Effect of pH on High-Salt-Thorium Fluoride Titration DONALD F. ADAMS and ROBERT K. KOPPE Division
o f Industrial Research, Washington State Institute of Technology, State College ofwashington, Pullman, Wash. ml. of the sodium chloride solution to control dissociation and suppress interference (8), and 2.0 ml. of the acidified alizarin indicator. -4djust the pH of the resultant solution to 2.90 with 0.1S hydrochloric acid using a pH meter. Cse 0.1N sodium hydroxide, if necessary, in making the adjustment to 2.90. Transfer the pH-adjusted sample to a 100-ml. Sessler tube. Tit'rate the solution with acidified thorium reagent against a daylight fluorescent background using a 10-ml. microburet, Continue t,itration by the addition of small increments of thorium reagent until the color of the indicator matches the color of the permanent color standard. Standardize each new thorium reagent against the standard 10 y per ml. fluoride solution covering the titration range of 1 t o 100 y of fluoride. Plot the amount of thorium required in milliliters against the micrograms of fluoride. .L straight line may be drawn for the fluoride range up to about 50 y of fluoride. formula for t,he calculation of the total micrograms of fluoride found in the sample may be used for the straight-line portion of the curve:
Variations in pH of the final titration solution are shown to increase the variability with which microgram quantities of fluoride may be determined. A graphic correction of the observed titration volume based upon the pH of the final titration solution increases the precision of the method. The effect of small changes in pH of the final titration solution is also reduced by conducting the titration at 2.90 instead of at 2.70 as previously recommended.
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H E salt-acid-thorium fluoride titration method of William. as modified by Smith and Gardner (4-6) offers several advantages over the previously used thorium-alizarin titration ( 1 ) of microgram quantities of fluorides. The primary advantages of the modified titration procedure are the use of a single titration against a permanent color standard and the control 01 interfering ions and dissociation by the introduction of sodium chloride. The modified procedure was found to be more satisfactory than the back-titration procedure for routine fluoride analysis. However, it was observed upon utilization of this method over a period of time that the titration was not always as reproducible as might be anticipated. A study of the initial and final pH of titrated replicated samples indicated that the apparent fluoride content, as measured by the volume of thorium reagent required, was an inverse function of the pH of the final titration volume. The proposed modification is based on a study of the effect of the pH of the final titration values upon the observed quantity of fluoride.
(h41. of thorium for sample - ml. of thorium foe blank) X (thorium titer, fluoride/ml.) X (total vol. of sample)/(vol. of aliquot titrated) = y of fluoride Calculations above the straight-line portion of the curve must be made by reference to t'he titration curve. EXPERIM EYTS L
Five replicated samples of fluoride at) five different concentraadjusted to an initial tion levels-0, 20, 40, 60, and 80 ;-were pH of 2.6. Similarly, replicated samples at each of the five concentrations were adjusted to an initial pH of 2.7, 2.8, 2.9, 3.0, and 3.1. These samples ivere titrated as outlined in the procedure, Folloivirig completion of each titration, the pH of the final t,itration volume \vas determined. The volume of thorium reagent used was plotted against the final observed pH. -4s evidenced by the family of curves in Figure 1, the apparent fluoride concent.ration as indicated by the volume of thorium used is inversely related t'o the p€I of t,he final titration volume. Furthermore, as the pH becomes more acid, the titration syst,em becomes more sensitive t'o small changes in pH. The sensitivity of the system for fluoride decreases as the p H becomes more alkaline.
REAGENTS
Acidified thorium nitrate solution. Dissolve 0.268 gram of thorium nitrate tetrahydrate in distilled water, dilute to nearly 1 liter, adjust the pH to 2.90 with 1.ON hydrochloric acid using a p H meter, and complete dilution to 1liter. Acid-indicator solution. Dissolve 0.020 gram of monosodium alizarin sulfonate (Alizarin Red S) in water, add 16.1 ml. of 1.0-V hydrochloric acid, and dilute to 200 ml. Salt solution, 5A' sodium chloride. Hydroxylamine hydrochloride solution, 1%. Hydrochloric acid solutions, 1.0 and 0.1K. Sodium hydroxide solution, 0. I S . Permanent color standard. Prepare a stock solution containing 6.0 ml. of 0.65A- hydrochloric acid, 52.0 ml. of 3.66% cobaltous chloride (CoC1~.6H20),and 4.0 ml. of 0.10% potassium chromate, and dilute t o 100 ml. Dilute 5.0 ml. of this stock solution t o 100 ml. with distilled water in a Nessler tube. Keep the Kessler tube containing the permanent color standard tightly stoppered when not in use. Sodium fluoride solution. Dry reagent grade sodium fluoride in an oven at 110" C. for 24 hours. Cool in a desiccator, weigh 0.221 gram, dissolve, and dilute to 1 liter TTith disfilled water. Dilute 100 ml. of the stock solution to 1 liter to obtain a solution containing 10 y of fluorine per milliliter. Store both stock and dilute fluoride solutions in wax-lined or polyethylene bottles.
2.5L" 0
PROCEDURE
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The folloi~ingprocedure is used for determining 1 to 100 y of fluoride in solutions containing sodium hydroxide.
