V O L U M E 2 2 , NO. 1 1 , N O V E M B E R 1 9 5 0 sample having the largest specific surface. I t is drawn through the origin because for zero surface there should be zero adsorption. The slope of this line indicates that 1 mg. of adsorbed Daxad covers 0.864 square meter of zinc oxide surface. From this value and the extrapolated value of the weight of Daxad adsorbed per gram of pigment the specific surfaces of the various zinc oxides have been calculated (column 4). This straight line indicates that the electron microscope and this adsorption method are ‘measuring the same surface characteristics and the deviations indicate that both methods give an accuracy of 0.1 square meter per gram of zinc oxide unless both methods have a corresponding error for any one pigment. An exception should be noted in the case of pigment 2. This is an acicular zinc oxide and the highly irregular shapes revealed by the electron microscope make evaluation of their dimension difficult. The authors feel that in this case the adsorption method gives the better value. The weight of Daxad adsorbed plotted against the specific surface as determined from electron microscope data gives a straight line. When a similar plot is made against the specific surface data determined by BET nitrogen adsorption, erratic behavior results. These different behaviors might be explained by assuming that the nitrogen adsorption method is measuring a different surface characteristic’ than is measured by the other two methods. DISCUSSION
This study indicates that the specific surfaces of zinc oxide pigments can be determined by the adsorption of a surface-active wetting agent from aqueous solution. As an analytical method it is less tedious than the usual methods. For routine analytical work the procedure can be shortened to a determination of three points on the flat portion of the curve (Figure 1). If an interferometer is not available, equally precise results can be obtained by gravimetric methods, using a semimicrobalance and following the procedure suggested by Ewing ( 2 ) . The accuracy of the method depends upon the accuracy of the determination of the concentration differences and upon the
1455 reproducibility of the attainment of either B monomolecular adsorbed layer or some function of it. The interferometer is a precise instrument; actual readings consist of drum readings where one unit = 0.000256% solids, or in the ordinary runs of 50 ml., one unit = 0.128 mg. of Daxad. This corresponds to 0.11 square meter of surface per gram of sample. The interferometer can be read easily to +0.5 unit on the drum usually by averaging ten different readings), and the precision IS therefore more than adequate for this type of determination. In working with solutions of high concentration4.30% in the case of Davad -the interference patterns are difficult to match, and on the authors’ instrument a mismatch of patterns gives a constant difference of 20 drum units. This introduces a serious error and can be eliminated by the use of less concentrated solutions and some degree of care in removing samples from the centrifuged tubes. The reproducibility of the adsdrbed film was investigated by ten consecutive determinations of the specific surface of sample 6 ; all gave results that were in agreement within the experimental error. Maximum value was 4.4, minimum value 4.1, average value 4.2, and average deviation t0.05 square meter per grain.
6
ACKNOWLEDGMENT
The authors are indebted to C. E. Barnett, D. G. Brubaker, M. L. Fuller, and C. W. Siller of the Yew Jersey Zinc Company of Pennsylvania for their cooperation in furnishing the pigmenta and the electron microscope and nitrogen adsorption measurements. LITERATURE CITED
(1) Brubaker, D. G., private communication. (2) Ewing, W. W., J . Am. Chem. Soc.. 61, 1317 (1939). (3) Ewing, W. W., and Rhoda, R. N., ANAL.CHEM., 2 0 , 7 4 3 (1948). (4) Green, H. J., J. FranklinInst., 204,713 (1927). ( 5 ) Harkins, W. D., and Gans. D. M., J . Am. Chem. SOC.,53, 2804 (1931); J . Phus. Chem., 3 6 , 8 6 (1932). (6) Harkins, W. D., and Jura, G.. J . Am. Chem. Soc., 66, 1366 (1944). and Fuaek, J. F., Ihid.,68, 229 (1946). (7) Smith, H. A4., RECEIVED April 1, 1950.
Volumetric Determination of Sulfate Ion Using Barium Ion and a Standard Disodium Dihydrogen Ethylenediamine Tetraacetate Solution J. R. MUNGEKI, R . W. NIPPLERz, AND R. S. INGOLS Engineering Experiment Station, Georgia Institute of Technology, Atlanta, Ga. S E V E R A L volumetric procedures for determining sulfate ion concentration are described in the literature, but each appears to have flaws, either because of time-consuming steps or because of inaccuracies in the values obtained with low sulfate ion concentrations. Thus, sulfate ion concentrations (1) can be determined with benzidine hydrochloride, but the determination involves a filtration step and a blank correction for low sulfate ion concentrations. Barium chromate ( 4 ) can be double-precipitated with barium sulfate and the sulfate ion determined by subtraction of the excess chromate ion from the known amount added, hut the precipitate must be removed by filtration before the titration step is possible and a calibration curve must be drawn to obtain accurate results. The internal indicator, disodiuni tetrahydroxyquinone (f), can be used for the titration of, the sulfate directly with standard barium chloride solution, hut because of the relatively slow formation of the barium sulfate precipitate at low sulfate ion concentrations, the technique is limited to use with 1 2
Present address, Red Stone Arsenal. Huntsville, Ala. Present address, Georgia State Board of Health, Rome, Ga.
