SEPTEMBER, 1935
INDUSTRIAL AKD ENGISEERING CHEMISTRY
(4) Conley, J. E., Fraas, F., and Davidson, J. RI., Bur. Mines, Rept. Investigations 3167 (1932). (5) Davidson, J. hf., and Fraas, F., I b i d . , 3237 (1934). ( 6 ) Hill, A. E., J . A m . Chem. Soc., 56, 1071-5 (1934). (7) Storch, H. H., 1x0. ESG.C'HEX., 22, 934-41 (1930). (8) Storch, H. H., and Clarke, L., Bur. Mines, R e p t . Investigations 3002 (1930). (9) Storch, H. H., and Fragen, N., I b i d . , 3116 (1931).
Protective Films on Ferrous Allov s FLORENCE FENWICK United States Steel Corporation,
1095
(10) Storch, H. H., and Frapen, N., 1x0. EXG.CHEM.,23, 991-5 (1931). (11) Wroth, J. S., Bur. RIines, Bull. 316, 15-20 (1930). RECEIVED March 14, 1936. Presented before the meeting of the American Institute of Chemical Engineers, Viilmington, Del., 3Iay 13 t o 16, 1935. Published b y permission of the Director, U. S.Bureau o f Mines. (Xot subject t o copyright.)
Influence of Chloride Ion
upon Electromotive Behavior
Kearny, N. J.
T
HAT the ordinary structural metals can be safeguarded from corrosive influence only by the interposition of a barrier between metal and environment is now generally accepted. This barrier may be a film developed more or less spontaneously, as in the case of the stainless steels or of aluminum; or it may be an applied coating, whether of another metal or a paint or resin or of some kind of cement, which is itself resistant to corrosion in the environment in which the metal is to be used. It would be desirable to have a rapid, reliable method of comparison of t,he effectiveness of such films in order to replace, or at least to supplement, the only way now certain of yielding significant results-namely, to expose the various specimens for a period of months or years to the particular environment they will encounter in service. The method to be described offers possibilities in this direction; a t least it distinguishes, in a way which parallels general experience, between the films formed spontaneously on fairly sharply defined groups of ferrous alloys such as ordinary carbon steel, copper-bearing steel, and the stainless steels (the latter fall into subdivisions according to the heal, treatment to which the samples had been subjected). It is published in the hope that it, or its obvious modifications, will be tried by others 80 that there may be a wider range of evidence as to the usefulness of a measurement of the permeability of a film or coating to ions in solution as a possible measure of relative resistance to corrosion of a metal with such a film or coating in a similar environment. The method is essentially an electrometric titration wherein a chloride solution is slowly added to the solution in which is immersed the metal; this metal, with its film or coating, acts as one electrode, and the other is a suitable standard electrode appropriately connected. The sudden unmistakable change in the observed potential marks an abrupt change, not in the specific ion concentration of the solution in which the electrode is immersed, but in the character of the surface of the electrode itself The concentration of chloride ion in the solution when the sudden change (which may amount to as much
as 1 volt) occurs is reasonably reproducible for similar samples similarly prepared, changes with the mode of preparation of the a m , and differs systematically from one type of alloy (hence, of film) to another. These essentially reproducible differences seem to show that this general method offers a means of characterizing the film or coating on the metal, with respect to its relative permeability to chloride ions and possibly, therefore, to other ions, since chloride ion is one of the most active, as \Tell as the most prevalent, of all ions in corrosion processes.
