Spectrophotometric Determination of 1-Naphthylamine-Application to Sea and River Waters

The present work includes a detailed investigation of a new spectrophotometric method for the determination of 1-naphthylamine. The method is based on oxidative coupling reaction of the compound with catechol in the presence of potassium chromate as oxidant in acidic medium to form a stable purple–red coloured product which shows maximum absorption at 530 nm. The molar absorptivity is 3685 l.mol.cm. Beer's law is obeyed over the range of (160) ppm. The method is applied to the recovery of 1naphthylamine from sea and river water.


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INTRODUCTION 1-Naphthylamine is used in the preparation of dyes.It is also used for toning photographic prints (Snell,1963).The techniques available for the determination of 1naphthylamine are mainly spectrophotometry.The commonly used methods for the determination of the intended compound can be grouped into three types.Type I methods involving treatment of 1-naphthylamine with nitrite in acidic medium to form a diazonium cation; this is subsequently coupled with an activated aromatic nucleus in either acidic (Norwitz and Keliher,1985) or basic medium (Al-Hemiry,2004), depending on the nature of the coupling component, to form a coloured azo dye for measurement of 1-naphthylamine.Type II methods comprising the coupling of 1-naphthylamine with diazotised reagents to form an intensely-coloured azo dyes.
Consequently ,the literature methods revealed that still there is a lack for a more rapid method for determination of 1-naphthylamine.The proposed method is based on oxidative coupling reaction of the compound with catechol in the presence of potassium chromate as oxidant in acidic medium.

EXPERIMENTAL Apparatus
All spectral measurements were performed on Shimadzu UV-visible recording spectrophotometer (UV-160), where as absorbance measurements were carried out on CECIL-CE-1021spectrophotometer.pH measurements are carried out using a Philips PW 9420 pH meter.

Reagents
All Chemicals used are of the highest purity available.Pyrocatechol (2.72×10 -3 M) solution.This solution is prepared by dissolving 0.03g of pyrocatechol in distilled water in 100-ml volumetric flask.
Hydrochloric acid solution: A diluted (0.05 M) was used.1-Naphthylamine (10000 µg ml -1 ).0.1g is dissolved in ethanol, and the volume is then made up to 10-ml in a volumetric flask.This solution, when stoppered tightly and kept in the refrigerator (≈5°C) is stable for at least 3 months.
Working 1-naphthylamine 50 µg ml -1 solution.a 0.5-ml volume of the stock 1naphthylamine solution is transferred into a 100-ml volumetric flask, and diluted to the mark with distilled water.This solution is stable for at least 1 week.

RECOMMENDED PROCEDURE
2.5 ml of pyrocatechol (2.72×10 -3 M) was added into a series of 25-ml calibrated flasks and 2.5 ml of potassium chromate (4.6×10 -3 M) followed by the addition of increasing volumes of (50 µg ml¯¹) 1-naphthylamine solution then 1.5 ml of (0.05 M) hydrochloric acid.The solutions were diluted to the mark with distilled water and the reaction mixture was allowed to stand for 5 minutes.The absorbance of each solution was measured at 530 nm versus blank prepared in the same manner but without 1-naphthylamine.

RESULTS AND DISCUSSION
It was found that the reaction of studied 1-naphthylamine with pyrocatechol in the presence of potassium chromate, a highly coloured purple-red condensation product was obtained with a maximum absorption at 530 nm .
Initial studies were directed towards the optimization of the experimental conditions in order to obtain a good sensitivity, stability and reproducibility of the coloured product.
The influence of various reaction variables on the coloured product was tested to establish the most favourable conditions for the determination of 1-naphthylamine.

Study of the Optimum Reaction Conditions
The various parameters affecting and related to the above mentioned coloured product have been studied and optimum conditions have been selected.

Effect of reagent concentration
This effect was studied by placing different volumes of pyrocatechol (2.72×10 -3 M) into a series of 25-ml calibrated flasks, followed by 2.5 ml potassium chromate (4.6×10 -3 M), followed by 5 ml of 1-naphthylamine (50 µg ml -1 ) and 1.5 ml of hydrochloric acid solution(0.05M).The solutions were diluted to the mark with distilled water.The absorbances were measured at 530 nm versus blank.The results obtained in Table ( 1) indicate that the use of 2.5 ml of (2.72×10 -3 M) pyrocatechol reagent gave the maximum colour intensity.

