(Methods f o r Analysis of Titanium Alloys)
Determination of Phosphorus in Titanium Alloys MAURICE CODELL AND JAMES J. MIKULA Pitman-Dunn Laboratories, Frankford Arsenal, Philadelphia, Pa.
T
HE gravimetric and volumetric methods commonly used for
determining phosphorus in steels and irons were found to be unsatisfactory when applied to the analysis of titanium and titanium alloys. Not only did these procedures prove to be excessively time-consuming, but interferences were encountered from alloying elements. A colorimetric method for the determination of phosphorus, based upon the blue color obtained by the reduction of molybdiphosphoric acid, was therefore selected for developmental work in connection with titanium, because of its recognized simplicity, accuracy, and speed. Many articles have been published concerning the reduction of molybdiphosphoric acid to a blue color which is proportional to the amount of phosphate ion incorporated in the complex. Much of the early work on the heteropoly blue method has been summarized by Woods and hiellon (8). The reaction of molybdenum blue for determination of phosphorus has been utilized in biochemical analysis for many years. The means of obtaining the blue compound by employing various reducing agents is summarized by Snell and Snell(7) and by Yoe (9). A survey was made of available methods for the determination of phosphorus, and attempts were made to adapt various procedures to titanium. Multiple precipitation of titanium with ammonium hydroxide was tried, and phosphorus was precipitated from the solution after filtration. Considerable quantities of phosphorus were obtained from the solution even after the fourth precipitation. Attempts were made to determine phosphorus gravimetrically by quantitative precipitation as the ammonium phosphomolybdate compound. Precipitation of phosphorus as the phosphomolybdate compound is greatly retarded by the presence of titanium and fluorides. The effects of fluorine can be overcome by the addition of boric acid which forms harmless fluoboric acid (6). It was found that precipitation could be effected in the presence of a considerable quantity of titanium by adding a large excess of molybdate reagent to a boiling solution containing a sufficient amount of ammonium nitrate. Boiling in the presence of ammonium nitrate, however, caused hydrolysis of titanium, This difficulty was overcome by removing the sample from the hot plate after boiling and adding a boiling solution of ammonium nitrate, followed immediately by the addition of molybdate reagent. An overnight setting period is required for complete precipitation. In order to obtain a more ideal composition of ammonium phosphomolybdate, the precipitate was dissolved in ammonia, the acidity and ammonium nitrate concentration were adjusted, and the precipitation was carried out below 45' C. While the results obtained M ere considered acceptable, they tended to be high. I t was found that tungsten and vanadium interfered. Tungsten frequently occurs in titanium metal both as a contaminant and as an alloying metal. Methods of removing tungsten are extremely difficult and impractical to apply in this analysis. Because of the obvious disadvantages inherent in this procedure, it was considered advisable to seek a different approach to the problem. The determination of phosphorus as the magnesium ammonium phosphate compound was then attempted in accordance with the procedure outlined by Lundell and Hoffman (6) for phosphate rock. Results appeared to be fairly satisfactory when applied to titanium, although Epperson ( 1 ) states that the presence of citric acid causes low results. The length of this procedure, however, is prohibitive, and the sensitivity of the method did not appear to be adequate for the quantity of phosphorus which may be ex-
pected to be found in titanium. Attempts to increase the sensitivity bv increasing the sample size were unsuccessful because quantities of titanium much in excess of 1 gram prevent the precipitation of magnesium ammonium phosphate. d colorimetric procedure based on the work of Hill ( 3 ) was attempted. This method utilizes the molybdenum blue reaction and is carried out in a nitric acid solution. Sodium fluoride is added to suppress the ionization of nitric acid and to complex ferric ions. Solution is attained by dissolving the sample in nitric acid. I t was necessary when applying this procedure to titanium, to use a mixture of nitric and hydrofluoric acids in order to effect solution without the possibility of losing some phosphorus. Xitric acid alone will not dissolve titanium, and hydrofluoric acid alone might result in loss of phosphorus as phosphine. Boric acid was added to form fluoboric acid and thus prevent the attack on glassware in subsequent operations. Silica showed a definite interference by increasing the final color intensity. Since silica remains in solution as fluosilicic acid and is not removed during the course of the given procedure, it was found necessary to carry out the procedure in platinumware and to remove silica as the tetrafluoride by adding sulfuric acid to the solution and evaporating the sample to fumes. The addition of boric acid was no longer necessary, since all the fluoride was removed from solution by this procedure. After dilution the solution is oxidized with potassium permanganate in order to ensure the complete conversion of phosphorus to the ortho form. Manganese is then reduced to the divalent state by the addition of sodium nitrite. The