Details of Method The metal specimen, usually in the form of strip 2 em. wide or of a cylindrical rod, is suspended in a 400-cc. beaker containing 200 cc. of a solution, usually a passivating solution of potassium dichromate, which is well stirred by a motor-driven stirrer. Electrical connection with the second electrode is made through a bridge tube, oontaining this same solution and so arranged as to prevent any transfer of chloride ions from the vessel; the vessel usually contains a saturated solution of potassium chloride into which dips one arm of the bridge tube and the arm of a silver-silver chloride, 0.1 M potassium chloride half-cell of the form described by Willard and Fenwick ( 3 ) . The potential of this cell, as chloride is slowly added from a buret to the beaker containing the variable electrode, is observed in tjhe usual way and recorded directly; a negative sign indicates that the variable electrode is more noble than the silversilver chloride electrode, and vice versa, in accordance with the convention as to sign adopted by Lewis ( 2 ) and his eo-workers (to refer the readings to the standard hydrogen electrode, add -0.287 volt to the stated values). The preparation of the metal specimens raises many questions which can be answered only by detailed investigation of how differences in mode of preparation affect the result of the electrode titration. The mode of preparation adopted vas either t o secure a fresh surface by fracture of a rod or t o scour the strip specimen with fine emery paper, followed by a wash with ether; the specimen was then coated with paraffin to obviate a metal-liquid-air interface when it, was immersed, leaving bare an area usually about 1 sq. cm. The result mas not affected by the method of securing a fresh surface used or by the size of the bare area, so long as it was not in contact with the air; if any part of the bare area was in contact with air, the results were quite irregular.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1096
X o special precautions were used to purify the reagents, except t h a t t h e standard hydrochloric acid was made up from cons t a n t - boiling acid; nor was any attempt made to control the temperature closely because the result proved to be practically unaffected by any d i f f e r e n c e i n temperature between 8' and 40' C. In carrying out the titration, the chloride s o l u t i o n was a d mitted s l o w l y with frequent tapping of the g a l v a n o m e t e r key, the flow being stopped as soon as a significant change in potential was observable. The recorded I I I 8 I6 24 e position of the abrupt CC. I D _M HCI change in potential is believed accurate FIGURE1. EFFECTOF ADDITIOSOF 1.0 POTASSIUM CHLORIDE SOLUTION t o within 0.1 cc. of the chloride solution; ON THE POTENTIAL OF (A) 0.T4-CARBOIV the reproducibility of STEEL, S T E E L .4ND (B) LOW-CARBON the break is surprisIN CONTACT WITH A POT.4SSIUN ingly good for like DICHROV.4TE SOLUTION INITIALLY samples treated simi(1) 0.4 121, (2) 0.1 Af, (3) 0.04 M , AND larly. Back-titration (4) 0.01 1 v with diehroma t e does not in general yield consistent results; in other words, the concentration of chloride required just to break down a given film is reproducible, but the concentration of dichromate to complete the rebuilding of a film already broken down is much less reproducible, presumably because the resulting film is not precisely the same from one case to another. It was found preferable to have only one specimen in the solution a t a time; when several test samples were suspended simultaneously in the solution, it was observed that a somewhat smaller amount of chloride sufficed to bring about the abrupt change in potential. This is presumably due to the presence of a larger concentration of iron 3, ion which favors the establishment of a more reversible potential.
of chloride required is regularly less, though the ratio of concentration of chloride to dichromate in the final solution becomes greater. When an electrode which had been titrated past this break was removed, washed carefully and thoroughly, and then titrated a second time, the break always occurred only after somewhat more chloride had been added, but it was much smaller than before. This procedure illustrates the difficulty of securing identical actual surfaces, even in adjacent specimens from the same piece of material. Moreover, the previous treatment of the high-carbon steel! prior to its preparation as an electrode, influences its behavior in this, as in other respects; for instance, specimens which had been cooled slowly from 800" C. and consequently comprised more than one crystalline phase, showed the break at a distinctly lower amount of added chloride than did specimens from the same rod which had been quenched from the same temperature and therefore had a different structure. Such differences are t o be expected, by reason of the different make-up of the steel, and they depend undoubtedly more upon differences in actual structure, however brought about, especially a t the surface, than upon mere differences in chemical composition as usually reported. I n the test described there is a decided difference in electrode behavior of material A (high-carbon steel, quenched) and B (lowcarbon steel) ; the film on A withstands a higher chloride concentration than that on B. Or it may be said that the film on the low-carbon steel is more sensitive to, or more easily permeated by, chloride ion. This statement in no wise implies that there would always be such a difference between lower and higher plain-carbon steels; to justify such a statement more experimental work would be necessary. However, the behavior as an electrode of wrought iron, lowcarbon steel, ingot iron, an open-hearth rimming steel (0.06 per cent carbon), a pipe steel with 0.18 per cent carbon, 1.4 per cent chromium, and 0.7 per cent molybdenum, and a 5 per cent chromium steel was so nearly alike as not t o be definitely distinguishable by this test as carried out. On the other hand, the copper-bearing steels behaved somewhat differently; the change of potential was much less regular (prob-
I
I
A
B
Effect of Chloride Addition on Potential of Steel Electrodes Immersed in Potassium Dichromate A large number of titrations of electrodes of various ferrous materials, carried out as described, with several compositions of both the original and the titrating solution, were made. The general form of the t i t r a t i o n curves is apparent from Figure 1. The general picture is that, as the chloride solution is added, the potential of the variable electrode changes a t first very slightly and then abruptly, and characteristically becomes much less noble with a final relatively slow further falling off in nobility. Typical numerical data for these same two materials are given in Table I ; each line is for a separate specimen. I n ostensibly identical cases, though the actual potentials and the magnitude of the break may differ, the position of the break is reasonably reproducible. This break occurs, for a given material treated in a prescribed way, a t a point which depends upon the initial Concentration of dichromate; as this concentration is made smaller, the amount
I
VOL. 27, N O . 9
,
;
I 4
I
I 8 cc. 10 .
