ANALYTICAL SCIENCES AUGUST 1993, VOL. 9 483

Spectrophotometric Determination of the Alcohol Content of Alcoholic Drinks with Bis(O, O'-dipropyl dithiophosphato)nickel(II)1

Yoshiaki SASAKI, Shoji TAGASHIRA, Yoshiko MURAKAMI and Sayuri KAI

Department of Chemistry, Faculty of Science, Yamaguchi University, Yoshida, Yamaguchi 753, Japan

Bis(O, O'-dipropyl dithiophosphato)nickel(II) is soluble in ethanol and absorbs light at 524 nm (molar absorptivity, e, is 91 mol-' dm3 cm-') and 688 nm (E is 74 mol-' dm3 cm-'). In water-ethanol mixed solvents, however, part of the complex dissociates into nickel(II) and 0, O'-dipropyl dithiophosphate ions, and the absorbances at 524 and 688 nm depend on the water content. This property of the complex can be utilized for the spectrophotometric determination of the alcohol content of alcoholic drinks. The method proposed is as follows: 6 cm3 of sample drink cooled to 15°C and 14 cm3 of a 14.3 mmol/ dm3 ethanolic solution (its temperature being adjusted to 25° C) of a nickel(II) complex are mixed. After the mixture is warmed to 25°C, the absorbance is measured at 524 or 688 nm. The alcohol content is calculated by using calibration graphs. This method is simple, rapid, accurate and suitable for the analysis of shochu, whisky, brandy and vodka. This method is also applicable to the analysis of sake, wine and beer, provided that a distillation step as a pretreatment is introduced into its procedure.

Keywords Spectrophotometric determination,

phato)nickel(II)

alcohol content, alcoholic drinks, bis(0, O'-dipropyl dithiophos-

The alcohol content of alcoholic drinks is defined by the liquor tax law of Japan as being the volume

percentage of ethanol in drinks at 15° C.2 Redox titration with dichromate and a specific-gravity

measurement have been commonly used as the official methods2 for determining the alcohol content of alcoholic drinks. However, both methods require a

preliminary time-consuming distillation step to separate the ethanol from any interference coexisting in the samples. Gas chromatography is adopted as one of the official

methods by the AOAC (Association of Official Analytical Chemists).3 Further, a method using a flow-injection analysis system equipped with a gas-diffusion unit and an oxidation-reduction potential detector was recently reported.4 These instrumental analyses are useful and accurate, but require more or less expensive instruments. The ethanolic solution of bis(0, O'-dipropyl dithio-

phosphato)nickel(II) ([Ni{(C3H70)2P(S)S}2]) assumed a violet color. However, the intensity of the color was small in the presence of water in the solution, because

part of the amount of the nickel complex dissociated into nickel(II) and 0, O'-dipropyl dithiophosphate ((C3H70)2-P(S)S-) ions. The decrease in the color intensity depended on the water content. This property of the complex was, therefore, applied to spectrophotometric analysis for the alcohol content of alcoholic drinks.

Experimental

Reagents Potassium 0, O'-dipropyl dithiophosphate was

synthesized and purified according to a method described in a previous paper.5 Bis(O, O'-dipropyl dithiophosphato)nickel(II) was

prepared by the following procedure. A 2 mol/dm3 aqueous solution of potassium 0, O'-dipropyl dithio-

phosphate was slowly added to a 1 mol/ dm3 aqueous solution of nickel nitrate at ice-bath temperature. The precipitated complex was collected by filtration, washed with water and dried under a vacuum. The crude com-

plex was recrystallized from ethanol and dried under a vacuum. The melting point of the purified complex was 65° C. Ethanolic solutions of the complex were pre-

pared daily. Nickel(II) complexes of 0, O'-diethyl dithiophosphate

and 0, O'-dibutyl dithiophosphate were also prepared and tested. However, the former lacked thermodynam-ic stability in ethanol, and the latter was oily and intractable at room temperature. All other chemicals used were of analytical reagent

grade.

Apparatus A spectrophotometer (Hitachi, U-2000, Japan) with

1 cm quartz cells was used for all absorption measure-

484 ANALYTICAL SCIENCES AUGUST 1993, VOL. 9

ments.

Results and Discussion

Absorption spectra of the nickel(II) complex in water-ethano~ mixed solvents Figure 1 shows the absorption spectra of 10 mmol/ dm=

bis(0, 0'-dipropyl dithiophosphato)nickel(II) in 0 :100 -50 : 50 v/ v water-ethanol mixed solvents. The nickel(II; complex exhibited two absorption maxima at 524 anc

688 nm in the visible region. The molar absorptivities in ethanol were 91 (at 524 nm) and 74 (at 688 nm) mol-1 dm3 cm 1. The spectra were little affected in shape, but

gradually changed in absorbance upon increasing the water content in a mixed solvent. At a water content of more than 50v/ v%, the absorption spectra coincided with that of nickel(II) nitrate dissolved in the same mixed solvent. This fact lead to the conclusion that a part of the amount of the nickel(II) complex dissociates by degrees into nickel(II) and 0, 0'-dipropyl dithiophosphate ions upon increasing the water content in the mixed solvent, because of an increase in the dielectric constant of the solvent. The color intensity was stable for at least 3 h.

Effect of water content The effect of the water content in the final solution on

the absorbances at 524 and 688 nm was examined

(Fig. 2). The absorbance decreased with an increase in the water

content, and reached the minimun value at 50v/ v%. The curves crossed each other at 30v/v%, due to absorption at 688 nm by the nickel(II) ion, which was released by dissociation. The absorbance is sensitive to the water content over

the range of 10 to 30v/ v%. This range was used for the determination of the alcohol content.