Figure 1. Effect of pH on volume of standard thorium required in high-salt-sodium fluoride titration
Isolate the fluorides from the original samples by the usual procedure of either single or double distillation (1-3, 7 ) , the distillate being neutralized with sodium hydroxide. Transfer an aliquot of the distillate of not more than 80 ml. into a 150-ml. beaker. If a smaller aliquot than 80 ml. is required t o obtain a fluoride concentration below 100 y, add sufficient distilled water to the aliquot to make the aliquot volume up t o 80 ml. ildd 1.0 ml. of the hydroxylamine hydrochloride to reduce any chlorine that might be present in the distilled sample ( I ) , 5.0
It is therefore proposed that the titration be carried out at an initial pH of 2.90 rather than the 2.70 as outlined by Smith and Gardner. This change represents a compromise between the minimum fluoride titer of the thorium reagent and the maximum changes in apparent fluoride concentration with small variations in p H of the final titration volume. 116
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V O L U M E 28, NO, 1, J A N U A R Y 1 9 5 6 Forty samples in the concentration range of 20 to 80 y of fluoride and p H range of 2.9 were titrated according to the analytical procedure outlined above. The fluoride content of each sample was then calculated in two manners: The actual volume of thorium nitrate required to titrate each sample was used, regardless of the pH of the final titration volume; and the volume of thorium nitrate was corrected for variation in pH of the final titration volume from the recommended 2.90 and used as the basis for calculation. Table I summarizes the comparative results obtained by the two methods of calculation. The mean per cent recoveries obtained by the two calculation procedures show excellent :tgreement. However, the range when no correction is made for variations in p H of the final titration volume is somewhat greater than that obtained when the suggested correction for pH variation is made. Comparison of the standard deviations obtained by the two calculation procedures indicates a marked increase in precision by correcting the volume of thorium nitrate used to a standard pH according to the proposed procedure. CORRECTIOS O F . w P . m E w FLUORIDE LEVEL
Table I.
Determination of Known Quantities of Sodium Fluoride Corrected for p H 40 20-80 99.7 93.8-103.7 2.8 2.83-2.94
ZTncorrected for pH 40
Number of samples Concentration range, y Mean recovery, Range of recovery. % Standard deviation, R Range of pH
20-80 100.4 92.5-107.8 7 7
2.83-2: 9;
upward or downward, until it intersects the ordinate representing a p H of 2.90. The actual volume of thorium nitrate which would have heen required by the given quantity of fluoride a t a p H of 2.90 is then determined at the vertical intersection of the point :it 2.90 and the abscissa. LITERATURE CITED
Chemists, Washington, D. C., “Official Methods of Analysis,” 7th ed., 1950. Remmert, L. F., Parks, T. D., Lau-rence,A. >I., and McBurney, E. H., ANAL.CHEM.25,450 (1953). Rowley, R. J., Grier, J. G., and Paraons, R. L., Ibid., 25, 1061 (1953). Smith, F. A., and Gardner, D. E., Arch. Biochem. 29, 311 (1950). Smith, F. A., and Gardner, D. E., J . Dental Research 30, 182 (1951). Smith, F. .I.,and Gardner, D. E.. U. S . Atomic Energy Commission, Bull. AECD-2161, 1948. Killard, H. H., and Winter, 0 . B.. ISD. ENG.C m x , ANAL. ED. 5 , 7 (1933). Williams, H. *4., Andust 71, l i s (1946).
(1) hssoc. Offic. Agr.
(2)
In order to improve the reproducibility of the modified Williams titrat’ion on replicated samples, the titration volume of
(3)
thorium reagent required for a given sample must be correct’ed for the effect of variations on pH from 2.90 of the final tit’rat’ion solution. A graphic correct,ion may be made by reference to a family of curves such as 8hoJT.n in Figure 1. The point of intersection of the final pH and the observed volume of thorium is first located within this family of curves, and t’he point’ is then moved in a manner parallel with the family of curves, either
(5)
(4)
(6)
(7) (8)
RECEIVED f o r review June 13, 1955. Accepted October 17, 1955.
Velocity Barrier to Eliminate Absorption of Carbon Dioxide during Titrimetric Procedures ARTHUR M. CRESTFIELD Department of Physiological Chemistry, University of California School of Medicine, Berkeley, Calif.
Absorption of acidic and basic gases from laboratory air is a difficulty associated with titrations involving solutions of low buffer capacity. RIethods of enclosing the solution in an inert atmosphere are generally cumbersome and not adaptable to the routine treatment of many samples. A simple and convenient new method permits access to the titration vessel at all times. A flat stream of nitrogen is directed horizontally across the top of the titration vessel so as to form a seal of rapidly moving nitrogen through which the acidic and basic gases may not diffuse. This velocity barrier is useful in the assay of ribonuclease activity.
Nozzle
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Vessel
Figure 1.
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Titration
Lucite nitrogen nozzle
URISG the development of a micromethod for the quanti-
tative determination of ribonuclease based upon the acidic groups which are liberated from the substrate (ribonucleic acid), it became necesbar) to carry out titrations on a small scale (2 ml.) on solutions of low buffer capacity ( 0 . 3 ~eq. per pH unit), with the assurance that no absorption of acidic or basic gases from laboratory air would occur. Several arrangements for the use of nitrogen were tried. The most efficient and convenient arrangement was found to be a nozzle constructed from Lucite, as shown in Figure 1. This nozzle fixes the position of the titration vevel with respect to the stream of nitrogen. The stream of nitrogen functions in
two ways to prevent the absorption of laboratory gases: (1) An aspirator-type action removes the gases from between the liquid and the streaming nitrogen, replacing them with nitrogen and water vapor from the sample; and ( 2 ) a velocity barrier action decreases the probability that a molecule of carbon dioxide, for example, could diffuse through the rapidly moving stream and into the titration vessel. The efficiency of the first action depends upon the velocity of the stream and its orientation with respect to the titration vessel. For this reason the titration vessel is shown to have a fixed position with respect to the efferent
