Supercentrifuge Data (2)
Table 111.
Diameter Range, P
Wt.
73
Diameter Range, P
0-0.085 41.8 0.152-0.161 (av. 0.05) 0.114-0.118 12.3 0 . 1 6 1 4 . 1 7 2 0.132-0.138 19.9 0.172-0.186 0 . 1 3 8 4 . 1 4 4 2 . 9 0.186-0.204 0.315-0.456
Wt. % 2.4 7.4 1.3 6.7 5.3
This lends further support to the hypothesis that the state of aggregation of the solid must be carefully considered when adsorption data are interpreted. Finally, the supercentrifuge data (Table 111) indicate a surface area of 55 sq. meters per gram, which lends additional proof to the aggregation hypothesis. When the normal Langmuir curves for the adsorption experiment are drawn (l/e against l/Ceq, where 8 = fraction of surface covered and C,, = equilibrium concentration of adsorbing ion in solution) the equilibrium constants obtained are in good agreement for all experiments reported (wet or dry) (1.854 X lo3 for stearate and 1.825 X los for cetyltrimethylammonium bromide). These constants were based on the following equation: S+P==S
where S = surfactant, P = pht,halocyanine blue, and S. . .P = the adsorption complex.
...P
For very small values of reduces t o
e, this equation
If the value of adsorption at 8 = 1 for the press cake is uscd as the maximum surface for both the press cake and the dried material, a different slope is obtained for the Langmuir plot of the dried material. K calculated from this slope is about one half of the K value for press cake (as expected). The theoretical implications of this are under investigation at present and will be reported later. At present, it appears possible that such information may be indicative of the free energy of aggregation, and hence, of the surface free energy of the solicl. ACKNOWLEDGMENT
The authors gratefully acknowledge the assistance of David Stewart, Jr., Hunter C. Craig, and H. J. DeHoff, v h o carried out some of the espcriments.
They are indebted to t,he iLIicroscopical Laboratory of the Organic Chemicals Division for the electron micrograph, to R. G. Fessler for his cooperation and for suggesting comparisons with values obtained by ultracentrifuging, and to M. C. Davis, Lederle Laboratories Division, American Cyanamid Co., Pearl River, N. Y., for ultracentrifuging the samples reported here. Appreciation is expressed to the American Cyanamid Co. for permission to publish. LITERATURE CITED
(1) Bikerman, J. J., “Surface Chemistry,” pp. 287-94, Academic Press, New York, 1958. (2) Bini, L., American Cyanamid Co.,
Bound Brook, N. J., personal communication. (3) Brunauer, S., Emmett, P. H., Teller, E., J. Am. Chem. SOC.60, 309 (1038). (4) Harkins, IT, D., “Physical Chemistry of Surface Films,” pp. 135-8, Reinhold, S e w York, 1952. (5) Loukomsky, S. A., O’Brien, S. J., Proc. Am. Sac. Testing Materials 46, 1437 (1946). (6) Robinson, M. T., Klein, G. E., J. Am. Chem. SOC.74, 6294 (1952).
RECEIVED for review September 21, 1960. Accepted November 22, 1960. Presented in part before the ilnalytical Group, North Jersey Section, ACS, at the Meeting-inMiniature, Seton Hall University, South Orange, N.J., January 26, 1959.
Spectrophotometric Determination of Traces of Calcium in Sodium Visible and Ultraviolet Methods Using Sodium Naphthalhydroxamate D. K.
BANERJEE, C. C. BUDKE, and F. D. MILLER U. S. Industrial Chemicals Co., Cincinnati 37, Ohio
Research Division,
b A sensitive spectrophotometric method for the determination of traces of calcium in reactor grade sodium has been developed. Calcium is precipitated as calcium naphthalhydroxamate, and then dissolved in EDTA solution. An equivalent amount of the highly colored naphthalhydroxamate ion i s released whose color is proportional to the amount of calcium present. Aluminum, copper, iron, manganese, potassium, tin, titanium, and vanadium do not interfere. Strontium interferes only when present in appreciable amounts. Chloride and hydroxide do not interfere. Sulfate, fluoride, nitrate, oxalate, and phosphate are without effect even in 5-mg. amounts. The reagent has a sharp ultraviolet absorption peak a t 339 mp which gives a tenfold increase in sensitivity. 418
ANALYTICAL CHEMISTRY
Indirect procedures are also described which lead to a considerable saving of time. The method should b e applicable to many other types of material with little or no modification.