solution is diluted to ,z definite volume, and an appropriate aliquot is treated with a reagent consisting of sodium fluoride, stannous chloride, and ammonium molybdate. The solution is finally heated for a few minutes in a uater bath, and the intensity of the blue color developed ir read on a spectrophotometer. A stable color could not be obtained by using the quantity of sodium fluoride which was recommended for steels (3). I t was therefore necessary to increase the sodium fluoride concentration of the sample in order to complex titanic ions, since the molar quantity of 0.5 gram of titanium is greater than that of 0.5 gram of iron. Because of its low solubility, the amount of sodium fluoride required to complex the titanium could not be completely incorporated in the reagent. The additional sodium fluoride necessary was therefore added to the solution of the sample. An attempt to eliminate the necessity of adding fluoride salt to the sample was made by preparing a reagent using potassium fluoride, Lvhich is considerably more soluble than sodium fluoride. The blue color developed when potassium fluoride was used did not appear to be as stable as the color obtained with sodium fluoride, and the quantity of potassium fluoride used in preparing the reagent had to be more critically controlled. APPARATUS AND REAGENTS
Spectrophotometer, Universal Coleman Model 14 or equivalent apparatus having optically matched cuvettes. I n this procedure optically matched, round cuvettes having a 19-mm. light path were used. Cuvettes having other dimensions may be used by suitable adjustments in the amount of sample and the reagents used. Platinum crucibles or evaporating dishes, 3C-ml. capacity or larger, with platinum covers. Volumetric flasks, 250-ml. capacity. Beakers, 250-ml. capacity. Sitric acid, concentrated C.P. grade. Hydrofluoric acid, 48% BCS reagent grade.
1444
V O L U M E 25, NO. 10, O C T O B E R 1 9 5 3 Sulfuric acid, concentrated C.P. grade. Sulfuric acid, 1 to 6. Potassium permanganate, 3%. Sodium nitrite, 3%. Sodium fluoride, C.P. grade. Standard monobasic ammonium phosphate solution. Dried C.P.monobasic ammonium phosphate (0.1857 gram) is dissolved in 1 liter of TTater. One milliliter equals 0.05 mg. of phosphorus or 0.01 % phosphorus for a 0.5-gram sample. Ammonium molybdate, 8% solution. Sodium Fluoride Solution, 3%, containing 0.2% stannous chloride. Three grams of sodium fluoride and 0.2 gram of stannous chloride dihydrate are dissolved in 100 ml. of water. This solution is not stable on standing because the tin is s l o ~ l yoxidized. Erratic results will be obtained if this solution is not fresh. I t is therefore recommended that this solution be prepared the day it is to be used. Plastic Bottles, approximately 500-ml. capacity. Bottles in which hydrofluoric acid is shipped are very convenient. Plastic Dropper. A dropper can be made by heating and drawing out the end of a piece of saran tubing, '/*-inch bore, '/Isinch wall, and attaching a rubber bulb to the other end. The over-all length of the tube should be sufficient to prevent any of the liquid from entering the bulb of the dropper. PREPARATIOh OF CALIBRATION CURVE
Several representative-size aliquots of standard monobasic ammonium phosphate solution, covering the desired range of 0.05 to 0.60 mg. of phosphorus, are placed in platinum crucibles, and an additional platinum crucible is carried through as a blank. To each crucible 0.5 gram of phosphorus-free titanium drillings is added. The standards are carried through the entire procedure beginning with the addition of 3 ml. of concentrated nitric acid The calibration curve is prepared by plotting the logarithm of the transmittancy values obtained against the concentration of phosphorus per 250 ml. of solution, using water as the reference cell.
1445 Table I.
Analysis of Titanium Metal
Phosphorus, '70 Recovered Added (ar.) 0.0033 0.0105 0.0593 0.1004 0.2970 0.4976
0.005 0.010 0.060 0.100 0.300 0.500
Table [I. Element Tungsten Nickel Silica Chromium Chromium Molybdenum Aluminum Vanadium Iron
Number of Determinations
0.0008 0.0012 0.0021 0.0011 0.0071 0.0131
Analysis of Synthetic Titanium Alloys _ Phosphorus _ _ _ _ Added, % Recovered, %
~
Amount Added, %, 1 3 3 6 15 10 5 2.5 20
0.060 0.060
0.100 0.100 0.100
0.100 0.100
0,100 0.100
0.061 0.059 0.102
0.098 0.100 0.099 0.099 0.099 0.100
and measure the transmittance a t 650 mp against water. From the calibration curve, determine directly the percentage of phosphorus present in the sample. The calibration curve will accommodate values up to 0.12% phosphorus. In cases of samples with higher phosphorus content, the sample may be brought within the limits of the curve by diluting an aliquot of the sample with a solution of phosphorusfree titanium which has been prepared by the same procedure used for the sample. An aliquot of this diluted solution is then processed as described in the procedure. A blank should be run with each set of determinations, and a correction should be made for any phosphorus present due to reagents. RESULTS
PROCEDURE
To a 0.5-gram sample of titanium drillings in a platinum crucible, add 3 ml. of concentrated nitric acid. From a plastic dropper, cautiously add 5 or 6 drops of hydrofluoric acid and immediately cover with a platinum cover. Heat slightly in order to initiate reaction. After the reaction has ceased, add 5 or 6 drops of hydrofluoric acid. Continue in this manner until all the titanium is in solution and a total of approximately 3 ml. of acid ha3 been added.
