Determination of Combined Acetic Acid Content of Cellulose Acetate Gravimetric Method GIUSEPPE GARETTO and ALFRED0 RUFFONI Research Department, Rhodiatoce S.p.A., M i l a n , Italy
-4 gravimetric method for the determination of the
glassn are and burets of precision corresponding to those certifiecl by the Sational Bureau of Standards were used. I n the gravimetric method normal filter paper, washed by water and dried to constant weight, was employed. The reagents were not specially controlled, as their influence is without importance in this case. The same considerations apply t o the glassware. Drying was carried out in a thermostatically controlled oven; the temperature of 105' C. was kept to within 1 2 ' C. The saponifications were carried out a t room temperatui e (about 20" C.) in 500-ml. flasks with narrow necks and groundglass stoppers. The flasks were rotated on a roller iiiixer a t 60 r.p.m. Test Methods. VOLTUETRICMETHOD. The cellulose ncet:ite Tvas dried for 5 hours at 105' C. and cooled in a desiccator. -12.5-gram sample was placed in a 500-ml. flask, and 100 nil. of 0.5.V sodium hydroxide solution were added. The stoppered flask was placed on a roller mixer for 48 hours a t 20" C. Then the flask was opened, the stopper and the neck were thoroughljwashed, and 0.5.V sulfuric acid solution was added until there was an excess of about 1 ml. (phenolphthalein as indicator). The well-stoppered flask mas allowed to stay on the roller mise1 for 5 hours, and it was then titrated until a pink color appeared and remained after vigorous stirring. I blank test was made under the same conditions. GRAVIMETRIC METHOD. The same technique was used as for the volumetric method. At the end of the titration the contentof the flask were quantitatively passed through a folded filter of 130-mm. diameter. The cellulose was washed with 500 ml. of water a t 20" C. using small quantities a t a time, and then with 200 ml. a t TO" C. The filter was allowed to stay 12 hours in the oven a t a temperature of 50" C., was placed on a weighing glass, and was then replaced in the oven at 105' C. until a constant weight was achieved (about 2 hours).
per cent of combined acetic acid content of acetonesoluble cellulose acetate has been established. The method is based on weighing the cellulose regenerated from the ester by means of complete saponification in an aqueous-alkaline medium. Such a method gives results w-hich compare favorably with those obtainable by the usual volumetric methods. It is anticipated that this new technique will be applicable to cellulose acetates having solubility properties different from those of the products examined here.
D
URING the past 50 years nunierous papers have been pub-
lished relating t o studies carried out both in research institutes and in industrial laboratories on the determination of the percentage of combined acetic acid in cellulose acetate. For an extensive and up-to-date documentation on this subject, the reader is referred to Kriiger ( 5 ) ,Heuser ( d ) , and Dor6e ( 2 ) , and to the report of a subcommittee of the Division of Cellulose Chemistry ( 1 ) of the AMERICAN CHEMICAL SOCIETY. The Eberstadt method (S) and the 0 s t method (6) have stimulated most of these studies. Both methods involve the volumetric determination of acetic acid-with previous alcohol-alkaline saponification (Eberstadt method) or acid hydrolysis and distillation (Ost method) The work presented here describes a new gravimetric method which has given very good results in the determination of the percentage of combined acetic acid of the secondary acetonesoluble acetate having a 54 to 55% acetic acid content. R7ith this method the percentage of combined acetic acid is obtained from the weight loss of the ester by its transformation to cellulose by complete saponification. The saponification is carried out in an aqueous-alkaline medium. The regenerated cellulose is collected on a filter, washed well, dried to a constant weight, and weighed accurately. If A is the weight of the dried cellulose acetate, and C is the &eight of the dried cellulose, the acetic acid content, t (expressed as acetyl value), is given by the following equation: t =
Table I. Comparison of \-ohmetric and Gravimetric >lethods of Determining Combined Acetic Acid in Two Samples of Cellulose Acetate Acetic .4cid Content, % Volumetric Gravimetric method method
Test h-0. 1 2 3
4 5 6
4300 ('4 - C ) 42 B
Av.
Sample -4 53.94 53.94 53.97 53.96 53.94 53 95 33.95 Saiiiple B 55.68 55.68 55.70 55.68
while the acetic acid content, T (expressed as acetic acid), is given by 6000 ( A - C) T = 42 A
2;. 66
.68 5 5 68
JD
.4r. EXPERIMENTAL
a
Samples. Two samples of cellulose acetate having the following properties have been examined. Sample A. American cellulose acetate of the type used in the production of plastics. It contained about 54% of combined acetic acid, and was in powder form which could be passed through a sieve of 840-micron mesh. Sample B. Italian cellulose acetate of the usual type for use in rayon production. It contained about 55.5% of combined acetic acid, and was in flake form. After being milled in a Forples mill it became pulverized and could be passed through a sieve of 840-micron mesh. Materials. I n the volumetric method (control test) reagents free of carbonates and of carbon dioxide were used, as the influence of such factors on the results is well known. Neutral
54.00 54.00 63.84 54.00 54,OOQ 5 4 ,OOU 53.9T 55.77 55.81 55.75 55.85 55,88a
...
55.81
Infrared dried.
The final drying t o constant weight can also be carried out by using an infrared radiating lamp of 0.250 kw., the distance between the lamp surface and the bottom of the weighing glass being 20 cm. This type of drl-ing was found t o be more rapid (about 1 hour). E X P E R I M E N T A L DATA
The results obtained in tests carried out by the same operator a t different times are given in Table I.