C
is a critical impurity in reactor grade sodium for which a limit of 15 p.p.m. has been set. The purpose of this study mas to develop a rapid and sensitive method for calcium which would be free from interference from the other impurities found in sodium. The development of sensitive colorimetric methods for calcium has been hampered by the scarcity of reagents that form colored complexes with calcium. Existing procedures cannot be applied directly to sodium due to the ALCIUM
large quantities of salts present in solution (5, 8, 10-12). Sodium naphthalhydroxamate has been known as a fairly specific precipitant for calcium for some time, forming the extremely insoluble calcium naphthalhydroxamate (2, 3). It has also been used for its nephelometric determination (4). Amin (1) adapted this reagent for a colorimetric method by dissolving the calcium precipitate in excess EDTA and measuring the color of the equivalent amount of naphthalhydroxamate ion released. Application of this principle t o the traces of calcium present in sodium permitted the determination of a minimum of 10 pg. of calcium in 1- t o 1.5-gram samples. There was no interference from the large amounts of salts formed by solution of the sodium or the other impurities pres-
ent. The discovery of a strong absorption peak in the ultraviolet greatIy extended the potential of the method. Indirect procedures based on the measurement of excess reagent have also been developed to reduce the time of analysis. EXPERIMENTAL
Apparatus. A Becknian D U spectrophotometer rvith 1- and 5-cm. matched cells was used for all absorbance measurements. Reagents. Reagent grade chemicals and deionized water are used throughout. XAPHTHALHYDROXANIC ACID. Dissolve 50 grams of naphthalic acid anhydride (Chemical Intermediates and Research Laboratories, Inc., P.0. Box 146, Cuyahoga Falls, Ohio) in 300 ml. of boiling concentrated HKOI. Cool and collect the crystals by filtration. Recrystallize this material from boiling "03. Boil 8 grams of the purified anhydride, 3.6 grams of hydroxylamine hydrochloride, 4 grams of Sa2COa,and 200 ml. of water under reflux for 2 hours. Add n'azCOB until the solution is strongly alkaline. Filter the scarlet red solution through Khatman No. 42 filter paper. Cool and precipitate the naphthalhydroxamic acid by adding dilute HC1. Filter the crystals and wash thoroughly with water. Recrystallize the acid twice from absolute ethyl alcohol. The l$hite, crystalline acid should have a melting point of 282' to 28.2' C. The above method of preparation has been described (6, 7'). CALCIUM SOLUTIONS. Transfer 0.1000 gram of CaC03 to a 250-ml. beaker containing 50 ml. of Lvater. Add concentrated HC1 drop by drop until solution is complete. Transfer the solution to a 200-ml. volumetric flask and dilute to volume with water. This solution contains 200 pg, of calcium per ml. Use dilutions of this solution to make standards containing 1, 10, and 50 pg. of calcium per ml. SODIUM ~APIITIIALHYDROXAMAlli: SOLUTION.Transfer 0.090 gram of naphthalhydroxamic acid to a small beaker and add 4.5 nil. of 0.1N KaOH. Swirl to dissolve and add 1 drop of concentrated KHaOH. Dilute to 100 ml. with water. Filter the solution if it ia slightly turbid. This reagent is quite stable. 0.1M. Dissolve EDTA SOLUTION, 37.22 grams of disodium (ethvlenedinitril6)tetraacetate dihydrate in water and dilute to 1 liter. BUFFER SOLUTION. Dissolve 13.8 grams of NH4Cl in 100 ml. of water. Add S8 ml. of concentrated N H 4 0 H and diIute to 250 ml. with water. BUFFEREDRASH SOLUTIOX.Dilute 50 ml. of buffer solution to 500 ml. with water. Calibration Curves for Direct Procedure. To set up a calibration curve covering the range of 0 to 70 pg. of calcium per 25 ml., pipet 0, 1, 3, 5, and 7 ml. of the calcium solution containing 10 pg. of calcium per ml. into clean 40-ml. centrifuge tubes and dilute to 15 ml. with water. Add 2 ml. of buffer
