Determination of Small Amounts of Chlorides in Titanium Sponge MAURICE CODELL AND JAMES J. R;IIKULA Pitman-Dzcnn Laboratories D e p a r t m e n t , Frankford Arsenal, Philadelphia, Pa. In titanium casting operations, even low concentrations of chloride cause spattering of molten metal and interfere with the production of satisfactory castings. It is therefore necessary to have a procedure by which the chloride content of titanium sponge can be accurately determined. A n accurate and fairly rapid method for determining chloride in titanium sponge has been developed which is satisfactory for chloride concentrations in the range 0.001 to 0.209%. The production of titanium sponge is increasing rapidly and methods for quality control must be capable of handling large numbers of samples simultaneously. This procedure is well adapted for use in routine analysis and can be carried out with apparatus common to most laboratories.
PHOTOhIETRIC procedure for the determination of small amounts of chlorides in titanium sponge was chosen, because the absolute amounts of chloride present are so small. Khen an ammoniacal solution of silver chloride is treated with a soluble sulfide, a colloidal suspension of silver sulfide is formed. The resulting color is amber and the amount of silver can be quantitatively determined by photometric means ( 7 ) . Kuroda and Sandell ( 4 ) developed a satisfactory method for determining small amounts of chlorine in silicate rocks based on this procedure. Many other applications of this procedure have been made in biochemical analysis. Titanium was originally dissolved by digesting with sulfuric acid, but this required several hours. It was found that solution could be effected rapidly through the use of hydrofluoric acid. The excess hydrofluoric acid was converted t o the harmless fluoboric acid, This procedure proved entirely satisfactory. Vhen chlorides are precipitated from solution containing titanium, sufficient acidity must be maintained to prevent hydrolysis of titanium (6). The coagulation of silver chloride in the presence of titanium cannot be hastened by any of the usual procedures. Heating causes rapid hydrolysis of the titanium, and shaking by mechanical means appears to retard coagulation. Considerable efforts were made to separate silver chloride by centrifuging, but in many cases results were low because of “creeping” of very fine silver chloride particles and their subsequent loss on decantation. It was found most practical to permit the precipitate to settle overnight in a dark place. The silver chloride could be readily filtered the following morning, redissolved with ammonium hydroxide, and treated with sodium sulfide to form a colloidal suspension of silver sulfide which is quantitatively determined by use of a spectrophotometer. Ions forming insoluble silver salts, such as bromides and iodides, must be absent. It has been the authors’ experience that at least 10 to 15 minutes are required after the addition of the sodium sulfide for full development of the color, which remains quite stable for about 1 hour, then gradually begins to deepen. The silver sulfide sol formed by the addition of sodium sulfide to the ammoniacal solution was found to be unstable on longer standing in the concentrations of chloride reported here. A cloudy solution resulted in most cases on standing overnight. Attempts were made to stabilize the silver sol by using gelatin as a protective colloid in accordance with the instructions of Snell and Snell ( 7 ) but no appreciable difference could be observed. Equally accurate results were obtained with or without the addition of gelatin solution. It was found advisable to take all transmittance
readings a t approximately the same time interval after the addition of the sodium sulfide, owing to the instability of the color. Low results were obtained by taking readings beforelfull color development, while readings taken much later yielded high results. Repeated tests showed that readings taken between 15 to 30 minutes after the addition of the sulfide yielded the most accurate results. The calibration curves and all results reported here are, therefore, based on readings taken between 15 and-30 minutes after the sulfide addition. SCOPE
This method is satisfactory for determining chlorides in titanium sponge when the chloride content lies within the range of 0.001 to 0.200%. CONCENTRATIOY RANGE