Standard Deviation, %
The results in Table I indicate the accuracy of the method. One half gram of phosphorus-free drillings from cast titanium was used in these analyses. Phosphorus was added to the titanium in the form of monobasic ammonium phosphate solution of known concentration. The calculated concentration of phosphorus in the reagent blanks ranged from 0,000 to 0.003% in 250 ml. of solution, using water as the reference cell.
It is essential that the reaction go to its completion after each addition of hydrofluoric acid and that the portions specified not be exceeded; otherwise the violence of the reaction will result in loss of sample. After the crucible has cooled, add 5 ml. of concentrated sulfuric acid, cover, and take to light fumes on a hot plate. Fume the sample for 3 to 4 minutes, remove from the hot plate, and allow to cool. Using 7 ml. of sulfuric acid (1 to 6) wash the sample from the crucible and cover into a 250-ml. beaker. Heating the crucible may be necessary to ensure complete transfer of the sample. Wash the crucible and cover a few times with Jmter, transferring all washings into the beaker. (The solution may occasionally appear to be slightly cloudy a t this point owing to partial hydrolysis of titanium; however, this will cause no interference since the solution is filtered later.) Cover the beaker n-ith a watch glass, place on hot plate, and bring the solution to boiling. From a graduate add 1.5 ml. of 3y0 potassium permanganate. Boil the sample for 2 minutes, then add 1.5 ml. of 3% sodium nitrite. Continue to boil an additional 45 seconds, remove from hot plate, and allow the sample to cool. Wash the sample into a 250-mI. volumetric flask, add 2.0 grams of sodium fluoride to the flask, dilute to the mark with water, and shake well t o dissolve the fluoride completely. ( A delay much in excess of 1 hour a t this point in completing the procedure mill lead to high results due to the attack of fluoride on the volumetric flask. This danger of silica interference may be eliminated by transferring the solution to a plastic container where it mag be stored indefinitely.) Filter off any turbidity. Immediately before use. mix 20% by volume of ammonium molybdate solution and 80% by volume of the sodium fluoride-stannous chloride solution. To a suitable aliquot of sample in a test tube, add an equal volume of this newly prepared reagent. Heat for 3 minutes in boiling water, cool,
Oo2001
1 vi l/iI
I
I INSTI"*L#T
COLLYAY U N l Y E l l l L § P E C T R O P I ( O T O I € T I I
MODEL NO I4
In Table I1 the elements listed were added to titanium in the form of their soluble salts in proportions nhich were equal to 0.5 gram of alloy. DISCUSSION
Several investigators have reported interference due to alloying elements in steel. Hague and Bright ( 2 ) have found interference from quantities in excess of 2% chromium, 15% copper, 5 % vanadium, and 35% ni~kel. Katz and Proctor ( 4 ) were able to
1446
ANALYTICAL CHEMISTRY
determine phosphorus in alloys containing up to 28% chromium. The purpoae of the n ork reported here was to develop a method which shows no interference from alloying elements in concentrations which would be expected to occur in commercial titanium alloys. S o interference was observed with titanium samples containing 15% chromium, 10% molybdenum, 5% aluminum, 2.570 vanadium, 1% tungsten, 20% iron, 3% silica, and 3% nickel (Table 11). Synthetic alloy samples xvere prepared by adding the various elements to the titanium in the form of their soluble salts. In order to obtain a suitable working range, a curve was constructed at 650 mp. The molybdenum blue compound was found to follow Beer’s law a t this wave length for the concentrations of phosphorus plotted on the cali1,ration curve (Figure 1 ) . Good color stability has heen observed up to 1 hour, after which the blue color begins to deepen owing to reduction pf excess molybdenum. The concentrations of reagents specified in the procedure may be varied somewhat. The limits established by Hill ( 3 ) were found to be applicable to titanium. Variations up to 10% in acid concentration, 2.5% in fluoride concentration, and MYo in sodium nitrite concentration may be tolerated, while wider variations are permitted in ammonium molybdate and potassium permanganate Concentrations.
The sample may be diluted with either sulfuric acid or nitric acid. The molybdenum blue color appeared to be somem-hat more stable in sulfuric acid, although excellent results can be obtained with either acid. LITERATURE CITED (1) Epperson, A. IT.,J . Am. Chem. SOC., 50,332 (1928).
