ANALYTICAL EDITION
234
sand, now used as filler, the acidity produced by these mixed fertilizers could easily be neutralized. Those manufacturers who use limestone or produce base-forming fertilizers would then get credit for the added value of their products and the farmer would be protected in that he would know the acidor base-forming properties of the fertilizers. Moreover, economy would result fro& the saving of freight on the sand or inert material now used as filler in mixed fertilizers. ACKNOWLEDGMENT The author wishes to express his appreciation for the cooperation given by T. B. Leith, State Chemist, and the following fertilizer companies in furnishing many of the samples of fertilizer used in this study: Armour Fertilizer Works, The Baugh and Sons Company, Gulf Manufacturing
Vol. 5 , No. 4
Company, F. S. Royster Guano Company, and the VirginiaCarolina Chemical Corporation.
LITERATURE CITED (1) (2) (3) (4)
Britton, H. T. S., “Hydrogen Iong,” Van Kostrand, 1929. Burgess, P. S., Rhode Island Agr. Expt. Sta., BUZZ. 189 (1922). Frear, D. E., J.Bid. Chem., 88, 675-81 (1930). Lipman, J. G., Blair, A. W., and Prince, A. L., Soil Sci., 19,
57-75 (1925). (5) MacIntire, W. H., and Shaw, W. M., IND. ENG.CEIW., 24, 1401-9 (1932). (6) MacIntire, W. H., and Shuey, G . A., Ibid., 24, 933-41 (1932). (7) Parker, F.W., Am. Fertilizer, 76, 2, 13 (1932). (8) Pierre, W. H., J. Am. SOC.Agron., 20, 254-69,269-79 (1928). (9) Pierre, W. I-I.,Ibid.,in press (1933). RECEIVED April 17, 1933. Publiahed with the approval of the Director of the West Virginia Agrioultural Experiment Station as Scientific Paper 128. Contribution from the Department of Agronomy and Genetics.
Colorimetric Determination of Fluoride in Water Using Ferric Chloride MARGARET D. FOSTER, United States Geological Survey, Washington, D. C. COLORIMETRIC method for the determination of fluoride in water, recently described ( I ) , is based on the fact that the intensity of the color produced with thiocyanate by a given amount of iron in the presence of fluoride is less than that produced in the absence of fluoride, by an amount depending on the quantity of fluoride present if there is an excess of iron. By determining colorimetrically the excess of iron reacting Kith ammonium thiocyanate, the quantity withdrawn by the fluoride from a given amount of iron may be found by difference and its equivalent in fluoride read from a curve which has been made by plotting definite amounts of fluoride against the iron they withdraw from the amount of iron used in the determination. In a volume of about 75 cc. 0.375 mg. of iron (5 cc. of a standard ferric chloride solution containing 0.075 mg. of iron per cc.) produces about as deep a color as can be read in a Schreiner colorimeter and was taken as the amount of iron to be used. On this amount of iron 0.25 mg. of fluoride produces a marked fading and even as little as 0.025 mg. produces a fading that can be detected in a colorimeter. This degree of sensitivity permits the use of small volumes of water, 50 or 100 or even 25 cc,, if the fluoride content of the sample is more than 4 parts per million.
larger quantities. Under the present condition, however, the iron withdrawn from reactivity with thiocyanate-that is, the bleaching effect of a given amount of fluoride-is definite and reproducible although not a straight-line function. I n the presence of 0.45 mg. of fluoride the color is more yellow than red and it is inadvisable to extend the curve beyond this point. TABLE 11. DETERMINATION OF FLUORIDE IN ARTIFICIAL WATERS ---so4
..
..
TOTAL FLUORIDE AS-TOTAL -C1 C1 Added Found SO4 CaClz MgCh NaCl MQ. Mg. MQ. Mg. Mg. Mg. Mg. 7 . 0 26.5 16.5 2 . 0 45.0 0.35 0.32 0.20 0.20 0.025 0.026 0 . 0 0 0.00
..
1.5
8.5
10.0
2.5
..
17.7
20.2
0.35 0.32 0.20 0.20 0.025 0 . 0 2 0.00 0 . 0 0
..
3.0
1.0
4.0
16.0
,.
4.0
20.0
0.35 0.35 0.25 0.24 0.025 0.025 0.00 0.00
..
3.1
16.8
19 9
9.2
..
16.5
25.7
0 . 3 5 0.325 0.20 0.20 0.025 0.025 0.00 0.00
..
