tri
termin
n of
JOSEPH H. FINLEV ntml Research Laboratories, Air Reduction Co., Inc., Murray Hill, N. J.
50 ml. of approximatel 0.2N HCl and boiled gently under r e d x for 1.5 hours. A few milliliters of the control is tested (8) for starch by the addition of 1 drop of REAGENTS AND APPARATUS 0.1N iodine solution and is compared with a similar volume of 0.2N HC1 Boric Acid Solution. Forty grams tested with a drop of the iodine solution. of reagent or U.S.P. grade dissolved in The hydrolysis of the sample is con1000 ml.of distilled water. tinued, if necessary, until the control Iodine Solution. Resublimed iodine alcohol with iodine in the presence of (heated for the same period) yields a 12.7 grams, dissolved in a concentrated negative test for starch. The sample boric acid. ~ p e c ~ r o p ~ o ~ o m mease~r~c solution of 25 grams of potassium iodide is then filtered into a 1-liter volumetric urement is made at the absorption in water and diluted to 1 liter with disflask and the pulp thoroughly washed maximum, 690 mp, and Beer's law tilled water. with hot water. The filtrate is cooled applies in the concentration range Standard Polyvinyl Alcohol Stock and diluted to 1 liter with water. corresponding to 0 to 1 mg. of polySolution. Fully hydrolyzed polyvinyl An aliquot (20 ml. or less) of the alcohol, 1.000 i0.001 gram, (previously vinyl alcohol per 50 ml. of solution. prepared solution is transferred into a 7 washed with pure methanol to pH ~ n ~ e r ~ ~ ~due e n ctoe the blue starch50-ml. volumetric flask and diluted and dried to constant weight), is dig iodine complex is eliminated by a prewith water to approximately 25 ml. solved in 100 ml. of distilled water by treatment involving acid hydrolysis of The solution is now treated with 15.0 heating on a steam bath and after coolml. of boric acid solution which is starch. The effect of various ing, diluted to exactly 1liter with water. introduced with continual swirling of stances which may be encountered Stock solutions thus prepared are stable the mixture. Finally, 3.0 ml. of iodine in paper sizing and coating formulafor at least 2 weeks. solution is added and the mixture tions has been investigated. Constant Temterature Bath. Regudiluted t o 50.0 ml. with water. The latedat25' 1 .C, flask and its thoroughly mixed contents Blendor. Wanng or equivalent. are placed in a water bath maintained at HE increasing use of polyvinyl All measurements were made with a 25' f '1 C. Beckman Model B spectrophotometer alcohol in paper coatings has As soon as the mixture has attained using 1-cm. matched Corex cells. necessitated the development of a the temperature of the bath, its absorbmethod for its determination in the ance is measured at 690 mp against L PROCEDURE presence of starch and various other reference solution prepared by diluting substances. A sensitive spectrophoto15.0 ml. of the boric acid and 3.0 ml. From 1 to 5 grams of the paper of iodine solution to 50.0 ml. with metric method was considered particusample, cut into small pieces, are water. larly desirable because of the relatively weighed to within a few milligrams. The concentration of PVA is deterdilute solutions which may result from The optimum sample size corresponds mined by reference to a previously to 10 to 50 milligrams of PVA. the extraction of paper samples. prepared calibration curve relating If the absence of starch has been It has been reported that polyvinyl milligrams PVA (per 50 ml. of test established by a separate qualitative alcohol (PVA) forms stable colored solution) and absorbance at 690 mp. (iodometric) test the sample may be complexes with iodine in the presence Preparation of Calibration Curve. transferred to a blendor for disintegraof boric acid (6). The presence of a A solution containing 0.100 mg. of tion in 50 to 100 ml. water as suggested characteristic intense absorption band PVA per ml. is prepared from the by Harvey, Forshee, and Fletcher (i?) standard polyvinyl alcohol stock soluin the visible spectral region suggested or digested by maceration with a glass tion by diluting 10.0 ml. to 100.0 analytical application and led to the rod, with equal effectiveness (8). The ml. in a volumetric flask. The calidevelopment of the method described disintegrated sample is transferred to a bration data are obtained by trans250-ml. flask with additional water and herein. Starch also forms a colored ferring aliquots up to 10 ml. of the heated for 15 to 30 minutes just below complex under the same conditions latter solution into 50-ml. volumetric the boiling point, then decanted and with an absorption maximum at a flasks, diluting each to 25 ml. with filtered into a 1-liter volumetric flask. wave length 110 mp shorter; its conwater, and adding 15.0 ml. of boric acid After washing with hot water, the pulp siderable absorption at the wave length and 3.0 ml. of iodine solution to each, on the filter is transferred to a suitable suitable for measurement of PVA followed by final dilution to 50.0ml. vessel and digested for a few minutes The calibration curve is derived from necessitates the prior conversion of any with 50 ml. of approximately 0.1N the absorbance values at 690 mp (measstarch present (by acid hydrolysis) HCI, then refiltered. This operation ured at 25' C.) plotted against the into noninterfering sugars. is repeated and the pulp finally washed quantity of PVA per 50 ml. Polyvinyl alcohol-iodine solutions with 200 ml. of hot water. The filtrate is cooled and diluted to 1 liter with have been successfully employed for water. the qualitative detection of small DlSCUSSlON If starch is present, two identical amounts of boric acid (4), while polyaamples are weighed and transferred vinyl alcohol-boric acid solutions have Basis of Colorimetric Method. into two 250-ml. Erlenmeyer flasks; Polyvinyl alcohol forms a green one of the samples serves as a hydrolysis 1 Present address, FMC Corp., Princeton, N. J. control. Each sample is treated with complex with iodine in the presence
b A spec~ropho~o~e~ric method has been developed for the determination of polyvinyl alcohol in paper and coating formulations which may also contain appreciable concentrations of starch and other substances. The
been adapted for use as an indicator in iodometric titrations in place of starch
*
i
1
O ' T 0.6 0.5
0.2
1
1
t-
W A V E LENGTH (MU)
Figure I . Spectral absorption of PVA-boric acid-iodine system at various PVA concentrations. PVAf 50 ml. 1. 2. 3. 4.