400
V O L U M E 2 7 , N O . 3, M A R C H 1 9 5 5 C0NCLI;SIOY
401
s
It will lie seen from Table I that the new method al1on.s the conil)iiied acetic acid cont,ent of secondary acetone-soluble wllulore acetate to be determined with great accuracy. Small tlifterences in the results ohtaineti by the new method and by the volumetric method can he justified by experiment,al errors and by the fact that salts and ot,her n-ater-soluble impurities, a1n:iys present in the commercial products, give iiworrect results in a tli:miet,rically oppoFite \my. T h e gravimetric. method can be used to help establish the :icc.ur:icy of other methods including those using saponification, : i n t i to analyze mixed esters :ind other esters.
LITERATURE CITED
Division of Cellulose Chemistry, Committee on Standards and Methods of Testing, A r a ~CHEM., . 24, 400-3 (1952). Dorbe, C., “Methods of Cellulose Chemistry,” pp, 285-9, Chapman & Hall, London, 1947. Eberstadt, O., dissertation, Heidelberg, 1909. Heuser. E., “Chemistry of Cellulose,” pp. 277 -9, Wiley, Sew York, 1947. Kruger, D., ”Zelluloseaaetate,” pp. 218-28, T. Steinkopf, Dresden, 1933. Ost, E., and Katayama, T., 2. a v g r w . rophosphate but not triphosphate at pH 6.5. The of the hexammine- and tris(ethy1enediamine)cobalt phosphates, precipitates dried at 110’ C. are C O ( ~ ~ ) ~ H Z P @ ~ O . ~silicates, € I Z O and su1f:iter is shown in Table I. Sulfate and silicaate and Co(en)s€IPzO~.Orthophosphate, trimetaphosphate, were included hecause thrir separation from t,he phosphates is and tetrametaphosphate are not precipitated. Trioften a problem. Tris( propylencdismine)cohalt( I11) chloride phosphate can be precipitated from a mixture which was also tested, but it did not preripitate any of the phosphatrls contains pjrophosphate, but some of the latter is coexcept the polyphosp1i:rte n-ith the longest chnin length. Trisprecipitated and some of the triphosphate is left in (ethylenediamirie)cobdt(III) chloride showed promise as a spc~solution. The distribution of plrophosphate and trirific precipitant. for of the phosphates and other anions testetl phosphate between precipitate and solution was deteronly pyrophosphte :inti triphosphate precipitated anti the.sc mined by phosphorus-32-tagged phosphates. Co(en)iprecipitates n-ere otitained at different p H values. With hcJxCIS may prove a valuable reagent for triphosphate, amniinecol~alt(II1)chloride, liyrophosphate and triphosp1i:itt’ notwithstanding the influence of plrophosphate on the both precipitated at d l pI-1 values, indicating that i~ niethotl precipitation of triphosphate. In contrast to Co(en)ifor one in t,he prcsence of the other was unlikely. VI?, hexamminecohalt(II1) chloride [Co(”&CIj] prechloride ( I ) iCo( SHa)6C‘l:i cipitates PjOlo----- and l’*O;----, instead of IbP3Materials. Hr?;snimir~ec~~balt(III) and tris(ethylenediamine)cohalt(III) chloride ( 1 4 ) [Co(en)3CI3] and HPzOi---, and the yield of both is increased were prepared according to puhlished procedures. Tris(prop.1h! increasing plI. Orthophosphate is also precipitated, enediamine)cot)alt(III) chloride [ C ~ ( p n ) ~ C l was a ] prepared i i ? . so Co(”&C13 is not a potentially valuable reagent. t>hesame method as Co(en)aCI~,making allowance for the different molecular weight of propylenediamine. -411 were recrystallized The triphosphate precipitate dried at 110’ C. is Na[Coand dried a t 110” C. hefore use. Their identities 1T-ei-e verified hy (Y C11)613(P3010)2. analysis. Sodium triphosphate v a s prepared from a commercial product liy salting it out of aqueous solution n-ith alcohol four times and, E S ~ l I M I S C C O B A L T ( I I I chloride ) has been suggehted as a reagent in qualitative microchemical tests for p~ rophosphate (3) and triphosphate ( I O ) . Recently the amperoTable I. Precipitations with Excess Reagent“ metric titration of pyrophosphate m ith this reagent has been CO(SH~)OC b II Co(en)~CIs b tiescrihed (6). T h e possibility of a stepwise variation in com~pH PH position, size, and charge made Kerner cations an attractive Sodium Pnlt in Solutiirn 12.3 7.5 L.5 12.5 7 . 5 4.5 class to investigate as specific precipitants for the various phosPolyphosplintrs phate anions. 1 I> .., = -1 1 5 = 31 I, This background led to a study of heuamminecobalt(II1) Tr.iphosphate X Pyrophosphate 0 and the related tris(ethy1enediamine) and (propylenediamine)0 Orthophosphate cohnlt(II1) ions as precipitants for use in the determination of 0 Tetrametaphosphate 0 Trimetaphosphate pyrophosphate and triphosphate in the presence of each other 0 Sulfate 0 Silicate, SiOt/KaiO = 2 . 0 and in the presence of orthophosphate, trimetaphosphate, and a I n 50 ml. of nierlianicnlly aeitated solution a t room temperature there other phosphates. l I o s t of this paper deals with the precipitawere 3 00 millinioles of cobalt re&ent and 0.250 gram of sodium salt except tion of phosphates, especially pyrophosphate and triphosphate, for triphosphate (1.65 niillirnoles) and sulfate, orthophosphate, pyrophosphate (2.50 millimolr-). T h e pH wns maintained constant with S a O H -,1 hexammine- and tris(ethylenediamine)cobalt(III) chlorides, or HCI. b X = precipitate; 0 = no precipitate; L = an oily liquid separated. the description of resulting precipitates, and the coprecipitation of pyrophosphate with the tris(ethylenediamine)cohalt( 111)
H
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