12
M HCI
cc. 1.0 _M HCI
OF ADDITION OF 1.0 FIGURE 2. EFFECT
(c)
iv HYDROCHLORIC ACIDSOLUTION O N
THE POTENTI.4L OF 12 P E R C E N T CHROMIUM S T E E L A N D (D) T H E S A M E H E 4 T - T R E A T E D S O AS TO PRECIPIT.4TE C 4 R B I D E , I N CONTACT WITH A POTASSIUM D I C H R O l l l T E SOLUTION INITIALLY (1) 0.01 hf,(2) 0.0013 -%f, (3) 0.0008 M , (4) 0.0004 AXD (5) WATER
M,
SEPTEMBER, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
ably an indication that the film has some power of healing itself) but finally showed a sharp break. Stainless 18-8 steels may or may not exhibit a rapid change in potential as the potassium chloride is added; in all cases the change appeared only a t a much higher chloride concentration than with the other ferrous materials tested, and it was gradual and relatively small, again indicating that the film has marked selfhealing properties under these conditions.
TABLEI. TITRATION OF PREPARED ELECTRODES OF STEELS IN POTASSIUM DICHROMATE SOLUTIOX TO IMMERSED INITIALLY WHICH1 -21 POTASSIUM CHLORIDE Was SLOWLY ADDED
Steel
.An
Bb
Initial K?Cr?OConcn .\foles/litrr 0.4
Vults 0.24
ro1ts 0.50
0.1
-0.07 -0.12 0.00
0.50 0.35 0.29
fZB,nE KCI a t
Concn. Ratio of KCI/
Break
X2Cr20: at Break
KC1 19.9
XiZZiwZts 730
0.25
9.2 8.0 9.5
560 450 300
0.48 0.40 0.48
Position of Break cc. I .1f
0.04
0.02
0.25
7.0
50
0.88
0.01
-0.03 0.07
0.40 0.40
2.9 2.5
330 210
1.45 1,25
0.4
-0.22 -0.02
0.30 0.34
7.1 8.7
475 270
0.09
-0.20 0.07
0.25 0.40 0.38 0.29
2.9 3.3 2.2 1.3
160 500 570 275
0.15 0.17 0.11 0.07
0.04
0.08 0.10
0.38 0.30
0.9 1.2
235 100
0.09 0.15
0.01
0.04 0.14
0.20 0.30
0.7 1.3
130
0.35 0.65
0.1
a b
Observed Potential After break Initial
0.08
-0.18
An 0.74-carbon steel quenched from SO00 A low-carbon steel.
80
1097
chloride to the dichromate solution. It appears therefore that, though chromate is less strongly oxidizing than dichromate, the film formed on iron in its slightly alkaline solution is somewhat more resistant to chloride penetration.