high sulfate ion concentrations such as those normally found in boiler water. Schwarzenbach (3, 10-12)recently published a series of articles on the formation of soluble organic complexes with divalent and monovalent metallic ions, in which the divalent metal ion concentrations are reduced to values which are useful for analytical and industrial purposes. His technique (3, f0) for determining hardness in water has been readily accepted, and confirming articles indicating success in its use have been kublished (2, 5-7, 9). Two of the confirming articles give compIete and slightly modified procedures and data ( 2 , 6). From the original literature, it is evident that the disodium salt of ethylenediamine tetraacetic acid (Versenate) can be used for determining the concentration of the barium ion while using the dye, Eriochromeschwarz T, as the indicator. It should then be possible to add a standard barium chloride solution to a sample being analyzed for sulfate ion concentration and to determine tllv escesb barium ion. Because the standard barium ion solution is :idded to the sample in excess in the first step, it is possible to
1456
ANALYTICAL CHEMISTRY
Table I. Recovery of Sulfate Ions Using Versenate Titration of Excess Barium Ions Sulfate Added P.p.m.
Sulfate Recovered P.p.m.
Error P.p.m.
%
Reproducibility (Standard Deviation) P.p.m.
Table 11. Analysis of Natural Waters by Versenate Sulfate Ion Concentrationa Sample
No.
Direct P.p.m.
Amount Added P.p.m.
Amount Recovered P.p.m.
5.0 5.4 9.6 19.1 8.0 8.0
Difference P.p.m.
20.3 25.9 5.6 20.3 25.9 5.6 20.3 30.1 9.8 21.4 40.6 19.2 40.5 49.6 9.1 60.8 70.9 10.1 All values are average of a t least six determinations. 1
2 3 4 5 6
Deviation P.p.m.
+0.6 +0.2 +0.2 f0.1
+1.1 +2.1
precipitate the barium sulfate completely even a t very low sulfate ion concentrations before making the titration for the excess barium ion. It is not necessary to remove the precipitate of barium sulfate before making the titration for the excess barium ion, so that this titration step can be started very shortly after beginning the procedure. The major usefulness of this procedure will be found only in a laboratory also using the Versenate procedure for determining the total hardness, because the value of the combined calcium and magnesium ion concentrations is required for calculating the sulfate ion concentration. TECHNIQUE
Reagents. Standard Versenate solution, 0.02 N . The reagent that includes magnesium chloride may be used (6). Buffer solution, 8.25 grams of ammonium chloride and 113 ml. of concentrated reagent ammonium hydroxide in 1 liter pH 10.0 when 10 ml. are added to 50 ml. of sample. Barium chloride solution, 0.020 N . Calcium chloride solution, 0.020 N , from calcium carbonate as described in ( 1 ) . This is the primary standard for the Versenate, barium, and magnesium chloride solutions. Magnesium chloride solution, 0.02 N . Standard hydrochloric acid solution, 0.020 N . Indicator, Eriochromeschwarta T, 0.4% in alcohol (8). Procedure. Determine hardness with Versenate as described by Biedermann and Schwarzenbach ($,IO)and modified by Diehl, Goetz, and Hach (6). Determine alkalinity with the standard hydrochloric acid solution (not the usual sulfuric acid solution). To a third aliquot of 50 ml. of sample, add standard acid equivalent to the alkalinity (or slightly more?, in order to destroy carbonates. Boil the sample and add 5 or 10 ml. of the standard barium chloride solution (depending on the estimated sulfate ion concentration), and allow the mixture to boil for a few seconds. Cool the flask, add 10 ml. of buffer and 5 drops of indicator, then titrate with the standard Versenate solution. The first end point may not be used, because its accuracy will be poor even when the standard solution containing some magnesium ion is used (6). The addition of a small amount of standard magnesium ion solution and a second end point with extra Versenate solution will give good accuracy, because the indicator is more sensitive to the magnesium ion. In order to minimize the end-point error, it is recommended that one approach the same color change used for the determination of the hardness. Calculations. The sulfate ion can be calculated by the formula:
(H
+ B + M - T ) X 48 lo00 = p.p.m, of sulfate
titration of barium ion plus magnesium ion, and sample used.
