Anal. Chem. 1985, 57, 1523-1526
1523
Determination of Carbonates in Detergents by a Carbon Dioxide Gas Selective Electrode Satoshi Takano,* Yukihiro Kondoh, a n d Hiroshi O h t s u k a Tochigi Research Laboratories, Kao Corporation, 2606, Akabane, Ichikai-machi, Tochigi 321-34, J a p a n
A rapld and convenlent method was developed for the determlnatlon of carbonates In detergents by uslng a carbon dloxlde gas selectlve electrode. A PO-mL detergent sample solution contalnlng 0.3-8 mg of sodium carbonate Is transferred into a 50-mL centrlfugal glass tube wlth a small Teflon-covered splnbar, and 5 mL of sodlum acetate-hydrochlorlc acld buffer of pH 2.0 Is added to the tube. The electrode Is qulckly Immersed In the solutlon and tlghtly sealed with a rubber stopper wlth arbitrary headspace. The solution Is stlrred, and the potential reaches a stable reading wlthln 12 mln. There are no Interferencesof the lngredlents of detergents, and the accuracy is within 3% relatlve. The proposed method is also appllcable for the determlnatlon of calclum carbonate.
Carbonate is one of the important ingredients in household detergents. The content is hitherto determined by the Schrotter method ( I ) , chelatometry (2),gas chromatography ( 3 , 4 ) ,and automated analyzer method (5))and so on (6-9). However, none has presented an entirely convenient and rapid method. Recently, a convenient method was developed in our laboratory. In this method, carbon dioxide gas generated from detergent solution by acidification is circulated and trapped completely in barium hydroxide solution, and excess barium hydroxide is titrated with hydrochloric acid (6). On the other hand, many ion or gas selective electrodes are now commercially available (IO),and their advantages are ease of measurements, low cost, sensitivity, selectivity, etc. However, in the soap and detergent industry, ion selective electrodes are not used for various official test methods, since surfactants interfere with the measurement of various species by ion selective electrodes. Several studies on the interference of surfactants were reported (11-14); however there were no entirely useful methods with no interference effects from surfactants. In our continuous studies on the determination of carbonates in detergents, the C02 gas selective electrode (pC02 electrode) was also investigated. This paper describes a rapid and convenient method for the determination by measuring C02 gas dissolved in detergent sample solution after acidification without the interferences of the ingredients of detergents. EXPERIMENTAL SECTION Apparatus. All experiments were carried out with a CE-235 C02 gas selective electrode (Toa Electronics, Ltd., Tokyo, Japan) with a rubber stopper (ca. 30 mm height) at 70 mm height from the bottom. Potentiometric measurements were performed by using an Orion Ionalyzer 901 potentiometer in conjunction with a Hitachi 056 recorder. A centrifugal glass tube, 100 mm long, 27 mm i.d., and a 300-mL round-bottomed flask were used as measurement vessels. Reagents. All the reagents used were of reagent grade. Sodium carbonate stock solution, 0.02%, is prepared by dissolving 0.20 g of anhydrous sodium carbonate in 1000 mL of freshly prepared deionized water. Each standard sodium carbonate solution for
