Table VI.
Sample Mineral oil diethyl carbonate
+
Determination
Calcd. 0
of Oxygen by Fast Neutron Activation Oxygen content, %
Chemical analysis GR&DC Commercial lab ... 0.03, 0.12, 0.06
0,252 ... 0.556 ... 1.094 ... 5.028 Heavy gas oil . . . 0.30,'0.33 Fuel oil distillate . . . 0.22, 0.24 Furnace oil distillate . , . 0.26, 0.23 Light furnace oil . . . 0.14,0.12 Furnace oil ... 0.20, 0.19 Mineral oil dibutyl 0.727 ... Carbitol Errors are standard deviations.
+
0.10, 0.11, 0.31, 0.37, 0.34, 0.43, 4.94, 5.04, 0.20, 0.20
0.47 0.51 1.08 5.12
Neutron activation" 0 f 0.004 0.261 f 0.007 0.550 f 0.007 1.10 f 0.01 5.04 f 0.07 0.176 f 0.005, 0.178 =k 0.005
0.25, 0.20
0 . 1 3 1 r t 0.005, 0.134=tO0.O05
0.14, 0.18
0.077 f 0.003, 0.071 f 0.003
0.17,0.22 0.14, 0.18
0.152 f 0.005, 0.151 f 0.005 0.117 f 0.005, 0.115 =k 0.005
0 67, 0.80
0.7'25 f 0 007, 0 72 f 0 01
Neutron activation results are given in Table VI for a group of synthetic samples containing known amounts of added oxygen and a group of typical petroleum product samples. The poor agreement with chemical analysis loses most of its significance when the unreliableness of the chemical method in the range below 1%total oxygen, as exemplified by the data in the third and fourth columns of Table VI, is taken into consideration. The two activation analysis values given for the last six samples
are from runs made several days apart In using different sample bottles. all runs, the samples were cycled a t least twice; blank runs were made with helium. CONCLUSIONS
The fast neutron activation method allom oxygen to be measured faster and with a higher precision and accuracy than any other method yet developed. Oxygen concentrations greater than 100 p.p.m. can be routinely measured to
within 10% in less than 10 minutes; a t 30 p.p.m., the relative standard deviation on a result is estimated to be about 25%. T o lower the limit of detection and to improve the accuracy of analysis for samples with less than a few hundred parts per million of oxygen, a sample bottle containing less oxygen than the one employed in this work is required. ACKNOWLEDGMENT
The authors thank J. M. Orange for assistance in constructing the accelerator, transfer system, and counting equipment and in obtaining the data used i n this paper. LITERATURE CITED
(1) Coleman, R. F., Perkin, J., Analyst 84, 233 (1959). (2) Ibid., 85, 154 (1960). ( 3 ) Dinerstein, R. A., Klipp, R. IT., AYAL.CHEJI.21, 545 (1949). (4) Lbov, A. A , , Naumova, I. I., At. Energ. USSR 6, 468 (1959). (5) hIeulen, H. ter., Chem. Weekblad 19, 191 (1922). (6) Oita, I. J., Anal. Chim. Acta 22, 439 (1960). 17) Schutze. AI., Naturwissenschaften 27, k22 (1939). (19391. ' ' 822 (8) Steele, E. L., Pvleinke, R.W.,ANAL. CHEN.34, 185 (1962). (9) Unterzaucher, J., Ber. 73B, 391 (1940). (10) Veal, D. J., Cook, C. F., ANAL. CHEW34, 178 (1962). R E C h I V E D for review September 17, 1962. Accepted October 24, 1962. Division of Fuel Chemistry, 141st PIIeeting, ACS, Washington, D. C., March 1962.
Determination of Phosphorus in Hypereutectic Aluminum-Silicon Alloys by a Neutron Activation Method ROBERT BLACKBURN' and BRIAN
F. G. PETERS
Tube Investments Research Laboratories, Hinxfon Hall, Cambridge, England
A method to determine small amounts of phosphorus in hypereutectic aluminum-silicon alloys is described. The method consists of the thermal neutron activation of the alloy followed by its complete dissolution in 1 to 3 hydrofluoric-nitric acid mixture. The phosphoric acid so formed is converted to the molybdophosphoric acid complex which i s extracted into isobutyl alcohol. The isotope P32 is then determined by a P-counting technique using a liquid counter. Extraction of phosphorus is highly efficient and selective, and the loss of phosphine on dissolution in the acid mixture used i s negligible.