and place a thin stirring rod in each tube. Add 3 ml. of sodium naphthalhydroxamate drop by drop while stirring. Place the tubes in a beaker of boiling water for 3 minutes and stir occasionally. Cool the tubes, remove the rods and place them in order on a clean watch glass. Centrifuge the tubes a t 2500 to 3000 r.p.m. for 3 minutes. Carefully decant the clear solution and place the stirring rods in the proper tubes. Add about 10 ml. of wash solution to each tube and stir well. Centrifuge and decant the clear solution carefully. Add 5 ml. of EDTA and 2 ml. of buffer and stir. Heat the tubes in hot water until the precipitate dissolves completely. Cool, transfer the solutions to 25-nil. volumetric flasks, and dilute to volume with n-ater. Measure the absorbance a t 410 mp using 5-em. borosilicate cells with water as the reference solution. A blank with no calcium carIied through the entire procedure should show negligible absorbance. Plot the absorbance us. micrograms of calcium per 25 ml. For a curve covering the range of 0 to 300 pg. of calcium per 25 ml., use 0, 1, 2, 4, and 6 ml. of the standard solution containing 50 pg. of calcium per ml. Proceed as described above using 5 ml. of naphthalhydroxamate reagent and 1-cm. borosilicate cells for the absorbance measurements. To cover the range of 0 to 7 pg. of calcium per 25 nil. use 0-, l-, 3-, 5-. and 7-ml. aliquots of the standard containing 1 pg. of calcium per ml. Proceed as described above using 5-em. silica cells for the absorbance measurements at 339 mp. Determination of Calcium in Sodium. Carefully trim a sample of sodium with a stainless steel spatula to remove surface oxides and impurities. Cut off a 1- to 1.5-gram sample and transfer i t to a tared weighing bottle. Obtain the weight of the sample. Transfer the sample to a borosilicate evaporating dish containing 30 ml. of methanol. When solution is complete, add concentrated hydrochloric acid drop by drop until the solution is acid to litmus. Place the solution on an asbestos-covered hot plate and evaporate carefully to dryness. Add 15 ml. of water to the residue and dissolve the salt. Transfer the solution to a 40-ml. centrifuge tube. Proceed Rith the precipitation and color development as described under preparation of calibration curves. Determine the absorbance of the sample against a water reference. Subtract the absorbance of a blank run through the entire procedure. By using the appropriate standard curve, convert the value obtained to parts per million of calcium in the sample. Calibration Curves for Indirect Procedure. For a calibration curve covering the range of 0 to 250 pg, of calcium per 25 ml., transfer 0-, 2-, 3-, 4-, and &mi. aliquots of the standard solution containing 50 pg. of calcium per ml. to 25-ml. volumetric flasks. Dilute the solutions to about 15 ml. and add 2 ml. of buffer and 4 ml. of sodium naphthalhydroxamate reagent.
