Table 111. Four-Month Reproducibility Study of Silica Determinations in 36.5% Seawater by the Procedure in the Experimental Section Sample 1 Sample 2 Sample 3 Average concentration 10.22 62.81 117.85 (pmoles Si02/l.) Absorbance range 0.1010.6101.139(raw data) 0.106 0.632 1,174 Number of analyses 9 9 8 Standard deviation 0.27 0.31 0.31 (Mmoles SiOnil.) Relative standard 2.6 0.49 0.31 deviation (7;)
sorptivity in the procedure in the “Experimental” section can be adequately duplicated in a single 1-1 electrolyte solution of the same ionic strength, and there is no marked additional effect due to any specific ion. Second, on each occasion curves of salt-factor against salinity were prepared for the standards in sodium chloride solutions and in seawater solutions. From these curves, the salinities of sodium chloride giving the same salt-factors as seawater salinities were obtained (Figure 5 ) . Two conclusions may be drawn from the figure in conjunction with Table 11. First, equal salinities of seawater and sodium chloride d o not result in equal salt-factors because ionic strength apparently controls the salt-factor. Second, silica standards prepared in sodium chloride solutions are nevertheless suitable for analyzing natural waters. For brackish to marine waters, the equation of the line in Figure 5 gives the appropriate salinity of sodium chloride (Sracl) to use for samples of a given natural salinity (S): S X ~(g C NaCl/kg I solution) = 1 . 1 5 S (%o>
there was no evidence of “aging” of standards, and thus fresh silica standards put into a salinity series gave saltfactors compatible with the trend of‘the other salt-factors of the series. The observed differences in the salt-factor on four different occasions (Table 11) illustrate the long-term variability to be expected in the determination of this parameter. In our case the temperature of the laboratory varied between 30 and 24 “C, and temperature changes probably caused the differences. The salt-factor should be measured under the conditions of the analysis of samples. Long-Term Reproducibility. Table I11 gives a quantitative evaluation of the long-term reproducibility of the procedure in the “Experimental” section. The salinities of the samples were unchanged during that period. The higher relative standard deviation for Sample 1 is probably due to the higher relative error of the spectrophotometer at low absorbance (26). N o trends or effects due to age of reagents, age of standards, or temperature were observed. The concentration range of the three samples-10 to 120 p M SO?-covers most of the range in natural seawater: 0 to 180 pA4 (27). These data show that use of this metol-sulfite procedure can give excellent reproducibility if standards are run with each batch of samples and the time between formation and reduction of molybdosilicic acid is carefully controlled. The short-term reproducibility is given by the average and standard deviation of the differences between 50 recent duplicate measurements: 0.2 i 0.2 p M over a concentration range of 0 to 150 p M . RECEIVED for review May 15, 1972. Accepted September 20, 1972. Various portions of this work were supported by ONR Contract N00014-68-A-0215-0003 and by NSF Grant NO. GA-23414.
(1)
The standards were prepared at the beginning of this experiment and were replaced as needed during the experiment. In agreement with results of the fourth experiment (below),
(26) R. P. Bauman, “Absorption Spectroscopy.” John Wiley & Sons, Inc., New York, N.Y., 1962. p 379. (27) F. A. J. Armstrong, “ChemiLal Oceanography.” Vol. I. J. P. Riley and G. Skirrow, Ed., Academic Press, London, 1965, p 409.
Mold for Casting Aluminum Spectrographic Standards H. L. Redfield and W. S. Wagoner Kaiser Aluminum & Chemicul Corporation, Spokane, Wash.
