Quantitative Determination of the Phase I/Phase II Ratio in Sodium Triphosphate by Raman Spectrometry Jan Bus Unilever Research Vlaardingen, Olivier van Noortlaan 120, Vlaardingen, The Netherlands
Sodium triphosphate (STP) is used as a builder in washing powders and it is to be expected that this situation will not change until an acceptable substitute has been developed. An important step in the production of washing powders is the spray-drying of the slurry which contains all the ingredients. The properties of this slurry depend largely on the ratio of phase I and phase 11, the two crystal modifications, in which STP can occur. This is explained in the following way: Phase I is stable a t temperatures >410 "C ( I ) and metastable a t room temperature, while phase I1 is stable a t room temperature. This difference in stability is the cause of the much higher hydration rate (to Na5P3OIr6 H20) of phase I than that of phase 11. STP.GH20 dissolves slowly and the rate of formation from STP determines certain important slurry properties ( e . g . , viscosity). Hence, the phase I/phase I1 ratio is an important parameter for the factory or the pilot plant. Three different methods exist for the determination of phase I and phase I1 concentrations in STP samples. 1) The temperature rise test (2, 31, in which the temperature increase of a well defined mixture of STP and glycerol is measured after the addition of a known amount of water. It is assumed that in this medium only STP-I reacts with water which causes the temperature rise. The method is easily performed but not very accurate, possibly because this assumption is incorrect. 2) The infrared spectrometric method described by Corbridge and Lowe ( 4 ) by which the phase I content is determined from the difference in absorbance a t 707 and 797 cm-'. As hexahydrate also absorbs a t 707 cm-l, the method can only be used for samples containing no hexahydrate which, however, is never the case in commercial samples. A second drawback is the possibility of phase transitions during the pressing of the potassium bromide disks. 3) X-Ray diffraction methods which utilize the intensities of the characteristic diffraction lines of the various components in the mixture. Mabis and Quimby ( 5 ) were the first to describe an application for STP mixtures, but modified procedures are used as well (6).The X-ray method gives serious problems for samples containing STP6H20, due to the usually severe preferred orientation of the hexahydrate crystallites (6). In this paper, the results of an investigation of Raman spectrometry (7, 8) as a method for the quantitative analysis of STP samples are presented. EXPERIMENTAL Materials. The three STP samples were prepared as described by Troost ( I ) and the remaining materials purchased from Merck, (1) (2) (3) (4) (5) (6) (7) (8)
S. Troost. Thesis Groningen, 1969, 14. J. D. McGilvery, Amer. SOC. Test. Mater. Bull., 191, 45 (1953). J. Seiffarth and D. Zobel. Chem. Techno/.,20, 490 (1968). 0.E. C. Corbridge and E. J . Lowe. Anal. Chem., 27, 1383 (1955). A. J. Mabis and 0. T. Quimby, Anal. Chern., 25, 1814 (1953). A . A . W. A . van Eulem, this laboratory, private communication 1973. P. J. Hendra and C. J . Vear. Analyst (London),95, 321 (1970). H. A. Szymansky, Ed., "Raman Spectroscopy," Plenum Press, New York, N.Y., 1967.
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Darmstadt (Na3P04.12H20 and N a ~ P 2 0 ~ . 1 O H ~ O and ) from Riedel-de Haen AG, Seelze-Hannover NaiPliO: / . Na.lP0, was prepared by dehydration of Na3POc12H2O at 100 "C in vacuo. The weighed components were gently mixed in an agate mortar. Thorough mixing, if necessary, must be carried out in a closed ball mill. Grinding in an open mortar lowers the phase I/phase I1 ratio. Apparatus and Procedure. The spectra were measured on a Cary 82 Laser Raman Spectrometer of the Cht-mica1 Tt.chnology Department of the University of Technology, Eindhoven. The 514.5-nm argon ion line of a Spectra Physics Model 165 laser was used. The small positive lens which normally focuses the laser beam at the sample had to be removed to obtain ii larker spot on the sample surface. The focused beam may accidentally hit a relatively large crystal of only one of the components which would lead to erroneous results. M o s t of the measurements were repeated after rotating the sample about 90" along an axis perpendicular to the sample surface. The standard Raman sample holder for solids u'aa dipped into the phosphate powder until the conical opening was completely filled, and pressed against a piece of glass in order to obtain a flat surface. Only a few milligrams of the sample are needed. The sample preparation time is less than one minute. If the sample is homogenized in a ball mill for one minute, the total time for sample manipulation becomes about three minutes. The measurement of the spectrum took about 15-20 min in our experiments but this might be different for other types of Raman spectrometers.