Mix the solution and heat the flasks in a boiling water bath for 3 minutes. Cool the flasks and dilute to volume uith water. Transfer the solutions to dry 40-ml. centrifuge tubes and centrifuge for 3 minutes a t 2500 r.p.m. Using 1-cni. borosilicate cells measure the absorbance of the blank at 410 mp after setting the instrument a t zero absorbance with the supernatant liquid from each centrifuged standard. This is done by adjusting the slit width. Plot absorbance of the blank us. micrograins of calcium per 25 ml. in the standards. Use a I to 10 dilution of the sodium naphthalhydroxamate reagent for a curve covering the range of 0 to 50 pg. of calcium per 25 ml. Pipet 0, 1, 2, 3, 4, and 5 ml. of the standard solution containing 10 pg. of calcium per nil. into 25-m1. volumetric flasks. Dilute to about 10 ml. and add 2 ml. of buffer. Add 10 ml. of diluted reagent, swirl, and place the flasks in a boiling water bath for 3 minutes. Cool and dilute to volume. Your the solutions into dry 40-nil. centrifuge tubes and centrifuge a t 2500 r.p.m. for 3 minutes. Using 5-cm. borosilicate cells, measure the absorbance of the blank a t 410 nip after setting the instrument a t zero absorbance with the supernatant liquid from each centrifuged standard by adjusting the slit. Plot absorbance of the blank us. micrograms of calcium per 25 ml. in the standards. For a calibration curve in the ultraviolet covering the range of 0 to 10 pg. of calcium per 25 nil., prepare a 1 to 5 dilution of the sodium naphthalhydroxamate reagent. Transfer 0, 0.2, 0.4, 0.6, 0.8, and 1 ml. of a standard solution containing 10 ug. of calcium per ml. to 10-nil. centrifuge tubes. Adjust the volumes to 1 ml. with water. Add 0.5 ml. of buffer and 1 ml. of diluted reagent. Rfix the solutions by gently tapping the tubes. Stopper the tubes and allow them to stand for 15 minutes. Centrifuge the solutions a t 2500 r.p.m. for 3 minutes. Pipet 1-ml. aliquots of the clear supernatant solutions into 25ml. volumetric flasks and dilute t o volume with water. Using 5-cm. silica cells, measure the absorbance of the blank a t 339 mp after setting the instrument a t zero absorbance for each standard. This is again done by adjusting the slit width for each standard. Plot absorbance of the blank zs. micrograms of calcium per 25 ml. in the standards. DISCUSSION
Calibration Curves. For the analysis of commercial and high purity sodium, curves covering the two ranges of 0 to 70 pg. per 25 ml. and 0 to 300 pg. per 25 ml. were used. The day t o day reproducibility of these curves was good. There was no need to run a standard curve with each set of samples as is so often the case. This reduced the analysis time for samples considerably. The curves were checked only when a new batch of naphthalhydroxamate reagent was prepared. VOL 33, NO. 3, MARCH 1961
419
Interferences. The information in literature on metals t h a t interfere by forming insoluble precipitates with naphthalhydroxamate is rather meager. According to Beck (S), large quantities of the other alkaline earths and some heavy metals interfere in gravimetric and volumetric procedures. Amin ( 1 ) reported interference from magnesium when 20 pg, mere present with 100 pg. of calcium in a colorimetric method. I n the present work 100 pg. of aluminum, copper, iron, manganese, tin, and vanadium gave no precipitate with the reagent under the conditions used for calcium. Titanium a t the same level gave a gelatinous white precipitate of hydroxide at the high p H of the buffer used. V'hen small amounts are present, the precipitate can be filtered off before the naphthalhydrosamate is added to precipitate the calcium. Iron also gave a precipitate of hydroxide which dissolved when the EDTA was added. Since the levels of the metals examined were well above those found in commercial and reactor grade sodium, no difficulty is experienced. If the method is applied to other types of materials, the presence of large amounts of titanium would be undesirable. This situation is not likely to be encountered with most high purity materials for which the method is suitable. KO precipitate formed with the reagent in the presence of 300 p g . of barium and magnesium and 5 mg. of potassium. This was true even when salt was added to simulate the conditions present in a solution of sodium. Strontium a t a 50-pg. level
Table I. Precision of Method for LowCalcium Sodium (Low range curve) Sample Ca Ca Std. Dev.; Wt., Found, Found, P.P.31. P.P.R.I. Grams Hg. 15 13 15 16
1.4578 1.3937 1.3303 1.2030 1.3695 1.5508 1.3590 1.2780
18 19 15 15
Sample
10.3 9.3 11.6 13.3 13.1 12.3 11.4 11.7
1.3
I
Mi0
325
W V E LENGTH,m)J
Figure 1. Absorbance naphthalhydroxarnate
curve
30 pg. of Ca per 25 ml. 320 to 600 rn& 1 -cm.cells
gave a yellow precipitate and 11-ould interfere. The absorbance obtained with 300 pg. of strontium was equivalent to 140 p g . of calcium. Among the anions, chloride and hydroxide obviously have no effect. No interference was noted from 5mg. amounts of fluoride, sulfate, nitrate, oxalate, and phosphate. This quantity of anion corresponds to a concentration of 0.5% when a gram sample of sodium is taken for analysis. Sampling. As i t is quite possible t h a t there is some segregation of calcium in sodium, a n a t t e m p t n-as made t o determine t h e effect of t h e method of sampling on t h e precision of analysis. A series of small samples were taken from a block of high calcium sodium and analyzed. I n a second series, a 2-gram sample of t h e same material was dissolved and diluted to volume and aliquots of this solution were taken for analysis. Although the averages of the two sets were l i 0 and
Table II. Recovery of Added Calcium Ca, Micrograms Sample n't., In Grams sample Added Found
Recovd.
