A study on the thermal stability of sodium dithionite using ATR-FTIR spectroscopy

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A study on the thermal stability of sodium

dithionite using ATR-FTIR spectroscopy

Vijaya Lakshmi Vegunta

This work has been carried out at INNVENTIA AB

Master Thesis in Fibre and Polymer Technology KTH Royal Institute of Technology Stockholm, Sweden

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1. Abstract

Sodium dithionite (Na2S2O4) is a powerful reducing agent. It has therefore been suggested to be used as an additive in kraft pulping to improve the yield. However, sodium dithionite easily decomposes and it is thus important to determine the effect of different conditions. The aim of this thesis has been to investigate the thermal stability of sodium dithionite under anaerobic conditions using ATR-FTIR spectroscopy under different conditions, such as heating temperature, concentration of the solution, heating time and pH.

The stability of sodium dithionite was found to decrease with increasing heating temperature, concentration of sodium dithionite, heating time and pH. Sodium dithionite was found to be relatively stable at moderate alkaline pH:s 11.5 and 12.5, while a rapid decrease in stability with time was noted at higher heating temperatures and concentrations of sodium dithionite.

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2. Introduction

Sodium dithionite (Na2S2O4) is a strong reducing agent with great industrial significance. Due to its reducing ability it finds use in fields such as printing and dyeing of textiles, production of synthetic rubber, synthesis of stabilizers for polymeric materials and as a biochemical reductant. Sodium dithionite also has valuable environmental remediation applications. It is being used to treat heavy metal waste in aquifers and other environmental significant treatments.

A study of Jayme and Wörner (1952) showed that improved strength, higher bleachability and improved lignin degradation in kraft pulping were seen in the system when using sodium dithionite as a pulping additive. Due to its reducing properties sodium dithionite is used as a bleaching agent in textile and paper industry.

Sodium dithionite may be used for the preservation of hemicelluloses in pulping and as it will not introduce new elements to the recovery system and it may be used without changing the system. Sodium dithionite efficiently reduces carbonyl functionalities of functionalized aldehydes and ketones to corresponding alcohols in good yields without affecting other functional groups in water or dioxane system at 85 °C (De Vries and Kellogg 1980; Sigh et al. 1998). An experimental research has been done by using sodium dithionite as pulping additive in a pilot scale and the results were promising (Wang et al. 2014). It is cost effective when compared to other alternative additives, such as sodium borohydride (NaBH4).

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6 2.1 Kraft pulping

This section describes an overview of the kraft pulping process. Kraft pulping is currently the dominant chemical method for pulping. The pulp produced using this process is stronger and darker and requires chemical recovery to compete economically. The main objective of kraft pulping is to liberate wood fibers from the wood matrix to obtain wood pulp. This is achieved by removing lignin and preserving the molecular weight of carbohydrates as much as possible. The kraft pulping process involves digesting of wood chips at elevated temperatures and pressure. The typical out-line of this process is wood handling (chipping, debarking, and screening), cooking in digester (kraft cooking), and oxygen delignification, washing of pulp, multi stage bleaching and drying of pulp.

In kraft mills, the wood chips are fed into digester together with the cooking liquor. The cooking liquor is white liquor which is a mixture of sodium hydroxide (NaOH) and sodium sulphide (Na2S). The active species in this cooking process are hydroxyl (OH-_{) and hydrogen sulphide (HS}-₎ ions. The hydroxyl ions are consumed during kraft pulping and neutralize acidic groups. There is only a slight decrease in hydrogen sulphide ion concentration during pulping. The first step in kraft cooking is an impregnation step, where the cooking liquor is transported through the surface of wood chips and followed by the penetration and diffusion into the wood chips. Depending on the amount of hydroxyl and hydrogen sulphide ions present in the liquor, cooking temperature and time; different compositions of pulp can be obtained (Kleppe 1970; Aurell and Hartler 1965).

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7 2.2 Stability of sodium dithionite

The stability of sodium dithionite (Na2S2O4) in aqueous solution is discussed in this section. The anhydrous solution of sodium dithionite is combustible and decomposes exothermically if subjected to moisture. Under anaerobic conditions at room temperature, alkaline (pH 9-12) aqueous solutions of dithionite decompose slowly over a matter of days. Therefore, sodium dithionite is more stable and effective as a reducing agent in alkaline solution (Wayman and Lem 1970).

The following generally accepted formula shows decomposition of sodium dithionite in aqueous solutions at pH 7.

This reaction leads to several side reactions. However, there is a little agreement on the importance of products (Spencer 1967).

