Pyrohydro lytic Determination of ChI oride in Titanium Sponge A. R. GAHLER and GALEN PORTER Metals Research laboratories, Electro Metallurgical Co., Division of Union Carbide and Carbon Corp., Niagara Falls, N.
b A rapid and precise method for the determination of chloride in titanium sponge consists of pyrohydrolysis of the titanium sponge for 30 minutes a t 1000" C. in a nickel or quartz apparatus and titration of the chloride collected in the distillate. A single determination may b e carried out within 45 minutes.
A
accurate, and precise method is needed for the determination of chloride in titanium sponge. The usual methods described in the literature, including gravimetric, titrimetric, amperometric, and spectrophotometric procedures ( I , 2, 5), require from 3 to 50 hours of elapsed time for a determination. Dissolution of the sample is timeconsuming, and, if hydrofluoric acid is used, the procedure requires much attention by the analyst. It was evident that to develop a rapid method, an entirely new approach to the problem was necessary. An interesting and novel method for the determination of halides in inorganic compounds involves pyrohydrolysis. The pyrohydrolytic method for halides, as described by Warf (6) and later by Warf, Cline, and Tevebaugh {7), consists of passing steam over the sample in a platinum tube a t 1000" C. 'The halide is collected and determined i n the distillate. Although fluorides could be quantitatively pyrohydrolyzed, Warf and coworkers found that good results were not obtained for chlorides. However, it was felt that by suitable modifications, chloride in titanium sponge could be determined quantitatively by pyrohydrolysis in much less time than by conventional procedures involving dissolution of the sponge. The following study showed this to be true. RAPID,
APPARATUS
The pyrohydrolytic apparatus was similar to that of Susano, White, and except that the equipment was Lee (4, constructed entirely of nickel. This apparatus gave continuous service for 9 months before replacement was necessary. The apparatus may also be made of quartz, using a Vycor ball point (T 35/20) rather than a machined metal ball seal as a sample entrance. A 3-liter flask was used for the steam generator. Nickel boats (31/2 inches long, 6 / 8 inch wide, and inch deep) were used 296
ANALYTICAL CHEMISTRY
to contain the samples during pyrohydrolysis. To prepare the apparatus for use, steam generated from distilled water was passed through both the nickel and quartz apparatus at 1000" C. until the distillate indicated the absence of acidic substances. EXPERIMENTAL
An attempt was made to determine the chloride concentration indirectly by titrating the hydrochloric acid in the distillate with standard sodium hydroxide solution, but low results were obtained. It was found necessary to titrate the chloride, because some sodium chloride and nickel chloride are collected in the distillate along with the hydrochloric acid. Analysis of the distillate after pyrohydrolysis in an allnickel apparatus showed that 80% of the chloride was present as hydrochloric acid and 20% mas present as sodium chloride and nickel chloride, Even in an all-quartz apparatus using a nickel boat, nickel chloride was found in the distillate. The temperature for the pyrohydrolysis must not fall below 900" C. for chloride in sponge formed by either the magnesium- or sodium-reduction process. At 850" C., only 45% recovery of the chloride results (Table I). For this reason, the temperature is maintained a t 1000" C., well above the minimum temperature for 100% recovery. It was noted that, a t 900" C. and above, the titanium was converted t o titanium dioxide, whereas below 900" C. the sponge was only partially converted to the oxide. At 1000" C., 93% of the chloride in a sample containing 0.15% chloride is evolved during the first 15 minutes of the pyrohydrolysis, and 6% comes over during the next 1.5 minutes. Less than 0.5% of the chloride is found in the distillate in a third 15-minute period. Therefore. no more than 30 minutes is required for complete pprohydrolysis of titanium sponge. navis (S) passed oxygen along with the steam over the metal in determining fluoride in zirconium. It mas not necessary to use oxygen in the steam for chloride in titanium sponge. hccelerators, such as U308, which are required in determining chlorides in pure salts, were not required for promoting the volatilization of chloride from the sponge. Up to 3.0 grams of UaOa
Y.