Effect of temperature A rise in temperature brings about a volume expansion

of the test solution, a decrease in the dielectric constant of the solution and a variation of the stability constant for the complex. Therefore, the effect of the temperature on the absorb-

ance was examined. The test solutions were prepared at

Fig. 1 Absorption spectra of 10 mmol/dm3 bis(O,O'-dipropyl dithiophosphato)nickel(II) in water-ethanol mixed solvents.

Water content (v/v%) in mixed solvent: a, 0; b,12.5; c, 25.0; d, 50.0. Temperature: 25° C; reference: water.

.. u~~.• V V11LUl14, . ~ r ~v

Fig. 2 Relation between the water content in a water ethanol mixed solvent and the absorbances at 524 (~) and 688 (0) nm of 10 mmol/dm3 bis(O,O'-dipropyl dithiophosphato)nickel-

(II) at 25° C against water.

Fig. 3 Effect of the temperature on the absorbance of 10 mmol/dm3 bis(O,O'-dipropyl dithiophosphato)nickel(II)

at 524 nm in water-ethanol mixed solvents. Water content

(v/v %) in mixed solvent: 0 (s), 12.5 (0), 25.0 (U); reference: water.

ANALYTICAL SCIENCES AUGUST 1993, VOL. 9 485

25° C by mixing proper volumes of water, ethanol and an ethanolic solution of the nickel(II) complex. After the temperature of the test solutions was adjusted to 15, 20, 25 or 30° C, the absorbances were measured. As can be seen from Fig. 3, the rise in temperature

resulted in an increase in the absorbance at 524 nm. The absorbance increment per 10°C rise in temperature was within 3% for solutions of less than 12.5v/ v% water content, but increased to 20% for that of 25v/v%. A similar result was obtained at 688 nm. The measure-ment was carried out at 25° C in further experiments.

Determination procedure and calibration graph Considering the results mentioned above and the

definition of the alcohol content according to law2, the following procedure is recommended for determining the alcohol content.

A 6 cm3 sample of a drink previously cooled to 15° C and 14 cm3 of a 14.3 mmol/ dm3 ethanolic solution (its temperature being adjusted to 25° C) of the nickel(II) complex are mixed; after the mixture is warmed to 25° C, the absorbance is measured at 524 or 688 nm against a control solution consisting of 6 cm3 of the sample drink and 14 cm3 of ethanol. Calibration graphs were made by using 6 cm3 of

standard samples having an alcohol content of 0 - 60v/ v% at 15°C. Both of the calibration graphs obtained at 524 and

688 nm curved slightly (Fig. 4). Therefore, a non-linear regression analysis was applied, and the following equations were obtained:

A = 5.02 X 10-5c2 + 7.03 X 10-3c + 0.122

(at 524 nm, r = 0.9994),

and

A = 4.35 X 10-5 c2 + 5.26 X l03 c + 0.121

(at 688 nm, r = 0.9993),

where, A, c and r are the absorbance, the alcohol content

(v/ v %) of the sample drinks and the correlation coeffi-cient, respectively.

Analytical results on some alcoholic drinks The proposed method was applied to some alcoholic

drinks. The results are given in Table 1. The results concerning shochu, whisky, brandy and

vodka were in good agreement not only with the indicated values on the labels of the drinks, but also with the results obtained by the redox titration method with dichromate.2 The proposed method is simpler and more rapid than the dichromate method. On the other hand, the precipitate was deposited

immediately after the addition of an ethanolic solution of the nickel(II) complex in the analysis of wine, sake and beer. The results obtained from the supernatant solution contained seriously negative errors. However, reliable results could be obtained by introducing a distillation step as a pretreatment before this method. Precipitation also occurred upon the addition of pure ethanol to the drinks. The decrease in the dielectric constant may cause the deposit of certain components contained in the drinks. A negative error may arise from some interaction, such as adsorption, between the

precipitate and the nickel(II) complex.

In this study, we did not attempt to examine the effect

of each component coexisting in the drinks. However,

we have clarified the usefulness and limitations of the

proposed method by analyzing several drinks which are commercially available.

Fig. 4 Calibration graphs at 524 (~) and 688 (Q) nm for the determination of the alcohol content.

Table 1 Analytical results on the alcoholic drinks

The values given are mean and 95% confidence interval from five determinations. a. Continuous distillated. b. Simple distillated. c. Obtained value is minus number. d. By the

procedure containing distillation step.

486

References

1. Analytical application of 0, O'-dialkyl dithiophosphate IX. For part VIII, see Y. Sasaki, S. Tagashira, Y.

Murakami, M. Yoshida and K. Hayashi, Anal. Sci., 8, 67 (1992).

2. "Kokuzeichou Shotei Bunsekihou Chuukai (Analytical Methods Recommended by the National Tax Administration Agency, in Japanese)", 2nd ed., p. 14, Japan Brewing Association, Tokyo, 1967.

ANALYTICAL SCIENCES AUGUST 1993, VOL. 9

3. "Official Methods of Analyses of the AOAC", ed. K. Helrich, Vol. 2, 15th ed., p. 708, Association of Official Analytical Chemists, Arlington, 1990. 4. H. Ohura, T. Imato, Y. Asano, S. Yamasaki and N.

Ishibashi, Anal. Sci., 6, 541 (1990). 5. Y. Sasaki, S. Tagashira, Y. Murakami, K. Okada and K. Hayashi, Bunseki Kagaku, 38, 707 (1989).

(Received March 10, 1993) (Accepted May 13, 1993)