10
ANALYTICAL CHEMISTRY
A
alloys are of considerable technical importance in casting processes, both eutectic and hypereutectic alloys being utilized. K h e n casting such alloys in the hppereutectic region, homogeneous crystallization of the melt is ensured by the addition of small amounts of phosphorus which reacts with the aluminum t o produce aluminum phosphide, the latter constituting an efficient nucleating agent for the silicon. As the amount of phosphorus retained in the alloys is of considerable metallurgical significance, it is necessary to have an accurate method for the determination of this element in these LURIINUM-SILICOX
materials. The phosphorus is usually present to the extent of less than 0.01% in the preience of 20% silicon and i t is particularly important with such a small concentration that any losses of phosphorus, the bulk of which is associated with the silicon, are avoided. Such losses are usual in the case of metals containing phosphorus, as phosphine is readily liberated by the action of many reagents, particularly those which produce nascent hydrogen. Even simple machining 1 Present address, The Royal Military College of Science (Physics Branch), Shrivenham, England.
operations cause considerable loss of phosphorus, either by phosphine evolution or by preferential loss of the phosphorus-rich silicon, and for analytical purposes it is advisable to use relatively massive lumps, produced by percussion, not filings or turnings obtained by a shearing action. -4 number of general methods for the determination of phosphorus have been described (2, 8 ) but, as Kuhn (5) has pointed out, these are unsatisfactory in the particular case of aluminum-silicon alloys. The method adopted by Kuhn consists of absorbing the phosphine liberated during hydrochloric acid attack of the alloy in brominated hydrobromic acid, the whole operation being carried out in a closed system. The siliconrich residue is then dissolved in hydrofluoric-nitric acid reagent, and the phosphorus contents of the two fractions are determined by the conventional molybdenum blue method. This procedure, although producing consistent results, is extremely laborious. Moreover, since the results for a number of hypereutectic alloys analyzed a t the laboratories of the British Aluminium Co. Ltd. by an adaption of Kuhn's method showed no correlation with values quoted by other workers for samples having a closely similar metallurgical history, it was desirable to devise an independent method for the determination of phosphorus in alloys of this type. I n particular it was desired to check whether the hydrofluoric-nitric acid mixture, used in both Kuhn's method and the present work, liberates phosphorus as phosphine. Use of this reagent to effect total dissolution of the samples in one operation in an open system would obviously simplify the determination considerably.
Figure 1 . phine
In the present work use WBS made of the thermal neutron activation of phosphorus by the reaction P31(ny)P32. Owing to the low sulfur content of the alloys and the low flux of fast neutrons in the irradiation facility used, the yield of P32 from the fast neutron reaction S32(np)P32 was calculated to be negligible. This conclusion is supported by the work of Foster and Gaitanis (4). Because the alloys under consideration contained as muc has 20% silicon, the possibility of producing phosphorus by the following sequence was considered. laSi30(nr)laSi31
14Sis1 p'15P31( t l 2
= 2.62
S. C. G.
hr.)
(2)
The activation cross-section of Sim is, however, low and in the neutron flux (5 X n/sq.cm.jsec.) used in the present work the total number of stable atoms of phosphorus produced in the course of a 50-hour irradiation would not exceed -1012 per gram of alloy. Comparison of this figure with the number of phosphorus atoms already present in the alloy (-10'8) shows that any errors introduced by this cause will be negligibly small. EXPERIMENTAL
Beta Counting.
T h e radioactive
P32may be detected by very simple Geiger-counting equipment, and in the present work a halogen-quenched liquid counter of 7.5-ml. capacity was used. Since a solvent extraction method was utilized, liquid counting presents obvious advantages, and the possibility of error due to coprecipitation phenomena during solid source preparation is removed, T o test for evolution of phosphine on dissolution of the alloy, the apparatus in Figure 1 was employed.
Apparatus for detection of evolved phosW.