(2) Hague, J. L., a n d Bright, H . A , , J . Research .Vatl. Bur. Standurds, 26, 405-13 (1941). (3) Hill, E. T.,- 4 9 . 4 ~ . CHEM., 23, 1496-7 (1951). (4) Katz, H . L., a n d Proctor, K. L., I h i d . , 19, 612-14 (1947). (5) Imidell, G. E. F., and Hoffman, J. I., I n d . Etu. Chena., 15, 44 (1923). (1;) Lundell, G. E. F., a n d Hoffman, J. I., J . Research .VatZ. B w . Stnndards, 19,59-64 (1937). ( 7 ) Snell, F. D., a n d Snell, C. T., “Colorimetric Methods of dnalysis,” Vol. 1, New York, D. Van Nostrand Co., 1936. (8) Woods. J. T., and Mellon, hf. G., IND.ENG.CHEM.,. ~ N A L . ED., 13,760 (1941). (9) Toe, J. H., “Photometric Chemical Analysis,” Vol. 1, New York, J. Wiley & Son-8, 1928. RECEIVEDfor review September 20, 1952. Accepted December 22, 1982. Presented before the Pittsburgh Conference on Analytical Chemistry and Applied Spect,roscopy, Pittsburgh. Pa., March 2, 1953.
(Methods j o r Analysis of Titanium Alloys)
Colorimetric Determination of Small Amounts of Boron in Titanium Alloys M.4URICE CODELL AND GEORGE YORWITZ Pitman-Dunn Laboratories, Frankford Arsenal, Philadelphia, Pa.
S
MALL amounts of boron have a marked effect on the mechani-
cal strength and grain structure of titanium alloys; consequently, the determination of small amounts of boron in titanium alloys is a matter of considerable importance. Hitherto, no methods specifically developed for the determination of boron in titanium alloys have been published. In an attempt to apply standard procedures to the determination of boron in titanium alloys, various methods were considered. The volumetric methods were found to be rather untrustworthy for the small amounts of boron (O.OOO0 to 0.10%) which would ordinarily be encountered in titanium alloys. Among the volumetric methods that were considered were the titration with alkali in the presence of mannitol after a prior separation of the boron as methyl borate (8, 13), the titration with alkali in the presence of mannitol after a separation of the interfering elements with calcium carbonate (10, 18), and the potentiometric titration after prior distillation of the boron as methyl borate (20). The gravimetric method, whereby the boron is weighed as calcium borate (9),was also found to be inaccurate for small amounts of boron. Colorimetry seemed to offer the best chance for a successful attack of the problem; therefore, the various colorimetric reagenta available for boron were considered. Quinalizarin (2,19) and carmine ( 7 )seemed undesirable because of the intense color of the reagent itself. Curcumin (1, 11) and turmeric (6, 21) leave something to be desired because of their limited range. Procedures using turmeric paper (14, 16) are relatively inaccurate. The colorimetric reagent deemed most suitable waB 1,l-dianthramide. The use of this reagent for boron was first proposed by Ellis, Zook, and Baudisch (4). It was subsequently applied to the determination of boron in aluminum alloys by Brewster (3). Neither the method of Ellis, Zook, and Baudisch nor the method of Brewster is directly applicable to the determination of boron in titanium alloys because of the in-
terference from titanium and other metals that might be found in titanium alloys. Ellis, Zook, and Baudisch applied their procedure to the determination of boron in plants, a type of material that contains relatively little inorganic salts. Brewster in his method for aluminum alloys used a very small sample. This eliminates the interference due to aluminum but makes the method inapplicable to the accurate determination of small amounts of boron (less than 0.01%). In view of the marked interference of titanium with the colorimetric determination of boron by dianthramide, a prior separation of the boron was clearly necessary. Various separations were considered. Separations involving the use of sodium hydroxide, ammonium hydroxide, or calcium carbonate were rejected because of occlusion of boron by the precipitate (12). Also, filtration of gravimetric precipitates can definitely lead to high results due to boron picked up from funnels, filtering devices, or filter paper. Winsor (21) has shown, in this regard, that very significant amounts of boron can be picked up from filter paper. The use of a mercury cathode for separating boron from other elements (16, 17) was not applicable, because titanium does not deposit into the mercury cathode. In view of the unsatisfactory nature of the above separations for boron, the only feasible solution of the problem was to use the methyl borate distillation method. However, before the dianthramide colorimetric procedure could be applied to the methyl borate distillate, many problems had to be overcome. First of all, the manner of distillation of the methyl borate had to be worked out. I t n a s decided to use a sulfuric acid solution of the sample in the distillation, The use of hydrochloric acid was considered unsatisfactory because the hydrochloric acid would distill over with the alcohol and cause difficulty by making the distillate very acid. On neutralizing the excess acid prior to driving off the alcohol a considerable quantity of salts would be formed. These salts