20.0
20.0
1.4
1.2 29.6
32.2
0.35 0.20 0.025 0.00
4.0
1.5
..
5.5
6.3
2 . 0 27.5
35.0
0.35 0.34 0.20 0.20 0.025 0.026 0.00 0.00
16.4
13.5
2.6
32.5
13.0
..
7.6
20.6
0.35 0.20 0.025 0.00
0.31 0.18 0.02 0.00
0.35 0.20 0.025 0.00
0.36 0.21 0.03 0.00
TABLEI. FLUORIDE FOUND BY SUBTRACTINQ IRON WITHDRAWN ... BY SULFATE FROM TOTAL IRONWITHDRAWN SO4 AS NaaSO4
MO. 5.0 10 25 50 100 50 75 100 10 25 50 75
Fe TOTAL Fe WITHDRAWN Fe LEFT WITHDRAWNBY S 0 r FOR F Mg Mo. Mg 0.020 0.029 0.049 0.030 0.019 0.049 0.065 0.017 0.082 0.105 0.010 0.115 0.170 0.160 0.010
.
.
FLUORIDE Added Found Mo Mg. 0.025 0.03 0.025 0.02 0.025 0.02 0.025 0.01 0.025 0.01
.
AS---
Cas04 MgSO4 NazSOa Mg. Mg. MQ. 7.0
0.175 0.191 0.212
0,105 0.140 0.160
0.070 0.051 0.052
0.10 0.10 0.10
0.08 0.06 0.06
..
,.
2.5
2.5
10.6
3.2
7.6
21.4
0.159 0.195 0.217 0,212
0.03 0.065 0.105 0.140
0.129 0,130 0,112 0.072
0.20 0.20 0.20 0.20
0.17 0.17 0.14 0.075
4.7
4.4
10.9
20.0
3.2
..
2.2
5.4
I n Figure 1 fluoride is plotted against iron withdrawn. The amount of iron withdrawn does not increase directly with increase in fluoride. Smaller quantities of fluoride cause, proportioDately, a much greater bleaching than
0.33 0.20 0.020 0.00
0 . 23 05 0 . 23 04 0.025 0 . 0 3 0.00 0 . 0 0
July 15, 1933
INDUSTRIAL AND ENG INEERING CHEMISTRY
235
The possibility that iron in solution in the sample might increase the color with ammonium thiocyanate and counteract the effect of fluoride is negligible. Examination of more than 650 analyses of waters from all parts of the United States indicates that practically all alkaline waters when analyzed contain in solution less than 0.10 part per million of iron-that is, less than 0.005 mg. of iron in 50 cc. The details of the procedure follow: Neutralize the alkalinity of 50 cc. of the sample of water contained in a comparison tube with 0.05 N nitric acid. Do not add an indicator but calculate from the alkalinity determination the amount of acid necessary t o neutralize the volume of water used. If the alkalinity is high, use a stronger solution of nitric acid (0.02 N ) in order t o keep the volume of the neutralized sample within 60 cc. Add 5 cc. of ferric chloride solution (1 cc. = 0.075 mg. of iron, with 30 cc. of N hydrochloric acid to a liter) plus the amount necessary (by reference t o the curves) to counteract the effect of the sulfate and chloride present and 10 cc. of ammonium thiocyanate (24 grams in a liter). Make the volume up t o about 75 cc. and mix well. Compare in a Schreiner colorimeter with a standard containing a known amount of iron (2 t o 5 cc. of ferric chloride solution, the amount depending on the depth of color of the sample) and 10 cc. of ammonium thiocyanate solution in the same volume. Subtract the amount of iron found from 0.375 t o get the amount withdrawn. Its equivalent in fluoride may be read from the curve (Figure 1). The ammonium thiocyanate should be added t o the sample and standard at the same time and the comparison made immediately. The color of the standard should not be over 50 er cent greater or less than the color of the sample. If the cogr of the sample is too weak t o be comFIGURE 1. IRON WITHDRAWN FROX REACTIVITY WITH pared with a 2-cc. standard-that is, if the sample contains more THIOCYANATE BY DIFFERENT QUANTITIES OF FLUORIDE than 0.40 mg. of fluoride-a smaller sample should be taken. The sample used for the determination should not contain more than 50 mg. of sulfate or 100 mg. of chloride. If it conThe iron withdrawn from reaction with thiocyanate by tains less than 2.5 mg. of sulfate and/or 5.0 mg. of chloride, the other acidic constituents of natural waters was determined iron in excess of 0.375 need not be added. in the same manner as for fluoride. Most natural waters do not contain enough phosphate or borate in the volume used for the determination t o interfere with its accuracy. 