(1)
Glass wool filter Sample l e a d shielding castle Geiger tube
G a m m a Counting. A thallium activated sodium iodide crystal (1- X 1inch) coupled with a n 11-stage multiplier phototube was used. As the activity decayed, advantage was taken of the greater sensitivity (but slightly diminished resolving power) of a (1.5- x 1.5-inch) well-type crystal. Irradiation. Pieces of aluminumsilicon alloy, each about 50 mg. in weight, were broken from larger lumps of three samples by means of a percussion mortar. These, together with about 200 mg. of animoniu,m phosphate, ( N H 4 ) 2 H P 0 4 , were irradiated in close proximity in the same can in a thermal neutron flux of 5 x njsq. cm./sec. for 50 hours in the B.E.P.O. reactor a t the Atomic Energy Research Establishment, Harwell. Because all the elements involved have low thermal neutron absorption cross-sections, self-screening effects may be ignored, and the small volume occupied by the specimens minimizes any effects due to spatial. inhomogeneity in f l u density. G a m m a Spectrometric Examination. Four hours after withdrawal from the reactor small pieces of the alloy were qualitatively examined by a scintillation counting technique. T h e energy spectrum indicated t h e presence of the y-emitting radionuclides listed in Table I. I n a few cases it was necessary to resort to simple half-life analysis to effect complete identification. After a period of 5 days, the isotopes NaZ4, Mn66, Sb'22, Cu64, and As76 were no longer present in detectable quantities, although the longer-lived Fe69 ( t l / 2 = 45 days) and Sc4'j ( t l l ~= 84 days) were still present. A small quantity of the irradiated standard was also examined gamma spectrometrically, the only significant ?-emitting impurity being sodium. T h e activity due to this was allowed to decay away before the standard was used. The work was divided into three parts: to check that there n-as no loss of phosphorus as phosphine a t the dissolution stage, to measure the efficiency of the extraction procedure, and t o determine the phosphorus content of three aluminum-silicon alloys. Evolution of Phosphine on Acid Dissolution. T h e reagent used t o bring about complete dissolution of the silicon-rich alloy was a 1 to 3 mixture of hydrofluoric acid (sp. gr. 1.13) and nitric acid (sp. gr. 1.42). Figure 1 shows the apparatus used to test for evolution of phosphine. After 5 days, when the shorterlived radioactive components had decayed, 50 mg. of the alloy were placed in the side tube, and about 10 ml. of reagent were placed in the conical flask. After the evacuation of the whole apparatus, a background count was taken on the Geiger-counting equipment. The alloy was then allowed to fall into the reagent, t h e liberated gases passing into the annulus of the Geiger tube. The count rate was unchanged when hydrofluoric-nitric acid mixture was used, but on subVOL. 35, NO. 1, JANUARY 1963
* 11
stituting hydrochloric acid a high count rate was observed demonstrating the release of phosphine. On addition of inactive aluminum to the resulting active chloride solution, further release of phosphine was observed, this presumably being due t o the reduction of phosphate to phosphine by the nascent hydrogen (3, 7 ) . It was estimated from the size of the apparatus and the efficiency of the Geiger tube that less than half of 1% of the phosphorus in the sample, evolved as phosphine, would be detectable, and it is therefore concluded t h a t the oxidizing conditions in t h e hydrofluoric-nitric acid mixture are sufficiently severe to prevent loss of phosphorus as phosphine. Activities which might have invalidated this test-e.g., those due to the elements arsenic, antimony, and germanium, which also form volatile hydrides-had already been shown by y-spectrometry t o be absent or t o have decayed. Examination of Solvent Extraction Method. To ensure t h e efficiency of t h e solvent extraction method used, a n aqueous solution of t h e irradiated ammonium phosphate standard was prepared, the concentration being such as t o give a reasonable counting rate with t h e equipment available. T o eliminate the effect of valency changes due to (ny) recoil processes, the standard was dissolved in a n aqueous bromine solution, the solution being boiled to convert the
Table I.
y-Emitting Radionuclides Detected
Energy, m.e.v.