calibration of the pC02electrode is prepared by diluting the stock solution. Sodium acetate-hydrochloric acid buffer of pH 2.0 is prepared by mixing 200 mL of 1M sodium acetate, 210 mL of 1N hydrochloric acid, and 590 mL of deionized water. Potassium chloridehydrochloric acid buffer of pH 0.85 is prepared by mixing 100 mL of 1M potassium chloride and 100 mL of 1N hydrochloric acid. When 10 mL of the latter buffer is diluted 36 times with water, pH of the solution becomes 2.0. All solutions are renewed every day in order to avoid contamination of COz gas from air. Calibration Procedure. A 20-mL standard sodium carbonate solution, 15-400 mg/L (1.42 X lo4 to 3.77 X lom3M), is transferred into a dry 50-mL centrifugal glass tube with a small Teflon-covered spinbar, and 5 mL of sodium acetate-hydrochloric acid buffer of pH 2.0 is added to the tube. The pC02 electrode is quickly immersed in the solution and tightly sealed with a rubber stopper in order to prevent C02 gas leak as shown in Figure 1. The electrode and tube are placed in a cylindric wire netting electrically grounded in order to suppress pulse noise. The solution is stirred, and the potential is allowed to reach a stable reading (12 min). Calibration curve is obtained by plotting each potential reading in millivolts vs. logarithm of concentration of sodium carbonate solution. Analytical Procedure. A 20-mL detergent sample solution containing 0.3-8 mg of sodium carbonate is transferred into a 50-mL centrifugal glass tube with a small Teflon-covered spinbar. The subsequent procedure is similar to that of the calibration procedure. The concentration of sodium carbonate in the detergent sample solution is calculated from the calibration curve. Direct Sampling Procedure. For the determination of calcium carbonate, weigh a sample containing 5-250 mg of calcium carbonate in a 300-mL round-bottomed flask, whose real volume is 370-380 mL, with a small Teflon-covered spinbar. The sample is dissolved in 350 mL of water, and then 10 mL of potassium chloride-hydrochloric acid buffer is added. The pC02 electrode is quickly immersed in the solution and tightly sealed with a rubber stopper. The subsequent procedure is similar to those mentioned above. RESULTS AND DISCUSSION The pCOz electrode used in this study is a Severinghaustype electrode (15))which consisted of a glass pH sensing electrode coupled with a gas permeable Teflon membrane. Many attempts to use ion selective electrodes in the presence of surfactants concluded that surfactants interfered with the measurement of various ions (11-14). These facta let us try to determine the content of carbonates in detergents by measuring C 0 2 gas concentration of headspace of sample solution after acidification. Preliminary investigation of this idea gave unsatisfactory results as shown in Figure 3D,which did not indicate a stable potential reading within 20 min. These results suggest that analysis of C02 gas dissolved in water has to be investigated. A few commercially available electrodes were examined, and one of those was chosen as a most suitable one. By using this electrode, three typical measuring methods are designed as shown in Figure 2. The open system shown in Figure 2A gave an inadequate potential trace with a maximum as shown in Figure 3A, since COz gas generated by acidification was gradually lost to the atmosphere. The sealed system shown in Figure 2C, however, did not give a stable potential reading within 20 min (Figure 3C), whereas the sealed system with an arbitrary headspace shown
0003~2700/85/0357-1523$01.50/0 @ 1985 I American Chemical Soclety
1524
ANALYTICAL CHEMISTRY, VOL. 57, NO. 8, JULY 1985 TO ion meter
Y
I
0
IO Time
Flgure 1. Apparatus for the determination of carbonates: (1) pC0, electrode, (2) rubber stopper, (3) centrifugal glass tube, (4) Tefloncovered spinbar, (5)magnetic stirrer, (6) cylindric wire netting, (7) sample solution.
&
2
mln
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Flgure 3. Responses of the pC0, electrode to three typical methods shown in Figure 3. (D shows the response in the analysis of CO, gas concentration of headspace of sodium carbonate solution after acidification.)
C
B
A
20 (
-20
3
0
?
