ards containing 0 to 0.2 pc. of HTO per liter. Thirteen separate gas generations and rate-of-drift measurements of a 0.052 pc. of HTO per liter standard required a drift time of 3535 ==I 194 seconds per 100 mv. The 0.01 pc. of HTO per liter sample, after gas generation, was detected in the amount of 18 d.p.m. of tritium in the 1-liter chamber. This represents a current measurement of 10-17 ampere above background for the instrument used in this analysis.
strument is required. Control samples, containing small quantities of tritium, were analyzed after every four to six unknown samples. Considering the evacuation and gas generation time, and the number of control samples required, one instrument was used t o analyze four to six samples per day. Since the initial expense of the rate-of-draft equipment is small when compared with the Tri-Carb equipment, the rate-of-drift method is desirable for small loads. ELECTROLYSIS
Table IV.
Sample 1 2 3
4
Accuracy by the Rate-ofDrift Method
pc. per Liter Experimental Actual value value 0.022 0,019 0.031 0.024 0.045 0.045 0.062 0,062
Four unknown, low-level samples were analyzed by the rate-of-drift procedure. Drift times were interpolated onto a standard curve to determine tritium. These results are shown in Table IV. Frequent recalibration of the in-
Water and urine samples were electrolyzed by a procedure similar to that of Brown and Grummitt '$). A cast iron cell of 300-ml. capacity was used as the cathode. A nickel screen inside of the cell served as the anode. Samples of water, containing 6% potassium carbonate, were poured into the cell. The solution was cooled by a coil containing tap water. After 36 hours of electrolysis at 25 amperes, the volume was reduced to 10 to 12 ml., with a 12to 14-fold increase in tritium concentration. An automatic cutoff device, based on current conductivity, was installed. Tritium at levels of 5 X IOp4 pc. of HTO per liter was determined within a 2-day period by electrolysis, followed by either scintillation counting or rate-of-drift measurement.
LITERATURE CITED
(1) Bell, C. G., Jr., Hiyes, F. N., North-
western University Conference, August 1957, Pergamon Press, New York, 1958. (2) Brown, R. M., Grummitt, W. E., Can. J . Chem. 34,220-6 (1956). (3) Davidson, J. D., Feigelson, P., Intern. J. Appl. Radiation and Isotopes 2, 1-18 (1957). (4) Day, F. H., Attix, F. H., Natl. Bur. Standards (U. S.) No. 2080 (December 1952). (5) Furst, M., Kallman, H., Brown, F. H., Nucleonics 13, 58 (1955). (6) Hursh, J. E., Ed., "Chyyical Methods for Routine Bioassay, 38-58, AECU-4024, University of 'Kochester, Rochester, N. Y., November 1958. (7) Kinard. F. E.. Rev. Sci. Instr. 28, 293 (1957). (8) Libby, W. F., Phys. Rev. 69, 671 (1946). (9) Natl. Bur. Standards (U. S.), Handbook 69. (10) New England Xuclear Corp., Boston, Mass., Proc. Symposium on Advances ~
in
Tracer Applications of
(October 1958).
Tritium
(11) Okita, G. T., Spratt, J., LeRoy, G. V., Nucleonics 14, 76 (1956).
(12) Packard Instrument Co., Inc., Operation Manual, Tri-Carb Liquid Scintillation Spectrometer Model 314, La Grange, Ill., 1958. (13) Werbin, H., Chaikoff, I. L., Miles, R. I., Espt!. Biol. Med. 102, 8 (1959). RECEIVEDfor review July 28, 1960. Accepted November 4, 1960. Information contained in this article was developed during the course of work under Contract AT(O7-2)-1 with the U. S. Atomic Energy Commission.
Radiotracer Method for Determination of Adsorption of Surfacta nts on Cop per Pht haIocya nine WILLIAM SEAMAN and GEORGE 1. ROBERTS Organic Chemicals Division, American Cyanamid Co., Bound Brook,
b A radiotracer method is reported for determining the adsorption of sodium stearate and cetyltrimethylammonium bromide on copper phthalocyanine b y the use of the carbon-14tagged compounds. By means of an indirect calculation the method solves the difficulty posed b y inability to obtain a supernatant liquor free of dispersed solids with an ordinary laboratory centrifuge. Differences in adsorption values, and surface areas calculated therefrom, are found between undried samples and samples dried b y heating. The significance of these differences is discussed in relation to the surface areas found b y other methods of measurement and to the Langmuir adsorption equilibrium constants. 414