Effect of Polarizing the Electrode The experiments already described indicate that iron, when immersed in an oxidizing environment such as a (dichromate solution, ceases to react by reason of the formation of an isolating oxidic film or barrier; but thrlt the introduction of hydrogen ion or chloride ion in effect destroy3 thiq barrier and redores the activity of the iron itself, the syjtem bezoniing electrochemically reversible with reqpxt to iron and iron ion. Presumably hydrogen and chloride ions are especinlly effective in penetrating the barrier film because of their small pize and high mobility. Let us now consider some precisely similar experiments, except that the electrode is subjected to a polarizing electromotive force by means of an auxiliary platinum electrode and a battery with appropriate rheostat. If the iron electrode is anodically polarized (attached to the positive terminal of the battery) the fihn should tend to build up. This implies greater resistance to penetration by chloride ion; but any effect in this direction is opposed by the added electromotive force which tends to push chloride ion inward,
0.11
I I I
c. .3 -
Series of experiments entirely similar, except that the titrating solution was 0.1 M potassium chloride, showed that the break is less abrupt than with the 1 M potassium chloride solution, but it occurred a t the same ratio of potassium chloride to potassium dichromate in the solution. This led to a decision to use a 1 M solution for titrating since nothing is gained by employing a more dilute solution. A large number of entirely similar titrations were run with 1.0 A4 hydrochloric acid as titrating agent, and a few with 1.0 M perchloric acid. As between these two acids, there seem to be no significant differences in behavior of the several alloys, except perhaps in the case of stainless steel which became steadily inore noble as the perchloric acid was added; the hydrochloric acid caused a break, though again only a t a concentration considerably higher than for the other steels examined. With hydrochloric acid as titrating agent, the behavior of the several materials is similar to that observed n-ith potassium chloride, except that the break occurs a t a lower concentration of chloride and is, in general, sharper and larger in amount, so that the specimens finished by being in a less noble state than when potassium chloride was used. This difference is especially marked with the stainless steels and the copper-bearing steels which, with potassium chloride, decreased in nobility gradually. A few experiments were made with high-carbon steel A in which the initial solution was 0.1 M potassium chromate, instead of dichromate as in all of those considered so far. I n this case, addition of neutral chloride was almost without effect on the potential; addition of hydrochloric acid first made the electrode more noble but later brought about a sharp break just as in the course of the addition of a neutral
c
,'/*-'
-
-
I
2-
Y
l-
1 0-
-.I
-
I
I
I 6
I 8 cc. ID
1 HCI
I 0
I 1
2
I 1
4
FIGURE3. GENERAL EFFECTOF ADDITIOS OF 1.0 M HYDROCHLORIC ACID SOLUTIOV ON THE POTENTIAL OF (I) PLAIN-CARBOX STEELS,(11) COPPER-BEARING STEELS, AND (111) STAINLESS STEELS,INITIALLY IY CONTACT WITH 0.1 M POTASSIUM DICHROMATE (FULLLISE) OR WITH W.4TER (BROKEN LISE) and iron ion outward, through the film. The net effect of anodic polarization under the conditions specified proved to be a somewhat increased sensitiveness to chloride ion. On the other hand, if the iron electrode is cathodically polarized (attached to the negative battery terminal), the film has less tendency to form, and chloride ion is moved away from, and iron ion is moved toward, the metal surface. Consequently
INDUSTRIAL AKD ENGINEERING CHEMISTRY
1098
the effective potential of the metal should then be little affected by addition of chloride ion t o the solution, as proved to be the case. A few typical data are given, where the initial solution in all cases was 0.1 M potassium dichromate: Electrode Material
Polarization
Polarizing Position E. m. f . of Break
Volts -4
Anodically Cathodically
B
1.5 0
1.5
Anodically
0.5
Cathodically Anodically Cathodically
6.5
1.5 1.5
Chan e for 1 KC1 a t Break
8,.
Concn. Ratio of KCI/&Cr20? a t Break
cc. 1 M KC1 MillivoIts 3.2 9.0 13.5
1200
0.16 0.45 0.68
-31.. 4-9
!300 % -""
0 09
3.6 2.8 No break
450 100
1500 0
__
n _ . ix
0.18
0.14
..