8
is milliliters of
RESULTS
A number of waters with known sulfate ion concentrations were prepared and analyzed with the Versenate technique as outlined above and the results were compared with the theoretical values. The results of this comparison, as shown in Table I, indicate that the technique gives an agreement within approximately 1 part per hundred a t the higher concentrations used. A study of the time involved showed that six samples were analyzed in an hour by the Versenate method, and that the results obtained included data on total hardness, alkalinity, and sulfate ion concentration. The accuracy of the Versenate increased as the amount of sulfate ions increased, but reasonable recovery waa obtained with concentrations as low as 5 p.p.m. This accuracy was checked in natural waters by the addition of a known increment to a second aliquot; the results from this study, which are presented in Table 11, show a good agreement in the values obtained first directly and then by difference from the increment. The Versenate technique is not completely free of interference. In order to use it in the presence of copper, manganese, cobalt, and nickel ions, the modifications given by Diehl, Goetz, and Hach (6) for total hardness should be used, For concentrations of copper up to 10 p.p.m., Betz and Noll ( 2 ) recommend a different buffer system; they indicate that manganese up to 2 p.p.m. does not interfere with the accuracy of the test, although it does change the color of the indicator when enough is used. The modifications have been checked and found to work for the suifate ion determination. DISCUSSION
The over-all accuracy of this technique depends on the accuracy of the prelimiiiary hardness titration as well ap the final titration after addition of the barium chloride solution. In both titrations, an adequate concentration of dye is essential. Although it is not necessary for the usual determination of hardness, it has been found that the titration for determining the total hardness can be made more accurate by the incorporation of the back-titration with magnesium chloride as recommended for the final titration with barium. Back-titration of the sample with magnesium chloride is recommended because the color change for the initial end point when using the barium ion alone is. slow in forming, and can easily be passed. Upon addition of the magnesium chloride, the wine-red color is restored, and the final end point can be reached quickly and accurately. As might be expected from the data in the articles by Schwarzenbach, the magnesium chloride gave a much sharper end point than did calcium chloride in the back-titration. The addition of more indicator does not sharpen the end point. The addition of a small amount of sodium chloride as recommended in the turbidimetric determination-of sulfates (19)does not sharpen the end point nor increase the rate' of precipitation but tends to reduce the reproducibility of the results. Previous workers (8, 6) and the authors have found that sodium chloride does not interfere in the calcium and magnesium ion determinations, so that the cause of these poor results is not understood. If a sample is known to have a large sulfate ion concentration, the range of the technique can be extended by the addition of a larger portion of barium chloride, or a smaller aliquot of sample may be used. If a large number of samples a t high sulfate ion concentrations are to be analyzed, the concentrations of the reagents may be adjusted as necessary. ACKNOWLEDGMENT
~
where H is milliequivalents of Versenate re uired for total hardness, B is milliequivalents of barium chloriie solution added, M is milliequivalents of magnesium ion solution used to sharpen the end point. T i s the total milliequivalents of Versenate used for the
The work described in this paper waa supported in part by a grant-in-aid from the National Institutes of Health of the United States Public Health Service for research on rapid procedures for the analysis of water and sewage.
V O L U M E 2 2 , N O . 11, N O V E M B E R 1 9 5 0
1457
LI1'ER.ATURE CITED (9) ltossum, J. R., and Villarrus, P., Water and Sewage W o r k s , 96, 391 (1949). (1) Am. Pub. Health Assoc., "Standard Methods for Examination (10) Schwarzenbach, G., and hckernlann, H., Helo. C h i w Acta, 31, of lyater and sewage," 9th ed., xew york, Aqmerican public Health Association, 1946. 1029-48 (1948). Schwarzenbach, G., and Biedermann, W., Ibid., 30, G78-87 (2) Beta, J. D., and Noll, C. A , , J . ~ m ate^ . worksA ~ ~42,~49~ . (11) , (1947). (1950). (12) Schwarzenbach, G., Biedermann, W., and Bangerter, F., Ibid., (3) Bicdermami, W., and Schwarzenbach, G., Chitnia, 2, 56 (1948). 29, 811-18 (1946). (4) Cantina, E. C., Soil Sci., 5 , 361-8 (1946). (13) Sheen, R. I., Kahler, H..L., and Ross, E. hl., IND.EN. CHFX., ( 5 ) Canners, J. J., J . A m . Water Works Assoc., 42, 33 (1950). ANAL.ED.,7, 262 (1935). ( 6 ) Diehl, H.. Goetz. C. A.. and Hach, C. C., Ibid., 42, 40 (1950). (7) Ingols, R. S., Filter Press, 4, S o . 8, 5 (1949). RECEIVEDJanuary 27, 1950. Presented before the hleeting-in-~liniature, ( 8 ) J. A m . Water Works Assoc., 42, 39 (1950). Georgia Section, AVERICASCHEMICAL SOCIETY,November 1919
Determination of Acetyl in Pectin E. L. PIPPEN, H. JI.
