Indirect Spectrophotometric Determination of Formaldehyde with Sulphur Dioxide in Air Samples using p-Aminobenzene Sulphonic Acid

 

Prasanna K. Sharma*and Surendra K. Rajput

Department of Chemistry and Biochemistry, Govt. Nagarjuna P.G. College of Science, Raipur,  492010 (C.G.), India.

*Corresponding Author E-mail: sharma.prasanna@rediffmail.com

ABSTRACT:

 

 

INTRODUCTION

Formaldehyde is one of the most important air pollutants results from the partial oxidation of organic matter, particularly fuels, and is a normal constituent of automobile and aero-engine exhaust products^1,2. It is also found in the waste gases from incinerators as well as in the effluents from plastic, textiles and other industries³ and also widely used in hospitals as disinfectants in carpet and furniture glues and in a large number of other products. It is produced on a substantial scale of 6M tons/y. Formaldehyde (FA) irritates the mucous membrane and causes eye irritation. In view of its widespread use, toxicity, and volatility, exposure to formaldehyde is of significant consideration for human health⁵.  Its Threshold Limit Value is 3 mg/m³,  Henry’s law constant (Pa.m³/mole, at 25^oC) is in the range 2.2x10^-2 to 3.4x10^-2, Log Octanol/water partition coefficient (Log K_w)  is in the range  0.75 to 0.35 and Conversion factor is 1.2 mg/m³ (see⁴).

 

A large number of reagents have been proposed for the spectrophotometric determination of formaldehyde in air samples^6-12, in tobacco, cigarettes saples¹³and pharmaceutical samples^14-16  Trace amount of formaldehyde was detected by Flow injection Analysis method²³ .

 

West and Gaeke¹⁷ developed a method using pararosaniline and formaldehyde for the estimation of sulphur dioxide, and this method has been modified by several researchers for determination of formaldehyde^18-19. Kniseley andThroop²⁰ recommended p-aminobenzene sulphonic acid for the spectrophotometric determination of sulphur dioxide, as it is chemically similar to pararosaniline but contains only one amino group. This method is more sensitive and the reagents are available in higher purity.

 

The aim of the present investigation is to overcome the existing inadequacies, like instability of colour and interference of co-pollutants and provide reliable, accurate and cost effective procedure in determining formaldehyde in which the formaldehyde reacts with p-aminobenzene sulphonic acid and SO₂ resulting  in the formation of a pinkish-red colour dye. We successfully applied the method to determine formaldehyde in different environmental samples. (Table-4)

 

EXPERIMENTAL:

Apparatus: A Systronic UV-VIS Spectrophotometer (Model 104) with matched quartz cells was used for all spectral measurements. A Systronic pH meter (Model 331) was used for pH measurements. A Remi C-854/4 clinical centrifugal force of 1850 g with fixed swing out rotors was used for centrifugation. Midget impingers of 35 ml capacity were used for air sampling and calibrated rotameters were used for measurement of the air flow-rates.

Reagents: All the reagents used were of AnalR grade, and double-distilled deionised water was used throughout the experiment.

 

Standard formaldehyde solution: One ml of 30-40% formaldehyde solution diluted to 100 ml and further, 0.005M hydrochloric acid was added to give a 6 µg/ml working formaldehyde standard.

 

Sulphite solution: 0.005M Prepared by dissolving anhydrous sodium sulphite in distilled water.

p-Aminobenzenesulphonic acid (E. Merck, Germany):  0.05% (m/v) solution of the reagent was prepared by dissolving 500 mg of the p-aminobenzene sulphonic acid in 100 ml of ethanol¹².

 

Procedure : Two midget impingers, each containing 10ml of 0.005M hydrochloric acid, were connected in  series and then to a rotameter and a vacuum pump. Air was drawn through the system at ca. 1.5 to l min¹². Then the solutions were quantitatively transferred to a 25 ml calibrated tube and 1 ml each of the p-aminoazobenzene sulphonic acid and sulphite solution was  added. The solution was thoroughly mixed and kept for 20-25 min. The acidity was then adjusted between 0.4 and 1.2M with hydrochloric acid and the solution was made up to the mark with double distilled water. The absorbance of the pink dye was measured at 510 nm against distilled water and the amount of formaldehyde was computed from a calibration graph prepared in a similar manner after correction for the absorbance of a reagents blank measured at the same wavelength.