The overall redox equilibria reaction for anaerobic oxidation of sodium dithionite to bisulphite is described by the following formula (Mayhew 1978).

The stabilization of dithionite ion should be achieved by adding formaldehyde to the sample which forms hydroxymethane sulfonate or formaldehyde bisulphite (HOCH2SO3) and hydroxymethane sulphinate or formaldehyde sulphoxylate (HOCH2SO2-_{). Formaldehyde can} stabilize sulphite ion in aqueous solution through the formation of the hydroxymethane sulfonate (De Carvalho and Schwedt 2001). The following reaction describes the stabilization of sodium dithionite solution in formaldehyde.

Sodium dithionite is mainly stabilized by sodium polyphosphate, sodium carbonate and sodium salts of organic acids.

The stability of dithionite can be determined by different methods namely iodometric, potentiometric, spectrometric titrations, ion chromatography and electroanalytical methods (Fox 1974).

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ion is accelerated by increasing acidity, higher concentration of the dithionite ion, temperature and other sulphur containing species (Kilroy 1980 ; Wayman and Lem 1970).

Considering the stability of sodium dithionite in water, it dissolves in water and forms sodium hydrogen sulphite (NaHSO3), sodium hydrogen sulphate (NaHSO4) and sodium thiosulphate (Na2S2O3) (BASF AG 1988). Based on the pH range, sulphur dioxide (SO2), hydrogen sulphide (H2S) and sulphide (S2-_{) are present in aqueous solution. Sodium dithionite transforms into} sodium sulphite (Na2SO3) and sodium thiosulphate (Na2S2O3) without the presence of air, and forms sodium sulphite (Na2SO3) and sodium sulphate (Na2SO4) via oxidation with air. Hydrolysis is faster at lower pH values (Camacho et al. 1995).

2.3 Decomposition of sodium dithionite

The decomposition and behaviour of sodium dithionite in solution has been studied over long periods of time. Since early 20th_{century there are numerous studies carried out on the} decomposition pathway and kinetics of dithionite by several workers. The generally accepted reaction is as follows and the formed decomposed products are sodium thiosulphate (Na2S2O3) and sodium bisulphite or sodium hydrogen sulphite (NaHSO3) (Burlamacchi et al. 1969).

From this reaction other stable sulfur species such as sulphate (SO42-_{), dithionate (S2O6}2-_{) and} hydrogen sulphide (H2S) are also formed in traces. The rate of this reaction is greatly affected by pH. The rate is slow in alkaline medium and rapid in acidic conditions (Kovács and Rábai 2002). Decomposition products of sodium dithionite under various conditions are described by the following equations at temperatures between 0 o_{C to 35}o_C.

Under aerobic conditions:

Under anaerobic conditions:

In strong alkaline medium:

In weak alkaline and weak acidic medium:

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In strong acidic medium:

The decomposition of dithionite undergoes a number of protonation and decomposition pathways that compete with each other (Kovács and Rábai 2002). Trithionite (S3O62-_{) is} produced in a 9:1 ratio related to bisulphite (HSO3-_{) and thiosulphate (S2O3}2-_{). The produced} trithionite (S3O62-_{) can be disproportionate into dithionate (S2O6}2-_{) and elemental sulphur or it} reacts with water and forms bisulphite (HSO3-_{) and hyposulphite (S2O2}2-_{) anions. When sodium} dithionate undergoes thermal decomposition, it decomposes in an autocatalytic way. After a short induction period the radical hyposulfite (SO2-_{) ion present in the solution under goes rapid} reaction and forms sodium thio sulphate (Na2S2O3) and sodium hydrogen sulphite (NaHSO3) (Rinker et al. 1960).

Under aerobic conditions the formation of hydrogen sulphite (HSO3-_{) and hydrogen sulphate} (HSO4-_{) lowers the pH of the media and accelerates the process of decomposition. Therefore, to} keep the solutions of dithionite stable it must be cooled and kept in alkaline state using NaOH solution with oxygen excluded.

Due to complex nature of dithionite (S2O42-_{) the analysis of the dithionite ion has been a difficult} problem to solve. From previous studies an attractive procedure for determining dithionite (S2O42-_{), bisulphite (HSO3}-_{) and thiosulphate (S2O3}2-_{) contents in the product by carrying three} operational distinct iodometric titrations has been described. However, the data presented is quite insufficient to support this method and it is also impracticable (Wollak 1930).