Half life Isotope Parent Cr5o days Cr51 Cue4 Cu63 0.51 12.7 hr. As76 As75 0.55 26.5 hr. As76 Asii 0.64 26.5 hr. 0.57 2.74 days SblZ2 Sb121 0.69 2.74 days Sblz2 Sb121 2.60 hr. Mn56 Mn56 0.84 Sc46 0.89 84.0 days Sc46 Sc45 1.12 84.0 days Sc46 Fe68a 1.28 45 days Fe59 Na2' Na235 1.37 15.0 hr. Mn56 Mn55 1.77 2 . 6 hr. 9 a The 1.10-m.e.v. Fe59 -,-peak was not detected owing to obliteration by the 1.12m.e.v. Sc46 activity. b NaZ4is also produced by the reaction A12'(na)NaZ4. 0.32
28
phosphorus to the extractable orthophosphate form and drive off excess bromine. To a 10-ml. aliquot of this almost neutral solution were added 5 ml. of perchloric acid and 8 ml. of 5% ammonium molybdate solution. A pale yellow-green color developed immediately, and after standing for 5 minutes the solution was extracted with 20 ml. of isobutyl alcohol in a separating funnel. After separation of the aqueous layer, the organic phase was washed with 10 ml. of distilled water, the washings being combined with the aqueous phase which was then re-extracted with a further 20 ml. of alcohol, and re-washed. The two organic extracts were discarded, the aqueous extracts and washings being made up t o a volume of 7 5 ml. The activity remaining in the aqueous phase was found, by liquid Geiger counting of aliquots of the standard solution and aqueous extract, to be less than 1.8% i o.13yOb, this result being the mean of eight separate determinations. The possibility that this residual activity may have been due t o a nonextractable material other than phosphorus was checked by carrying out a third extraction which removed virtually all the activity. This fact, coupled with the y-spectrometric examination of the standard, suggests that the activity produced in the standard is due solely t o phosphorus, and t h a t the chemical treatment of the standard was sufficient to ensure that all the active phosphorus produced was converted into the extractable orthophosphate form. Effect of Elements Present in Sample. A solution was prepared by dissolving 500 mg. of inactive aluminum-silicon alloy in 30 ml. of a 1 t o 3 mixture of hydrofluoricnitric acids, t h e volume being made up t o 100 ml. with distilled water. A 10-ml. aliquot of this solution was then added t o 1 0 nil. of phosphate solution, t h e pH adjusted t o -4 with ammonium hydroxide, and the extraction procedure outlined above was repeated. The efficiency of the extraction does not appear to be affected by the presence of aluminum or fluoride ions, and the presence of a large excess of boric acid (added to prevent fluoride attack of the glass walls of the counting tube) also had no effect.
II.
Phosphorus Content of Alloys Examined Mean Specimen %P A. No phosphorus deliberately 0.0017 added t o the melt 0.0017 0.0017 0.0018 B. Phosphorus added to melt 0,0063 0.0065 0.0065 n0.0068 0068 0.0064 C. Phosphorus added t o melt 0.0069 0.0065 0.0067 0.0066 Table
32
ANALYTICAL CHEMISTRY
Irradiation No. 1
No. 2 No. 2 No. 1 No. 1 No. 2 kNo. o. 2 No. 1 No. 2 No. 2
Determination of the Phosphorus Content of Hypereutectic AluminumSilicon Alloys. The irradiated samples of alloy (-50 mg. each) were dissolved in 5 ml. of 1 to 3 hydrofluoric-nitric acid mixture contained in a small platinum dish. Since the reaction tends to be violent, i t was essential to keep the dish covered to prevent spray losses. The resulting solution was then washed into a beaker with 10 ml. of distilled water, 0.5 gram of boric acid was added, and the solution mas warmed until the latter dissolved. The solution was then adjusted to a pH value of 4 with ammonium hydroxide, 5 ml. of perchloric acid was added. followed by the addition of 8 ml. of ammonium molybdate ( 5 % ) . The resultant pale yellow solution was allowed to stand for 5 minutes and was then extracted twice with isobutyl alcohol as outlined above. The organic phase was made up to a suitable volume, and 7.5-ml. aliquots were counted. B solution of the ammonium phosphate standard was then prepared, extracted, and counted in the same way as the test solutions. Comparison of the specific activities of the test specimens and of the standard allowed calculation of the percentage of phosphorus in the alloys. The standard could not be directly counted in aqueous solution because the stopping power of the water for the P32beta particles is about 10% greater than that of the alcohol and consequently too low a count rate is observed. Gamma-spectrometric examination of 2-ml. tlliquots of the organic extract showed no evidence of any of the elements listed in Table I. RESULTS AND DISCUSSION
The samples examined contained amounts of phosphorus in the range 0.001% to 0.01%. The results of two independent irradiations are shown in Table 11, these results confirming those from the adaption of Kuhn's method mentioned earlier. The limited number of specimens available made a statistical study of the results invalid, but the reproducibility appears t o be acceptable. Throughout the determinations the source strengths and Geiger counting characteristics were such that the paralysis-time correction was less than 475, and the background correction less than 0.5%, of the total count rate. The statistical uncertainty in the latter was less than 0.3%, this figure being within the scatter of the results quoted. Examination of the counting and analytical errors suggests that the total uncertainty in the results for phosphorus contents in the range examined does not exceed +5%. The formation of the molybdophosphoric acid complex and its extraction into isobutyl alcohol lead t o a satisfactory recovery (98.2%) of the phosphorus present in the test samples.
