Improvements in Flame Photometric Determination of Sodium in Portland Cement J . J. D I O I O Y D AND LEONARD BEAN .Vational Bureau of Standards, Washington, D . C . Extensive use of these methods a t the Kational Bureau of Standards for research, routine acceptance testing, and the analysis of comparative samples disclosed several anomalous facts about the determination of sodium. First, a t least two brands of low sodium cement, when analyzed by any one of three model 52h Perkin-Elmer flame photometers>using the ASTM procedure, showed negative sodium contents. Second, consistent differences between the results obtained by several Perkin-Elmer instruments were noted Tvhen a series of comparative samples were analyzed. Third, one particular Perkin-Elmer Model 524 instrument consistently gave results several hundredths of 1% lower for sodium than the Beckman instrument with the old Model 11300 flame photometry attachment ( 6 ) , as well as with the new Model 9200 attachment. This investigation v-as undertaken to try to explain these phenomena. It was hoped that’ knowledge of the basic factors relevant to the analysis viould be increased and the accuracy of the method improved to the point where its use as a referee method would be warranted. Noreover, i t was hoped that a procedure could be developed that ~ - o u l dpermit the use of a t least some of the other flame photometers now available commercially in addition to the Model 52 Perkin-Elmer.
The sodium content of portland cement is routinely determined by flame photometry. Some flame photometers were found consistently to indicate sodium contents of less than zero for certain cements. In addition, different brands of flame photometers almost never gave identical results for any cement. A study of the method showed that flame photometers having wide effective band widths gave low, often negative results, because of the inhibiting effect of silica on the emission of calcium oxide in the region of the sodium line. Increased accuracy was obtained by using an auxiliary multilayer interference filter to narrow the effective band width of the instrument or by removing the silica from the cement prior to the determination of the sodium. Both techniques together converted the current routine procedure into one which gave results substantially identical with those obtained by the gravimetric J. Lawrence Smith method, when using the Barclay, the Beckman DU, or the Perkin-Elmer flame photometer.
OPT of the portland cements manufactured in this country contain from 0 to perhaps 1%each of sodium and potassium ouides. There was little interest in these minor constituents until it became knova that the alkali in cement can react with certain types of aggregate to give unsound concrete (15). Since that time there has been a continuing interest in these constituents and a corresponding interest in the development of more rapid and accurate methods for their determination. Until very recently, gravimetric procedures v-ere invariablJu-ed, notably modifications of the Glaze ( 8 ) , Berk-Roller ( d ) , and ,J. 1,awrence Smith (9) methods. Even the more rapid of these procedures were very time-consuming, and the most accurate required much patience and skill on the part of the analyst. I n general, the prerision and accuracy to be expected from ordinary analyses were not good. I n 1948, Pritchard ( I S ) first proposed the use of flame photometry for the determination of sodium and potassium in portland cement. This suggestion received ready acceptance, and additional development work was carried out by Pritchard, Bogue, and Bean. This resulted, in 1949, in the adoption by the American Society for Testing Materials of a tentative standard method ( 1 ) and a studv of the subject by Eubank and Bogue ( 7 ) . T h e 1952 rcvision of thr federal specification on methods of testr ing cement included essentially the same procedure ( I O ) . All this work was done with the various models of the PerkinElmer flame photometer. I n addition, the requirements of both the 1949 ASTM and the 1952 federal specifications are fulfilled only by the use of one of the Perkin-Elmer ?\lode1 52 series of instruments. There has long been a feeling, ho\vever, that i t would be desirable to have methods available which would permit the use of other instruments. I n 1951, Diamond and Bean (6) modified the ASTN Perkin-Elmer method to adapt i t to the Beckman D U spectrophotometer with the ;\lode1 11300 flame photometry attachment. This method has been incorporated in the U. S. -4rmy Corps of Engineers’ “Handbook of Concrete and Cement” (14).
