Determination of colloidal electrolytes: amperometric titration of dodecylquinolinium bromide with inorganic anions

Analytrca Chrmlca Acta, 236 (1990) 325-329 Elsevler Science Pubhshers B.V , Amsterdam

325

Determination of colloidal electrolytes: amperometric titration of dodecylquinolinium bromide with inorganic anions RYSZARD Instrtute of Chemistry, N Copermcus

MIKULSKI

Unrversrty, ul Gagarma 7, PL-87 100 Tonoi (Poland)

(Received

3rd March

1990)

ABSTRACT

The posslblhtles of the amperometnc tltratlon of aqueous solutions of dodecylqumolmmm bronude m 0 1 mol dmm3 potassium bronnde as supportmg electrolyte \nth potassmm dlchromate, potassmm hexacyanoferrate(II1) and ammomum molybdate were mvestlgated Titrations were carned out m which dlchromate, hexacyanoferrate(II1) and molybdate amons were determmed by titration of colloidal electrolyte solutions Keywords

Colloidal

electrolytes;

Dodecylqumohmum

brormde

New polarographic techniques, some with detection limits of 10-8-10-9 mol dm-‘, have been developed and apphed to the determination of oxygen in gas mixtures and oxygen and metal ions in biological fluids and water. Polarographic methods are now widely used [l-4] in the determination of organic compounds including naturally occurring mycotoxms (e.g., fungal metabolites), compounds used to improve crop yields (e.g., insecticides and fungicides) and environmental pollutants (e.g., nitrosamines, detergents, azo dyes). The main rivals to voltammetric methods for the determination of traces of organic species are UV-visible spectrophotometry, fluorescence spectroscopy and chromatograpmc methods such as liquid chromatography (LC). With limits of detection of ca. 5 X 1O-5 mol dme3 for d.c. polarography and 1 X lop5 mol dmp3 for sampled d.c. polarography, these two methods are similar in sensitivity to UV-visible spectrophotometry. Differential- and normal-pulse polarography have 0003-2670/90/$03

50

0 1990 - Elsevler

Science

Publishers

limits of detection of ca. lo-’ and lo-’ mol dmp3, respectively, which are similar to those for fluorescence spectroscopy and LC techniques. Amperometric titration provides more precise results than other polarographic methods. In the case of titration, many separate readings are made on each sample, thus reducing the error in the final result. The widespread occurrence of colloidal electrolytes has led to a need for analytical methods for then determination, especialy when present in low concentrations. Wang [5] described a simple titration method applicable to the determmation of cationic surfactants. Methyl orange and azure A were used as primary and secondary dye, respectively. This method, however, cannot accurately measure ionic surfactants at low concentrations, e.g. below 1 mg dmp3. Waters and Kupfer [6] described a titrimetnc method for the determination of cationic detergents m biodegradation test liquids. The method is not suitable for the analysis of natural waters which contain high levels of B.V

326

natural substances that react with disulphine blue. Wang et al. [7] described a direct titrimetric procedure for catiomc surfactants with poly(vinylsulphunc acid) potassium as the titrant. Le Bihan and Courtot Coupez [8] analysed fresh water and sea water for catiomc detergents by a method based on atormc absorption spectrometry of the copper-detergent complex. The British Standard method [9] for cationic detergents is based on the addition to the sample of an excess of a standard anionic detergent and determination of the excess of the latter by a methylene blue spectrophotometric method. Linhart [lo] described a method for separating detergents mto cationic, anionic and non-ionic types on a nnxed bed of a cation- and an anionexchange resin, followed by a polarograpmc detection procedure. The method is based on the lowermg of the polarographic maximum of oxygen by the surfactants and on the fact that in milhmolar potassium chloride solutions it is not the molarity but the percentage by weight of the surfactants that is proportional to this reduction in height. This proportionality (which is rectilinear over the range O-100 mg dme3) applies equally to anionic, cationic and non-ionic surfactants. Zvonaric et al. [ll] described a polarographic method for the determination of the surfactant activity of sea water. Hart et al. [12] reported an indirect polarographic method for determimng linear alkylbenzene sulphonates in sewage and tap water. The method is rehable for concentrations of > 0.5 pg dme3. Czermawski and co-workers determined dodecylammonium bromide [ 13-161 by amperometric titration m 0.5 mol dme3 potassium chloride as supportmg electrolyte at a mercury drop with Cr, Of ~, AsO;, H,PO< and SOianions and dodecylpyridmium bromide [17] with [Hg(SCN),]*anions. The relative errors were I 3%. The inorgamc anions can also be titrated with colloidal electrolyte solutions under the same conditions. An aqueous solution of dodecylquinohmum bromide (DDQB) gives a polarograpmc wave at negative potentials and forms poorly soluble compounds with some inorgantc anions. On the basis of this phenomenon, the possibilities of the amperometric titration of DDQB solutions with