The presence of fluoride ion does not affect the extraction of the complex, and hence i t is not necessary to fume off the hydrofluoric acid which is required for dissolution of the hypereutectic aluminum-silicon alloy. A mixture consisting of 1 to 3 hj.drofluoric-nitric acid does not cause loss of phosphorus as phosphine a t the dissolution stage, and this reagent may safely be used in open systems for the quantitative estimation of phosphorus in these alloys. The radioactivation-solvent extrsction method outlined above appears to be highly specific for phosphorus, a t least where the impurities in the alloy are not present in concentrations Sone of the greater than O.l%,. radioactive impurity atoms listed in Table I were rxtractecl into the organic phase to an extent greater than 0.00170. The extraction of silicon was not investigated, as the activity induced in this element was small and quickly decayed. I n the present work the sulfur content of the alloy was so low as t o render the effect (of the S3*(npjP3? reaction negligible, but in the case of materials containing appreciable amounts of sulfur a check on the magnitude of this effect would be necessary. This could be effected by separately irradiating two samples of the same specimen, together with sulfur-free phosphate standards, at high and low neutron fluxes. The proportion of fast t o thermal neutrons in a reactor normally increases with the thermal flux, and hence the specimen irradiated at higher flux would have a n apparently higher phosphorus content because of the above reaction. A correction could then be applied t o allow for the sulfur contribution. It has already been shown that, under the conditions used, the error introduced by the sequence in Equations l and 2 is negligibly small even
when the silicon t o phosphorus ratio is as high as lo3. In theory the ultimate sensitivity is set only by the length of the irradiation, and the thermal neutron flux and amount of sample available. The longer the irradiation time, however, the greater is the P32 contribution by the silicon, and the thermal flux cannot normally be increased without a corresponding increase in the fast neutron flux rendering the S 3 2 ( n p ) P 3 2effect more acute. Self-screening and flux inhomogeneity effects limit the size of the sample for irradiation. The ultimate sensitivity in practice is therefore rather dependent on the composition of the alloy but, by the present technique and using similar materials, it should be possible t o determine a phosphorus content as low as 1 pg. with reasonable accuracy. Conventional spectrophotometric methods for the determination of phosphorus (as reduced phosphomolybdate) are difficult t o apply t o alloys of the present type and suffer from the disadvantages that the intensity and absorption wavelength of the blue color produced is strongly dependent on time, temperature, acidity, and the method of reduction ( I ) , and fluoride ions exert a strong depressant effect on its formation, necessitating their removal. Moreover, in the case of high percentage silicon alloys, there is some danger of interference from the silicomolybdate complex which has an absorption band in the same wavelength region as the phosphorus complex. Although the silicon may be removed by fuming, and the extraction of the phosphomolybdate complex into a suitable solvent before development of the blue color is said t o produce more reliable results (6),spectrophotometric methods are not wholly satisfactory. The activation method is relatively free from interferences and its success
depends only on the efficiency of the solvent extraction stage. I n addition the technique has the important advantage over the conventional spectrophotometric method in that only a small amount of sample is needed. Thus, in the present work, about 50 mg. was a typical specimen weight, whereas conventional methods require about 1 gram. Smaller pieces may be used but errors due to sampling an unrepresentative specimen and high relative loss of phosphine on fracture of the sample may become important. I n the present work, only hypereutectic alloys were examined but the method may be extended t o other aluminum base alloys. ACKNOWLEDGMENT
The authors thank J. H. G. Thomson of the Research Laboratories of the British Aluminium Co. Ltd. for suggesting the problem and for supplying samples of the alloy, and C. H. Bovington for carrying out much of the gamma spectrometry. They also thank the Chairman of Tube Investments Ltd. for permission t o publish the results. LITERATURE CITED
( 1 ) Boltz, D. F., Mellon, hl. G., ANAL. CHEM.1 9 , 8 7 3 (1947). (2) Cripps, F. H., U. K . At. Energy Authority Rept., CRL/AE49 (1950).
(3) Ephraim, F., “Inorganic Chemistry,” 6th Eng. ed., p. 637, Oliver and Boyd, London, 1954. (4) Foster, L. bl., Gaitanis, C. D., AKAL. CHEM.27,1342 (1955). ( 5 ) Kuhn, V., Fonderie, 149, 279 (June 1958). (6) Lueck. H.. Boltz. D. F.. ASSL. CHEY.
on Inorganic & The Vol. 8. D. 806, Lonamans, London, I
^
1958. (8) Wadelin, C., Mellon, M. G., ANAL. CHEY.2 5 , 1 6 6 8 (1953).
RECEIVED for review June 18, 1962. Accepted October 15, 1962.
VOL. 35, N O . 1, JANUARY 1963
13