EXPERI3IEiYTAL WORK
I n current flame photometric procedures for the determination of sodium in cement, the sample solution contains all the constituent,s of the cement, except the negligibly small acid-insoluble residue, dissolved in 5 to 05 hydrochloric acid. The stan(lard solutions contain, in addition to the alkalies, 6300 p.p.m. of calcium oxide in 5 to 95 hydrochloric acid. The other constituents of portland cement are not’ included in the standards. Eubank and Bogue ( 7 1 showed that these standards give reasonably good results, and several years of esperience by the cement industry have confirmed this. The assumption was macle, however, that the errors in which this laboratory was interwted were caused by interference by one or more of the other constituents of cement. Since the silica content, of cement is about 21%) next in importance to t,he 63% of calcium oxide present, it received the greatest attention. The assumption was also made that some instrument characteristics w-ere involved because any error due to solution int,erference$ 11-ould otherwise be identical xvith all instruments. To determine the effect of silica on the emission of sodium and calcium a t the sodium wave length, four solutions were prepared. Silica iT-as introduced into two of these solutions by the hydrolysis of tetraethyl orthoeilicate. Since ethyl alcohol was the other product of the hydrolysis, an equal amount was added to the other two solutions to eliminate it as a variable. One solution contained 50 p.p.m. of sodium oxide; a second 50 p.p.m. of sodium oxide plus 2100 p.p.m. of silicon dioxide; a third, 6300 p.p.m. of calcium oxide: and a fourth, 6300 p.p.m. of calcium oxide plus 2100 p.p.m. of silicon dioxide. A11 four solutions contained the same amounts of ethyl alcohol and iyerp made up in 5 to 95 hydrochloric acid. The emissions of these four solutions a t the sodium wave length (589 mg) were compared with that of the 100-p.p.m. standard routinely used in the current ASTI1 and federal specification methods. This solution contained 100 p.p.m. each of sodium and potassium oxide?, plus 6300 p.p.m. of calcium oxide in 5 to 95
1825
1
ANALYTICAL CHEMISTRY
1826 hydrochloric acid. The solutions were compared on a PerkinElmer Model 52A instrument with air-propane flame and on a Beckman Model 9200 instrument with oxyhydrogen flame and a 2-mp band width, with the results shown in Table I. The silica depressed the calcium emission by about 40 % and enhanced the sodium emission slightly. The same 40% depression was noted a t the adjacent wave length for maximum calcium emission, indicating the same effect a t all wave lengths in that band. The silica stock solution was examined flame photometrically and was found to be sodium-free and to have no emission of its own a t the sodium wave length.
Table IV. Effect of Yarying Slit Width on Apparent Sodium Oxide Content of Four Portland Cements4 Effective Band Width, I l l p 1
10
20
2
10
0,080 0.14 0.20 0.27 0.50 0.57 0.020 0.044
-0.002 +0.12 + O . 074 +0.25
0.10 0.14 0.24 0.27 0.54 0.59 0.040 0.048
0.052 0.14 0.16
+o.
0.40 0.52 0.008
$:-0.069 : ;I
20
X a z 0 Determined, % Oxyhydrogen burner .Oxyacetylene burner
Cement A Si02 in
0.12
Si02 out B SiOzin Si02 out C S/Oz in s102 out D SiOrin SiOz out
0.16 0.27 0.28 0.59 0.60 0.048 0.051
+0.31 i-0.54 -0.038 +0.042
0.25
0.023
-0.029 10 +0.027
+0.22 I
10.018
Beckman D U spectrophotometer with Model 9200 flame photometer; lime-acid standards. a
Table I.
Effect of Silica on Emission of Sodium and Calcium at 589 mp Flame Photometer, Scale Reading Perkin-Elmer Beckman
Solution 100 p.p.ni. standard
50 p.p.m. of sodium
Table V. Effect of Vary-ing Slit Width on Apparent Sodium Oxide Content of Four Portland Cementsn Effective Band Width 2 Alp
Cement
50 p.p.m. of sodium, plus silica Calcium Calcium plus silica Distilled water
A
10 A l p
20 M
0.15
0.15
Apparent S a g 0 determined, 70
0.14
p
B 0.28 0 27 0.28 C 0.59 0.60 0.62 D 0.048 0.049 0.052 a Beckman D U spectrophotometer with model 9200 flame photomelrr; synthetic cement standards; oxyhydrogen burner.