Coming to the kinetics of sodium dithionite solution, the decomposition in aqueous solution under anaerobic conditions is a second order (sum of exponents in rate law is equal to two; rate law equation for second order reaction (d[A]/dt = - k[A]2_{)) with respect to sodium dithionite} concentration. The decomposed products formed are sodium thiosulphate (Na2S2O3) and

sodium hydrogen sulphite (NaHSO3) (Meyer 1903).

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Sodium dithionite present in the form of powder can decompose in air on contact with small amounts of water to produce intense heat and burns with flame. The aqueous solution of sodium dithionite decomposes slowly in cold and rapidly in warm conditions. Initially thermal analysis in nitrogen showed that decomposition of sodium dithionite takes place by evaluation of heat and sulphur dioxide as reported by earlier workers (Janzen 1972; Lister and Garvie 1959; Erdey et al. 1965). As the temperature increases the concentration of dithionite ion decreases (Wayman and Lem 1970; Camacho et al. 1995).

Coming to effect of pH, sodium dithionite decomposes rapidly under weak acidic conditions (especially under warm conditions). In an alkaline solution, the decomposition reaction is much slower and the products formed are thiosulphate (S2O32-_{) and bisulphite (HSO3}-_{) (Greenwood} and Earnshaw 1984) as well as a small amount (2-4%) of sulphide (S2-_{) and sulphur (S). The} disproportion occurs slowly at pH greater than or equal to 8 and rapidly in acidic conditions. Decomposition of sodium dithionite in alkaline solution is accelerated by thiosulphates (S2O32-₎ and polysulphides (Sn2-_{). In addition of strong acids to dithionite solution it changes from yellow} to red in colour. After a short time, complete decomposition occurs with precipitation of sulphur. A weak alkali at pH range from 8-13 stabilizes dithionite solutions and can then be kept for weeks below 10 o_{C with no presence of air or oxygen. The aqueous solution of sodium} dithionite in the presence of air readily decomposes into sulphite (SO32-_{) and sulphate (SO4}2-_{) at} room temperature, with or without stabilizer. Dithionite ion decomposes rapidly in acidic conditions and at higher temperature even under anaerobic conditions (Cleghorn and Davies 1970; Selwyn and Tse 2008).

2.5 Possible reaction mechanisms

The dithionite ions react with water and decompose into sulphur containing species. The following reaction describes the decomposition of sodium dithionite with water.

The main decomposition products are bisulphite (HSO3-_{) and thiosulphate (S2O3}2-_{) ions. The} actual decomposition process is more complicated than it appears in the equation. This is due to that various sulphur containing species react with one another to form other sulphur containing species, such as hydrogen sulphide (H2S), polysulphides (H2Sx) and elemental sulphur (S). These new species can react with dithionite ions and further decrease the dithionite ion concentration (Selwyn and Tse 2008).

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The overall reaction of decomposition of sodium dithionite in various aqueous solutions has been shown in the following reaction:

The possible mechanism could be the initial and slowest stage of reaction with water and is explained by following reaction:

This is followed by a rapid reaction of sulphoxylate ion (HSO2-_{) with dithionite ions and forms} thiosulphate and sulphite:

3. Purpose of the study

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4. Experiment

4.1 Materials

The materials used in this study were sodium dithionite (Na2S2O4) (≥ 85% purity) supplied by Merck KGaA, Darmstadt, Germany and sodium hydroxide (NaOH) (≥ 99.0 % purity) supplied by Merck KGaA, Darmstadt.

4.2 Sample preparation

The sodium dithionite solutions were prepared under anaerobic conditions. For that a box purged with nitrogen gas was used. The solutions were prepared in 50 ml volumetric flasks (purged with nitrogen) by mixing sodium dithionite and deionized water (purged with nitrogen). The pH of the solutions was adjusted using sodium hydroxide. For pH 9, deionized water (purged with nitrogen) was used for preparing the solutions. For the pH:s 11.5, 12.5 and 14, the pH was adjusted by adding required amounts of sodium hydroxide. The prepared solution was transferred to an autoclave (a Teflon bottle with a steel cap) previously purged with nitrogen and kept in an oil bath for heating with experimental heating times according to run order. After heating the autoclave, it was placed in a water bath with temperature of 10 °C for cooling. The sample solution was cooled for 40 min. After cooling the sample solution, 2 drops of sample solution was placed on the ATR-FTIR crystal and a spectrum was recorded also under nitrogen atmosphere.