 

           

      P-amino benzene                                         formaldehyde 

         Sulphonic acid

                                                                                     Pink Red Colour

Reaction for formation of coloured species.

 

RESULTS AND DISCUSSION:

 

Fig 1: Absorption spectra: A, reagent blank; B, reagent plus 5 µg of formaldehyde in 25 ml.

The pink coloured dye formed in the proposed reaction shows maximum absorbance at 510 nm.All spectral measurements were carried out against distilled water. The absorption spectra are shown in fig.1.The reagent blank shows negligible absorbance at this wavelength.

 

Effect of acidity: The effect of acidity, before and after addition of sulphite solution, was studied. It was found that 0.02M hydrochloric acid was essential before the addition of sulphite solution, but up to 0.1M hydrochloric acid could be used without change in the absorbance. The acidity range of 0.02M to 0.1M hydrochloric acid was necessary before the addition of sulphite solution and 0.4M HCl was for full colour development after the addition of sulphide solution. It was also found that the absorbance was constant by increasing hydrochloric acid concentration up to 1.2M.

 

Effect of time and temperature:  Under optimum condition, the time necessary for the full colour development was found to be 20 min, and the absorbance remains constant up to 90 minutes. The determination was carried out at room temp. 30 to 35°C.

 

Effect of Reagent concentration: It was observed that a minimum 1 ml. of p-aminobenzene sulphonic acid and sulphide solution was required for full colour development. By increasing the quantity of p-aminobenzene sulphonic acid, the absorbance of both the reagent blank and the sample solutions increased. The amount of sulphur dioxide present should be ten times the stoichiometric amount needed for the formaldehyde level. A large excess of sulphur dioxide does not affect the absorbance of the solutions.

 

Analytical Data: The adherence to Beer’s Law was studied by measuring the absorbance value of the solutions for various formaldehyde concentrations. A linear curve was obtained by plotting the absorbance against the concentration of formaldehyde, and thus obeying  Beer’s Law in the range of 1.8-9.6 µg formaldehyde per 25 ml of the final solution. The apparent Molar absorptivity and Sandell’s sensitivity were found to be 1.5×10³ l mol^-1 cm^-1 and 0.002µg cm^-2, respectively. The reproducibility of the method was checked by seven replicate measurements, each containing 5 µg of formaldehyde per 25 ml of final solution over a period of seven days. The Standard Deviation and R.S.D. were found to be ± 0.002 and 1.8%, respectively. The low R.S.D. value and the range of error at 95% confidence level in terms of absorbance were ± 0.019 indicating good precision of the method.

 

Effect of foreign species: To assess the validity of the method, the effect of foreign species commonly found with formaldehyde was studied. The tolerance limits for different foreign species are shown in Table 1. Several common organic species, viz, other aldehydes and hydrocarbons, do not interfere with the present method. The interference from nitrogen dioxide is removed by the addition of 1 ml of 5% potassium hydroxide solution, and that of sulphite can be removed by passing the air sample through a tube containing lead acetate solution.

 

Collection efficiency: The Wilson’s procedure²¹ was used to investigate the collection efficiency of the absorbing solution²². Purified air was passed through an evaporation chamber preheated to 60-70^oC. Known amounts of formaldehyde were added from a micro burette and then allowed to evaporate in the chamber. The formaldehyde present in the air stream was collected in the absorbing solution, achieving 95% collection efficiency in the first impinger with a flow rate of 1.5l/min. and 5% in the second.  (Table-2)

 

Application of the method for the analysis of formaldehyde in the Air and Automobile Exhaust: The method has been successfully applied for analyzing formaldehyde in air and automobile exhaust. The samples were introduced to the sampling train through a sintered funnel to avoid the entrance of particulate matters. Automobile exhaust (scooter) was sampled from the inner part of the muffler. No comparative analysis was made. The results are presented in Table 2 and 3.