-1study was then made of the effect of varying the effective band width of the light measured by an instrument. This was done by determining the apparent sodium content of four cements using the Beckman DU flame photometry attachment (Model 0200) a t three different slit widths. Since it was recognized that flame temperature might be important, both the oxyacetylene and the oxyhydrogen flames were used. The cements used in the study were ASTM comparative samples, having a range of sodium contents. Their chemical compositions, as determined by a single analysis of each using routine federal specification methods, are given in Table 11. Their sodium and potassium contents %ere determined by the J. Lawrence Smith procedure ( 9 ) , with the results shown in Table 111. Two sets of samples were prepared as follows. One-gram samples of each cement were dissolved in 5 ml. of concentrated hydrochloric acid, filtered, and diluted to 100 ml. as specified in the current routine methods. For the second set, WAVE LENGTH,
Table 11. Chemical Composition of Cement Samples" Cement, % Constituents
.4
B
C
D
rnp
Figure 1. Wave Length 1's. Intensity Curves for Tw-o Perkin-Elmer Flame Photometers Used to Determine Effective Band Width at 590 mu
0. Perkin-Elmer @.
0.
See Table I11 for alkali values.
Table 111. Alkali Content of Four Cements Determined by J. Lawrence Smith Method .4
NazO in Cement, % B C
D
0.14 0.16 0.12 0.12 0.12 0.11 0.12 .4v. 0 . 1 3
0.27 0.24 0.25 0.26 0.27
0.65 0.58 0.56 0.56 0.54 0.57
0.062 0.051 0,057 0,042 0.041 0.065
0.26
0.58
0.051
0.30 0.29 0.33 0.31 0.30 0.29 0.29 A v . 0.30
0.63 0.64 0.66 0.64 0.63 0.64
1.06 1.06 1.03 1.03 1.06
0.14 0.18 0.16 0.15 0.16 0.16
0 64
1 .04
0.16
0.24
..
..
Kz0 in Cement, 70 1 .03
..
..
...
...
No. 446, model 52C Perkin-Elmer No. 72, m o d e l 52A
1-gram samples were dissolved in hydrochloric acid, the silica was removed by a single dehydration and the cement was dissolved in 2.5 ml. of concentrated hydrochloric acid, filtered, and diluted to 100 ml. These t v o sets of samples -.ere designated ae SiO, in and Si02 out, respectively. The standard solutions used were the lime-acid standards, as specified in the ASTM and federal specification methods. Since the effective band nidth of the Beckman DU spectrcphotometer was known to be 33 nip per mm. slit width a t 590 mp (Z), slit widths were selected to give band widths of 1 or 2, 10, and 20 mp. The photocell, range, and sensitivity settings were made as needed to balance the instrument. From an examination of the results shown in Table IV, several conclusions may be drawn. It is evident that the greater the band width, the lower the apparent sodium content, and that the elimination of silica minimizes the effect of slit width but does not eliminate it. Both effects are more pronounced for the oxyacetylene flame than for the somewhat cooler oxyhydrogen flame. In order to assess the importance of the constituents of cement other than lime and silica, a set of synthetic cement standardwas made up. These were made from reagent grade chemicale
1827
V O L U M E 25, NO. 1 2 , D E C E M B E R 1 9 5 3 and contained typical concentrations of silica, alumina, ferric oxide, magnesium oxide, and sulfur trioxide in addition to lime and acid. The samples were made up with silica in, and alcohol was added to compensate for that in the standards. The results shown in Table V indicate that the effect of slit widt)h can be essentially eliininakd by t,he use af such synthetic standards. However, the values obtained with the synthetic standards are substantially identical n-ith those obtained a t small slit widths for samples with SiO, out. Since the Perkin-Elmer flame photometer does not have a variable slit width, these experiments could not be duplicated with it. However, various att,empts were made to confirm them indirectly. Some of the results of these attempts are shown in Tables 1'1 and VII. One difference betveen the models is that the 52A uses a motor-driven chopper unit to interrupt the light beam from the flame, and t,he 52C and the modified 52A use a vibrating reed for the same purpow. The slit widths were measured directly by means of feeler gages or by a precision glass-tuhing gage. They were found to lie substantially identical in all instruments. Hom*ever, direct measurement of the effective band n-idths of the instruments ihowed a range of 8 to 16 nip. These measurements were made as follows.
-4solution containing sodium, but no calcium, was atomized into the flame. Data on intensity, or meter reading, us. wave length were obtained on the resulting light. A divided circle was substituted for the wave-length dial to permit more precise reading of \Tave-length increments, and this divided circle was calibrated in trims of wave length in the region of the sodium line by measuring in terms of its units the 5800and 6000-A. marks on the instrument panel. The wave length 2 3 . intensity data were plotted on cross-sectional paper, as illustrated in Figure 1. The two points on the curve where the intensity values were one half the maximum were then noted. The wave-length interval hetn-een these points was taken as the efFective band Tvidth. From Table VI it can be seen that these instruments of different band width gave different apparent sodium contents for cement D. The sodium values were obtained flame photometricslly by the current federal specification method (10).