SaCl Xa
1.2095 1.1000 1,2719 1.1863 1.3047
420
ANALYTICAL CHEMISTRY
14 13 15 14 15
for
30 40
50 100 200
47 54 70 120 223
31 41 55 106 208
171 p.p.ni., respectively, the scatter of values was greater for the small samples when compared to the aliquots taken from the large samples. This was reflected in the difference in standard deviations which was significant a t the 97.5% level. This clearly indicates that taking aliquots from a solution of a large sample is the preferred sampling procedure when the low range curve is used. Better precision resulted when large samples of highcalcium sodium were used in conjunction with the high range curve. Precision a n d Recovery. Results obtained with a typical sample of reactor grade sodium are shon-n in Table I. The excellent precision is probably a result of the large samples used to get a n adequate amount of calcium in solution. Segregation effects TTere thus reduced to a minimum. No standard samples were available to check the accuracy of the procedure. However, good recoveries were obtained when calcium was added to prei-iously analyzed samples of sodium chloride and sodium. The result's of analyses with different calcium leyels using both the loa- range and high range calibration curves are slion-n in Table 11. Ultraviolet Absorption. .I single absorption peak at 410 nip was reported for naphthalhydroramate bj- dniin (I). The molar absorptivitj- a t this wave length \vas calculated to be 1250 for this anion. In the course of our work, a wave length scan with a Beckman DK spectrophot'oiiieter revealed a strong absorption peak i n t'he ultraviolet at 339 mp (Figure 1:. Use of this peak resulted in a tenfold increase in sensitivity as shon-n in Table I11 and a molar absorpt'ivity of 14,000. The reproducibility of tlie straight-line curve obtained in the range of 0 to 7 pg. of calcium per 25 nil. is shown in Figure 2. The scatter in the points is probably due to difficulties in centrifuging the minute amounts of precipit'ate formed by the t'races of calcium present and the slight solubility of calcium naphthalhydroxamate in the buffered wash solution. The solubility of the salt in aqueous solution at 25' C. was found to be 0.000543 gram per 100 nil. I n the wash solution used in the procedures described. the solubility a t 25' C. dropped to 0.0002'71 gram per 100 ml. The 10 ml. of n-aTh solution used 11-ould dissolve 27 pg. of tlie calcium salt or 2.3 pg. of calcium. This high sensitivity curve should permit the use of the procedure in the clinical field and other instances \There a simple micromethod is required. One of the more obvious applications would be the analysis of high purity chemicals and reagents. S o illustrations of such applications are presented here as they are beyond the scope of this paper.
1.2.
Table 111. Comparison of Visible and Ultraviolet Absorption of Naphthalhydroxamate
~
t
Concn. of Ca, pg./25
'.O
111.
n 10 30 50
02
I/
0
Absorbance,
3lp
339
410
n
n
0.025 0.078 0.135
(3.280 0.840 1.50
339 410
_ .
11.2 10.8 11.1
by using the suggested indirect procedures which would make them attractive for routine control. ACKNOWLEDGMENT
I
t
2 3 4 5 6 MICROGRAMS CALUUM PER 25ML.