450 4: Nitrate present in quantities as great as 75 mg. in the volume used has no effect on the color; 100 mg. effect a fading which 400 4C can just be detected in the colorimeter and which is equivalent to only 0.01 mg. of fluoride. For this reason nitric acid 3 5 0 , I5 was chosen to neutralize the alkalinity of the water sample. The quantities of sulfate and chloride likely to be found in water, however, have a decided bleaching effect on the 330 30 intensity of color produced by 0.375 mg. of iron with thiocyanate. The effect of sulfate and chloride is the same 2 5 0 ; 25 whether.they are present as calcium, magnesium, or sodium salts. I n the earlier paper this effect of sulfate and chloride 200 2c was corrected by subtracting, by reference to the curves, the iron withdrawn by the amounts of sulfate and chloride I50 15 in the unknown from the total iron withdrawn. On further study, however, this was found to give erroneous results 100 1 9 particularly for the larger concentrations of sulfate (as shown in Table I) because, in the presence of sulfate and 50 .5 chloride, the effect of the fluoride is not on 0.375 mg. of iron, on which the curve is based, but on 0.375 mg. minus o < the amount taken by sulfate and chloride. All three are competing for the iron. If, however, the amounts of iron FIGURE 2. IRON WITHDRAWN FROX REACTIVITY WITH withdrawn by the amounts of sulfate and chloride in the THIOCYANATE BY DIFFEREXTQUANTITIES OF OTHER sample are added to the sample’in addition to the 0.375 mg. ACIDICRADICALS FOUND IN WATER of iron regularly added, the effect of the sulfate and chloride It is advisable that each analyst work out his own curves for is counteracted and the effect of the fluoride is on 0.375 mg. fluoride, sulfate, and chloride, of iron. This was proved by a number of tests in which the The accuracy of the method was checked by adding 0.025, effect of different amounts of sulfate and chloride in samples 0.20, and 0.35 mg. of fluoride t o artificial waters containing the containing no fluoride was counteracted in this way (shown sulfate and chloride salts of sodium, calcium, and magnesium different combinations and amounts. The results are shown in Table 11). The resultant red color was equivalent, in in in Table 11. all tests, to 0.375 mg. of iron. If, however, the sample contains much more than 50 mg. of sulfate or 100 mg. of One hundred and thirty samples of ground and surface chloride, this does not hold true. So much excess iron must water from 28 states, tested by this method, ranged in be added that the bleaching effect is on 0.5 or 0.6 mg. of fluoride content as follows: 0 t o 0.5 part per million, 50; iron and not on 0.375. 0.5 to 1.0 part, 38; 1.0 to 2.0 parts, 25; 2.0 to 3.0, 11; and
236
ANALYTICAL EDITION
more than 3.0, 6. The highest fluoride found, 5.6 parts, was in a sample from a hot spring in California. If preferred, standards made by adding known quantities of fluoride, 0.10 to 0.40 mg., to 5 cc. of ferric chloride solution and 10 cc. of ammonium thiocyanate in the same volume might be used, preparing the samples as already described. This affords a direct determination of fluoride against fluoride standards, By using such fluoride standards, and by adding to the standards the quantities of sulfate and chloride (as sodium 'salts) contained in the volume of sample used, this method may be applied to the determination of fluoride in sea waters, brines, or any water containing more than 2500 parts per million of sulfate or more than 5000 parts of chloride. For such waters, however, 10 cc. of ferric chloride must be added to both sample and standard instead of the 5 cc. used when the effects of sulfate and chloride are counteracted by the addition of ferric chloride. This is necessary in order to have enough iron left for the fluoride after the sulfate and chloride have been satisfied.
Vol. 5, No. 4
This modification is recommended only for highly concentrated waters. It is not advisable to use it in preference to that in which the effects of these constituents are counteracted by the addition of ferric chloride for waters containing less than 2500 parts per million of sulfate or 5000 parts per million of chloride because (1) if 5 cc. of ferric chloride solution are used in sample and standard, the iron left for fluoride after the sulfate and chloride have been satisfied may be so reduced as greatly to restrict the fluoride range and sensitivity; (2) if 10 cc. are used the color in water containing low sulfate, chloride, and fluoride would be too deep to read; and (3) a different set of standards might be required for each sample. (1) Foster, M. D.,
LITERATURE CITED J. Am. Chem. Soc., 54, 4464 (1932).