Table VI. Characteristics of Four Perkin-Elmer Flame Photometers Serial Yo, 72
Model
s2.4
25.5
52A modified
466 258
52C
EntrancP Slit Width, Inch 0 090
Exit Slit Width, Inch 0.019
0 . 029 0.02Y 0,029
0,020 0.018 0.020
52A modified
Effective Band Width,
Apparent KazO in Cement D,
IN#
%
15.2tight 1 6 . 0 loose
+0.01 -0.02
10.7 9.5 8.3
+0.03
0.00
+o.
04
Table VII. Apparent Sodium Oxide Content of Four Portland Cements Determined under Various Conditions by a Perkin-Elmer Flame Photometer" A
Na2O in Cement, % B C
D
Condition hi,ecification method Instument loose Instrument tight
0.10
0.20 0.22
0.49
0.51
-0.02 tO.O1
0.06
$io! o u t , instrument tight
0.125
0.26
0.60
+0.04
Synthetic standards Instrument loose Instrument tight S o alcohol in sample
0.12 0.13 0.10
0.26 0.26 0.23
0.59 0.58
0.52
+0.035 +0.055 +0.023
Special filter Si02 in Si02 out
0.11 0.13
0.26 0.28
0.58 0.58
+0.05 +0.045
Orax-imetric average
0.13
0.26
0.58
iO.051
(1
Xodel 5 2 4 serial N o . 7 2 .
At the start of this investigation the Model 52A instrument (Serial 30.72) consistently gave values of -0.02% of sodium oxide for sample D, and two similar instruments in other laboratories also gave negative results. Several attempts were made to improve the performance of instrument i 2 , first by fastening a metal plate over the exit slit so as to decrease its width by one half or more. This attempt did not succeed, mainly because the instrument became unstable, the meter needle fluctuating over a range of perhaps 15 scale divisions. Because of this instability, the next step of proportionately decreasing the width of the entrance slit was not taken, and the exit slit was restored to it? original state.
x 2o
t WAVE LENGTH, rnr
Figure 2. Wave Length us. Transmittance Curves of Colored Glass Sodium Filter and Its Filter Components Used in Barclay Flame Photometer
I t was assumed that the instability was caused partly by the necessity of increasing the gain of the instrument to an unstable region of the amplifying circuit and partly by the mechanical oscillation of the exit slit and the image of the entrance slit with respect to each other caused by the large amount of vibration from the motor-driven chopper unit used in the 52h model. I t was possible to decrease appreciably this vibration by recementing one of the rubber motor mountings that had apparently broken loose and tightening all the others. This resulted in an appreciable improvement in performance as shown by determination of the sodium content of samples A, B, C, and D (Table VII). However, only a slight decrease in effective band width accompanied this improvement (Table VI). Table VI1 shows that the results obtained on the Perkin-Elmer instruments parallel those obtained on the Beckman (Tables IV and V). The elimination of silica increases the apparent sodium content. The same improvement is obtained by the use of synthetic standards. In addition, the use of synthetic standards essentially eliminates the effect of vibration on the results just as their use eliminates the similar effect of change in slit vidth with the Beckman instrument. Finally, the effect of alcohol is appreciable, and it must be added to the sample solution when silica from tetraethyl orthosilicate is used in the standard. From the data presented it is evident that in order to give acceptable results for the determination of sodium in cement, a flame photometer must be so designed that the sodium light impinging on the photocell is not appreciably contaminated by light from the adjacent calcium band. I t \$as known to the authors that several unsuccessful attempts had been made to determine the sodium content of portland cement using optical filter type flame photometers. Because of the insight into the problem obtained from the data presented above, it seemed possible that this situation could be corrected by relatively simple modifications of the instruments or the procedure.