Therecommended concentration range is from 0.10 to 2.00 mg. of chloride in 100 ml. of solution and from 0.01 to 0.10 mg. of chloride in 10 ml. of solution using matched 13-mm. cells. In this procedure optically matched cells having a 13-mm. light path were used. Cells having other dimensions may be used, if suitable adjustments are made in the amounts of sample and the reagents used. APPARATUS AND REAGENTS
Apparatus. Spectrophotometer, Universal Coleman Model 14 or equivalent apparatus having optically matched cuvettes. Selas crucibles, No. 2001. Plastic beakers of approximately 250-ml. capacity viith covers (polyethylene or polystyrene). A 500-ml. suction filtering flask with an adapter suitable for holding a KO.2001 Selas crucible and having a drawn-out stern which can fit into the neck of a 10-ml. volumetric flask. Volumetric flasks, 100-ml. and 10-ml. Reagents. Standard sodium chloride solution. Dissolve 0.1648 gram of dried C.P. sodium chloride in 1 liter of distilled water. 1ml. = 0.10 mg. of chloride. Dilute nitric acid, specific gravity a proximately 1.20. Dilute 403 ml. of concentrated nitric acid gpecific gravity 1.42) to 1 liter with distilled water. Hydrofluoric acid, 48%, ACS reagent. Boric acid, C.P. grade. Silver nitrate solution, approximately 0.1 AT. Dissolve 4.25 grams of C.P. silver nitrate in 250 ml. of distilled water. Nitric acid wash solution, 1%. Dilute 5 ml. of concentrated nitric acid (specific gravity 1.42) to 500 ml., in a wash bottle with water. Ammonium hydroxide solution, 1 t o 1. Dilute 250 ml. of concentrated ammonium hydroxide (28%) with 250 ml. of water. 1972
V O L U M E 2 4 , NO. 1 2 , D E C E M B E R 1 9 5 2 Sodium sulfide solution, approximately 0.1 M . Dissolve 2.5 grams of sodium sulfide nonahydrate in 100 ml. of distilled water. PREPARATION O F CALIBRATION CURVES
A. Chloride Concentrations between 0.01 and 0.20%. a.Place representative sizedaliquotsof standardsodium chloride solution, covering the desired range 0.10 to 2.00 mg. of chloride, in 200-ml. beakers, and carry through an additional beaker as a blank. b. To each beaker add 60 ml. of distilled water, 5 ml. of dilute nitric acid, and 4 ml. of silver nitrate solution. Mix thoroughly and let stand in a dark place overnight. c. Filter the precipitate on a Selas crucible. Wash beaker twice with small portions of nitric acid wash solution; finally, remove any remaining precipitate with a policeman, and wash twice with small portions of water. d. Discard the filtrate and wash the flask thoroughly. Dis~ o l v ethe precipitate completely by using 20 ml. of 1 to 1 ammonium hydroxide and wash several times n-ith water. e . Transfer the filtrate t o a 100-ml. volumetric flask, add 1 m!. of sodium sulfide solution, dilute to volume with water, and mix. f. After 15 minutes, transfer a suitable portion of the solution to an absorption cell and measure the transmittancy at 415 mp, using as a reference cell the blank carried through from above containing all the reagents. Plot the logarithm of the transmittancy values obtained against the concentration of chloride per 100 nil. of solution.
1973 Transfer to a 200-ml. beaker, add 5 ml. of dilute nitric acid, and heat carefully on a hot plate until the solution becomes colorless. Stir constantly and avoid excessive heat because of the danger of hydrolysis. Dilute to 60 ml., add 4 ml. of silver nitrate solution, and allow to stand overnight in a dark place. Continue in accordance with A , c, as in the preparation of the calibration curve. For samples containing 0.01 to 0.20% chloride continue in accordance Kith A , d, e, and j . For samples containing 0.001 to 0.01% chloride, continue in accordance rn-ith B, c. From the proper calibration curve determine directly the concentration of chloride present in the sample. Run a blank with each set of determinations to eliminate any possible error due to varying amounts of chloride in the reagents from one determination to the other. RESULTS
The results listed in Table I indicate the accuracy of the method. h 1-gram sample of chloride-free cast titanium drillings was used in each of these analyses. Chloride R-as added to the titanium in the form of sodium chloride solution of known concentration. The calculated concentration of chloride in the reagent blanks remained constant at 0.01% in 100 ml. and 0.001% in 10 nil. of solution, using distilled water as the reference cell. DISCL-SSIOK
Table I. Accuracy of AIethod CI Added, ?c 0.0010 0.0020 0.0040 0.0060 0.0080 0.0100 0.015 0.022 0.033 0.047 0.055 0.070 0,087 0,099 0.105
0.120 0.155 0.178 0.199
Avera, e C1 Foun2, %
Standard Deviation. %
0.0010
....