0.25 mg.
0.45 0.55 0.90
of boric acid. The wave length of maximum spectrophotometric absorbance of the system is 690 mp, as illustrated in Figure 1 covering the spectra for concentrations of PVA from 0.2 to 0.7 mg. (per 50 ml.) in the range 600 to 808 mp. A large number of absorbance measurements a t 690 mp on solutions containing from 0 t b 1 mg. of PVA per 50 mi. and the quantities of boric acid and iodine stipulated in the procedure indicate that Beer's law applies within this concentration range. In the absence of boric wid, PVAiodine solutions exhibit no appreciable absorbance in the range 500 to 800 mp, a n interesting consequence of which i s that starch may be determined spectrophotometrically a t 580 my by the procedure of Browning, Bublitz, and Baker (I) in the presence of PVA. The converse however is not true since it has been established in the present study that starch absorbs very nearly as strongly a t 690 mp in the presence of boric acid and iodine as in the presence of iodine alone, thus eliminating the possibility of determining PVA in the presence of starch by direct spectrophotometric means. Stayco M, a typical paper coating material, one of five varieties of hypochlorite-oxidized thin boiling corn starches produced by A. E. Staley Mfg. eo., was used in this study. At equal concentrations of 1.0 mg. per 50 ml. thc starch-boric acid-iodine absorbanee a t 690 my is about 27% of that of the PVA-boric acid-iodine system a t the same wave length. These spectral data indicate the necessity for eliminating the interference due to thc presciirc of starch by the hydrolysis of the latter to substrincw which do not react \\ith iodine. Thc e
ANALYTlCAl CHEM!§TRY
conditions of acidity, temperature, and time required to effect this hydrolysis do not appear t o affect the polyvinyl alcohol in any way. ect of Dilution. It was necessary t o dilute the test solutions with water to within a definite volume range prior to the addition of boric This effect is illusPolyvinyl alcohol ed by either boric acid or iodine (or the combination) unless sufficient water is present before the addition of these reagents. The data of Figure 2 indicate that the absorbance values become consistent for the concentrations of polyvinyl alcohol covered if dilution to 20 to 30 ml. precedes the addition of the
1
0.1
01 0
ffect of Boric Acid Concentration. The effect of increasing amounts of 3.8% boric acid solution is represented in Figure 3, which indicates a general increase in absorbance with increasing amounts of boric acid. However, in the range of 10 to 20 ml. of 3.8% boric acid, the rate of increase of the absorbance is relatively small. On the basis of these data, 15 ml. of 3.8% boric acid was chosen as the fiscd quantity to be added. This value represents a compromise between the maximum amounts which could be added (22 ml.) to a 50-ml. volumetric flask, and the minimum quantity of 10 ml. a t which point the rate of absorbance increase has just started to level off. Since the maximum solubility of boric acid a t room temperature is approximately 5%, the 3.870 solution employed is approximately 75% saturated with respect to boric acid. This appears to be a safe working value as no boric acid precipitation has been noted to dutc. It also provides an adequate quantity of boric acid
I
5
I IO
I
I
15
4
6 ML.
Figure
3.
I
with respect to the sensitivity of detection of polyvinyl alcohol. Edect of Iodine Concentration. A rapid increase in absorbance occurs up t o about 1 ml. of added iodine solution. As the amount of iodine added is increased further, the absorbance values tend to assume a fairly constant masiinum value (Figure 4). On the basis of these data, 3 ml. of 0.1N iodine solution appeared to be a suitable quantity for the purposes of this analytical method. Effect of Temperature. For accurate work it is recomniended that color development for both samples and calibration standards be carried out a t a constant temperature. e.g., the 25' (2, used in the development of
0
IO
aoRic
ACID
Effect of
i
PVA/50 rnl. 1. 0.50 mg. 2. 0.70 3. 1.0
I 2
I
Figure 2. Effect of dilution of test solution prior to addition of reagents
1
0
I
20 25 30 35 T O T A L VOLUME I M L . 1 BEFORE A D D I T I O N OF H3B03 A N D I2
1
I
12 14 ADDED
I
I
16
I8
boric acid concentration
PVA/5Q ml. 1 . 0.50 mg.