RESULTS A N D DISCUSSION R a m a n S p e c t r a of the Phosphates. Raman spectra were measured of Na5P3010-I, NaSP:3Ol"-II, NasPsOlo. 6H20, Na4P207, Na4P2Oj.lOH~0, Na:IP04, and Na:qP04. 12H20. Inspection of the STP spectra reveals that the best region for a quantitative determination of the phase I/phase I1 ratio is that between 900 and 1040 cm-'. Figure 1 shows these spectra of the three individual STPs and an approximate 1:l:l mixture. Anhydrous Na:jPOd shows two bands a t 1018 and 936 cm-l (intensity ratio 14:100) and Na3P04. 12H20 gives bands a t 1034,1009,971, and 943 cm-' (intensity ratios 24:25:55.5:100). In the commercial STP samples, no sodium orthophosphate bands could be detected. The situation is quite different for sodium pyrophosphate. In the spectrum of the dehydrated sample, a very sharp band is present a t 1032 cm-l (halfwidth 7 cm-l) and the hydrated sample gives a broader band at 1026 cm-' (halfwidth 15 cm-I). One or both of these bands are present in spectra of many samples with variable, but relatively small intensities. See the band a t about 1030 cm-' in the two (bottom) spectra of Figure 1 which probably corresponds to a decomposition product of STP. If there is no difference, or only a small difference in intensity per standard quantity for both types of sodium pyrophosphate, then this band can be used for measuring the amount of pyrophosphate in STP mixtures. This was not investigated in the present experiments. STP-I, w h i c h is probably not completely free of' its hexahydrate, has a very strong peak a t 1014 c m - ' which is well separated from the Raman lines of STP-I1 and STP.6H20 (Figure 1). The sample of STP-I1 might also contain a small amount of hexahydrate; both have a %L? c.rr-' band
ANALYTICAL CHEMISTRY. VOL. 46, NO. 12, OCTOBER 1974
1011,
STP-I
1
STP-Baq
992
980
900
940 AD
1000
900
Figure 1. Raman spectra of three STPs and a 1:l:l mixture of them in the 900-1050 cm-' range
and the broad 965 cm-l band of STP.6H2O might be hidden in the high-frequency side of the 947 cm-l band of STP-11. T h e Phase I/Phase I1 Ratio of STP. If the mixtures contain only S T P - I and STP-11,the ratio between the two can be calculated directly from the ratio of the peak heights or areas by means of a calibration line. Due to the presence of small amounts of STP.6H20 in many samples (Figure 2; shoulder a t -965 cm-l), it is necessary to use peak heights for the calculation. From the spectrum of the pure sample, it can easily be concluded that the 965 cm-l peak of STP-6H20 contributes less than 5% of its maximum height to the signal a t 947 cm-I. Moreover, in all industrial samples, the hexahydrate content is much smaller than the phase I1 content. The 965/947 cm-l peak intensity ratio is approximately the same as the STP-Gaq/STP-II ratio (Figure 1, mixture spectrum). The error in the 947 cm-' peak height will be smaller than 2% if this ratio is smaller than 0.40. In a first approximation, the contribution of the hexahydrate signal to the 947 cm-' peak height can be neglected. Tangential base lines are drawn from about 920 cm-l to about 975 cm-' and from about 975 cm-l t o about 1045 cm-]. In the exceptional cases where the minimum between the pyrophosphate peak and the phase-I peak is lower than this line. the tangent is drawn along this minimum; hence from about 975 cm-l to about 1024 cm-l Now the peak heights ( I ) are measured. For ease of calculation, the difference in peak height per unit weight for the 1014 cm-I peak (STP-I) and the 947 cm-' peak (STP-11) is corrected fffor with the constant factor 0.326. If A = 0.326 I1014 and R = 1 9 4 5 , then 100 A / ( A B ) is the percentage of S T P - I of the total amount of dehydrated STP. The results are given in Table I. A time interval of about one month lies between the measurements shown above and below the dotted line. The deviations from the mean value of duplicate measurements are less than 2% in nearly all cases which makes this method very attractive. The standard deviation is 2.3%.