mixed with the sponge did not affect the recovery of chlorides. Complete recovery of chloride resulted when the steam flow rate varied from 5 to 10 ml. of condensed water per minute. For ease in observing the end point of the titration, the rate of flow should not exceed 10 ml. per minute. so that not more than 300 ml. of distillate is collected. The recovery of chloride was found to be independent of sample size in the range of 1 to 5 grams. It was not practical to use a sampIe size greater than 5 grams, because of the dimensions of the nickel boat. The effect of sponge particle size was studied by riffling 5 pounds of sponge into three sizes: -20 mesh, f20 -8 mesh and +8 to 4 mesh. Chloride in each sponge size was determined by both the pyrohydrolytic method and a gravimetric method similar to that proposed by Thompson (J), which involves dissolution of the titanium sponge in hydrofluoric acid. The results in Table I1 show that particle size up to 4 mesh, which was the maximum size of any of the sponges tested, does not affect the recovery of the chloride in titanium sponge. It is possible that chloride in large, very dense pieces of titanium may not be recovered quantitatively by the pyrohydrolytic method because of difficulty in oxidizing the entire sample to titanium dioxide. Reduction
Table I. Effect of Pyrohydrolysis Temperature on Recovery of Chloride in Titanium Sponge
Chloride Recovery
T:mp., C. 700 800 850
%
25 25 45 100 100
900 1000
Table
II.
Effect of Mesh Size on Chloride Recovery
Chloride, % Gravimetric Screen Size Pyrohydrolysis method
-20 4-20 to - 8 +8 to 4
0.18 0.19 0.22
0.16 0.21 0.21
of the particle size is recommended in this case. Xickel boats, after an initial pyrohydrolytic treatment \\-lien nen , give blanks corresponding to less than 0.001% chloride, so that determination of a blank is not necessary. REAGENTS
Silver Sitrate (0.05LV).Drl crushed silver nitrate crystals at 130" C. for 1 hour Dissolve 8.495 grams of silver nitrate in 1 liter of water. Potassium Thiocyanate (0.05S). Dissolve 3.859 grams of potassium thiocyanate in 1 liter of water. Standardize with the silver nitrate solution. Ferric Ammonium Sulfate Indicator. Dissolve 400 grams of ferric ammonium EUlfate in 1 liter of water. Benzyl illcoho1 (chlorine-free). Eastman Kodak. RECOMMENDED PROCEDURE
Transfer 1 to 5 grams of titanium sponge to a nickel boat and spread the metal evenly throughout the boat. For sponge containing less than 0.1% chloride, do not use less than 2 grams. Place an 800-ml. beaker containing 25 ml. of water under the exit tube of the pyrohydrolytic apparatus, so that the tube is below the surface of the solution. With the steam passing through the apparatus a t a rate of from 5 to 10 ml. of condensate per minute, insert the nickel boat containing the sponge into the hot zone of the nickel tube. Close the entrance of the system as quickly as possible after introduction of the sample. Stir the water solution a t the condenser tip until the initial gassing has stopped (about 1 to 2 minutes). Samples of sponge containing much dust or small particles evolve a smoke during the first few minutes, which is not absorbed by the solution, but no chloride is lost. Continue passing the steam through the apparatus for 30 minutes. This should result in a distillate of 200 to 300 ml. During the last 15 minutes of the pyrohydrolysis, lower the beaker so that the surface of the condensate is just below the condenser tip. Rinse off the condenser tip with distilled water into the 800-ml. beaker. Add to the condensate 5 ml. of nitric acid, 10.0 ml. of standard silver nitrate solution (0.05N), 4 ml. of indicator, and 9 ml. of benzyl alcohol. Stir the solution and titrate with standard potassium thiocyanate solution (0.055) to the appearance of a faint red color.
Table 111. hydrolytic
Reproducibility of PyroMethod for Chloride in Titanium Sponge Av. ~- .
Value",
% Range, Sample Detns. Chloride % 7 0 045 0 008 WA48 S o . of
U
0 0 0 0
003 005 007 012
0 016 0 017 0 039 WA64 Averages are reported t o an additional WA46
WA65
12 12 9
0 083 0 148 0 li8
Q
decimal place to shol? range.