2. 1.0
20
the present method. Deviations in absorbance values were noted as the temperature was varied, the absorbance increasing a t lower temperatures. The change in absorbance with temperature was eatablished as approximately 2% per degree C. Effect of Time. The color develops immediately (at 25' C.) and is stable for a t least 4 hours. Effects of Acids and Bases. PVA solutions containing various concentrations of hydrochloric acid up to 0.025 mole per liter yielded absorbance values identical with those of the same PVA concentration prepared without any hydrochloric acid. Up to 0.015 mole of added sodium hydroxide per liter was also without effect on the absorbance values of PVA solutions.
Table I. Determination of PVA in Synthetic Samples
Ratio of Starch to PVA
1
0
0
1 2 3 4 ML. 0.1 N. IODINE AODED
% Recovery
1 ,0220 0.2119 0.0261
101.6 105.S 103.2 111.7 111.8
0.0142 0.0145
~~
~~
Table 11.
Determination of PVA in Sized Paper
% PVA
Sample
Reputedly present"
1 2 3
1.80-1.90 1.80-1.90 1.10-1.20
Comments
Found 1.65-1.70 1.69-1.72 0.91-0.92
No starch or clay present No starch or clay resent
Both starch and cl& present
Based on composition of paper coating formulation and concentration applied to paper
Table 111.
1 -
PVA, Grams Recovered
1 ,0057 0.2001 0.0236 0.0127 0.0130
1:l
10:1 100:1 500:1 1000: 1
a
0.6 O"
Added
Substance Calcium stearate Calgon Titaninm dioxide Potassium alum Kelcosol algin Protein (Adpro 112A) Rosin Casein (Nat'l. LT34) Clay (predis ersed) Calcium cartonate
Effect of Diverse Substances Present in Paper
Ratio of Substance to PVA 100:1 100 100 100 10 100 100 100 20 100
Added 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0
PVA, Mg. Recovered 10.20 10.35 7.71-8.02 9.60 10.03 9.95 8.46 9.92 8.90 9.72
% Recovery 102.0 103.5 77.1-80.2 96.0 100.3 99.5 84.6 99.2 89.0 97.2
5
Figure 4. Effect of iodine concentration 0.70 mg. PVA/JO mi.
Effect of Molecular Weight of Polyvinyl Alcohol. Calibration curves
prepared with a sample of high molecular weight polyvinyl aloohol (as indicated by a high 4% solution viscosity) corresponded to those prepared with medium viscosity insterial. Further work, however, is indicated along these lines. If possible the same grade of PVA should be employed in the preparation of the oalibration data as is present in the sample undergoing analysis. Order of Addition of Reagents. Experiments have confirmed that it is necessary to follow the prescribed order of addition of reagents. Slightly low results were obtained when iodine was added prior to the boric acid. RESULTS
The applicability of the method to the analysis of synthetically prepared samples of polyvinyl alcohol and starch is illustrated in Table I. Examples of the applicability of the method to the
analysis of paper sized with polyvinyl alcohol are presented in Table 11. Some uncertainty exists as to the actual amount of polyvinyl alcohol present in the latter samples, since the calculated weight per cent represents an average value for a large quantity of paper undergoing sizing treatment. Effect of Diverse Substances. Substances commonly present in paper as sizings, pigments, binders, and dispersants were investigated for possible interference in the determination of polyvinyl alcohol by the method outlined herein. Known quantities of these substances were added to standard polyvinyl alcohol solutions and refluxed for several hours as required by the procedure in the presence of starch. The effects on the recovery of polyvinyl alcohol are noted in Table 111. While the exact nature of the interferences caused by titanium oxide and clays is not known, it appears likely that adsorption of polyvinyl alcohol may occur in the presence of these substances which have very large surface areas and known absorptive properties. This may explain the low recoveries obtained in their presence (Tables TI and 111).
Somewhat high recoveries were obtained when extremely high concentrations of starch were present (Table I). In ordinary applications of the procedure this would cause little difficulty since such high ratios of starch to polyvinyl alcohol are not normally encountered. ACKNOWLEDGMENT
The authoi expresses appreciation to
L. Barnes, Jr., and C. A. Wamser of Air Reduction Research Laboratories for their aid and encouragement in the preparation of this paper. LITERATURE CITED
(1) Browning, B. L., Bublitz, L. O., Baker, P. S., Tappz 35,418(1952). (2) Harvey, J. L Forshee, B. W., Fletcher, D. G., Idid. 42,878(1959). (3) Monte-Bovi, A. ,f., Sciarra, J. J., Drua Standards 27.5 (1959). (4) Mbnte-Bovi, A: J.', Sciarra, J. J., Martorana, D., Ibid., 27, 15 (1959). ( 5 ) West, C. D., J . Cham. Phys. 17, 219 (1949).
RECEIVEDfor review April 3, 1961. Accepted July 31, 1961. VOL. 33, NO. 13, DECEMBER 1961
e
1927