Table 1: Parameters involved in experimental study are shown in the following table

Parameters Conditions

pH 9; 11.5; 12.5; 14

Temperature (°C) 80; 100; 120

Time (min) 10; 20; 30

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5. Method

5.1 ATR-FTIR spectroscopy

The ATR - FTIR spectra of the samples were recorded using a Varian 680-IR FTIR spectrometer (Varian Inc., Santa Clara (CA), USA). The software used was Varian Resolutions Pro (Varian Inc., Santa Clara (CA), USA) (Version 5.1) and ATR-FTIR system was operated in ATR mode. The Pike MIRacleTM_{ATR crystal consists of a single reflection diamond with w/Znse crystal plate (Pike} Technologies Inc., Madison (WI), USA). The ATR crystal was having contact area with diameter Ø2 mm and the penetration depth of the IR light of 2 µm.

For ATR sampling the IR beam was directed into the ATR crystal of refractive index of 2.4. The working pH range is 1 - 14 and long wave cut off is at 525 cm-1_{. This makes single reflection} diamond with w/Znse ATR crystal plate applicable for this experimental study. The IR beam reflects from the internal surface of the crystal and creates an evanescent wave. The created wave projects orthogonally into the sample which was in intimate contact with the crystal. Some part of the energy from the evanescent wave is absorbed by the sample and the reflected radiation is reflected to the detector. The graphical representation of single reflection ATR crystal can be seen in Figure 1: where dp is depth of penetration and ө is critical angle.

Figure 1: Graphical representation of a single reflection ATR

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14 5.2 Calculations

The stability of the sodium dithionite in the aqueous solution was evaluated from IR spectra. Two spectra were taken as an example, as illustrated in Figure 2, one was a reference spectrum of sodium dithionite solution having conditions: pH 9 and 0.6 M concentration at temperature 20 °C. The other spectrum was a dithionite solution spectrum having conditions: pH 9, 0.6 M concentration and heated at 80 °C for 20 min. The observed signal at 1051 cm-1_{was identified} as SO2 antisymmetric stretch of sodium dithionite. This signal at 1051cm-1_{was used for} evaluating stability of sodium dithionite solution.

The height of the signal was evaluated by drawing base line at the base of signal between two minimal points. A straight line drawn from base line to signal maxima was determined as height of the signal, as illustrated in Figure 2.

Figure 2: Determination of height of the signal from the baseline. Spectrum of reference sodium dithionite solution at pH 9 with 0.6 M concentration at 20 °C and spectrum of sodium dithionite solution at pH 9 with 0.6 M concentration at 80 °C heated for 20 min. (I) is height of the signal of experimental run and (Io) is the height of the signal of the sodium dithionite solution at 20 °C.

The determined heights of this signal at 1051 cm-1_{were used for calculating stability of sodium} dithionite solution according to Eq(1):

0 0.01 0.02 0.03 0.04 0.05 800 900 1000 1100 1200 A b so rb an ce Wavenumber (cm-1₎

Reference sodium dithionite solution (pH 9 ,0.6 M, 20 °C) Sodium dithionite solution (pH 9 ,0.6 M, 80 °C, 20 min)

1051

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where, Io is signal height of the reference spectrum sodium dithionite solution at 20 °C and I is

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6. Results and discussion

The effect of heating temperatures 80 °C, 100 °C and 120 °C, pH:s 9, 11.5, 12.5 and 14, concentrations 0.2 M, 0.4 M and 0.6 M and heating times 10 min, 20 min and 30 min have been investigated. The stability of dithionite is presented as the percentage of compound remaining. One could consider that the trends illustrating the stability of sodium dithionite in Figures 3-17 could appear somewhat different if additional conditions were used such as 90 °C or 100°C, 0.3 M or 0.5 M, pH 10 or 13.

6.1 Effect of heating time

Figure 3: Stability of alkaline sodium dithionite solutions at pH 9 as a function of heating time showing effects of heating temperature and concentration of sodium dithionite solution.

Figure 3 shows the stability of sodium dithionite solution at pH 9 under anaerobic conditions as a function of heating time. At pH 9 the stability decreased with increasing heating time of the sodium dithionite solution. The successive decrease in stability of sodium dithionite solution with increasing heating temperature, concentration of solution and heating time was seen.

0 20 40 60 80 100 0 10 20 30 Stab il ity o f d ith io n ite (r el ati v e % remai n in g )

Heating time (min)

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Figure 4: Stability of alkaline sodium dithionite solution of pH 11.5 as a function of heating temperature, heating time and concentration of sodium dithionite solution.