Table IV. Chloride Determination by Pyrohydrolytic and Gravimetric Methods
Sample WA48
WA46 WA65 \TA464
Pyrohydrolytic hlethod 0 0 0 0
Chloride Task Force
04.5 083 15 18
(Average) 0 048 0 096
0 16 0 18
+
Table V. Chloride Determination in Experimental Sponges b y Gravimetric and Pyrohydrolytic Methods
PyroGraviSample hydrolysis metric Difference 1 0.38 0.40 -0.02
RESULTS AND DISCUSSION
10 11
iifter investigation of the various factors of the pyrohydrolysis, the reproducibility and accuracy of the method were compared with other methods, The Panel on Methods of AnaIysis of the Metallurgical Advisory Committee on Titanium has issued several samples to various laboratories for cooperative work to develop methods of analyses for total chloride. These samples were useful for testing the pyrohydrolytic method, as it is not practical to make
12 13 14 15 16 17 18
19 20 21 22 23 24 25
0.37 0.55 0.53 0.19 0 22 o 18 o 22 0 21 0 22 0 34 0 24 0.25 0.19 0.19 0.31 0.29 0.29 0.17 0.13 0.11 0.08 0.10 0.10 0.21
0.37 0.54 0.57 0.21 0 21
o 0
16 21 0 21 0 24 0 38 0 22 0.28 0.20 0 17 0 31 0 25 0 30 0 16 0.14 0 10 0 07 0.11 0.09 0.22
+O.Ol -0.04 -0.02 +o 01
+n
~
~~~~
~
Table VI. Chloride Determination in Sponge by Pyrohydrolytic Method Using Nickel and Quartz Apparatus
standard additions of either solid sodium chloride or magnesium chloride to chloride-free titanium. The mesh 100. The reproducisize was -30 bility of the method is shown in Table 111, and a comparison of the results by the gravimetric method as tested by the Task Force on Chloride and the pyrohydrolytic method is given in Table IV. The pyrohydrolytic results on samples KA64 and n'A46 in Table 111 are an accumulation of the values obtained by three and four different analysts, respectively, rather than by one individual.
2 3 4 5 6 7 8 9
The method was further investigated by determining chloride by both a gravimetric method (6) and the pyrohydrolytic method in various types of experimental sponges prepared by sodium reduction of titanium tetrachloride. Several samples of unusually high chloride content were included to show the reliability of the method a t higher chloride concentrations. A comparison of the results from 25 samples is s h o m in Table V. S o systematic difference in the results by the two methods on this series of samples is evident. The feasibility of using a quartz apparatus for pyrohydrolysis rather than nickel nas also established, as. shown in Table VI. Quartz equipmenk ~ nickel tubing is particularly useful ' i hen is difficult to obtain, but quartz does not withstand prolonged use,
n2
+o oi
-0 02 -0 04 +O 02 -0.03 -0.01 +0.02
+o
04 -0 01
+0.01
-0.01
+o. 01 +o 01
-0.01 +0.01 -0.01
Chloride, % ' Sickel Q,uartz apparatus apparatus A
B C D E
0 0 0 0 0
29,O 28 22,O 22 19,0 19 38,O 38 26,O 25
0 0 0 0 0
28,O 31 21,O 23 20,o 18 40,O 37 25,O 25
CONCLUSIONS
A rapid and precise method for the determination of chloride in titanium sponge has been developed. An entire analysis requires no more than 45 minutes, less than 15 minutes of which is required for actual working time, compared with a minimum of 3 hours by other methods. Nore than one pyrohydrolytic apparatus may be run simultaneously by a single analyst. Kontechnical personnel can carry out the pyrohydrolytic determination without difficulty. The pyrohydrolytic method is applicable over wide chloride concentration ranges. It is especially useful for chloride concentrations of less than 0.05%, where the gravimetric procedure is less desirable because of the small difference in weight between the blank and the sample. Large and variable blanks, which are sometimes obtained in the gravimetric procedure from the use of hydrofluoric acid and from laboratory fumes because of long standing times, are not encountered in the pyrohydrolytic procedure. ACKNOWLEDGMENT
The assistance of John Kagdis, B. M. Brown, and members of the Analytical VOL. 29, NO. 2, FEBRUARY 1957
0
297
Department of the Metals Research Laboratories in collecting analytical data and the interest of R. M. Fowler are gratefully acknowledged. LITERATURE CITED
(1) Am. SOC.Testing Materials, Minutes
of Meeting, Titanium Task Group, Subcommittee VIII, Committee B-2, June 15, 1954.