Figlrre 2. Ultraviolet standard curves for calcium using the direct procedure 0 to 6 fig. of Ca per 25 ml. 339 mp
The authors acknodedgr the assistance of Mary J. Fuller in the synthesis and purification of the naphthalhydrosamic acid reagent. LITERATURE CITED
5-cm. cells
Indirect Determination of Calcium.
-4 considerable saving in time of analysis would result if t h e absoibance of t h e excess reagent could be used as a measure of t h e calcium. This n ould eliminate t h e steps of isolation, n-ashing, and solution of t h e centrifuged precipitate. Reilley and Hildebrand (9) recently suggested a new approach to photometric measurement for indirect methods u-hich permits absorbance reatlings in ii section of the sciilc n here the readingq are I\ ell separated, resulting in greater accuracy. This method is particularly useful n.lien Ion concentrations are being measured. For the calibration curve the instrument is sct for wro absorbance n-ith eac.11 standard by adjusting the slit and the absorbance of the blank is measured against tlic standards.
When applying this method in the present instance, the reagent concentration used for the direct procedure gave poor sensitivity in the range of 0 to 10 pg. of calcium per 25 ml. when the absorbance mas measured a t 339 mp. With 1 ml. of the reagent diluted 1 to 5, straight-line curves showing good sensitivity and precision were obtained. In the visible region, 10 ml. of a 1 t o 10 dilution of the reagent gave sensitive, reproducible curves in the range of 0 t o 50 pg. For the range of 0 to 300 pg. of calcium, 4 ml. of the original reagent was satisfactory. These reagent concentrations are not necessarily the optimum, but serve t o illustrate the indirect method. I n the absence of interfering colored ions or substances absorbing in the ultraviolet, a considerable amount of time can be saved
(1) Amin, A . R4., Chemist l n a l y s t 46, 31 (1957). ( 2 ) Beck, G , Anal. Chim. .4rtci 4 , 245 (1950). (31 Beck. G.. .lfzkrochemze Ler. Mzkrochim. Acta 35-36, 245 (1951). (4) Beck, G., Berli, W., dfikrochini. Acta _
I
1957. 24
(5)-Ga&on,
X., Jr., Forbes. R. B., ANAL.CHEM.21, 1391 (19491. (6) Graebe, G., Gfeller, E.. Bcr. deut. chem. Ges. 2 5 , 6 5 2 (1882). (7) Jaubert, G. P., Ibid., 28, 360 (3895). (8) Natelson, S., Penniall, E., ASAL. CHEM.27, 434( 1955). (9) Reilley, C. S . , Hildebrantl, G. P., I b d . , 31, 1763 (1959). (10) Tyner, E. H., Ibzd 20, T6 (1948). (11) Williams, M. B., lloser. J. H., Zbzd., 25, 1414 (1953). 1 (12) Young, A,, Sweet, T. R., Baker, B. B., Ibid., 27, 356 (1955). ~
RECEIVED for review October 21, 1960. Accepted December 9, 1960. Presented in part at the Pittsburgh Conference on Anal>-tical Chemistry and Applied Fpec-
troecopy, Pittsburgh, Pa., March 1959
Application of Pyrocatechol Violet as a Colorimetric Reagent for Tin W. J. ROSS and J. C. WHITE Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn.
b Pyrocatechol Violet has been applied as a sensitive reagent for the colorimetric determination of tin. This reagent forms a red-colored complex with quadrivalent tin a t pH 2.5. The molar absorptivity i s 65,000 a t its maximum absorbance of 555 mp. Bivalent tin does not form a complex with Pyrocatechol Violet. Interference i s produced b y those metal ions that
form colored species with the reagent in solution where the pH i s less than 5; these include zirconium, titanium, bismuth, antimony, gallium, and molybdenum.
D
the vast number of publications t h a t have appeared in t h e past decade describing new colorimetric ESPITE
reagents, few have been applied to the determination of tin. Sandell (5) describes reagents available now as generally unsatisfactory n-ith respect to sensitivity and specificity. Ditliiol is considered as the most useful ( 9 ) )although several recent publications have described methods using phenylfluorone (1, 7, 8). I n their investigations of the 1ii operVOL. 33, NO. 3, MARCH 1961
421