Figure 4 shows the stability of alkaline sodium dithionite solution under anaerobic conditions at pH 11.5 as a function of heating time. The sodium dithionite solution was found to be stable at all conditions of 80 and 100 °C. The stability of sodium dithionite decreases at the highest heating temperature and longest heating times at all concentrations of sodium dithionite solution. When it comes to temperature 120 °C after 10 min, the sodium dithionite solution starts to lose its stability and the stability decreases with increasing in concentration of solution and heating time.

0 20 40 60 80 100 0 10 20 30 Stab il ity o f d ith io n ite (r elati v e % remai n in g )

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Figure 5: Stability of alkaline sodium dithionite solution at pH 12.5 as a function of heating time showing effects of heating temperature and concentration of sodium dithionite solution. Figure 5 shows the stability of alkaline sodium dithionite solutions under anaerobic conditions at pH 12.5 as a function of heating time. The alkaline sodium dithionite solution at pH 12.5 was stable at all conditions studied except for the highest dithionite concentration of 0.6 M at 120 °C for heating time 30 min. The similar behaviour of sodium dithionite solutions with pH 11.5 is observed. It is evident that the stability was higher at pH 12.5 as compared to pH 11.5.

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Figure 6: Stability of alkaline sodium dithionite solution at pH 14 as a function of heating time showing effects of heating temperature and concentration of sodium dithionite solution.

Figure 6 shows the stability of alkaline sodium dithionite solutions under anaerobic conditions at pH 14 as a function of heating time. The stability of sodium dithionate decreases gradually with increasing heating temperature, heating time and concentration of sodium dithionite. The decrease in stability of sodium dithionite solution is rapid at higher concentrations, higher temperatures and heating time.

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20 6.2 Effect of heating temperature

Figure 7: Stability of sodium dithionite at pH 9 as a function of heating temperature showing effects of heating time and concentration of sodium dithionite solution.

Figure 7 shows the stability of sodium dithionite solutions under anaerobic conditions at pH 9 as a function of heating temperature. The stability of sodium dithionite solution gradually decreased with increase in heating temperature of the solution. At higher temperature and concentration of the sodium dithionite solution the stability decreases with heating time. The decomposition of sodium dithionite increases as the heating temperature increases at the highest concentration and heating time.

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Figure 8: Stability of sodium dithionite at pH 11.5 as a function of heating temperature showing effects of heating time and concentration of sodium dithionite solution.

Figure 8 shows the stability of alkaline sodium dithionite solutions under anaerobic conditions at pH 11.5 as a function of heating temperature. The sodium dithionite solution is stable at all conditions until 100 °C. The stability starts to decrease from 100 °C as the concentration of solution, heating temperature and heating time increases.

Sodium dithionite solution is unstable at highest heating temperatures (120 °C) with highest concentrations (0.4 M and 0.6 M)

The decomposition degree increases with increase in heating temperature, heating time and concentration of the solution. The degree of decomposition is higher at highest temperature 120 °C at all conditions and low until 100 °C at all conditions.

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Figure 9: Stability of sodium dithionite at pH 12.5 as a function of heating temperature showing effects of heating time and concentration of sodium dithionite solution.

Figure 9 shows the stability of sodium dithionite solutions under anaerobic conditions of pH 12.5 as a function of heating temperature. Sodium dithionite solutions are stable at this pH under most conditions but there was a rapid decrease in the stability at the highest temperature, concentration and heating time of the solution. Sodium dithionite at this pH 12.5 shows similar behaviour as pH 11.5. The stability decreases at the highest temperature 120 °C.

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Figure 10: Stability of sodium dithionite at pH 14 as a function of heating temperature showing effects of heating time and concentration of sodium dithionite solution.

Figure 10 shows the stability of alkaline sodium dithionite solutions under anaerobic conditions at pH 14 as a function of heating temperature. The stability of sodium dithionite decreases with increase in heating temperature, heating time and concentration of the solutions. The degree of decomposition is lower at lower concentrations, heating time and heating temperature of the sodium dithionite solution and higher at higher concentrations, highest heating temperature and heating time.

The degree of decomposition at pH 14 increases rapidly at the highest heating temperature of 120 °C with higher concentration 0.6 M as the heating time increases.

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24 6.3 Effect of concentration

Figure 11: Stability of sodium dithionite at pH 9 as a function of concentration of sodium dithionite solution showing effects of heating temperature and heating time.