1828
ANALYTICAL CHEMISTRY
Accordingly, experiments were performed on a Barclay flame a relatively large number of flame photometer results and photometer, designed primarily for use as an internal standard then compare these against single gravimetric values. Someinstrument for the analysis of biological materials. This instrutimes the single gravimetric value has been an average of a ment, which uses an air-propane flame, was selected as typical number of somewhat scattered values, perhaps with one or two of the group of optical filter, barrier layer photocell instruments stray values excluded from the average. At other times a single now available. For the isolation of sodium light it comes equipped determination has been reported. Because gravimetric values, with a combination of the Corning colored glass filters 3482 especially by the Smith procedure, are difficult and costly to (heat-resistant lantern shade yellow) and 9780 (colorimeter obtain, one is generally content with a few values. Furthermore, blue-green medium). The transmittance curves of these filters occasional wild values are the rule, generally accompanied by are shown in Figure 2, together with their transmittance when high blank values. These considerations have made flame used in combination. They transmit a maximum of 38.5% a t photometry popular for alkali determinations. Wild values 570 mp. They have an effective band width of 41 mp and transseldom occur, and precision is much better when using the flame mit about 24.5% a t 590 mp and 6% a t 620 mp. photometric procedure. Comparative samples A, B, C, and D were analyzed on the I n this study it was felt that a number of determinations by Barclay flame photometer using the direct intensity method. the best known gravimetric procedure were required in order to It was difficult to use the instrument in this way because of erestablish a criterion by which the various flame photometric techratic needle fluctuation. This behavior is predicted by the manuniques could be compared. The results shown in Table I11 are facturer, who strongly recommends use of the internal standard believed to be typical of what can be obtained by an experienced, method for quantitative work. The results are shown in Table competent analyst. VIII. Using the specification procedure ( 2 ) , the results for I n flame photometry calcium has a twofold interference with sodium are low by 0.35 to 0.51%. Eliminating the silica intersodium. First, it radiates a t the same wave length as sodium. ference raises all sodium results by about 0.30%, but they are This is a true calcium radiation (the lower wave.length region of still not generally acceptable. the calcium oxide bands with wave length maxima a t 603 and 622 Eliminating all other interferences by the use of the synthetic mp) and is not due to contamination with sodium salts of low cement standards seems to overcompensate slightly, and the alkali reagent calcium carbonate. The second calcium interresults for sodium, except for sample D, are 0.07% high. The ference is its depressant effect on the emission of sodium. These high results might involve experimental error caused by the poor two interferences are adequately compensated for in the present performance of the instrument when using the direct intensity routine methods for sodium in cement by the use of standard method, or they might be due to incomplete solution of the silica solutions containing 6300 p.p.m. of calcium oxide equivalent to in the cements invariably noted when these comparative samples the average amount in portland cement. It has been shown that (which had been stored for some time) were dissolved. no appreciable error is introduced by the normal variation in the An attempt was then made to improve the performance of the amount of calcium in cement ( 7 ) . Barclay and the Perkin-Elmer 52-4 (No. 72) by adding a FabryThe calcium interference discussed here is due to the depresPerot multilayer interference filter ( 5 ) , obtained from Baird sant effect of the silica in cement on the emission of the calcium Associates, Inc. This filter has a maximum transmittance of oxide band. .$lumina has long been known to have a similar 78% a t 591.7 mp and a band width of about 5 to 8 mp. .4 typical wave length us. transmittance curve is shown in Figure 3. The Barclay flame photometer was modified by replacing its Table VIII. Apparent Sodium Oxide Content of Four sodium filter with a combination consisting of the interference Portland Cements Determined under Various Conditions filter with Corning 3486 and 9780 colored glass filters to eliminate by Barclay Flame PhotometerG the side bands. This resulted in a remarkable increase in needle NazO in Cement, % stability and improvement in performance. The results for ceA B C D Lime-acid standard ments, both with silica in and out, are included in Table VI11 and -0.23 -0.10 0.05 -0.36 Si02 in +O.lS 0.38 -0.07 SiOr out +0.09 are substantially identical with those obtained with the PerkinElmer instruments, without filters, especially So. 255 (Table IV). Synthetic standard +O.l9 +0.32 0.63 +0.06 The interference filter x