The observations may be summed up in the statements that, if the formation of a film a t the surface of a steel is prevented, the presence of chloride ion has little effect upon its effective potential; whereas, if a film is present, the effective potential alters, abruptly and markedly, when a relatively small characteristic concentration of chloride ion is reached I
Effect of Chloride Addition on Potential of Electrodes Immersed in Dilute Dichromate Solutions and i n Distilled Water In view of the fact that the electrometric titration curve of the film formed on various stainless steels immersed in 0.1 M potassium dichromate was in all cases about the same, it was decided to make similar tests in which the initial solution was a very dilute dichromate, or even pure distilled water, in order to discover whether under these conditions any differentiation is possible. These tests were carried out exactly as described previously except that, on account of the high internal resistance of cells comprising such dilute solutions, it was necessary to make use of some form of electrometer; a thermionic electrometer (this instrument makes use of the single-tube bridge circuit described by DuBridge and Brown, 1 ) was constructed which proved very satisfactory. It had a sensitivity of better than 0.1 millivolt per mm. on the galvanometer scale for a current of about 10-15 ampere through the cell; such a small current should not in any way alter the a m , whereas the much larger current which would have been unavoidable with the ordinary potentiometer before the balanced setting is reached, might well have affected the ease of breakdown of the film,as indicated by the polarization experiments just described. A comparative test showed the results to be entirely comparable with those obtained by the earlier, more usual set-up. With these dilute initial solutions of dichromate, as with those already discussed, the break in the titration curve with potassium chloride is less abrupt and definite, but is more pronounced with hydrochloric acid as titrating agent; the latter was therefore used. These experiments show up marked differences in electrode behavior of materials immersed in very dilute solutions of dichromate, though in the stronger solutions little or no difference was observable. For instance, let us consider the pairs of specimens C and D, both of the same 12 per cent chromium steel and differing only in heat treatment; all had been quenched in oil from 890' C., but samples D were subsequently heated for 1 hour at 540" in order to favor the separation of carbides in the metal-a treatment which decidedly lessens the resistance of this alloy to atmospheric corrosionand all had then been treated in boiling 1 N nitric acid for 4 hours. When immersed initially in 0.1 M dichromate the position of the break was the same (about 10 cc. of 1 M hy-
VOL. 27, NO. 9
drochloric acid) for both C and D; for C the position of the break did not change down to 0.0008 M dichromate, whereas for D it occurred when about 1.5 cc. of 1 M hydrochloric acid had been added; in 0.0004 31 dichromate the break for C was a t about 5 cc., and that for D remained a t about 1.5 cc. A similar relative position of the break was observed when the specimens were immersed in aerated distilled water and titrated with potassium chloride; when titrated with hydrochloric acid the difference was less certain, but there were now two inflections. This result suggests that in this case the film was destroyed in steps, the potential a t the second break being characteristic of the bare metal; and that, as before with copper steel, this type of behavior indicates that the metal is then maintaining its protective film with difficulty. Figure 2 illustrates the behavior of these specimens. d similar sequence of phenomena was observed with 27 per cent chromium iron alloys, initially immersed in various concentrations of dichromate and in water, except that in the dilute solutions and in water the break occurred only after addition of an amount of acid greater than sufficed for the 12 per cent chromium iron alloy; and that in water no double break was observed, in line with the fact that 27 per cent chromium iron is considerably more resistant to corrosion than is 12 per cent chromium iron. The same holds true generally for the 18 chromium-8 nickel iron alloy, the most common stainless steel. In this case again, the alloy developed a less resistant film if it had previously been heattreated in such a way as to induce carbide precipitation. 411 samples of 18-8 tested showed a high resistance to permeation of the film, as measured by this test; but there were considerable differences in behavior between samples of different origin. This result indicates surface differences of some sort (which in service may, however, not be significant) as hetween the various specimens; the source and significance of these differences are now being investigated. Ordinary carbon steels, when immersed initially in very dilute dichromate solution or in water and titrated with hydrochloric acid, showed an immediate large decrease in nobility with the first drop of acid added. The general picture is evident from Figure 3, in which representative curves hare been drawn for several typical steels immersed initially in (a) 0.1 M potassium dichromate, (6) aerated water, and titrated with l -11hydrochloric acid. Curves I are typical for wrought iron, ingot iron, plain carbon steels of all kinds, and 5 per cent chromium steel; curves I1 for copper steels; and curves I11 for the group of stainless steels when properly heat-treated. There are differences within any one category depending on previous treatment and precise composition ; but these are in general smaller than the difference between the type curves.