Figure 11 shows the stability of sodium dithionite solution at pH 9 under anaerobic conditions as a function of concentration of sodium dithionite solution. At pH 9 the sodium dithionite is less stable at higher heating temperature and heating time at all concentrations studied. The stability of sodium dithionite solution is lower for all concentrations at the highest temperature 120 °C. 0 20 40 60 80 100 0.2 0.4 0.6 Stab il ity o f d ith io n ite (r el ati v e % remai n in g )

Concentration of sodium dithionite (M)

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Figure 12: Stability of sodium dithionite at pH 11.5 as a function of concentration of sodium dithionite solution showing effects of heating temperature and heating time.

Figure 12 shows the stability of sodium dithionites at pH 11.5 under anaerobic conditions as a function of concentration of sodium dithionite solution. At pH 11.5 sodium dithionite solution is stable under all conditions of concentrations until 100 °C. The stability decreases at all concentrations at the higher temperature of 120 °C and the longest heating time 30 min.

0 20 40 60 80 100 0.2 0.4 0.6 Sta b il ity o f d ith io n ite (r elati v e % remai n in g )

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Figure 13: Stability of sodium dithionite at pH 12.5 as a function of concentration of sodium dithionite solution showing effects of heating temperature and heating time.

Figure 13 shows the stability of sodium dithionite solutions at pH 12.5 under anaerobic conditions as a function of concentration of the solution, heating temperature and heating time. The sodium dithionite solution at pH 12.5 is stable with respect to all concentrations studied unless the temperature is higher than 100 °C. At higher concentration 0.6 M at highest heating temperature (120 °C) for longest time (30 min) the stability of sodium dithionite solution decreases. 0 20 40 60 80 100 0.2 0.4 0.6 Stabi li ty o f d ithi o n ite (r el ati v e % rema ini ng )

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Figure 14: Stability of sodium dithionite at pH 14 as a function of concentration of sodium dithionite solution showing effects of heating temperature and heating time.

Figure 14 shows the stability of sodium dithionite solutions under anaerobic conditions at pH 14 as a function of concentration of sodium dithionite. As the concentration increases the stability of sodium dithionite decreases. At the highest heating temperature 120 °C the stability decreases more rapidly with respect to increase in concentration of sodium dithionite solution.

0 20 40 60 80 100 0.2 0.4 0.6 Stab il ity o f d ith io n ite (r el ati v e % remai n in g )

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28 6.4 Effect of pH

Figure 15: Stability of sodium dithionite as a function of pH at heating temperature 80 °C in alkaline conditions.

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Figure 17: Stability of sodium dithionite as a function of pH at heating temperature 120 °C in alkaline solutions.

In Figures 15, 16 and 17, the change in the stability of sodium dithionite as a function of pH is shown. The sodium dithionite solution was less stable at weak and high alkaline conditions and relatively stable at moderate alkaline conditions with pH (11.5-12.5). The degree of decomposition for sodium dithionite solution increases with increasing heating temperature. The stability decreases with increase in heating temperatures.

The degree of decomposition of sodium dithionite is rapid in weak alkaline condition, pH 9 when compared to moderate alkaline conditions pH 11.5 and pH 12.5. It is due to presence of sodium hydroxide in solutions at pH 11.5 and pH 12.5. In case of pH 11.5 and pH 12.5 sodium hydroxide was added to adjust the pH of the sodium dithionite solution which also in turn stabilizes sodium dithionite. So the solution at pH 11.5 and pH 12.5 are relatively stable when compared to pH 9. At higher concentrations of sodium hydroxide, the stability decreases with increase in heating temperature, heating time and concentration of the solution. May be this is due to the hydroxide ions that catalyse the reaction to some extent, but the effect is not proportional to concentrations.

From previous work of researchers, the equilibrium between dimer and monomer species of dithionite is known (Burlamacchi et al. 1969). The dimer sulfur dioxide ion (SO2-_{) is more} reactive than dithionite ion (S2O42-_{). Due to presence of the low concentrations of sulfur dioxide} ion (SO2-_{) in equilibrium with dithionite ion (S2O4}2-_{) the decomposition reaction is slow (Lambeth} and Palmer 1973). The increase in decomposition is observed in the sodium dithionite solution

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at high heating temperatures. Maybe at high temperatures, the presence of high concentrations of sulfur dioxide ion (SO2-_{) in equilibrium with dithionite ion was leading to increase in degree of} decomposition of sodium dithionite solution.

At pH:s 11.5 and 12.5, sodium dithionite solution is mostly stable unless it is subjected to high heating temperature, heating time and concentration of the solution.