Effect of hydrochloric acid amount
The effect of the amount of hydrochloric acid was studied by placing into a series of 25-ml calibrated flasks, 2.5 ml of (2.72×10 -3 M) pyrocatechol, 2.5 ml of (4.6×10 -3 M) potassium chromate, followed by 5 ml of (50 µg ml -1 ) 1-naphthylamine and different volumes of (0.05 M) hydrochloric acid.The solutions were diluted to the mark with distilled water.The absorbances were measured at 530 nm versus blank.The results obtained in Table (2) indicate that the use of 1.0 -1.5 ml gave maximum colour intensity, 1.5 ml was selected in all subsequent experiments.

Effect of temperature
The effect of temperature on the absorbance of the coloured product was studied.This was implemented by placing into three 25-ml calibrated flasks, 2.5 ml of (2.72×10 -3 M) pyrocatechol, 2.5 ml of (4.6×10 -3 M) potassium chromate, followed by 5 ml of (50 µg ml -1 ) 1-naphthylamine, solution and 1.5 ml of (0.05 M) hydrochloric acid solution.The solutions were diluted to the mark with distilled water and the first flask was allowed to stand for increasing time at room temperature, the second was at 0°C and the third in water bath at 50°C.The absorbance was measured at 530 nm at different periods versus blank prepared in the same way but containing no 1-naphthylamine.The results obtained in Table (3) indicated that the absorbance of the coloured product was decreased when the reaction was carried out at 0°C or 50°C.Therefore, it is recommended that the reaction should be conducted at room temperature

Stability of the product
This was studied by placing 2.5 ml of (2.72×10 -3 M) pyrocatechol, into a series of 25ml calibrated flasks, followed by 2.5 ml of (4.6×10 -3 M) potassium chromate and 5 ml of (50 µg ml -1 ) 1-naphthylamine and 1.5 ml of (0.05 M) hydrochloric acid.The solution was diluted to the mark with distilled water and the absorbance was measured at 530 nm at different periods versus reagent blank.The results obtained in Table (4) show that the product needs 5 minutes to attain maximum absorbance and it remains stable for about 15 minutes.
The results indicate that order (I) gives higher absorbance of the product and therefore it was selected in all subsequent experiments.

Final absorption spectra
Using the optimum conditions described above, the 1-naphthylamine-pyrocatechol complex formed has an absorption spectrum ranging between 475 and 590 nm with a maximum absorption at 530 nm in contrast to the reagent blank which shows small absorption at λmax.Therefore, the 530 nm wavelength of maximum absorption has been selected for subsequent work.(2) blank against distilled water.

Quantification
A plot of absorbance versus determined concentration ,Figure (2) shows that Beer's law is being obeyed over the range of 1 to 60µg/ml of the solution.Negative deviation from Beer's law occurred beyond the upper determination limit.

Accuracy and precision of the method
To check the accuracy and precision of the method, 1-naphthylamine has been determined at three concentrations.The results are shown in Table ( 6) indicated that the method is performing well.

Nature of the product
The method of continuous variations and of mole -ratio have shown that 1naphthylamine and pyrocatechol react in a 1:1 mole ratio.

Application of the method Determination of 1-naphthylamine in river water
The present method has been applied to the determination of 1-naphthylamine in river water; the results of the determination are given in Table ( 7).

Determination of 1-naphthylamine in sea water
Sea water has been prepared (Henriksen,1965) and 1-naphthylamine has been assayed in sea water by the present method, and the results are shown in Table (8).

CONCLUSION
A new spectrophotometric method has been proposed for the determination of 1naphthylamine in aqueous solution.The method is based on coupling of 1-naphthylamine with pyrocatechol reagent in the presence of potassium chromate to form a coloured dye which exhibits maximum absorption at 530 nm.The molar absorptivity is 3685 l.mol -1 cm -1 .The proposed method is applied to the determination of 1-naphthylamine in river water and sea water.

Table 1 :
Effect of the concentration of reagent on pyrocatechol absorbance

Table 3 :
Effect of temperature on absorbance of coloured product

Table 4 :
Rate of reaction and stability of product

Table 5 :
Effect of order of addition on the absorbance of the coloured product

Table 6 :
Accuracy and precision of the method * Average for five determinations.

Table 7 :
Recovery of 1-naphthylamine in river water

Table 8 :
Recovery of 1-naphthylamine in sea water