure of the reproducibility of the instrument. The resistance required to compensate for this glass is 390 ohms, with variation from day to day not exceeding *3 ohms. OPERATIONOF PHOTRONIC COLORIMETER Fifteen minutes before use, the lamps of the set and the galvanometer are turned on. The variable resistance, V R , of Figure 1, is set a t 1000 ohms and the switch, Sw,is left open. Ten minutes later, the switch is closed and the slit, S1, is adjusted until the galvanometer shows no deflection. The set is then kept in balance by adjusting the slit until it remains in equilibrium. This requires only a few minutes, after which the set is ready for use. The switch is opened and into the box, Bz, a reference solution in a glass cell is introduced. The switch is closed and the set brought back to balance by means of the variable resistance. (The adjustment of the slits should not be disturbed during the measurements.) The resistance is recorded. The process is repeated with the unknown solution substituted for the reference solution, using the same glass cell.
The light, W , is a 50-watt, 110-volt mill-type bulb. The plate-brass light housing has a ground-glass covered opening, D, 0.5 inch (1.27 cm.) in diameter in each light path. Heavy, seamless brass tubes 2.06 inches (5.2 cm.) inside diameter connect the light openings with the solution cabinets, or boxes, B1 and BI, and like tubes, in turn, connect these with the photronic The ratio of the resistance reading of the unknown solution cells, P, P, mounted in the two extremities. The lenses, L, between the light and the boxes, render the light rays parallel to that of the reference solution bears a definite relationship and those beyond the boxes converge them slightly toward the to the concentration of the unknown solution. The method faces of the photronic cells. Slit 81has two moving parts and can be adjusted from a fully open position to completely closed. of establishing and interpreting this relationship will be disSlit L ! , has a sinele moving Dart which. when closed, will cover cussed below. aboucone-third f; the lighrpith. Glass light-filters may be The Model 594 Weston hos u b s t i t u t e d in place of the tronic cells are of photovoytaic ground-glass disks, D,Figure 1. type and in operation resemble For the fluoride determination o r d i n a r y voltaic cells. The positive poles are connected digreen glass filters, having a low rectly and the negative poles are t r a n s m i s s i o n in the red, inc o n n e c t e d through the resistcreased the s e n s i t i v i t y apances. The variable resistance preciably. is a decade box of 11,110 ohms in steps of 1 ohm. The fixed reWhere small differences are sistance has a value of 1000 significant, scrupulous cleanliohms. The galvanometer has a ness of the optical parts and sensitivity of 0.025 microamperes per scale division. The the cells must be maintained. resistance of the coil of the galThe condition of the apparavanometer is 1000 ohms. The tus in this respect can best be 110-volt alternating current cirdetermined by measuring frec u i t , n o t shown, includes a switch and a double outlet for quently the resistance of the the lamps of the colorimeter and I cell f i l l e d w i t h d i s t i l l e d the galvanometer, water. It was originally planned to the reference and unFIGURE 1. DIAGRAMOF PHoTRoNIC COLORIMETER, The switch, Xw, Figure 1, SHOWING ELECTRICAL CIRCUIT known solutions. For this purshould not be closed for any VR. Variable resistance pose two boxes, Bi and Bz Of P. Photronic cells length of time when the set is F R . Fixed resistance L Lenses Figure 1, were provided in the G Galvanometer B I ,Bz. Boxes far off balance as a condition set* To make such a measureSw: Double-polesingle-throw switch D Ground glass over openings ensues which, in effect, resernment, the set is balanced, as in- sl, SZ.Variable slits W Light 167
ANALYTICAL EDITION
168
bles polarization and from which the photronic cells recover rather slowly. The electrical circuit here presented offers several advantages: a balanced circuit, which largely eliminates effects resembling polarization; a "null" measurement, indicated by a sensitive galvanometer; recorded values in terms of ohms resistance rather than scale divisions; quickly established equilibrium; and a negligible drift,
DETERMINATION OF FLUORIDE IN NATURAL WATERS The procedure described by Armstrong ( I ) has been adapted for use with the photronic colorimeter. Only minor changes were found necessary. REAGENTS.Ferric nitrate solution, 1 cc. = 0.2 mg. of iron in 0.02 N nitric acid; acetylacetone solution, 0.35 per cent aqueous solution; standard fluoride solution, 1 cc. = 0.1 mg. of fluoride; salt solution, 29.2 grams of sodium chloride and 30.1 grams of magnesium sulfate in 1000 cc. of water; 0.1 N nitric acid; p-nitro henol indicator solution, 1 gram in 100 cc. of 70 per cent ethyl aEohol; approximately 1.0 N nitric acid; sodium hydroxide, carbon dioxide-free, approximately 0.5 N .