A simple mold and its use for casting small aluminum spectrographic standards is described. Standards so prepared fulfill the need for additional standards other than those commercially available and are useful in constructing working curves, evaluating interferences, and for analyzing experimental or proprietary alloys. The design of the mold i s unique in that a thin sheet metal single-trip disposable can contains the molten metal. Since the can has a low heat capacity, the metal will remain molten for a sufficient time to allow air bubbles and oxide to rise to the surface. Likewise, by spraying the can with water, very rapid freezing occurs. Uniformity is as good as commercial standards. Precision of replicate shots is shown for a number of elements. Good agreement is also obtained between the routine sample and the standard. IN RECENT YEARS the specifications for aluminum alloys have become increasingly more exacting, both as to major constituents and as to impurities. Likewise individual customerspeci140
fications have produced many modifications of registered alloys. Usually only nominal spectrographic standards are commercially available and these often do not cover a SURcient range to completely define all working curves. The need for more data for constructing working curves is especially apparent with computerized instruments where the polynomial expression for the function is derived. In a recent discussion concerning fitting analytical functions with digital computers for emission spectroscopy, the authors recommend a minimum of two standards per coefficient ( I ) . Thus for a second degree curve, six standards would be required, and even a first degree curve would require four standards. The necessity for special standards and a means for providing them has been inherent from the beginning of spectrochemical analysis. Several years ago three types of permanent (1) Vi.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1, JANUARY 1973
Margoshes ana S . Rasberrq, ANAL.CHEW.41, 1163 (1969).
molds were described for making small standards (2). While very satisfactory results were reported, it was felt that the principles of these molds could be extended to obtain even better results. It was to satisfy this need for additional spec. trographic standards for plant use that we undertook the design of a simple mold to produce such standards. Many considerations are involved in designing a mold for casting standards. Besides such considerations as simplicity, low cost, and ease of operation, which are necessary in order to be useful in a plant laboratory, there are also less obvious metallurgical factors to consider. The essential requirements are, of course, that cast standards give good repeatability between successive shots for the entire useable portion and that the results match a routine sample having the same chemical analysis. The most important fact to bear in mind at all times is that when a melt containing more than one constituent freezes, the composition changes continually along the solidus line of the phase diagram. In general, the greater the concentration of other constituents and the slower the rate of freezing, the more pronounced this segregation will be. For routine samples cast in approved sample molds and by approved procedure (3), the segregation is minimized and any mold for standards can probably only approach this degree of uniformity. Thus, precision is the first important criterion of acceptability for a standard. The next consideration after precision is that a standard and a routine sample of the same chemical analysis must have a similar spectrochemical response. It is not unusual for a standard to have an apparently different analysis for one or more elements than that obtained by chemical analysis. This may sometimes be explained by such things as a difference in grain size or shape or even a difference in smoothness of the machined surface. A change in spark time caused by the foregoing will often make a large error because of a different ratio of intensity of analyte to internal standard. For example, the analyte may be a volatile element and is depleted by prolonged sparking, or again background radiation may affect one line more than the other. There are many reasons, and by no means always recognized, which can cause a standard and routine sample to appear to have different analyses. The best check to see that a standard matches a routine sample is to cast a routine sample from the same melt at the same time as the standard is beingcast. For the usual sample mold, since the direction of heat flow is principally normal to the radii, the restrictions for analysis are depth of machining and location of annular area for sparking. For a standard, as usually cast, the direction of heat flow is along radii, and there should be no restriction as to depth of machining but there is the restriction as to the annular ring for sparking. For both sample and standard, the rate of freezing must be rapid and not too greatly different from each other. A fine grain structure is desired. EXPERIMENTAL
Equipment. The assembled mold is shown in Figure 1. ‘hemold consists of several principal parts as follows: !) G. P. (19641.
Koch, N. Christ, and J. L. Weber, ANAL.CHEM..36, 1957
5 ) “Spectrochemical Analysis of Aluminum & Aluminum Base
Alloys by the Paint to Plane Technique Using an Optical Emission Spectrometer,” ASTM E 227-67. Part 32, 1971 Annual Book of Standards, pp 729-40, American Society for Testing and Maleria18, Philadelphia, Pa.
Figure 1. Completely assembled mold
Figure 2. Manifold and base beside mold and sprue cup
.