ANALYTICAL CHEMISTRY
T
N. J.
determination of surfactant adsorbed from solution onto a suspended solid may be simple for systems in which complete settling of the solid adsorbent may be effected by filtration, gravity, or centrifuging. This may permit the determination to be made by difference in concentration before and after adsorption. For systems containing suspended solids which are difficult to remove a t equilibrium a less direct approach must be adopted. The need for such a method arises not merely for the determination of adsorption isotherms, but also for the determination of surface areas and derived physical-chemical information. For example, as has been found in the work reported, the surface area may be changed by the very process of drying HE
in preparation for determining it by adsorption of nitrogen. A determination by adsorption from an aqueous medium sometimes leads to a better surface area value. The method reported here is based upon the radiometric determination of surfactants-in this work, sodium stearate and cetyltrimethylammonium bromide. Carbon-14-tagged compounds served as tracers. With copper phthalocyanine dried at 60" C., and with wet press-cake samples of copper phthalocyanine at some low concentrations of surfactant, a negligible amount of pigment remains in suspension after equilibration and centrifuging. A direct approach to the determination is therefore possible. With wet press-cake samples at higher surfactant concentra-
.
tions considerable pigment remains dispersed after centrifugation. An indirect approach was therefore, worked out. The carbon-14 count on a weight aliquot of the supernatant dispersion comprises the count due to dissolved surfactant and also to surfactant adsorbed on the dispersed pigment. A determination of dispersed pigment solids is made on another aliquot of the supernatant dispersion. However, these aliquot values cannot be used to calculate concentrations in the whole of the supernatant dispersion without knowing the weight of the latter. This, in turn, cannot be calculated without knowing the weight of precipitated pigment plus adsorbate. The latter is, of course, one of the values to be determined. The weight of precipitated pigment plus adsorbate is accordingly calculated with the simplifying assumption that the weight of the whole supernatant dispersion is the same as the total weight of the equilibrium mixture minus that of the added pigment. A more correct second approximation is then calculated from the values so obtained. The second approximation for the determinations reported here was close enough to the first to make a third approximation unnecessary. EXPERIMENTAL
the cetyltrimethylammonium bromide about 13,000 counts per minute per milligram when determined on solutions such as those used in the actual adsorption experiments and in the same manner as for the activity in the supernatant liquor in those experiments. A portion of the cetyltrimethylammonium bromide showed no significant change in specific activity after recrystallization from isopropyl alcohol. In view of the known presence of homologous impurities in the stearic acid, this test vias not applied to it. The water used had been freed from ionic constituents by passage through cationic and anionic exchange resins. The same lot of chemically pure copper phthalocyanine of metastable crystal form was used for all experiments except those involved in the comparison n-ith values obtained by the ultracentrifuge. The water-wet press cake was prepared by diluting a concentrated sulfuric acid solution of the pigment with water, filtering, and washing free of residual acid. The dry product used was obtained by heating a portion of the press cake to constant weight a t 60' C. Adsorption Determinations. Glassware was rinsed before use with a solution of the inactive surfactant in order to minimize the possibility of only slowly reversible adsorption of the active species, due to possible temporary concentrations higher than those a t equilibrium.
The following details apply t o the Counting n a s carried out in the determination of adsorption of sodium Geiger region in nickel-plated cupped stearate. The experiments with cetylplanchets, 1 inch in diameter and 6/16 trimethylammonium bromide differed inch high, under an end-window Geiger in not requiring sodium hydroxide and tube, Amperex 200 CB, with a 1.4-mg. being run a t 21" C. instead of 55" C. per sq. em. window. [The use of glass planchets led to loss of ( ~ e t y 1 - l - C ~ ~ ) - because the greater solubility of the bromide did not necessitate heating. N,N,-V-trimethylammonium bromide.] The tube was contained in a lead shield. In a 50-ml. narrow-mouthed centri(Later work used a gas-flow counter fuge tube, tared with a rubber stopper with an ultra thin window, for more and a wire for closure, there were placed efficient counting.) about 5 mg. of active stearic acid, Reagents. Stearic-l-C14 acid and accurately weighed out on a micro(cetyl- 1 -C14)N,Ar,S-trimethylammobalance, a weighed quantity of inactive nium bromide Tvere purchased from stearic acid, 1 ml. of 0 . W sodium the Nuclear Chicago Corp. under a hydrovide for each 25 