FIGURE2. PHOTOGRAPH OF PHOTRONIC COLORIMETER PROCEDURE. Transfer an aliquot of 100 cc. of the water or an aliquot containing not more than 0.5 mg. of fluoride t o a 250-cc. beaker. Add one drop of p-nitrophenol indicator and 1.0 N nitric acid until colorless and 0.5 cc. excess. (Bring t o a boil and allow to boil 2 minutes, stirrin vigorously t o expel carbon dioxide. Remove from the flame anfcool to room temperature. While the samples are cooling, .prepare the ferric-acetylacetone reagent, 10 cc. for each determnation, as follows: 3 parts of acetylacetone solution, 2 parts of salt solution, 5 parts of ferric nitrate solution. Mix and allow to stand until ready for use. When the samples are cool, add carbon dioxide-free 0.5 N sodium hydroxide until a faint yellow color of p-nitrophenol develops. Adjust to colorless with 0.1 N nitric acid, adding one drop in excess. Make up to 110 cc. Add 10 cc. of the ferric-acetylacetone reagent and mix. After 10 minutes, compare in the photronic colorimeter as described above. With each set of determinations, include a sample of distilled water containing all the reagents but no fluoride, Calculate the ratio of the resistance reading of the unknown to that of the fluoride-free sample and read the fluoride concentration of the unknown from curves worked out in the calibration of the instrument. Solutions of known fluoride concentration covering the range from 0 to 5 p. p. m. were made up with distilled water and analyzed by the procedure described above. The results are shown in Table I. TABLEI. ANALYSIS OF PUREFLUORIDE SOLUTIONS FLU OR ID^
VARIABLE RESISTANCE READINQ Fluoride solution Blank"
P.p . m.
Ohms
Ohms
0 0.5 o 1.0 n
623 534
523 523 523 523 523
RATIO^
1.000 1.021 1.050 1.090 1.130 1 im 1.210 (I Resistance required to balance a solution of the reagents in distilled water, in this caae, t h e 0 p. p. m. fluoride solution. b Obtained by dividlng the figures of column 2 by the correaponding figures in column 3.
-.-._
It will be seen from these figures that for a change of 5 p. p. m. of fluoride the resistance changed 110 ohms or an
Vol. 6, No. 3
average of 22 ohms per p. p. m. Thus the indicated sensitivity for a change of one ohm is 0.05 p. p. m. of fluorine. The accuracy of the method is discussed below.
TABLE11. EFFECT OF SULFATE AND CHLORIDE ON DETERMINA TION OF FLUORIDE
ANION
RATIOSOB RESISTANCE IN SALT SOLUTIONS TO RESISTANCE IN DISTILLED WATER Fluoride, Fluoride, CONC~NTRATION0 p. p. m. 5 p. p. m. M.E./liter" 1.019 1.025
1.210 1.216 1.210 1.207
The effect of chloride has been investigated over a greater range than is reported in Table 11, and it is believed that the effect of this ion may be neglected in concentrations not exceeding 25 milligram equivalents per liter. Silica, borate, and phosphate, in amounts normally present in natural waters, likewise appear to be without measurable effect, Hydrogen ion is controlled sufficiently accurately by the procedure indicated above but it' should be emphasized, in this connection, that carbonates must be expelled. Sulfate appears to be the only ion commonly found in natural waters for which a correction must be made in the colorimeter readings. The reagents here differ somewhat from those suggested by Armstrong. The salt solution is an addition to the list. I n its absence, the bleaching effect of chloride and sulfate is much greater. Ferric nitrate in nitric acid is much more stable than ferric chloride. Such solutions have been in use for several months and appear to be unchanged.