The most important feature is a single usage, thin sheet metal cylinder to contain the casting. The cylinder is 28//ls inch i.d. by 4 inches high held together by a “stove pipe crimp”-hence the name “stove pipe mold‘-and is made from 26 gauge black iron. The important characteristics of this container are its low heat capacity and short path for heat transfer. This permits metal to remain molten until the cylinder is sprayed with water whereupon rapid freezing occurs. The second important feature of the mold is the spray manifold around the cylinder. This is so constructed as t o produce a uniform sheet of water directed toward the top of the cylinder containing the molten metal. An adjustable gap, set at an opening approximately 0.03 inch and at a downward angle of 20’ t o 2 Y , and using a flow rate of water approximately 5 gallons per minute, produces a satistactory cooling rate. The position at which the sheet of water strikes the cylinder can be adjusted by raising or lowering the cylinder. This is accomplished by raising or lowering a base plate supporting the cylinder, the base plate being held on threaded rods attached to the underside of the manifold. The manifold is suitably made from brass o r aluminum. Figure 2 shows the manifold with supporting base plate attached and with the assembled mold and sprue cup placed alongside. The third important feature of the mold is the sprue cup and sprue tube assembly. The design must be such that there
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1, JANUARY 1973
141
’
“Stove Pipe” Standards Per Cent by Weight
Z
Mg,%
Cr, Z
Pb,
Z
1.18 0.046 3.9
Bi. %
Sn,
%
1 0
2
0.51 0.007 1.4 1.0
ox
1.8
Y
1.4 0.11 0.002 1.8 0.17 0.008 1.0
6?51
1 13
Y
3003 s Y
6061 S
0.30 0.007 2.3 0.14
1.4 0.17 0.002 1.2 0.21
0.48
0.06
0.996 0.016 1.6
0.1
0.67 0.008
0.83 0.010 1.2 0.66 0.008
0.6
1.4
l.t
Figure 3. Detail of split sprue cup is sufficient metal contained in the cup for it to remain molten during the time the casting is freezing and thereby provide a head of molten metal for prevention of shrinkage cavities in the castine. The mrue tube has a smaller diameter to reduce heat flow &om the’cup and yet large enough t o feed the casting as it freezes. The cup is made from steel, is 2 inches i.d. by 4‘/%inches high, and has a wall thickness of approximately inch. The cup is split to facilitate disassembly and is held together in use by means of rings. The sprue tube is made from 1-inch steel conduit and is l’/s inches long. One end is flared for a steel washer so that the tube may he suspended fiom a shoulder inside the cup. Figure 3 shows details of the split sprue cup, sprue tube, and retaining rings. 142
0.57 0.008 1.4
For melting a charge of aluminum alloy, a crucible type furnace such as Hevi-Duty Type HDT-812 is used. A Dixon clay-graphite crucible size 8 contains the charge. Temperature is monitored using a chromelalumel thermocouple and suitable meter. An ASTM center pour (Type B) sample mold (3) .,is used to obtain a routine samDle for ComDarison. Procedure. Before the mold is assembled, the inside of the sheet metal cylinder is coated with a water dispersion of colloidal graphite. A suspension of coarse graphite is not satisfactory. The parts are then assembled using asbestos fiber packed around the sprue tube and Fiberfrax gaskets ‘--tween the cylinder and the sprue cup and also between the 1 “C CY]linder and a graphite plug at the bottom. Finally a loose, rolI1 of asbestos paper is placed around the inside of the sprue , cu p t o serve as insulation and facilitate disassembly after a ca!sting has been made. Before casting, it is very important; to remove all traces of moisture by thoroughly heating with a .. blast burner. I A charge of 1500 g is a convenient melt size. If master alloys are used t o produce a calculated composition, all the metal is conveniently melted at one time. However, if volatile elements, such as magnesium, are to be included, or if small additions of other metals are to be made, such additions are best made after the initial charge is molten or even immediately before pouring. Very small idditions are best wrapped in aluminum foil. If sodium or lithium metal is t o be added, it will be helpful t o place a foil wrapped “package” in a recess in the end of a graphite rod and hold the addition near the bottom of the melt. Even with these precautions, less than a SOX recovery of the sodium or lithium taken may be experienced. The melt, of course, must he at such temperature and held for such length of time as t o ensure complete solution of all solid phases and must be very thoroughly stirred with