1
W
-
1 2
Flgure 2. Three typical methods for the determination of COS!gas in water: (1) sample solution, (2) Teflon-covered spinbar, (3) rubber stopper, (4) O-ring. in Figure 2B indicated a stable potential reading within 10 min (Figure 3B). Therefore, the sealed system with arbitrary headspace was used in the subsequent studies. Effect of Volume of Headspace. Solubility of C02gas in water is high enough for the present experiment (ca. 1.7 g/L at 20 "C). However, effect of the volume of headspace on the C 0 2 gas concentration in water phase is studied. Twenty milliliters of sodium carbonate solution (50 mg/L) is transferred into reaction vessels with various volumes, and 5 mL of a buffer solution of pH 2.0 is added. Each potentiometric reading is plotted against the volume of headspace. As shown in Figure 4, potentiometric readings are independent on the volume of headspace up to 20 mL. Therefore, a centrifugal glass tube with a 50 mL volume was used and volume of the headspace was determined arbitrary as ca. 5 mL. Effect of pH of Buffer Solution. It is theoretically estimated that all carbonate species in solution are converted to COz gas by acidification below pH 4. Figure 5 shows the effect of pH of the buffer solution on the potential reading. Below pH 4 the potential readings of standard sodium carbonate solution (150 mg/L) are constant, and the blank value
-40
-60
-
0
5
IO
15
20
25
Volume o f headspace ( m l ) Flgure 4. Effect of the volume of headspace of sample solution on the potential reading.
is also constant between pH 1.5 and 10. On the basis of these results, a buffer solution of pH 2.0 is used in the present study. Response of t h e p C 0 2 Electrode. Figure 6 shows the response curves of the pC02 electrode to various concentrations of sodium carbonate solution. The time required to reach a stable potential increases with decreasing the concentration of sodium carbonate solution, and 12 min are required at the concentration of 10 mg/L. When the electrode is immersed in deionized water for reconditioning immediately after the measurement of sample solution, the potential reading gradually decreases as shown in Figure 6. However, it is not required to wait over 10 min for the next measurement. Keeley et al. (16) recommended a 0.1 M sodium phosphate buffer (pH 10) instead of deionized water for reconditioning of an electrode. Calibration Curve and Effect of Temperature. Figure 7 shows the calibration curves of sodium carbonate and the
ANALYTICAL CHEMISTRY, VOL. 57, NO. 8, JULY 1985
Table I. Effect of Ingredients of Detergents on the Determination of Sodium Carbonate by the Proposed Method
t20 -
-
sodium carbonate, mn/L 0.1% sodium solution carbonate reof each added covery, ingredient' solutionb 70
0-
>
-9
I
-
-20
u
ingredients sodium alkylbenzenesulfonate sodium poly(oxyethy1ene) alkyl ether sulfate soap poly(oxyethy1ene) alkyl ether poly(oxyethy1ene glycol) sodium sulfate sodium silicate sodium tripolyphosphate sodium pyrophosphate sodium nitrilotriacetate zeolite sodium perborate
-40 -
-60 2
4
6
8
1 0 1 2
PH
Figure 5. Effect of pH of buffer solution on the potential readlng: (0) standard sodium carbonate solution (150 mg/L); (0)blank solution.
c
>
-40
101.1 101.4
9.4 0 0 0 6.4 0 0 0 5.9 5.2
107.5 101.4 97.9 99.3 108.1 101.6 103.1 100.0 108.5 107.1
98.1 101.4 97.9 99.3 101.7 101.6 103.1 100.0 102.6 101.9
detergent
% sodium carbonate
n
coeff of variation, %
A
22.4 12.7 19.0
5 5 5
1.17 1.78 1.96
B 0
? ti
105.3 101.4
Table 11. Determination of Sodium Carbonate in Commercial Heavy Duty Powder Detergents
-20
E
4.2 0
Determinations are carried out by the automated analyzer method (5). *Sodium carbonate is added to 0.1% solution of each ingredient to become 100.0 mg/L.