a s added to the Perkin-Elmer 52.4 Special filter 0.00 +0.085 +0.195 0.49 Si02 in (KO. 72) by taping it over the exit slit. Because this is a monot0.13 1-0.27 0.53 +0.05 Si02 out chromator instrument, no auxiliary glass filters were needed to ~0.13 + O 26 0.58 +0.051 Gravimetric average eliminate the side bands. The filter caused some increase in Manufacturer prescribes 5 Values obtained b y direct intensity method. needle flicker, but improved the results obtained analytically, as use of internal standard method for quantitative work. shown in Table VII. The removal of silica still resulted in a slight, but negligible, increase in sodium value. Table IX. -4pparent Sodium Oxide Content of Four Portland Cements as DeThis filter did not have to be removed termined on Different Instruments Using Two Different Methods and LimeAcid Standards for the determination of potassium with NazO in Cement, 70 the Perkin-Elmer instrument. Because A B C D Instrument A B C D of the sideband present (Figure 3), it Si02 in Si02 out has no effect other than an insignificant Perkin-Elmer 52.4 (No. 72) 0.10 0.22 0.51 0.01 0.125 0.26 0.60 0.04 Perkin-Elmer 52A modified decrease in sensitivity a t the potassium 0.045 0.00 0.125 0.26 0.58 0.19 0.50 0.07 (N,o. 255) Perkin-Elmer 52A modified wave length (768 mp). None of the 0.59 0.06 0.125 0.275 0.54 0.04 0.23 0.105 (No.259) variations in technique mentioned in this Beckman D U , oxyacetylene 0.59 0.048 0.27 0.04 0.14 0.54 0.24 0.10 burner paper seemed t o have any significant Beckman DU. oxyhydrogen 0.051 0 048 0.15 0.28 0.60 0.27 0.59 0.12 burner effect on the potassium determination. Barclay' -0.23 -0.10 0 . 0 5 -0.36 0.09 0.18 0.38 -0.07 Filter + B a r c l a P +0.085 +0.195 0.49 0.00 0.13 0.27 0.53 +0.05 However, this was not investigated in Filter + Perkin-Elmer ( N O . detail. 72) 0.11 0.26 0.58 +0.05 0.13 0.28 0 58 0.045 ~~
DISCUSSION
There has been a tendency in investigations on flame photometry to obtain
Gravimetric average 0.13 0.26 5 Values obtai?ed b y direct intensity method. method for quantitative work.
0.58 0.051 0.13 0.26 0.58 0.051 Manufacturer prescribes use of internal-standard
V O L U M E 2 5 , NO. 1 2 , D E C E M B E R 1 9 5 3
1829
effect ( 1 2 ) , but in this study its effect has been lumped with those of the elements other than calcium and silicon. Since the emission of the calcium in the cement sample is depressed about 40% by the silica present and this effect is not compensated for in the lime-acid standards, the net result is an apparently lower sodium content. The amount of error depends on the ratio of light caused by calcium to that caused by sodium that is permitted to strike the measuring photocell. Since the light transmitted by a slit is directly proportional to its width for the sodium line and is proportional to the square of its width for the calcium oxide band (S), it is evident that the ratio of calcium to sodium light will increase with increasing slit width or effective band width. The variable slibwidth experiments performed with the Beckman flame photometer shoived this directly. The results were most nearly correct when the band x-idth was narrowest.
o
ee
l
00 L
4
60
40
20
0
400
500 WAVE LENGTH,
mo mir
Figure 3. Typical Wave Length z's. Transmittance Curve for Multilayer Interference Filter
The greatest error results from the use of instruments having a combination of colored glass filters as the sole means of isolating the sodium light. Figure 2 shows that the Barclay instrument transmits a t 620 nip about 25% as much light as a t 589 mp. So much calcium light is transmitted by these filters t h a t the light from the calcium in the cement, as inhibited by the d i c a present, even when added to light from the sodium present, is still considerably less than that caused by the calcium alone in the limeacid standard containing no sodium. Hence the negative results found for most cements. For cement analysis purposes, a colored glass filter Combination is a sodium-plus-calcium filter, rather than a sodium filter. Fortunately, filter-type instruments can be adapted to the determination of sodium in cement by the use of a narrow-band-pass multilayer interference filter plus colored glass filters to cut out the side bands. The following explanations are suggested as consistent with the data obtained on the various Perkin-Flmer instruments. The effective hand widths obtained by direct measurement ranged from about 8 to 16 mp, even though the slits themselves were identical in width. This could occur if there were slight differences between instruments in the bowing of the spectral lines, in their tilt with respect to the exit slit, in the placement of the prisms, or the focus of the entrance slit. The actual cause or rauses have not been determined. The observation that decreasing the vibration in a model 52il I'erkin-Elmer instrument improved its performance considerably, vhile decreasing its measured effective band width only slightly, is eyplained as follows. The sodium line and the calcium oside band coincide in a wave-length region of the wave length us. emission curve for calcium where the emission is low. increasing with increasing wave length, and where the slope of the r u r v e is continually increasing. I n other words, the calcium curve in this region is concave upward. Where there is no vibration of the spectrum and the exit slit with respect to each other, the wavelength interval passed by the