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6.5 Structural changes observed by infrared spectroscopy

All spectra used in this study are aqueous ATR-FTIR spectra of wavenumber range 4000 - 525 cm-1_{. Figure 18 shows the complete spectra of sodium dithionite solutions at pH:s 9, 11.5, 12.5} and 14, heating temperatures of 80 °C, 100 °C and 120 °C, and heating time 30 min. The spectra of sodium dithionite showed two strong and broad signals, at 3326 cm-1_{(O-H stretching) and} 1635 cm-1_{(O-H bending). The two signals observed are identified as characteristic signals for} water (H2O).

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Figure 19: FTIR spectra of sodium dithionite solutions in the spectral range 1200 - 800 cm-1

The region of the infrared spectra used for the investigations of chemical modification of the experimental samples was the part of finger print region, i.e. 1200 - 800 cm-1_{. Figure 19 shows} the spectral behaviour of sodium dithionite solutions in the wavenumber range of 1200-800 cm -1_{. Four signals were observed in this range. Pure sodium dithionite solution showing} characteristic peaks at 1051 cm-1_{and 912 cm}-1_{which were assigned for SO2 antisymmetric and} SO2 symmetric stretch of dithionite ion, respectively. Two other new signals observed were, at 995 cm-1_{and 912 cm}-1_{and these new signals were characteristic signals of decomposed} products, thiosulfate (S2O32-_{) and sulfite (SO3}2-_{), respectively.}

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33 6.6 Multivariate analysis

Figure 20: Scores scatter plot t1 vs t2 using SIMCA for the analyzing data with wave number range (4000 - 525 cm-1₎

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34 6.7 Decomposition products

Figure 21: Spectra of decomposed products of sodium dithionite at a heating temperature of 120 °C, for a heating time (30 min) and 0.6 M concentration of the sodium dithionite solution. In order to identify and determine the decomposed products, ATR-FTIR spectra were analysed. As a result of the decomposition of sodium dithionite produces various sulphur-oxygen containing compounds. Figure 21 shows spectra of decomposed products of alkaline sodium dithionite solutions with concentration 0.6 M, pH:s 9, 11.5, 12,5 and 14, at heating temperature of 120 °C, and a heating time of 30 min. Two kinds of decomposed products were seen depending on the pH of sodium dithionite solution.

Signals at 1051 cm-1_{and 912 cm}-1_{are seen for the spectrum of pure sodium dithionite solution.} The signals are identified as characteristic signals for SO2 antisymmetric and SO2 symmetric stretch of dithionite ion, respectively (Takahashi et al. 1982).

At pH range (9 - 12.5), the signals observed at 995 cm-1 _{showed the presence of sulphur} containing species. The decomposed product at this range is thiosulfate ion (S2O32-_{) (Holmen et} al. 1994). The signals with small intensities observed at 1108 cm-1_{and 1022 cm}-1_{were assigned} to S=O asymmetric stretch and S=O symmetric stretch of thiosulphate. The signal at 995 cm-1 was assigned to SO2 symmetric stretching for thiosulphate. This spectral information shows that decomposed product obtained was thiosulphate ion (S2O32-_{) (Strassburger and Breuer 1985).}

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New strong and broad signal appeared at 925 cm-1_{for pH 14 showing the presence of sulphite} (SO32-_{) (Takahashi et al. 1982). The signal at 925 cm}-1_{was assigned to S-O stretch of sulphite.} The decomposition reaction undergoes two mechanisms based on the pH; a disproportionation reaction mechanism and an electron transfer disproportionation reaction mechanism. Possibly at weak alkaline conditions the sodium dithionite undergoes a disproportionation reaction and at strong alkaline conditions it follows an electron transfer disproportionation reaction (Janzen 1972).

The decomposition of the sodium dithionite solution is mainly depending on the pH of the solution. The following reactions show the possible decomposed products of sodium dithionite solution under alkaline conditions.

In strong alkaline conditions (pH 14):

In moderate alkaline conditions (pH 11.5 and pH 12.5):

In weak alkaline conditions (pH 9):

Table 2: Infrared frequencies of decomposed products and sodium dithionite solution.