0
I
1525
\\\-
-60
-80 Time
(- min )
Figure 6. Responses of the pC0, electrode. The electrode is im-
mersed in deionized water at 20 min. +2G
0
- -20 >
0
? w
-40
-60
I 10-4
10-3 Na2C03
(
mol/]
)
Figure 7. Calibration curves of sodlum carbonate and effect of tem-
perature.
effect of temperature. All calibration curves gave linear relationships in the concentration range of 1 X lo-* to 1 x M, and increase in temperature gave a lower shift of calibration curve. Therefore, all experiments in the present study were carried out at 20 "C. Repeatability of the determination
C ~
~~
~
of 100 mg/L sodium carbonate standard solution is within 3%. Effect of Ingredients of Detergents. Interference effects of ingredients of detergents, especially surfactants, on the measurement of sodium carbonate are easily expected. To confirm these interferences, the potential of sodium carbonate solution (100 mg/L) was measured in the presence of 0.1% of various ingredients of detergents. Some of these ingredients indicated positive errors as shown in Table 1. However, it was found that these positive errors were not the interference of each ingredient, since the ingredients indicating a positive error contained a small amount of sodium carbonate. Content of sodium carbonate in these ingredients was determined by automated analyzer methods (5), and substracted from the apparent value obtained by using the pC02 electrode. The corrected recoveries, as shown in Table I, demonstrate that there are no interference effects of these ingredients on the determination of carbonates by using the pC0, electrode. Applications. The determination of carbonates in commercial heavy duty powder detergents by the proposed method were carried out, and the reproducibility was within 2% as shown in Table 11. The important factors to obtain good reproducibility are the control of temperature and the suppression of pulse noise by using a cylindric wire netting electrically grounded (Figure 1). In addition, sampling size reduction is also important in the case of powder detergents. Comparison of the results of the proposed method with those of the automated analyzer method (5) showed satisfactory coincidence as shown in Table 111. Direct Sampling Method. In the case of the determination of insoluble carbonates such as calcium carbonate, it is required that a sample is directly added in the reaction vessel. For these cases, a 300-mL round-bottomed flask is used as a reaction vessel instead of a 50-mL centrifugal glass tube in order to remove weighing errors. As described in the experimental section, 5-250 mg of calcium carbonate can be
1526
Anal. Chem. 1985,57,1526-1532
Table 111. Comparison of the Results of the Determination of Sodium Carbonate in Commercial Heavv Powder ” Dutv .” - --Detergents or Calcium Carbonate in Commercial Toothpastes by the Proposed Method with Those of Established Methods (5, 6 )
sample
sodium carbonate, % proposed method established method”
detergent A detergent B detergent C detergent D detergent E
11.6 0.8 19.0 5.4
toothpaste F toothpaste G
37.1 45.7
22.4
21.2 11.3
0.9 19.8 4.9
37.1 44.3 “Detergentsare analyzed by the automated analyzer method (5). Toothpastes are analyzed by the titration method (6). determined by adding 350 mL of water followed by 10 mL of potassium chloride-hydrochloric acid buffer solution. A calibration curve of calcium carbonate agrees well with that of sodium carbonate. Table I11 shows the determination of calcium carbonate blended as an abrasive in commercial toothpastes, the results are in good agreement with those of the titration method (6). Heavy duty powder detergent can also be analyzed by a direct sampling method with satisfadory repeatability (mean 8.49%, coefficient of variation 1.89% for n = 9). Therefore, the proposed method should be useful and applicable for the determination of various carbonates in detergents as a rapid and convenient method.
ACKNOWLEDGMENT The authors are grateful to Y. Ohhira of Toa Electronics, L a . , for his kind advice and valuable discussions. We are also indebted to R. Azuma and H. Ichikawa for their technical assistance. Registry No. Sodium carbonate, 497-19-8;calcium carbonate, 471-34-1; poly(oxyethy1ene glycol), 25322-68-3; sodium sulfate, 7757-82-6;sodium silicate, 1344-09-8;sodium tripolyphosphate, 7758-29-4; sodium pyrophosphate, 7722-88-5;sodium nitrilotriacetate, 10042-84-9;sodium perborate, 7632-04-4. LITERATURE CITED (1) (2) (3) (4) (5) (6) (7)
Japanese Industrial Standard K3362 (1974).