CALIBRATION Solutions of known fluoride concentration covering the range from 0 to 5 p. p. m. are made up with distilled water and analyzed by following the procedure described above. On one axis the fluoride concentration in parts per million is plotted and of the other the ratio of the resistances of the fluoride solutions to that of the fluoride-free solution. Such a curve is shown in Figure 3.
FIGURE3. RESISTANCE RATIOSAT VARIOUS FLUORIDECONCENTRATIONS IN PRESENCE OF SULFATE The only ion thus far encountered in the natural waters
of the West that measurably interferes with the fluoride determination is sulfate. It appears to be very difficult, if not impossible, to remove this constituent without a loss of fluoride, but, in view of the fact that the effect is approximately linear and additive, the correction for it is comparatively simple. Prepare a set of pure fluoride solutions as directed above, to which add 20 milligram equivalents of sulfate per liter, assuming
May 15, 1934
INDUSTRIAL AND ENGINEERING
a 100-cc. sample. Complete the determinations and plot the curve. It will be found to be below but nearly parallel t o the first, as reference to Figure 3 will show. In like manner complete additional curves to cover the range of sulfate likely t o be encountered. Then, knowing the sulfate concentration of the sample, the fluoride concentration can be determined by interpolation. For example, suppose the determined resistance ratio is 1.150. If sulfate is absent, the indicated fluoride concentration is 3.5 p. p. m. If there are 10 milligram equivalents of sulfate, the corresponding fluoride is 3.4 p. p. m., and for 20 milligram equivalents the fluoride is 3.25 p. p. m.
ANALYTICAL RESULTS Fluoride determinations on a series of natural waters are reported in Table 111. I n each case the fluoride was determined in duplicate. TABLE111. FLUORIDE IN NATURAL WATERS DUPLICATE DETERMINATIONBResistP. p. m. ResistP. p. m. ance Ratio fluoride ance Ratio fluoride
7 -
LAB.
No.
7887 7909 8031 8078 8080 8090 8146 8164 4
SULFATEBLANK" Y . E . / l . Ohms 523 10.8 12.3 522 3.2 522 522 3.0 2.5 522 522 0.5 2.0 522 52 1 2.5
539 537 548 549 526 544 563 549
1.031 1.029 1.050 1.052 1.008 1,042 1.079 1.054
540 1.033 0.45 537 1.029 0.40 548 1.050 0 . 9 5 549 1.052 1 . 0 526 1.008 0.10 543 1.040 0.80 564 1,080 1 . 7 549 1.054 1.05
0.40 0.40 0.95 1.0 0.10 0.85 1.7 1.05
CHEMISTRY
169
the determinations were repeated. The determined values were within *O,l p. p. m. of fluoride of the expected value. I n the second experiment, a large sample of a natural water was collected. The sample contained 11.6 milligram equivalents per liter of sulfate. Triplicate determinations by the method here described showed 0.40, 0.40, and 0.45 p. p. m. of fluoride. Duplicate aliquots were concentrated and distilled by the hydrofluosilicic acid procedure described by Boruff and Abbott (8). The fluoride in these distillates was determined colorimetrically by the procedure here suggested and found to be 0.40 and 0.40 p. p. m. Other aliquots were distilled as above and the fluoride in the distillate titrated with standard thorium nitrate solution following Boruff's procedure (2). The result in this case was 0.45 p. p. m. of fluoride. It is concluded, therefore, that the accuracy of the procedure is within *0.1 p. p. m. of fluoride. ACKNOWLEDQMENT The author wishes to express his appreciation to F. N. Cowperthwait, of the Weston Electrical Instrument Corporation, for helpful suggestions, and to Geo. B. Collins, formerly of this laboratory, for assistance in assembling the colorimeter.