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1. JANUARY 1973
Table 11. Uniformity of Routine Samples Si,
Alloy
Z
Pb-Bi
0.167
Sa
0,005
12% c u
3.0 0.140
S
0.00
Vb
V
4.97 0,096 1.9 0.306 0.006 2.0 0.408 0.004
355 S V
3003 S V
6063 S
h
V
1.0
6061
0.749
S
0.020
V
2.7
6351
1 . 13
S
0.013
V
1.2
5082
0.230
S
0.005
V
2.2
Fe, % 0.455 0.011 2.4 0.192 0.004 2.1 0.167 0.005 3.0 0.629 0.010 1.6 0.223 0.004 1.8 0.181 0,008 4.4 0.490 0.009 1.8 0.287 0.006 2.1
cu,
Per Cent by Weight Mn, Z Mg, %
7 0
~~
Cr,
Z
Pb,
Z
1.09 0.024 2.2 11.19 0.08 0.7 0.675 0.011 1.6
Bi,
Z
1.14 0.023 2.0
0.598 0.008 1.3
1.13 0.008 0.7
0.198 0,005 2.5 0.060 0,001 2.0 0.124 0.004 3.2
0.664
0.489 0.006 1.2 0.880 0.017 1.9 0.695
0.005
0.010
0.8 0.159
1.4 4.05 0.074 I .8
0.001
1.0
0.073 0.001 1 .o
0.131 0.001 1.0
s IS standard deviation. v is relative standard deviation.
a graphite rod. Immediately before casting, the crucible is skimmed. Just prior t o casting, the temperature is allowed t o drop t o within approximately 150 “C of the estimated freezing point. The purpose for this is to compromise between a high temperature within a sprue cup t o facilitate feeding the casting as it freezes and yet minimize the heat flowing from the cup t o the casting as the casting is freezing. This step affects both the uniformity and the soundness of the casting and, t o some extent, must be determined by trial for each alloy. When the desired temperature of the melt is obtained, a routine sample is poured and then the mold for the standard is filled. Some slight turbulence in the sprue cup is to be expected because of air escaping from the packing and this also tends to sweep any oxide to the surface. In approximately 60 to 90 seconds, the red color of the sheet metal cylinder will begin t o fade and the metal in the cup will become quiescent. It is now evident that the freezing temperature of the metal is approaching and water is now supplied t o the manifold at approximately 5 gallons per minute. Freezing in the cylinder occurs in a few seconds as judged by the lowering of the level in the cup; however, the metal in the cup itself remains molten for another minute or more and serves I Ofeed the casting and prevent the formation of shrinkage cavities. If desired, some adjustment of the freezing rate is possible by changing the gap spacing in the manifold or by using short bursts of water flow. When cool, the mold may be disassembled and the metal cylinder opened at the crimp and slipped off. With the sprue sawed otf. the lower portion becomes the standard. It is conveniently sawed into two 2-inch sections. RESULTS AND DISCUSSION
The proper evaluation of a standard should take into consideration both the uniformity of the standard itself and possible bias when compared with a routine sample. Routine samples taken using ASTM approved molds and procedure generally meet the accepted precision for spectroscopic methods--namely, 3 relative below 0.5 concentration and 1 relative above 0.5 (3). However, it is not unusual for a bias
z
z
to occur between sample and standard due to a number of metallurgical factors. Uniformity of a standard or a sample is here expressed in terms of both standard deviation and relative standard deviation, calculated as follows:
where s = standard deviation, X, = individual analyses, Ly = the average of the individual analyses, and n = the number of analyses. The relative standard deviation is simply : S
v = - .