0.0023 0.0035 0.0060 0.0079 0,0102 0.015
0.022
0.033 0.046 0.056 0,069 0.087 0.099 0.106 0.120 0.155 0.178 0.199
No. of Determinations
0.0002
0.0004 0.0001
0.0004
0.0013 0,001 0.001 0.002 0.001 0.000 0.001 0.000 0.001 0.’003
0 001
B. Chloride Concentrations between 0.001 and 0.01 %. a. From a buret measure exactly 50 ml. of standard sodium chloride solution into a 500-ml. volumetric flask. Dilute to the mark with distilled water and mix thoroughly. One milliliter will then contain 0.01 mg. of chloride. b. Place representative sized aliquots of this standard sodium chloride solution, covering the desired range 0.01 to 0.10 mg. of chloride, in 200-ml. beakers, and carry through an additional beaker as a blank. Continue in accordance with A , b andc. c. Break the suction and discard the filtrate. Place a 10-nil. volumetric flask inside the filtering flask and insert the drawn-out stem of the adapter into the neck of the volumetric flask. Add 2 ml. of 1to 1 ammonium hydroxide to the crucible. Allow sufficient time for the precipitate to dissolve completely. Connect the suction and wash several times with 1-ml. portions of distilled water until the total volume is approximately 8 ml. Remove the volumetric flask and to it add 0.1 ml. of sodium sulfide solution. Dilute to volume, and shake. Continue in accordance with A,f. d. Plot the logarithm of the transmittancy values obtained against the concentration of chloride per 10 ml. of solution. PROCEDURE
To a 1-gram sample of titanium in a plastic beaker, add 10 ml. of distilled water and 4 ml. of hydrofluoric acid, and cover immediately with a plastic cover. Carry a blank through, adding everything but the aample. After the titanium is completely dissolved (15 to 20 minutes), add 1 gram of boric acid, mix thoroughly, and dilute t o 20 ml.
Several colorimetric methods for the determination of chlorides exist. Clarke ( 2 ) determined chloride in water by adding acid, mercuric ion, and diphenylcarbazone to the chloride solution and measuring the excess of mercuric ion in terms of color intensity. Siggia (6) developed a colorimetric method for determining micro amounts of silver and silver halides based on a spot test described by Feigl ( 3 ) . The reaction .4gX Kp[Ni(CN)4] +.K[Ag(CN)j] Ni(CX)Z KX is carried out in a pyridine-ammoniawater system containing dimethylglyoxime and the color intensity of the resulting solution is measured. Baker and Reedy ( 1 ) developed a sensitive test for silver and the halides based on the production of a bright orange color when a solution of potassium iodide that has been saturated n-ith mercuric iodide is added to a silver chloride precipitate. They claim that this procedure is not quantitative, hovmw-, and the test must be made without diluting the reagent, as dilution causes the separation of a red precipitate of mercuric iodide. Any of the preceding colorimetric methods or many others could possibly be adapted to yield satisfactory results in the determination of chloride in titanium sponge. Horn-ever, no work along these lines has been attempted by the authors, as the procedure described above proved entirely satisfactory in precision, simplicity, and freedom from interferences.
+
+
+
ACKNOWLEDGMENT
The authors wish to express their appreciation to Herbert Engelbert for his assistance in performing some of the analyses reported here. LITERATURE CITED
(1) Baker, P. S., and Reedy, J. H., IND. ENG.CHEN.,AXAL.ED., 17,268-9 (1945). (2) Clarke, F. E., ANAL.CHEM., 22,553-5 (1950). (3) Feigl, F., “Qualitative Analysis by Spot Tests,” p. 254, New York, h-ordeman Publishing Co., 1939. (4) Kuroda, P. K., and Sandell, E. B., ANAL. CHEST.,22, 1144-5 (1950).
(5) McAlpine, R. K., and Soule, B. A., “Prescott and Johnson’s Qualitative Chemical Analysis,” p. 385, New York, D. Van (6) (7)
Nostrand Co., 1933. Siggia, S., ANAL.CHEM.,19,923-4 (1947). Snell, F. D., and Snell, C. T., “Colorimetric Methods of Analysis,” Vol. 1, p. 533, New York, D. Van Nostrand Co., 1936.
RIDCEIVED for review July 24, 1952.
Accepted September 12. 1952.