General Conclusions The evidence presented indicates a general parallelism between the relative amount of chloride required to bring about a breakdown of the film on ferrous materials, as shown in Figure 3, and general experience as to the relative resistance t o atmospheric corrosion of these three types of ferrous alloy. It is generally believed that all irons and carbon steels behave very much alike, that copper steels have a distinctly longer useful life, and that the high-chromium and chromium-nickel irons (the so-called stainless steels) are highly resistant. Further experience will, however, be required before entire reliance can be placed upon this electrode test as an indication of the useful life of a given sample of metal. Other methods of preparation of the surface of the specimen should be tried, for it is quite clear that the mode of preparation may well alter the constitution of the film and thus influence its permeability and usefulness. In any case, this electrometric test does
SEPTEMBER, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
1099
distinguish more or less quantitatively between films which to other rapid tests are indistinguishable, and holds out some promise therefore of making possible a more logical search for a material, or for the conscious preparation of a film, which
a film, as is the case in all the practically important instances of general corrosion.
will happen after the lnetai surface has become covered with
RECEIVED April
Literature Cited
9, 1935
Rare Elements in German Brown-Coal Ashes
WALTER FUCHb
\ew
Jersey igricultural Experiment Station, New Brunswick, S . J.
D
C R I S G an investigation on the production of activated carbon froin German brown cnal of the Rhine didrict, it wafound that some constituents of the ash exerted a catalytic influence on the velocity of combustion. T h e n the mineral constituent,5 were removed by treatment n-ith hydrochloric acid, a fair control of the acti\-ation procev became possible. In connection with another investigation concerning the possible use of brown coal as a fertilizer (S),a ccrtain favorable effect on plant growth could be traced back to the waterholuble substances in the coal, possibly the rare element's. Since the power plants in the brou-n-coal district near Cologne are producing as a waste material many tons of brown-coal ash daily, it seemed desirable to make detailed investigation of that ash. The ash taken for examination originated from ~
~ ~ ~ _ _ _ _ _ _ _ _ _ _ _ _ ~
~~~
TABLEI.
~
~~~
~
ELEMZESTS F~CSD IN GERMASBROI+-S-COAL ASHES
Gravimetric Analysis Per cent Ca Fe >I g Si A1 Na Ii lln P C , 0, H, N, 9
35 56 10.74 4.86 3.31 1.60 2.30 0.19 0 35 Traces Also present
Spectrophotograyhic -1nalysis 10-6 p e r cent
Zn Ti Au Ga Ge
10- 100 lo- 100 1- 10 Traces Traces
1. Lead, silver, mercury, bismuth, cadmium, coppel', platinum, osmium, iridium, tin, molybdenum, gold, tungsten. 2. Nickel, cobalt, manganese, zinc, iron, aluminum, chromium, titanium, vanadium, tantalum, columhium, thallium, zirconium. 3. Barium, strontium, calcium, magnesium, sodium. potassium, lithium. 4. Phosphorus, silicon, cerium, lanthanum, gallium ( 7 ) germanium (4);rhenium ( 6 ) . 5. Carbon, oxygen, hydrogen, nitrogen, sulfur. ~
Twenty-five of these element's were found in t'he coal, as shown in Table I. The amounts of the elements established by spectrographic analysis have been estimated in decadic intervals; in each case consideration was given to the intensity and, if possible, to the number of lines. These mineral constituents of brown coal may be of a primary or a secondary origin. It is possible to distinguish here several possibilities. The plants whose decomposition provided the material for the formation of brown coal, probably did not take up the mineral constituents in the same ratio as they were found in the soil, but rather acted as collectors. During the decomposition process, substances of an acid nature originated; the humic acids representing the bulk of the organic substance of the brown coal< are oxy-
TABLE 11. ASH, THE
A h f O U S T S OF EIGHTEEN ELEMESTS IN COAL EIRTH'SCILUST, .4ND VARIOUS FORMS OF
OROLNIC LIFE
brown coal of the Grube Fortuna near Cologne. The neceasary .pectrophotographic analyses were carried out by the author's assistant, J. Clermont (9). The ash was brought into solution by treatment with aqua regia and also by fusion. The analyses were made by the rnethodi of Gerlach and Scliweitzer (5) and Schleicher and Clerinont (9). Utilizing the experiences of the latter, the wlutions were subjected t o electrolysis under varying conditions; when the p H was varied, different electrolytic precipitates were obtained, and these as well as the other precipitates obtained in the work were finally examined by a spectrograph ( 8 ) . A sample of a-h was also analyzed in the usual way (1). The examination wac extended to include thr. following forty-five elements :
Ca Fe
E: Si h1 h-a
Ii P
Coal .ish
Earth s Crust
%
%
Organisms
%
35.6 10.7 4.8
0.36 3.31 1.6 2.3
25.7 7.5 2.6
0.2 Trace
2.4 0.1
10-1- 1 1
