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 , A N D 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 2.56, 2.57 2.9 3.01, 2.97 0.3 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-
ANALYTICAL CHEMISTRY
1458 tates of other simple sugars. The high results obtained for these compounds are probably due to the formation of acids, other than acetic, which are sufficiently volatile to enable them to appear in the distillate. On the other hand, consistent results were obtained on duplicate analyses of pectin acetates, for which the time of saponification was varied by as much as 1 to 20 hours. Thus, if any acids, other than acetic, are formed under the conditions specified, they are not sufficiently volatile to interfere with the method herein described for the analysis of pectin acetates. Arabitol pentaacetate served &s a suitable sthndard and the analysis of it gave consistent results which were close to, but never exceeded, the theoretical acetyl value. Thus the method is suitable for arabitol and pectin acetates and would presumably be
useful for other o-acetyl compounds which are readily saponified by dilute alkali near 20’ C. and do not give rise to acetic acid by side reactions or to volatile acids other than acetic acid. LITERATURE CITED
(1) Carson, J. F., and Maclay, W. D., J. A m . Chem. SOC.,68, 1016
(1946). (2) Clark, E. P., “Semimicro Quantitative Organic Analysis,” p. 73, New York, Academic Piess, 1943.
(3) Freudenberg, K.,and Harder, M., Ann., 433,230 (1923). (4) Henglein, F. A., and Vollmert, B., Makromol. C h a . , 2,77 (1948). (5) Liidtke, M.,and Felser, H., Ann., 549, 1 (1941). (6) Nelson, E.K., J . Am. Chem. SOC.,48, 2945 (1928). R E C E I V ~February D 6, 1950.
Determination of Chloride in Water-Addendum Preparation and Eflect of Mercuric Nitrate Reagent FRANK E. CLARKE, U.S . Naml Engineering Experiment Station, Annapolis, M d . A previous article ( I ) , the author described an improved IcuricNmethod for determining chloride ion by titration with mernitrate solution in the presence of diphenylcarbazonebromophenol blue indicator and controlled pH. The instructions
for preparing reagents were given briefly, assuming that they would be made in accordance with accepted laboratory practice. This prescribes acidification of the mercuric nitrate solution to prevent hydrolysis. There has been some concern that acidification of the mercuric nitrate might affect the accuracy of the chloride determination, which is influenced by pH. This note provides more information on the preparation of the mercuric nitrate reagent and shows that its acidification does not affect accuracy significantly. PREPARATION OF MERCURIC NITRATE SOLUTION
Mercuric nitrate does not dissolve readily in distilled water. Even in preparing 0.025 N solution, a large proportion of the crystals will remain undissolved and hydrolysis products may precipitate. The solubility is affected somewhat by the degree of hydration of the mercuric nitrate crystals. It is common laboratory practice to dissolve the mercuric nitrate residue by adding concentrated nitric acid slowly, while stirring vigorously. This procedure usually consumes more than the minimum effective quantity of acid. I t is better practice to dissolve the crystals directly in a small quantity of acidic water, and then to dilute the solution to volume. The quantity of mercuric nitrate required to make 1 liter of 0.1 N solution can be dissolved in 100 ml. of water containing 1.0 to 1.5 ml. of concentrated nitric acid. The quantity requiredfor a 0.025 N solution can be dissolved in 25 to 50 m]. of water containing 0.25 to 0.40 ml. of concentrated nitric acid. When prepared in this manner, either directly or by dilution from 0.1 N stock, the 0.025 N mercuric nitrate solution will have 6 pH of 2.25 to 2.50. It will be clear and remain clear for long periods of storage. Sediment which forms in storage can be filtered off, but the solution must be restandardized. EFFECT OF MERCURIC NITRATE ACIDITY ON CHLORIDE DETERMINATION
At the optimum pH, the mercury-diphenylcarbazone color complex forms after all the chloride ion is combined as weakly ionized mercuric chloride. I n weakly acid solutions the color complex will form in the presence of a large exceea of chloride ion. I n strongly acid solutions the tendency to form the complex is reduced, 80 that a large exceas of mercuric ion is required to develop the color. Between these two extremes there is a reaaon-
ably wide p H region (3.0 to 3.5) in which excellent accuracy can be obtained a t all chloride concentrations. It is most convenient to establish the optimum p H by adding nitric acid to the chloride solution before titrating it with mercuric nitrate (1). Because the mercuric nitrate solution contains some nitric acid, there is a slight increase in acidity of the chloride solution during the titration-for example, titration of 2 mg. of chloride ion in 100 ml. of solution will require 2.26 ml. of 0.025 N mercuric nitrate and reduce the p H from approximately 3.3 to 3.25. Titration of the maximum quantity of chloride ion recommended in (1) will require 22.6 ml. of 0.025 N mercuric nitrate solution and reduce the p H to approximately 3.05. The acid error in milliliters of mercuric nitrate solution consumed, and therewith milligrams of chloride, depends on the deviation from the optimum pH. It is essentially independent of the chloride concentration. The acid error aa percentage of chloride varies inversely with the total quantity of chloride present. It therefore is most significant in low chloride concentration$. Fortunately, these require small mercuric nitrate titrations, which yield insignificant p H reductions. At higher chloride concentrations, where larger quantities of mercuric nitrate yield greater p H changes, the lower percentage errors balance out the effects and maintain good accuracy. The pH-error graph in ( I ) was based on a 20 p.p.m. solution of chloride ion (2 mg.), which is a concentration frequently encountered in water work. It would not be applicable to chloride ranges far above or far below that concentration. The data in Table I11 (1) were obtained by first adjusting the chloride solution in accordance with the recommended procedure and then titrating with acidified mercuric nitrate solution. No correction was made for the additional acid introduced with that reagent. The data show that this procedure is adequate for all normal. analytical work. If extreme accuracy is desired, as in microanalysis, corrections can be made for indicator sensitivity and for excew acid introduced with the titrating solution. The indicator correction is determined by making a blank titration on chloride-free water containing the total quantities of indicator and acid which will exist in the sample a t the end of its titration. The effect of e x c w acid is overcome by neutralizing most of the excess before adding the indicator. Slightly less than the required amount of mercuric nitrate solution is run into the untreated chloride sample. The diphenylcarbazone-bromophenol blue indicator is then added, the P H of the solution is adjusted, and the titration is completed. These refinements are not required for most analytical work. I3TERATURE CXTED (1) Clarke,
F.E.,ANAL.CEDX.,22, 663-6
(1050).