(2) Codell, bl., Mikula, J. J., ANAL. CHEM.24, 1972 (1952). (3) Davis, P. C., U. S. Atomic Energy Commission, ISC-83 (May 10, 1950). (4) Susano, C. D., White, J. C., Lee, J. E., ANAL.CHEM.27, 453 (1955). ( 5 ) Thompson, J. XI., Zbid., 25, 1231 (1953). (6) Warf, J. C.. “Analytical Chemistrv of
the Manhattan “Project,” National Nuclear Energy Series, 1st ed.,
Div. VIII, vol. 1, pp. 728 ff., McGraw-Hill, New York, 1950. (7’) Warf, J. C., Cline, W. D., Tevebaugh, R. D., ASAL. CHEW26,342 (1954). RECEIVED for review September 18, 1956. Accepted October 19, 1956. Presented in part at Symposium on Analysis of Titanium and Its Alloys, Division of Analytical Chemistry, 128th Meeting, ACS, hlinneapolis, Minn., September 1955.
Simple Conversion for Automatically Recording Weight Changes with an Analytical Balance CLEMENT CAMPBELL and SAUL GORDON Pyrotechnics Chemical Research Laboratory, Picatinny Arsenal, Dover, N. 1.
This simple conversion of an analytical balance for the continuous automatic recording of weight changes, requiring no balance alterations, is based upon the hydrostatic principle that the displacement of liquid by a rod suspended into it from one end of the beam is proportional to the change in weight. A linear variable differential transformer i s used for electronically measuring and recording the beam deflections, and thus the changes in weight over ranges of micrograms to grams.
s
for continuously and automatically recording changes in weight with analytical balances (3) have been based upon a measurement of the beam deflection; the electromagnetic force required to restore and maintain the beam at its null point; or the position of a mechanical null point-restoring device. Photographic, mechanical pen linkage, or automatic potentiometric recording techniques have been most widely used for obtaining the change in weight curves. However, most of these techniques require an extensive modification of conventional balances or the construction of special apparatus. EVERAL METHODS
displacement of liquid by a rod suspended into it from one end of the balance beam as the sample on the opposite end of the beam undergoes a change in weight. This displacement, equivalent t o the change in weight, is a function of the rod diameter and the liquid density. It may be expressed as: W = +hd, where JV is the change in weight, r is the radius of the rod, h is the vertical displacement of the balance beam and therefore of the rod, and d is the density of the immersion liquid. By selecting the proper liquid and an appropriate rod diameter, it is possible to obtain any desired range of weight change for the balance beam deflection, which was found to be linear over a range of at least 5 mm. The recording system used by Tryhorn and Wyatt consisted of a light-source-tomirror optical lever, with the mirror attached to the beam so that vertical deflections of the light are proportional to balance beam displacements and
LINEAR VARIABLE DIFFERENTIAL TRANSFOR COIL,
therefore to changes in weight. These movements of the light beam were traced on photographic paper wrapped around a clock-motor-driven drum, so that the curve obtained was that of changes in weight as a function of time. With the widespread use of electronic recording techniques, the inherently inconvenient photographic methods have been superseded. I n order to adapt this hydrostatic balance principle to graphic recording with electronic pen-and-ink instruments, a linear variable differential transformer (1.v.d.t.) has been incorporated into the balance in place of the complicated optical system previously used for photographic recording. A linear variable differential transformer is a transducer which converts linear displacements into electrical signals linearly proportional to the movements involved. It consists of a transformer coil in which is placed a magnetic armature, free to move along the axis of the coil
7 ARMATURE DEMODULATON
PRINCIPLE
OF OPERATION
Tryhorn and Wyatt ( 5 ) constructed hydrostatic balance by means of which a conventional analytical balance could be used for automatically and continuously recording changes in weight. The operation, shown schematically in Figure 1, depends on the
RECORDER
8.
298
0
ANALYTICAL CHEMISTRY
Figure 1. Schematic diagram of converted analytical balance for automatic recording