M C C R E A D Y , AND
II. S. OWENS, W e s t e r n K5gional Reseurch Laboratory, Albany, C a l q .
a study of pectin acetates a t this laboratory, a simDURIKG method for the analysis acetyl in pectin was desired. of
Although successful methods ( 3 , 4,6 ) have been described for the determination of acetyl in pectin, the method presented here requires few operations, is economical of materials, and is rapid and accurate. The method is a modification of Clark's ( 2 ) procedure, devised to overcome the inconsistencies obtained when hot alkali is used for the saponification. The authors' experience has agreed with that of Liidtke and ~~l~~~( b ) ,who have shown that acids, other than acetic, are formed whea pectin is heated in alkali.
heating the distilling flask with a flame until the volume of liquid in the distilling flask was about 15 to 20 ml. Steam was then permitted to enter through the steam inlet tube by loosening the Screw clamp. The rates of steam inlet and application of heat to the distilling flask were adjusted so that the volume of liquid in the distilling flask remained at about 15 t o 20 ml. (Keeping the volume of liquid in the distilling flask low ensures a quantitative recovery qf the acetic acid in a 100-ml. distillate volume.) Distillation thus carried out until a distillate of 100 ml. was c o ~ lected. which !vas titrated writh 0.05 N sodium hvdroxide to an end point with phenol red as the indicator. A blink defermination was carried out by distilling, as described above, a mixture of 20 ml. of the magnesium sulfate-sulfuric acid solution and 20 ml. of distilled water. Titration of the distillate from the blank run usually requires 0.1 ml.
APPARATUS
The apparatus is identical to that described by Clark ( 2 )except for the distilling flask and condenser, which were modified slightly (Figure 1).
SCREW C L A M P 7
24/40
$
-7
MATERIALS
The pectin acetates analyzed were prepared by acetylation of commercial citrus pectin by the procedure of Carson and Maclay (1). Results for pectin are expressed on a moisture- and ash-free basis. The purity of other substances analyzed was established by their melting points, saponification equivalents, and specific rotations, where applicable. Figtire 1.
PROCEDURE
An accurately weighed 0.5-gram sample of the pectin acetate was placed in a 250-nd. Erlenmeyer flask and 25 ml. of 0.125 N sodium hydroxide were added. The flask was stoppered and the contents wcre stirred until all the pectin wasdissolved. The flask was then set aside at room temperature for at least 1 hour. ( A s a routine procedure, samples were permitted to stand in alkali overnight.) The contents of the flask were diluted to 50.0 ml. and a 20.0-ml. aliquot was withdrawn and placed in the distilling flask. This was followed by a 20-inl. aliquot of ('lark's ( 2 ) magrirsium sulfate-sulfuric acid solution and an ebullition tube. After the steam inlet tube was set in place, the rubber tubing was closed with the screw clamp and distillation wa? carried out by
Table I.
Comparison of Authors' with Henglein and Vollmert (4) Method
Subbtance Analgzrd Pectin acetate 13 Pectin acetate 12 Citrus pectin
Tahle 11.
Acetyl Found, % Method of Henglein Modifiration of and Yollrnert Clark's method 2.5 2.9 0.3
2.56, 2 . 5 7 3.01, 2 . 9 7 0.29, 0.30
Acetyl i n Suhstances Other than Pectin
Substance Analyzed Arabitol pentaacetate Galactose pentaacetate Glucose pentaacetate
Found 59.3, 59 3 65.2, 56.4 55,7,57.2
100 H L F L A S K
Acetyl. % Calculated 59.5
55,l 55.1
Diagram of Distilling Flask and C o n d e n s e r
Calculation. Net ml. of NaOH = (total ml. of XaOH required to titrate distillate) - (total ml. of NaOH required to titrate distillate of bla1:k run)
% acetyl
=
(net ml. of NaOH) (normality of NaOH) X (0.043) (100) weight of sample, grams, in 20.0-ml. diquat RESULTS
On duplicitte analyses on pectin, reproducibility of results within 0.1ri or better was consistently obtained. Further cxperiments to dctcrmine the accuracy and scope of the method were conduvted. Comparison with t8hemethod of IIenglein and Vollmert ( 4 ) (Table I ) shows that these two methods are in excellrrit agrccmcnt, for the analyses of pectin acetates. When substances other than pectin acetates \?ere analyzed (Table II), re sults for glucosr: and galactosc pentaacetates were higher than the theoretical acetyl values. \Vhile tk!e results of only two analyses of galactose and glucose peritaacetates are presented, other analyses of these compounds gave results which were consistently 1 to 2% higher than the theoretical values. I n the analysis of glucose, as well as the acetates mentioned above, the apparent acetyl content generally increased in direct proportion to the time of saponification. Consequently, the method, as described in this paper, is unsuitable for acetates of these sugars and presumably for ace-