Nagai, T.; Nihongi, T. Yukagaku 1971, 20, 816. Fujlta, M.; Asarni, A.; Inoue, S.; Hayashi, N. Yukagaku 1074, 23, 188. Zeman, I. J . Chromatogr. 1084, 286,311. Kawase. J.; Yamanaka, M. Yukagaku 1978, 27, 44. Hasegawa, A.; Ohtsuka, H.; Tsuji, K. Yukagaku 1984, 33, 380. Milwidsky, 9. M.; Gabriel, D. M. “Detergent Analysis”; Halsted-Wlley: New York, 1982. (8) Llenado, R. A.; Neubecker, T. A. Anal. Chem. 1983, 5 5 , 93R. (9) Tsuji, K. Bunsekl 1084, 178. (10) Pungor, E.; TBth, K. Analyst (London) 1970, 9 5 , 625. (11) Llenado, R. A. Anal. Chem. 1975, 47,2234. (12) Craggs, A.; Moody, G. J.; Thomas, J. D. R.; Birch, B. J. Analyst (London) 1980, 705, 426. (13) Frend, A. J.; Moody, G. J.; Thomas, J. D. R.; Birch, B. J. Analyst(London) 1983, 708,1072. (14) Huianicki, A.; Trojanowicz, M.; Pobozy, E. Analyst (London) 1982, 707, 1356. (15) Severinghaus, W.; Bradley, A. F. J . App. Physiol. 1058, 73, 515. (16) Keeiy, D. F.; Walters, F. H. Anal. Lett. 1983, 76(A20), 1581.
RECEIVED for review February 11, 1985. Accepted April 11, 1985.
Electrochemically Polymerized N ,N-Dimethylaniline Film with Ion-Exchange Properties as an Electrode Modifier Noboru Oyama,* Takeo Ohsaka, and Takashi Shimizu
Department of Applied Chemistry for Resources, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan
Poly(N,Ndlmethylanlllne) (PDMA) was prepared by electrochemical polymerlratlonof the correspondlng monomer and was found to have the structure of an lonene polymer wlth positively charged sltes as quaternary ammonlum groups In the polymeric backbone. I t Is demonstrated that the PDMA fllm has an anion-exchange character lrrespectlve of the pH of a solution, and coatlng electrodes wlth PDMA produces surfaces whlch strongly blnd multlply charged negatlve Ions.
Electrochemically initiated polymerization (sometimes referred to as electropolymerization or electrochemical polymerization) has recently received great attention in the modification of electrode surfaces, because of the potential applications (electrocatalysis (I, 2),protection of metals from corrosion (antiphotocorrosion) ( 3 4 , electrochromic display (7,8),energy storage (9),“ion gate” membrane (IO),etc.) of the resulting modified electrodes. This procedure also holds great promise for the synthesis of new organic conducting (metallic), semiconducting, or nonconducting polymers (e.g.,
poly(pyrrole),poly(thiophene), poly(aniline),and poly(pheno1)) (2,6, 10-20). The electrochemical, electrical, and physicochemical properties of the films prepared by electropolymerization of aniline, phenol and their derivatives, vinyl monomers, aromatic heterocyclic compounds, etc. have been extensively examined, together with the mechanism of electropolymerization for individual cases and the structures of the prepared films. It has become obvious that the properties and structures of the prepared films depend on the kind of the monomer used for polymerization as well as the experimental conditions used in their preparations (e.g., solvent, electrode material, supporting electrolyte, pH of electrolytic solution, and temperature). By the appropriate choice of a monomer and the experimental conditions for preparation, polymer films with a particular desired property can be prepared. In the present paper, we wish to report the preparation of poly(N,N-dimethylaniline)(PDMA) film by the anodic oxidation of NJ-dimethylaniline (DMA) and its electrochemical properties and structure. The PDMA is shown to be an “ionene polymer” with positively charged sites as quaternized
0003-2700/85/0357-1526$01.50/00 1985 American Chemlcal Society