slit will include the sodium light and some of the calcium light for some definite total of light. When the image and the slit do vibrate with respect to each other, the slit will pass a somewhat smaller amount of calcium light a t the lower wave-length end of its travel across the spectrum since the calcium curve has a fairly low slope in that region. At the higher wave-length end of its travel, it will pass a greatly increased amount of calcium light because of the fairly high slope of the curve. The net result of the vibration is an increase in the average amount of calcium light transmitted by the slit, and as far as the analytical results are concerned, vibration gives the same result as increased band width. The effective band width as measured, however, is affected very little by moderate vibration. The picture can be simplified by assuming that the image of the sodium line a t the exit slit is a rectangle of uniform light intensity and is being scanned by the rectangular slit opening of the same or a slightly narrower width ;it the wave length where the slit is half filled with light, the average light transmitted will be the same whether the image and slit are in relative motion or a t rest, unless the vibration caused relative motion equal to more than half the slit width. This is also true for most other settings, where the slit is partly filled with light. At the wave-length maximum, a t rest, the slit would be full of light. If there were enough vibration so that a t parts of its travel the slit would be only partly filled with light, the average light passed would be lower and a slightly lower maximum emission and hence greater effective band width would be measured. This is apparently the case for the particular instrument investigated. The vibration decrease caused by tightening the motor mount was enough to decrease appreciably the ratio of calcium to sodium light but to decrease the effective band width only slightly. The limited data obtained on the effect of the flame used indicate that the hotter the flame the greater the interference of silica and the narrower the slit required for acceptable results. The interference was greatest for oxyacetylene, less for ouyhgdrogen, and least for air-propane. CONCLUSIONS
The results obtained in this study are summarized in Table IX. From it, and from the data detailed elsewhere, the following conclusions can be drawn. Silica inhibits the emission of calcium in the region of the sodium wave length, 589 mp. This effect of silica introduces an error in the flame photometric determination of sodium in cement when silica, calcium, and sodium are present in the sample and only calcium and sodium are present in the standard. The magnitude of the error is a function of the ratio of light caused by calcium to that caused by sodium which is permitted to strike the measuring photocell. Therefore, the greater the effective band width of the monochromator device, the greater the error. The error can be decreased by removing the silica from the cement prior to the determination of the sodium or by narrowing the efkctive band width of the instrument by adding to its optical system a multilayer interference filter. The use of both techniques together converts the current routine procedure into one which gives results substantially identical with those obtained by the gravimetric J. Lawrence Smith method. The use of the modified procedure for referee analyses therefore seems to be warranted. Khen silica was eliminated from a cement sample before the flame photometric determination of sodium, excellent results were obtained with the three instruments tried: the Barclay with special filter, the Beckman DU with either oxyacetylene or oxyhydrogen flame, and the Perkin-Elmer 52 either with or without additional filter. When silica was not eliminated from the cement before the flame photometric determination of sodium (as in the present specification method) excellent results were obtained using the
1830
A N A L Y T I C A L CHEMISTRY
Beckman DU with oxyhydrogen flame and the Perkin-Elmer 52 when an auxiliary multilayer interference filter was added to its optical system. Results which were low, but perhaps acceptable for most routine purposes, were obtained with the Perkin-Elmer instrument without added filter, the Beckman DU with oxyacetylene flame, and the Barclay with special sodium filter incorporating a multilayer interference filter instead of its regular sodium filter. A Perkin-Elmer Model 52h instrument of the motor-driven chopper type was improved in performance by adjusting it to decrease its vibration. It seems reasonable to assume that filter instruments similar t o the Barclay-namely, the Process and Instruments, the Janke developed by Fox (11), and the Baird developed by White (16)could be used for cement analysis if a special sodium filter were provided and if the instruments were otherwise suitable. The Baird instrument is supplied with such a filter as optional equipment. The Process and Instruments apparatus may require modification of its stainless steel atomizer because of the corrosive effect of strong hydrochloric acid solutions. ACKNOWLEDGMENT