R MIKULSKI

potassium dichromate, potassium hexacyanoferrate(II1) and ammonium molybdate were investigated. The reverse titrations were also checked.

EXPERIMENTAL

Chemicals and apparatus All solutions were prepared by dissolving the appropriate amount of a compound in triply dlstilled water. The contents of potassium dichromate and ammonium molybdate [(NH,),Mo,O,, .4H,O] were determmed gravimetncally [18] and potassium hexacyanoferrate(II1) titrimetrically

[191Dodecylquinohmum bromide (C,,H,,NBr) was obtained by heating 0.1 mol of purified quinoline and 0.1 mol of dodecyl brormde at 388 K for 5 h under reflux m an atmosphere of nitrogen which was free from oxygen. After coolmg to 333 K, the dark red precipitate was dissolved in dioxaneacetone (9 + 1). After cooling to 277 K, orange crystals precipitated. Fractional recrystallization from dioxane-acetone (9 + 1) resulted in pale yellow crystals of the salt [20,21]. A standard solution of DDQB was prepared by dissolving a weighed amount of the compound in triply distilled water at 313 K. Working solutions of appropriate concentrations were prepared by dilution of this standard solution. Triton X-100 (Koch-Light) was used as a 2.5% (w/w) aqueous solution. Solutions of the chemicals did not show any polarographtcally active impurities under the experimental conditions. Prior to titration, the solutions were degassed m an ultrasonic washer for 30 min. DDQB solutions were stable for 1 week. The measurements were done with a PLP 225 C a.c.-d.c. type polarograph (Cobrabid, Warsaw). The mercury dropping electrode (DME) has been applied and a saturated calomel electrode (SCE) has been used as a reference electrode. The drop time is 3.4 s/drop, the mass of a mercury drop (m) is 0.1250 g. Procedure Various amounts (0.5-10 ml) of standard solution of DDQB were taken for trtrations, 10 ml of

DETERMINATION

OF COLLOIDAL

ELECTROLYTES

supporting electrolyte (0.1 mol dm-’ potassium bromide) and then four drops (ca. 0.1-0.2 ml) of the aqueous solution of Tnton X-100 being added. The amount of Triton X-100 m a sample should not be lower than 0.02% (w/v). The measurements were carried out in unbuffered aqueous medium. After each addition of 0.05 ml of titrant the solution was stirred for 2 min and left for 2 min in order to attain equtlibrium. The titration was performed at 298 K. The concentration of titrant reagent was 3-5 times higher than that of the titrant. The determinations were repeated five times and mean values were calculated. Dtrect titrations of DDQB with dichromate, hexacyanoferrate( III) and molybdate ions were carried out. The reverse titrations were done m such way that the morgamc anions were titrated with colloidal electrolyte DDQB.

RESULTS

AND

DISCUSSION

The essential factor for the amperometric titration of colloidal electrolytes is the establishment of the appropriate potential at which the titration can be done. According to the literature [15,16], these compounds can adsorb on the surface of a dropping mercury electrode (DME), hindering the reduction of inorganic ions. The investigated aqueous solutions of DDQB in 0.1 mol dmm3 potassium bromide gave severely distorted polarographic waves as a result of strong adsorption on the surface of the DME. Only after the addition of Triton X-100 at a concentration > 0.02% (w/v) were well shaped polarographic waves obtained, with an E,,, of -0.485 V vs. SCE. Tltratlon with potassrum drchromate Preliminary determinations at - 0.80 V showed that in a direct titration, i.e., when the titrated solution is DDQB and the titrant is potassium the ratio of the ions is 2 : 1 dichromate, (2DDQ+ : Cr@). In the titration curve one deflection is seen, which corresponds to the exchange of two K+ ions with two DDQf tons. The reverse titration, carried out at the same potential, gave the same ratio of reacting ions. The shape of the amperometric titration curve m the