+
(cm-1)
Figure 2. Raman spectra of three STPs with various relative amounts of STP-II and STPaGH20; a and b: -75% STP-It, c: -40% STP-II
T a b l e I. Measured H e i g h t s for the R a m a n Lines a t 1014 cm-I (X0.326) ( A ) a n d a t 945 cm-I ( B ) for Various K n o w n STP Mixtures. T h e Fourth C o l u m n Shows t h e Calculated P h a s e - I Percentages and the L a s t One, the Differences with t h e K n o w n Values STP-I, wt
c
5.2 13.9 25.0 50.3 65.9 79.7 5.2 25.0 50.3
R
A
0.69 0.69 1.86 1.53 3.46 2.99 7.04 7.56 7.26 5.93 6.90 7.42 7.19 0.63 0.59 2.74 6.37 6.32
10.73 9.65 10.50 10.56 10.64 10.46 7.49 6.78 3.85 3.08 2.78 1.88 2.00
-. ......
9.30 9.30 10.02 5.40 5.78
lOOA -_ (A
+ fC
I
6.0 6.7 15.0 12.7 24.5 22.2 48.5 52.7 65.3 65.9 71.3 79.8 78.2
+0.8
6.3 6 .O 21.5 54.1 52.2
+1.1 $0.8 -3.5
t1.5 +l.l -1.2 -0.5 -2.8 -1.5 +2.4 -0.6 0 +5.3
+0.1 -1.5
...........................
+4.1 +1.9
T o check the importance of the hexahydrate content, a mixture of 39.8 mg STP-I, 40.5 mg STP-11, and 39.8 mg STP-GHzO was measured. Using the procedure as outlined above, an experimental phase-I percentage of 50.6 was found. Calculation from the weighed amounts gives 49.6% (again of the total dehydrated STP), which justifies the use of the method described.
CONCLUSION The Raman spectrometry method for a quantitative determination of the percentage STP-phase I with respect to the total amount of dehydrated STP is very promising. The presence of STP.6H20 does not influence the result, if its quantity is less than 40% of the phase-I1 amount. This is always the case in STP made for detergents manufacturing.
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Curve resolution of partially overlapping peaks at 947 cm-1 and 965 cm-' may make it possible to quantify also the small amounts of STP.6Hz0.
ACKNOWLEDGMENT I wish to thank E. Strijks of the Chemical Technology Department of the University of Technology, Eindhoven, for measuring the Raman spectra, A. A. U'. A. van Eulem and N. J. Pritchard for supplying the phosphate samples, and Th. J. Liefkens for his skillful assistance. RECEIVEDfor review March 11, 1974. Accepted April 29, 1974.
CORRECTION Thermometric Titration Determination of Hydroxide and Alumina in Bayer Process Solutions
This paper by Eric VanDalen and L. G. Ward [Anal.
Chern., 45, 2248 (1973 I] contains two errors in the concentrations of the sodium aluminate solutions described on page 2249. In the Reagents section, the 10th word in the 13th line should read 4N and not 2N. In the caption for Figure 1, the 2nd line of the paragraph, "Titration Conditions," the 9th word should read 12 g/L, and not 24 g/l.