 

Comparison with other methods: Since the method was specially devised for the determination of formaldehyde in air, a comparison with other commonly used methods^6-12was worthwhile. Instability of the colour and interferences from co-pollutants are the main drawbacks of methods based on Schiff’s reagent⁶, Chomotropic acid^7,8, phenylhydrazine⁹, MBTH¹⁰ and p-aminoazobenzene¹². The comparison is summarized in Table 4.

 

Table 1.   Interference and tolerance limits* of other species in the determination of  4 µg of formaldehyde.

Inorganic   Tolerance Species       Limit (ppm)		Organic                           Tolerance Species                             Limit (ppm)
Cd2+       Zn2+ Mn2+         Pb2+          Hg2+          Cu2+           NH3            H2S             NO2+ **    Fe3+       VO3-	15 10 12 3.0 0.5 0.6 0.5 0.6 0.5 0.2 0.2	Ethanol Benzaldehyde Ethylamine Isobutyl methyl ketone Formic acid Nitrobenzene Acetic acid Acetone   Benzene Urea Toluene Aniline Acetaldehyde Phenol	20 2.5 2.5 2.0 1.0 0.5 0.25 0.25 0.25 0.25 0.20 0.20 0.10 0.08

*     Amount (mg) which cause a 2% error

 ** Masked up to this level by 1 ml of 5 % potassium hydroxide

 

Table 2: Analysis of Formaldehyde in the Air and Automobile Exahaust:

Sample	Sampling time Min.	Volume sampled (liter)	Formaldehyde found  (µg)
Air	60 60 120	90 90 180	22.0 36.0 67.0
Automobile exhaust (scooter)	2 2 5	3 3 7.5	44.0 51.0 78.0

 

Table 3: Analysis of Formaldehyde in Scooter Exahaust:

Vol. of sample taken	HCHO found in µg			Mean
	Ist set impinger	IInd set impinger	IIIrd set impinger
4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5	68.5 64.2 70.0 67.4 71.0 62.0 65.0 70.0	66.5 62.4 71.5 70.0 72.5 63.0 66.0 72.5	67.0 63.5 69.0 72.0 71.0 62.5 66.5 71.0	67.3 63.3 70.1 69.8 71.5 62.5 65.4 71.1

Table 4.  Comparison of some Spectrophotometric methods for the determination of Formaldehyde in air samples.

SN	Reagents	λ max, nm	Medium	Beer’s Law Range µg/25ml	ε l mole-1cm-1	Colour stability	Absorption efficiency %	Interferences
1.	Schiff’s reagent6	550	HCI	*	3.5 x 103	30 min	72	SO2, NO2 , Alcohol, Phenol
2.	Chromotropic acid7,8	278	H2SO4	5-100	1.57 x 104	24 hr	78-97	SO2, NO2, NO3 hydrocarbons
3.	Phenlhydrazine9	520	4M HCI	2-36	6.50 x 104	15 min	-	-
4.	Oxaldihydrazone Cu(11)11	620	pH 5.8-7.0	15-90	7.7 x 103	150 min	95-96	Sulphur compounds
5.	p-aminozobenzene12	505	pH 1.2-3.6	2-12	4.5 x 104	90 min	85	Organic and Inorganic compounds
6.	p-aminobenzene sulphonic acid (present method)	510	0.02-0.1 M and  0.4- 1.2 M HCI	1.8-9.6	1.5x103	60 min	95**	See table - 1

*    Beer’s law not obeyed

* * First impinger (100^oC with two impingers in series)

 

 

Acknowledgement:

The authors are grateful to Professor S. Nigam, Head, Department of Chemistry and Bio-chemistry and Professor Dr. K.N. Bapat Principal, Govt. Nagarjuna P.G. College of Science, Raipur, for providing laboratory facilities.

 

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Received on 14.01.2012         Modified on 10.02.2012

Accepted on 12.02.2012         © AJRC All right reserved