TABLE

1

Results of amperometrlc tltratlon of DDQB dlchromate solution (E = - 0 80 V vs SCE) Concentration of DDQB (mmol dme3)

0 288 104 1 92 2 66 3 29 3 84 4 74 5 76

Amount of DDQB used

Amount of DDQB found

(mg)

(mg)

1090 4 360 8 720 1308 17 44 21 80 30 52 43 60

1 105 4 360 8 807 1308 17 30 21 49 30 21 43 08

Error (%)

wth

a

potassmm

Standard dewatlon (n = 5) (mg)

+14 00 +10 00 -08 -14 -10 -12

0013 0 022 0 043 0 06 0 06 0 17 0 24 0.26

a The standard error (S,) [23] of the amperometrlc method, as found on the basis of the samphng dlstnbutlon, 1s 0 153 mg for the concentration range (0 7-7 8 mmol dmm3)

investigated range of concentrations (0.288-5.7 mmole drn- 3, was typical of this type of determmatton. The end-point was obtained by extrapolation of the two linear branches of the plot. The maximum difference between the lowest and the highest results m a separate series of determinations at concentrations up to 1 mol dme3 was < 7%. During the titration of solutions of concentrations > 1 mmol dmp3, the spread of results reached a maximum of 2.5%. The error of determmation m the mvestigated range of concentrations did not exceed 1.5%. The mean results of measurements for several concentrations of direct and reverse titrations are presented m Tables 1 and 2, respectively. TABLE

2

Results of potassmm dxhromate determmatlon wth DDQB solution (E = 0 80 V vs. SCE) Concentration of K,Cr,O, (mmol dme3)

0.412 0 784 1 50 3 80 6 18 7 80

Amount

of

Amount

K&W, used

K ,Cr,Q, found

(mg)

(mg)

1 212 2 425 4 850 14 55 29 10 43 65

1 230 2 454 4 850 14 55 28 81 43 39

of

Error

by tltratlon

(S)

a

Standard dewatlon (n = 5)

+15 +09 00 00 -10 -06

0 0 0 0 0 0

(mg)

a S, = 0 162 mg for the same range as m Table

1

034 026 060 09 17 30

R MIKULSKI

TABLE

3

Results of DDQB hexacyanoferrate(II1) Concentration of DDQB (mmole dm-‘)

104 1 92 2 66 3 29 3 84 4 74 5 76

determmatlon by tltratlon wth potassmm solutlon (E = -0 75 V vs SCE) Amount of DDQB used

Amount of DDQB found

(mg)

(mg)

4 360 8 720 13 08 17 44 21 80 30 52 43 60

4.277 8.650 13 18 17 26 21 47 30 76 42 99

Error (8)

a

Standard dewatlon (n=S) (mg) 0 0 0 0 0 0 0

-19 -08 +08 -10 -15 +08 -14

043 065 09 09 15 17 30

a S, = 0 153 mg for the same range as m Table 1

Tltratlon with potassnmz hexacyanoferrate(IZZ) Amperometrtc titrations of DDQB, both direct and reverse, were done with solutions of potassium hexacyanoferrate(II1) at -0.75 V vs. SCE. The end-point appeared at an ion ratio of 3 : 1 [3DDQ+ : Fe(CN)i-1. The experiment was carried out in the concentration ranges 1.04-5.76 mmol dme3 of DDQB and 0.716-7.52 mmol dmP3 of potassium hexacyanoferrate(II1). The error of measurement was < 2% and the spread m particular series of determinations did not exceed 5%. The mean values of the results for various concentrations of DDQB and potassium hexacyanoferrate(II1) are summarized m Tables 3 and 4. Tltratlon with ammonium molybdate The direct titration performed at -0.85 V vs. SCE gave a ratio of tons of 3: 1 [3DDQ+ :