Spectrochemical Method for Determination of Tellurium in Geological Materials lrena Schoenfeld and Armand Berman Soreq Nuclear Research Centre, Yavne, Israel
Tellurium is known as an element with a very low Clarke value, i.e., a low mean concentration in the Earth's crust. It is commonly associated with other elements of the sulfur group, but it occurs at lower concentrations. Several values for Se and Te content in geological materials are given in Ref. ( I ) . The determination of tellurium at such low concentrations is difficult and involves indirect procedures consisting of the formation of colored complexes of tellurium, their extraction and photometric (2-4) or catalytic ( 5 ) determination. For small amounts of tellurium, these methods require preliminary tedious chemical separation and are subject to the interference of many elements. The purpose of the present work was to develop a spectrochemical method for the determination of tellurium in geological samples. The two standards, Sulphide-Ore 1 (McGill University, Canada) and Basalt BCR-1 ( U S . Geological Survey, Washington) were chosen as experimental materials, although tellurium had not been previously reported in analytical data for these ores (6-8). The presence of sulfur in Sulphide-Ore 1 and selenium in BCR-1 (9), however, indicated that tellurium may have been present, but remained undetected because of its low concentration and the insensitivity of the analytical methods hitherto used. Therefore, preliminary separation of tellurium by distillation in air was undertaken. Volatilization is an attractive method because it offers enrichment of trace elements without the use of additional reagents. At a temperature of about 900 OC, the vapor pressure of tellurium oxide is high enough to allow its collection at atmospheric pressure. ( I ) "Losler-Lange Geochemische Tabellen," VEB Deutscher Verlag fur
Grundstoffindustrie,Leipzig, 1965. p 190. C. 0. lngamells and E. E. Sandell, Microchem.J., 3, 3 (1959). J. Jankovsky and 0. Ksir, Talanta, 5, 238 (1960). K. L. Cheng, Talanta, 8, 301 (1961). A. E. Hubert, U.S. Geoi. Sun/. Prof, Pap., 750-8, 188 (1971). (6) G. R. Weber, Geochim. Cosmochim. Acta, 29, 229 (1965). (7) F. J. Flanagan, Geochim. Cosmochim. Acta, 31, 289 (1967). (8) N. M. Sine, W. 0. Taylor, G. R. Weber, and C. L. Lewis, Geochim. Cos(2) (3) (4) (5)
mochim. Acta, 33, 121 (1939). (9) A . 0. Brunfeld and E. Steinnes, Geochim. Cosrnochim. Acta, 31, 283 (1967).
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0
EXPERIMENTAL The analytical procedure consists of the separation and subsequent spectral determination of tellurium. Separation of Tellurium. The separation of T e is based on distillation, using the apparatus shown in Figure 1. The inner walls of the quartz condenser are first lined with graphite by pouring a suspension of 2 parts (by weight) Aquadag (Aqua Deflocculated Acheson Graphite, Acheson Colloids Corp., Port Huron, Mich.) in 3 parts water into the quartz tube, closed a t one end. After 1 minute, the tube is emptied and dried slowly by means of a stream of hot air, in order to obtain a thin uniform layer of graphite. I t was experimentally established that a thickness of 0.2-0.3 mg/cm2 graphite is the most suitable. The tube is then tightly closed with a pure graphite electrode and baked in a high temperature furnace (Type K25A) a t 450 "C for 4 hours. T h e weighed sample (1-10 grams) is placed in a quartz vessel, which is then sealed to the condenser. The entire device is heated a t 900 O C in an electric furnace. After 1 2 hours, the heating is interrupted, the quartz condenser is cut from the volatilization vessel, and the graphite is removed quantitatively from the condenser walls with a special quartz spatula. The graphite is weighed, ground in an agate mortar, and packed into the same electrode that served previously as a stopper. This electrode is then excited in a dc arc. Spectral Determination of Tellurium. The sample and synthetic standards are excited under conditions given in Table I. The standards are prepared by mixing pure graphite (Specpure Johnson Mathey) with adequate amounts of oxides of Te, Se, Cd, Zn, Hg, In, Ga, Sn, Sb, Bi, Ag, and T1-i.e., all the volatile elements that can be present in sulfide and quartz sulfide ores. Photometric measurements were made with a non-recording Jarrell-Ash microphotometer. The photographic emulsion, Ilford Q-2, sensitive to short-wave radiation was applied in order to make use of the most sensitive lines, T e I2385 8, and T e I2142 A. The intensities ratio of these two lines is given in Ref. ( I O ) as 1 and in Ref. ( 1 1 ) as 1.27; under our conditions the mean ratio, experimentally established, is 1.46. The shorter wavelength line is probably attenuated by the blaze angle of our spectrograph (-3000 8,);however, it has the advantage of being interference-free compared with the T e 12385.76 A line, which can be subject to Cr 12385.74 8, interference. The use of both lines permits more controlled results. Harrison, "Massachusetts Institute of Technology Wavelength Tables," Wiley, New York, N.Y., 1939. W. F. Meggers, C. H. Coriiss, and 6.F. Scribner, "Tables of SpectralLine Intensities," Nat. Bur. Stand. Monograph 32, Pt. 1, U.S. Govt. Printing Office,Washington, D.C.
(10) G. H. (11)
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