X
100
Bias between standard and sample is determined by applying the “t” test. If the critical value is exceeded, a true difference is presumed t o exist.
xstd
x,,,,
where = average analysis of standard, = average analysis of sample, s p = pooled standard deviation, n = number of measurements of the standard and H I = number of measurements of the sample. The data in Table I are typical of the uniformity of these standards. It is t o be seen that the difference of a single sparking from the average of all values (relative standard deviation) is in the order of 1 to 3%. Some exceptions occurred with difficult compositions such as those with lead, tin, and bismuth. The data represent ten replicate sparkings which were made in the annular area lying approximately 3;’1F inch from the edge and l/? inch from the center. It is also important that the surface be machined to ASTM specified finish (3).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1, JANUARY 1973
143
~~
Table 111. Comparison of Standard us. Sample Alloy Pb-Bi
Si,
Std Sample
Z
Fey
Z
0.163 0.167
0.463
Std Sample
4.99 4.97
0.171 0.167
t
0.5
0.455
Per Cent by Weight Cu, % Mn, Z Mg, % 0.105 0.104
Cr,
Z
ta
355
6351
5
0,690 0.675 2.6 0.060 0.060
Std 1.13 0.484 0.666 Sample 1.13 0.490 0.664 t 5082 Std 0.217 0.269 0.124 0.159 Sample 0.230 0.287 0.124 0.159 t 7.2 5.8 Critical value for t for 18 degrees of freedom and 95 confidence level is 2.10.
The evaluations were made on the center portions after the standards were cut into two approximately equal sections. Several successive cuts were made for sparking and working toward the respective top and butt ends. In general, the ends are regions of least control during casting; hence, it is preferable to use the center portion and let it be the ends which are eventually discarded when the standard becomes too thin for further use. Table I1 shows the uniformity of routine samples taken according to approved procedures (3). The samples were lathed to a depth of approximately 0.05 inch and to a finely machined but not polished finish. They were then sparked 10 times each, allowing time to cool between sparkings. Again precision of replicate sparkings is in the order of 1 to 3 %. It is apparent that there was no significant difference in uniformity between the samples and the standards. In Table I11 is shown a comparison of sample 0s. standard. Both were cast from the same melt and a t the same time; hence, the actual composition was identical and any difference is only an apparent difference due to metallurgical effects. Sample and standard were sparked alternately and 10 times each and results averaged. In most instances, the agreement is very close. However it was of interest to test for differences which might be significant. Using the data obtained for calculating the uniformity of standards and samples in Tables I and 11, the “t” test was made. This revealed that in each of these samples, there was a true difference or bias. The bias is
144
BI, 3
Pb, % 1.18 1.09 5.4
1.19 1.14 4.0
0.570 0.598 8.4 0.655 0,695 9.8 3.99
0.122
4.05 2.1
0.121
small in all instances except for the standard containing lead and bismuth. While the uniformity of a standard and possible bias compared to a routine sample can be determined using approximate values obtained by comparison with other known standards, a final value based on chemical methods must be assigned to the standard. The chemical analysis should be performed using coarse chips taken from the same annular area used for sparking as noted above. If a significant bias between the standard and its matching routine sample was observed during the uniformity check, the chemical value obtained for the standard should be adjusted to an assigned value. The assigned value for the standard results in the correct analysis of routine samples. Good judgment would indicate that any bias adjustment should be small, and a large observed bias should cause rejection of the standard. The chemical methods used in our work were the ASTM methods ( 4 ) .
RECEIVED for review May 22, 1972. Accepted October 2, 1972. _
_
_
(4) “Chemical Analysis of Aluminum & Aluminum Base Alloys,” ASTM E 34-68, Part 32, 1971 Annual Book of Standards, pp 129-63. American Society for Testing- and Materials, Philadelphia. Pa.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1, JANUARY 1973
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