mg. of stearic acid, Byproduct Material License from the plus 1 ml. additional, and 15 ml. of United States Atomic Energy Comwater. The tube and contents were mission. heated to 55' C. in a water bath until The inactive stearic acid was a techa clear solution was obtained. A sample nical grade, with melting point 54of copper phthalocyanine containing 56' C. and equivalent weight by titraabout 0.5 to 0.7 gram of pigment solids tion with alcoholic Dotassium hvwas transferred to the tube, followed by droside 281; theoreticil 284.47. Tge about 30 ml. of water. The tube and inactive cetyltrimethylammonium brocontents were warmed to 55' C. in a mide was a technical grade which had water bath, the stopper was wired in been recrystallized once from isopropyl place, and the tube was shaken. The alcohol to the disappearance of an inoutside was dried, and the tube was flection in the curve of an aqueous soluallowed to cool in a desiccator and tion relating surface tension to concenweighed. tration. Elemental analyses for C, H, The stoppered tube was heated to PI;, and Br were in satisfactory agree55' C., shaken overnight a t 55' C., ment with theory. centrifuged a t that temperature and a t The active reagents were diluted with 3200 r.p.m. for 2 hours, unless the superinactive reagents by dissolving them natant liquor became clear before then, together in acetone and drying. Porand kept in a water bath a t 55' C. while tions of these preparations were used samples of the supernatant liquor were for further dilutions in the adsorption taken for analysis. experiments. The diluted stearic acid Samples of about 0.3 to 0.7 gram of had a specific activity of about 45,000 the supernatant liquor were weighed in counts per minute per milligram and
covered, tared, cupped nickel-plated steel planchets, dried under a lamp, and counted for 30 minutes. The accumulated count varied from about 6000 to 44,000. Self-absorption corrections were applied on the basis of a curve constructed in the usual manner in order to convert all readings to the same mass thickness a t which the specific activity had been determined. The total-solids content of the supernatant liquor was determined by drying about 15-gram portions a t 110' C. For supernatant liquors which retained more than a negligible quantity of pigment, as indicated by visual inspection, the dispersed pigment was determined. A 15-gram sample of the supernatant liquor, weighed in a tared centrifuge tube after cooling, was stirred at 55' C. with a sufficient volume (30 ml. or more) of a 4 to 1 (by volume) dilution of denatured ethyl alcohol (formula 3A) and water to give a clear, colorless supernatant liquor after the agglomerated pigment was precipitated by centrifuging. If the supernatant liquor had not become clear and colorless after this treatment, 10 ml. of 3A alcohol were added without redispersing the contents of the tube. The tube and contents were allowed to stand a t 55" C. for one-half hour additional and then centrifuged. The supernatant liquor was rejected and the extraction repeated twice. The tube and contents were dried to constant weight at 110' C. and the weight of pigment was calculated by difference. Calculations. All calculations for sodium stearate are expressed as stearic acid. Subscripts 1 and 2 represent values calculated as first or second approximations, respectively. Let VI = grams of supernatant liquor (the difference between the weight of the total contents of the centrifuge tube and that of the added pigment). Let E = milligrams of surfactant per gram of Supernatant liquor, calculated from the counting rate (corrected for background and self-absorption), the total milligrams of active and inactive surfactant added, the milligrams of active surfactant added, and its specific activity, and the grams of supernatant liquor taken for counting. Then Dl (milligrams of surfactant in the supernatant liquor) = ilflE. (-4more correct first approximation of M I n ould involve adding to it a neight of pigment calculated approximately from the pigment determination.) Let F1 = milligrams of pigment in supernatant liquor, calculated from W, and from the milligrams of pigment per gram of supernatant liquor. Let GI = milligrams of surfactant in the precipitated pigment, calculated by the difference between the total surfactant added and Di. For sodium stearate experiments: Let J1 = mg. of KaOH in the supernatant liquor, calculated from the difference between the total NaOH solids added and the NaOH associated with GI. Let K i(milligrams of solids in superVCL. 33, NO. 3, MARCH 1961
* 415
40
53
I00
150
from determinations on the same supernatant liquor averaged =t0.75% of the adsorption value, over a range of 0 to +1.78%, for 45 single determinations with cetyltrimethylammonium bromide ; and *1.38'%, over a range of h0.45 to =t2.30/,, for 18 single determinations with sodium stearate. Since the adsorption value is based on the difference between the concentration of surfactant in the supernatant liquor before and after adsorption, a high count in the supernatant suspension should lead to a poorer precision for the adsorption value. A rough correlation of this kind was found.