RECEIVED March 7, 1933. Presented before the Division of Water, Sewage, and Sanitation Chemistry a t the 85th Meeting of the American Chemical Society. Washington, I). C., March 26 to 31, 1933. Published by permission of the Direotor, U. 8. Geological Survey.
Determination of Fluorides in Illinois Waters C. S. BORUFF AND G. B. ABBOTT,State Water Survey, Urbana, Ill. An adaptation of the volumetric thorium preconcentration, drying, and disOTTLED tooth enamel has been found to be the most tillation of silicon fluoride from is caused by the concipitation dry residues in the presence of ingestion of reliable for the quantitative examination of sulfuric acid and silica (i7) with fluorides during the period the waters for fluorides. The fluoride content of a subsequent determination of the teeth are being formed (8, 18, soluble fluoride by one of many 19). Mottled teeth have been number of Illinois waters has been determined by m e t h o d s found in the literathe use of this method. found in p r a c t i c a l l y e v e r y F~~~ the data obtained it that the tUre, have been found tedious country. This condition is enand u n s a t i s f a c t o r y (23,27). demic over large areas in the waters containing the greater quantities of The technic is exacting. This southwestern part of the United fluorides are those which are found in and around m e t h o d w a s used in the first States and is frequently found in studies r e p o r t e d b y S m i t h , other s e c t i o n s (8, 11, 12, 1.4). the northeastern and northwestern part of the Lantz, and Smith (19). Illinois coal basin. All of these waters r u n high Studies already published have The method of Fairchild (9) largely l i m i t e d the p r i m a r y in nonincrustingsolids. T~ date the results has been modified and used by Sources Of thediet to have not shown fluorides to be characteristic of Churchill (r), Hale ( l i ) ,and that found in drinking waters (11-13, 18, 19). Bauxite, Ark., any one stratum since they are found in waters Smith (21). This method calls for the addition of a ferric salt and Oakley, I d a h o , have disassociated with rocks of Pennsyhanian, Silurian, which f o r m s ferric fluoride or and Ordovician age. carded abundant water supplies the complex FeF,j--- with the in favor of others containing less fluorides ( l a ) , and other cities fluorides present. The excess are considering similar action. The exact minimum concen- ferric ion is then caused to react with potassium iodide, and tration of fluorides that will cause mottled teeth is not known. the liberated iodine is titrated with thiosulfate. The reIt has been reported by some to be about 2.0 p. p. m, (21) liability of the method depends on the stability of the ironwhile others claim it to be about 1.0 p. p. m. (16). fluoride complex and on a quantitative reaction between Lack of agreement in results obtained in this and other ferric iron and potassium iodide. The reaction laboratories while analyzing waters for small quantities of 2Fe+++ 21- = 2Fe++ 1 2 fluorides, using methods reported in the literature, together with the fact that extremely little is known concerning the is reversible, and, if it is to be used in a quantitative defluoride content of Illinois waters, led the Water Survey to termination, the iodide must be added in large excess (26). initiate the following investigation. This is strictly a chemi- Fairchild and Churchill have not used sufficient potassium cal study; no attempt has been made by the authors to iodide. Upon testing the reliability of their methods, the correlate the presence or absence of mottled teeth with the present writers found that the amount of iodine liberated by fluoride content of the Illinois waters which have been ex- the reaction of 5.0 cc. of approximately 0.08 M ferric chloride with the 10 cc. of 5 per cent potassium iodide in distilled amined. Of the numerous methods found in the literature for the water, according to their procedures, required an amount of quantitative determination of fluorides, there seem to be only 0.025 N thiosulfate which varied from 13.7 to 15.2 cc. Others a few which might be utilized to determine the small quan- (6) have noted similar variations in this reaction. Increasing tities (0 to 15 mg. per 1.) found in waters. Gravimetric and quantities of potassium iodide (iodine-free) required incolorimetric methods are fraught with many difficulties and creasing volumes of thiosulfate to react with the liberated uncertainties (6, 9, 11, 16, 22, 24, 26, 27). Likewise, the iodine. Similar tests made by adding definite volumes (1
M
+
+