The authors express their appreciation to Col. 11.E. Freeman of the Walter Reed Army Medical Center for his cooperation in making available t o them a Barclay flame photometer, and to George Watton of the NBS Seattle laboratory and H. A. Berman
of the NBS San Francisco laboratory for the data obtained with
the Perkin-Elmer instruments 255 and 259, respectively. Analyses for the data in Table I1 were carried out by Lillie B. Jenkins, and those for Table I11 were carried out by Ethel J. Hackney. LITERATURE CITED
Am. SOC.Testing Materials, Designation C 228-49T (1949). Beckman Instruments, Inc., Bull. 91-F (1949). Ibid., 259, 6 (1951). Berk, A. A , , and Roller, P. S.,Concrete, 43,38-9 (1936). Billings, B. H., Photographic Eng., 2, 45-52 (1951). Diamond, J. J., and Bean, Leonard, Am. Soc. Testing Materials, Spec. Tech. P u b . 116, 28-32 (1951). Eubank, W. R., and Bogue, R. H., J . Research n’atl. Bur. Standards, 43,173-82 (1949). Federal SpecificationSS-C-l58c, “Cements, Hydraulic, illethods for Sampling, Inspection, and Testing,” par. 4.3.9.1 (rlpril 22, 1952). Ibid.,par. 4.3.9.2. I b i d . , par. 4.3.9.3. Fox, C. L., Jr., ASAL. CHEM.,23, 137-42 (1951). llitchell, R. L.. and Robertson, I. AI., J . SOC.Chern. I d . , 5 5 , 269-72T (1936). Pritchard, L. R., P i t and Q u a r r y , 41, No. 1, 83-5 (1948). U. S. Army Corps of Engineers, “Handbook for Concrete and Cement,” Method CRD-C, 244-51 (1951). U. S. Bureau of Reclamation, Denver, Colo., “Alkalies in Cement and Their Effect on Aggregaks and Concretes” (1942). White, J. E., Ax.4~.CHEM.,24,394-9 (1952). RECEIVED for revieiv April 16, 1953. Accepted August 26, 1953.
Rapid Photometric Determination of Cobalt in the Presence of Iron J. N. PASCUAL, W. H. SHIPMAN, AND WILSON SIMON c’. S. Naval Radiological Defense Laboratory, San Francisco 24, Calif. When cobalt is determined by use of nitroso K salt in the presence of iron, a depression of absorbance is observed. This phenomenon is successfully eliminated by theadditionof suitable alkali fluoridesto the sample, subsequent to the complete oxidation of ferrous iron with bromine. Several interferences are discussed and data are presented to substantiate and illustrate the techniques employed. Typical working curves are included along with complete analytical details. The precision of the method has been
A
LTHOUGH nitroso R salt has been used for many years in the detection and determination of cobalt ( r ) , the exact nature of the reaction is not definitely known. This reagent forms colored complexes with many cations. The cobalt nitroso R salt complex is stable to nitric acid, while all other cation complexes are destroyed by it (9). Iron, copper, cyanide, peroxides, persulfates, and reducing agents interfere in the determination of cobalt; small quantities of manganese, chromium, nickel, titanium, and vanadium are without effect (4, 7, 9). The exact mechanism of these interferences has not been reported. It has been observed that iron depresses the absorbance of the cobalt nitroso R salt complex ordinarily exhibited in its absence. Shcherbov claims that he can remedy this phenomenon by increasing the quantity of nitroso R salt (6). Low results were observed in the presence of ferrous iron originally in solution and with ferrous iron formed by the reduction of ferric iron by nitroso R salt.
established throughout its range. The standard deviation varies from 4=20/,in samples containing less than 20 micrograms of cobalt to less than =kly~ in samples containing more than 30 micrograms of cobalt. The procedure is suitable for the routine analysis of large numbers of both solid and liquid samples. It has been found suitable for the determination of cobalt in steel, and has been used at the U. S. Naval Radiological Defense Laboratory for a great number of other samples.
Because iron is almost always associated with cobalt in nature and is the principal interfering element in its determination, nearly all photometric procedures employ methods for the removal of iron. The most common methods utilize zinc oxide or phosphoric acid ( I S , 8, 9). Both procedures are rather lengthy and involve so much handling that chances of mechanical and chemical losses are increased. Overholser and Yoe ( 5 ) ,studying the use of o-nitrosoresorcinol as a reagent for the determination of cobalt, observed that, although citrates and tartrates did not completely eliminate the interference of ferric iron, potassium fluoride could be used for this purpose. Severtheless. these investigators did not recommend the use of o-nitrosoresorcinol for cobalt in the presence of iron. The method described here may be adapted to the determination of cobalt in steel, soils, minerals, and water after the samples have been dissolved by the appropriate methods.