0
200
EQUILiBRlUM CONCENTRATION OF SCCIUM STEARATE
AS MG. STEARIC 4ClD PER 00 GRAMS SOLUTION
RESULTS
Figure 1 . Variation of adsorption and pigment remaining dispersed (at 55" C.) after centrifugation, with concentration of sodium stearate (as stearic acid) in solution a t equilibrium X W e t press cake, adsorption
Figure 1 presents adsorption isotherms for wet copper phthalocyanine
0 Dispersed pigment Press cake dried at 60' C., adsorption
+ +
natant liquor) = D1 F1 J1 and Ll = grams of precipitated solids (as calculated by subtracting 0.001 K1 from the combined weight of pigment solids plus surfactant plus XaOH added). Then i l l 2 = total weight of contents of centrifuge tube - L1. Recalculate D1, F1, GI, J1, K l , and L1, using Mz in place of M1,to get second-approximation. subscript-2 values. (For experiments in which the amount of dispersed pigment left after centrifuging is negligible, calculation of a second approximation is unnecessary, since the weight of solution may be calculated accurately enough from the difference between the total weight and the weight of pigment solids, with only a negligible error due to the unknown weight of surfactant adsorbed.) hlilligrams of surfactant adsorbed per gram of precipitated pigment = G2/(grams of pigment solids added
-Fz).
In order to generalize this adsorption value to the whole sample of pigment, it is necessary to make the assumption that the pigment which remains dispersed after centrifugation has the same adsorption as the precipitated pigment. This value is also used to calculate the concentration of surfactant left in solu-
Table I.
7 37 87 137 187 237 287 487
416
UG
ANALYTICAL CHEMISTRY
,SO
253
ZCO
3ETrLTR h4ETeYLAMMOhlLM
BRCMIOE
PER IOC GRAMS SOLUTION
X Adsorption Dispersed pigment
tion a t equilibrium, apart from that held by adsorption on the dispersed pigment. Errors. With a n accumulated count for a determination of from 6000 to 44,000, t h e statistical counting error would range from a standard deviation of about +1.3 to &0.5%. The standard deviation of a single value from the mean of three values obtained
Adsorption Values, Mg. Surfactant/Gram Pigment Deviation from Mean Indirect Ultracentrifuge Av. =tAbs. 70 6.8 28.8 70.7 111.9 136.1 132.9 153.9 265.6
C i
Figure 2. Variation of adsorption and pigment remaining dispersed (at 21 o C.) after centrifugation, with concentration of cetyltrimethylammonium bromide in solution at equilibrium for wet press cake
Adsorption Values b y Indirect Calculation and Directly by Means of Ultracentrifuge Data
Initial Concn., Mg. Cetyltrimethylammonium Bromide/ 100 Grams Mixture
si3
LC i O U l L 3 9 'JM CCUCEh-Q&71CV
6.9 29.1 67.5 109.2 136.4 142.4 148.3 203.2
6.9 29.0 69.1 110.6 136.3 137.7 151.1 234.4
0.1 0.1 1.6 1.4 0.1 4.7 2.8 31.2
1.5 0.4 2.3 1.3 0.1 3.4 1.9 13.3
press cakes and for copper phthalocyanine dried at 60" C. with sodium stearate as the adsorbate. Figure 2 gives an adsorption isotherm for cetyltrimethylammonium bromide. I n the same figures there are curves for the variation with equilibrium concentration of surfactant of the per cent pigment remaining dispersed after 2 hours' centrifuging at 3200 r.p.m. There is some correspondence between the concentration a t which the inflection occurs in these curves and the concentration a t which the adsorption isotherm levels Off.
The inflection in the adsorption isotherm of Figure 2 is rather unusual. Hon-ever, the precision of the values is evidence of its reality. It may be explicable in terms of the occurrence of more than one type of adsorption for the system involved. The points in the adsorption isotherms have been calculated from the adsorption data for the precipitated pigment. There is no way of deriving
this information for the dispersed pigment unless it is assumed that the adsorption characteristics of the latter are the same as for the former, H ~ R - ever, Figures 1 and 2 indicate that, a t least for adsorption values below the plateaus of the adsorption isotherm, the proportion of pigment remaining dispersed is not great. Therefore, a change in adsorption capacity for the dispersed pigment of an improbable magnitude would be required to affect the adsorption values for the precipitated pigment significantly. Experiments carried out by ultracentrifuging the equilibrium mixture so as to leave only a negligible concentration of pigment dispersed in the supernatant liquor, eliminate the need for indirect calculations. A series of such determinations (see Acknowledgment) was carried out for the adsorption of (cetyl-l-C14))N,N,Nmethylammonium bromide on a sample of copper phthalocyanine (different from that used for the rest of this work). Adsorption values were also obtained on portions of the same equilibrium mixtures by the indirect method. Table I gives a comparison of the two sets of values. Reasonable agreement was found between them except for the last value. The lack of agreement for the value at the highest surfactant. concentration would seem to he too great to be accounted for by random errors and probably indicates that a t high concentrations of dispersed solids or dissolved surfactant the simplifying assumptions on x-hich the indirect method is based may not be valid.