Resistance, a t balance, of a solution of reagents in distilled water.
LITERATURECITED
To test the accuracy of the method, two experiments were undertasken. I n the first, the fluoride concentrations of a series of natural waters were determined, then to each of separate aliquots 2 p. p. m. of fluoride were added and
(1) Armstrong, W. D., IND.ENG.CHEM.,And. Ed., 5, 300-2 (1933). (2) Boruff, C. S., and Abbott, G. B., Ibid.,5,236-8 (1933). (3) Foster, M. D.,lbid., 5 , 234-6 (1933). RECEIVIUD September 23, 1933.
Use of Trichlorobenzene in Analysis of Graphite Greases FRANK M. BIFFEN,Foster D. Snell, Inc., 305 Washington St., Brooklyn, N. Y.
C
OMMERCIAL greases frequently contain chemically better solvents, make it a very satisfactory reagent for such inert materials, particularly graphite, from which it is extraction. The following procedure for determination of troublesome to separate such ether-insoluble materials as cal- graphite in greases containing calcium soap is illustrative of its cium stearate. To carry out the separation for gravimetric use. determination of graphite, a number of solvents have been PROCEDURE compared, some of them differing from the usual types. As Heat 5 grams of the grease with 75 cc. of trichlorobenzene to a result of this a method for extraction of calcium stearate about 165" C. Centrifugalize while still hot. Decant off the from graphite by trichlorobenzene has been developed. The bulk of the trichlorobenzene. Heat the residue with a further method is also applicable to other heavy metal soaps such quantity of solvent and repeat the operation. Shake the residue as aluminum stearate, magnesium stearate, etc., which are with a hot 50 t o 50 alcohol-benzene mixture, filter through a tared filter paper, and wash with the same mixture. Dry the even more readily soluble than calcium stearate. in the oven and weigh. There are given in Table I the series of solvents investi- residue If very great accuracy is required, after trichlorobenzene exgated, their boiling points, and other desirable solubility traction, shake with hot 50 to 50 alcohol-benzenemixture acidified data. These data show that a number of solvents will dis- with hydrochloric acid. Filter, and wash well, first with the solve this heavy metal soap, but that in no case is it highly mixed solvent, then with alcohol, and finally with hot water. soluble in the cold solvent, The high boiling point of tri- Dry and weigh. All filtrations are rapid. D 24, 1934. ohlorobenzene, combined with the fact that it is among the R E C E I V ~February QUALITATIVE SOLUBILITY OF CALCIUM STEARATE TABLEI. (0.5 gram of calcium stearate in 10 cc. of solvent) SOLVENT Acetone Benrene Butyl aloohol Carbon tetrachloride Ethylene glycol monoethyl ether Diethylene glycol o-Dichlorobeniene
Isopro yl alcohol P r o p y f k e dichloride Trichlorobenaene Trichloroethylene Toluene Turpentine Xylene Slightly soluble, some had dissolved.
BOILINQPOINT
At 20'
.4t 600
56 80 117 76 135 245
Insoluble Moderately Slightly Slightly Insoluble Insoluble
Insoluble Moderately Slightly Slightly Insoluble Insoluble
179
Slightly
Moderately
SOLUBILITY~
At boiling point
c.
Slightly Slightly 83 Insoluble Insoluble 97 Slightly Slightly 218 Slightly Slightly 87 Noderately 111 Moderately Moderately Slightly 156 Moderately Slightly 140 Moderately soluble, roughly 50 per cent had dissolved.
Insoluble Moderately Soluble but cloudy Soluble but cloudy Calcium stearate melts clear Moderately soluble Soluble but cloudy Separates rapidly on cooling Slightly Slightly. Completely soluble and clear a t 165' Slightly Moderately Soluble b u t cloudy Soluble but cloudy