whereas during the reverse (NW,Mo,OiJ~ titration at the same potential a ratio of 2 : 1 has been obtained [2DDQ+ : (NH,),Mo,O&]. The reason for the ratio obtained in direct titration may be the existence of DDQB as a micellar electrolyte. The solutions studied m this work were above the crttical micelle concentration. This aspect will be the subject of separate studies. The extrapolatton to obtain the end-point m the ranges of concentrations studied, i.e., 0.2885.76 mmol dmP3 m direct titration and 0.117-3.22 mmol dme3 in reverse titration, presented no difficulties. The percentage error of determinattons in both instances was < 0.5%. The maximum difference between the lowest and the highest results in a separate series of experiments was < 3% at concentrations up to 1 mmol dmP3, and ca. 2% at higher concentrattons. Calculated mean values of the measurements are given in Tables 5 and 6. It can be concluded that the smallest error is found in direct titration with use of the molybdate salt. All other results, acqmred m titrations with potassium dichromate and potassium hexacyanoferrate(III), are within the limits of errors permissible for thts type of determination. Thts amperometric method of titration of DDQB was developed because of the necessity for a rapid and precise method for the determination of the concentration of DDQB in aqueous solutions. N-Alkylquinolmium halides m aqueous soluttons above the crtttcal micelle concentration

TABLE

5

Results of DDQB determmatlon by tltratlon molybdate solution (E = -0 85 V vs SCE) TABLE

4

Results tltratmn

of potassium wth DDQB

Concentration of K,Fe(CN), (mmole dm- 3,

0 716 117 347 5 63 I 52

hexacyanoferrate(II1) determmatlon solution (E = -0 75 V vs SCE)

Amount of K,Fe(CN), used

Amount of K,Fe(CN), found

(mg)

(mg)

2475 4.9M 1.4.Ri 29 70 4.9 50

2438 4.9rnl 14.94. 29 4.0 49 90

Error a Standard dewatlon (8) (n = 5) (mg) -14 - 10 + 0.6. -10 +08.

’ .S, = 0 143 mg for the same range as m Table

1

0 030 (10.4.3 0 0.6. n 17 026

by

Concentration of DDQB (mmole drn-‘)

0 288 1 04 1 92 266 3 29 3 8.4. 4.74. 574

Amount of DDQB used

Amount of DDQB found

(mg)

(mg)

1 090 4 360 8 720 13 08 17 44 2180 30 52 4.3 6tl

1093 4 371 8 720 13 09 17 A4 2178. 30.61 43.4.0.

AS, = 0 113 mg for the same range

Error

with ammomum

a

(46)

Standard dewatlon (n=5) (mg)

0 026 +028 0 060 + 0.26 0 060 00 -coo8 006 0.06. r) 0. - (3.08:. Q 11 + 0. 30. n. 10. - Q 4.5 Q z3 as m Table

1

DETERMINATION

TABLE

OF COLLOIDAL

329

ELECTROLYTES

6

Results of ammomum molybdate determmatlon with DDQB solutton (E = - 0 85 V vs SCE)

by tltratlon

Concentration of ammonium molybdate (mmole dme3)

Amount of ammomum molybdate used (mg)

Amount of ammomum molybdate found (mg)

Error a Standard (W) deviation (n=S)

0 412 0 784 1.50 3 80 6 18 7 80

1212 2 425 4.850 14 55 29 10 43 65

1 230 2.454 4 850 14.55 28 81 43 39

+15 +0.9 0.0 00 -10 -06

(mg) 0 0 0 0 0 0

034 026 060 09 17 30

a S, = 0 162 mg for the same range as m Table 1

(CMC) show the properties characteristic of colloidal electrolytes, and are often uttlized as model compounds in chemical and btologrcal mvestigattons. The essential mformation about the possibility of neutralizatton of a catiomc surfactant which is toxic to the environment can be obtained by mvesttgattons of reactions between the surfactant and various morgamc anions and physicochemical conditrons for the reactions: inorganic anions with the catronic part (DDQ’). One can also obtain information on structural and micelle properties [22].

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