DISCUSSION
For predicting the properties of pigment dispersions, it is desirable t o have accurate data concerning the maximum amount of available interface. Many practical properties of a dispersion such as rheology, color intensity, and shade depend to a large extent on this value. The values determined by various methods do not always agree with one another. It is necessary, therefore, to use such values cautiously, since they may not correspond t o what is actually obtained by a given method of dispersion. The electron microscope can give a particle-size distribution curve and can distinguish small particles (-100-A. diameter). However, its use is timeconsuming and the field of viewing is often not representative of the entire sample. With an electron micrograph, however, the maximum surface area can be obtained by calculation. The supercentrifuge (6) has also been used to obtain particle-size distribution data. Its major disadvantage is that it does not differentiate between ultimate par-
Figure 3. Electron rnicrogroph of metastable crystalline form of copper phthalocyanine ticks and aggregates of these, which appear as large particles. Gas-adsorption methods, which have been used to give an average value of the surface or particle size, do not take into account the distribution factois. Solution-adsorption experiments suffer from the same disadvantage and, in addition, only relative adsorption values can he obtained when a binary system is used (1). Other methods such as line-broadening techniques from x-ray diffraction and light scattering have also been used successfully. The radiotracer method reported here resembles reported techniques in some respects, but none has mentioned the use of press cakes or filter cakes without previous complete removal of wetting liquid. One reason for this is the difficulty of completely separating solids from liquid in a dispersion by techniques such as centrifugation or filtration. The method reported here overcomes this difficulty. When a solid is synthesized in any particular medium, it exists as particles of diameter d. The substance has a surface free energy, F, which is a function of its particle size and its inherent interfacial free energy, y (which in turn is a function of the affinity of the solid for its surrounding medium).
F
= f(r,
a
When the medium surrounding the particles is removed, capillary attraction and energy cause the particles to agglomerate and, hence, reduce the surface. The net change of F in the process can be negative if the solid has greater inherent affinity for tbe final medium-e.g., air-than i t has for the initial medium-.g., water. The only case in which it can he positive is when the y term is positive enough to overcome the negative term induced by the change in surface. It is, therefore, unreasonable to expect values on a dried powder necessarily to correspond to those of the maximum surface by any accepted experimental technique in every case. The fraction of this madmum surface ob-
tained will depend upon its tendency to aggregate, the size of the poles in the aggregate, and the size of the adsorbing molecule. These postulates are illustrated by the data obtained in this work. For example, if the area per molecule of sodium stearate is taken as 25 sq. A. a t 50" C. (4),the surface area calculated a t the plateau is about 80 sq. meters per gram for the press-cake sample. [Harkins (4) gives a value of 22 t o 24 sq. A. at 20" C. and this has been increased to 25 sq. A. t o adjust for increased thermal vibration at 50" C.1 However, when the same calculation is made foi the dry solid, the surfare area is approximately one half of this value or about 40 sq. meters per gram. (A close packed monolayer for stearate ion is assumed in both cases because of the shape of the adsorption curve and because the pigment becomes more dispersed as the concentration of stearate is increased.) The maximum surface available from the electron micrograph (see Figure 3) is about 80 sq. meters per gram. When the method of Brunauer, Emmett, and Teller (3) is used, the values vary considerably, depending upon the temperature of outgassing (see Table 11). Table II. Surface Areas of Copper Phthalocyanine
s, sq.
Method Meters/G. Remarks Adsorption 79.G Presscske from solution Adsomtion 38.7* Drv from solution BET-Ne 39.1 Outgas, 25" C. adsorption BET-NS 19.0 Outgas, 100' C. adsorption Supercentrifuge 55.2 Press cakeb Electron 81.5 Press cake microscope dried( This assumes 25 sq. A. for area of stearate ion. This is approximated at 50" C. from 24 sq. A. at 20" C. k Press cake dispersed with 20% commercial surfactant. L. Calculated from average diameter of particles present in several fields and densityof 1.49 g./ml. (6).
VOL. 33, NO. 3, MARCH 1961
417
Table 111.
Diameter Range, P
Supercentrifuge Data (2)
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-