Name Chemical formula Frequencies (cm-1)

Dithionite S2O42- 1051 (SO2 antisymmetric stretch)

912 (SO2 symmetric stretch)

Thiosulphate S2O32- 995 (SO2 symmetric stretch)

1108 (S=O asymmetric stretch) 1022 (S=O symmetric stretch)

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7. Conclusions

ATR – FTIR spectra have been used for determining stability of sodium dithionite under alkaline conditions. The studies showed that the main factors having a significant effect on the decomposition and stability of sodium dithionite were heating temperature, heating time, concentration of the solution and pH. As the heating temperature, heating time and concentration increases the degree of decomposition increases. The decomposition was higher at weak (pH 9) and high (pH 14) alkaline conditions and was found to be relatively lower under moderate (pH:s 11.5 and 12.5) alkaline conditions. The decomposition was also low at lower concentrations of sodium hydroxide and higher at higher concentration of sodium hydroxide present in the solution.

The stability of sodium dithionite solution at certain pH decreases with increasing heating temperature, heating time and concentration of solution. In alkaline conditions, the stability decreased rapidly at weak (pH 9) and strong (pH 14) alkaline conditions and was rather low at moderate (pH: s 11.5 and 12.5) alkaline conditions. Sodium dithionite produces two different decomposed products based on the pH of the solution. Thiosulfate (S2O32-_{) is formed as a} decomposed product of sodium dithionite solution having pH range of (9-12.5) and sulphite (SO32-_{) is formed at pH 14. This showed that the aqueous solution of sodium dithionite thermally} followed two different chemical reaction mechanisms based on the pH of the solution.

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8. Recommendation and future work

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9. Acknowledgement

I am very thankful to my supervisors, Lennart Salmén, Jasna Stevanic Srndovic and Mikael Lindström for their guidance, support and fruitful discussions throughout the project. I appreciate that Mikael Lindstrom and Lennart Salmén gave me the opportunity to write my thesis at Innventia AB.

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Appendix 1: Scatterplots for each pH of sodium dithionite

solution using Simca

Multivariate analysis:

The multivariate data analysis of IR spectra was performed in SIMCA-14 1998-2015 (Umetrics AB). The methods used are principle component analysis (PCA).

PCA is a projection method that describes the systematic variation in a data table X with a few latent variables called scores (T). E is the residual variation in X. The weight of each variable in the latent variables called loadings (P). PCA is used to get an overview of spectral data, detect outliers and grouping of samples and also help to interpret some of the observed groupings. The appendix contains diagrams obtained using SIMCA-14 of alkaline sodium dithionite solution at each pH. This helps in clear understanding of the relationship between reponses’s Y of different groups within the same pH. The Figures 22 - 25 shows the grouping of experimental runs based on the experimental behaviour of sodium dithionite solution at each pH.

Figure 22: Scatter plot for pH 9 of wavenumber in the range of 4000- 525 cm-1

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Figure 23: Scatter plot for pH 11.5 of wavenumber in the range of 4000 - 525 cm-1

.

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Appendix 2: MODDE

Experimental design:

Experimental design is a methodology which allows the experimenter to systematically vary multiple factors with a context of one experimental design. The experimental design was mainly done on basing MODDE experimental design for designing the runs in the first place. The experimental runs are designed in order to reduce the number of experiments. Table 3 illustrates the design of run (a screening design: 4 factors, 1 midpoint, total 19 runs, 2 replicates). The results obtained are not enough to predict the stability of sodium dithionite solution. In order to study the behaviour of sodium dithionite more runs were added latter. Table 3: Experimental design using MODDE software.

Exp. no Run order pH Concentration(M) Time(min) Temperature(◘C)

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The significant factors affecting alkaline aqueous sodium dithionite solution decomposition under anaerobic conditions from MODDE experimental design can be seen in Figures 26 -29. The coefficient plot shows the coefficients related to scaled and centered variables. The plot expresses change in the response when the factors vary from low to high. Figures 26- 29 illustrate the significant effect of each factor on the stability of sodium dithionite solution at each pH.

Figure 26: MODDE Coefficients plot for pH 9, heating temperature (80 - 120 °C), heating time (10 - 30min) and concentration of the solution (0.2 M - 0.6 M).

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Figure 27: MODDE Coefficient plot for pH 11.5, heating temperature (80 - 120 °C), heating time (10 - 30 min) and concentration of the solution (0.2 M - 0.6 M).

Figure 28: MODDE Coefficients plot for pH 12.5, heating temperature (80 - 120 °C), heating time (10 - 30 min) and concentration of the solution (0.2 M - 0.6 M).

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Appendix 3: ATR-FTIR spectra of sodium dithionite solution at

each pH

Figure 30: FTIR spectra for pH 9 at different concentrations of the sodium dithionite solution (0.2 M - 0.6 M), heating time (10 - 30 min), and heating temperature (80 - 120 °C).

Figure 31: FTIR spectra for pH 11.5 at different concentrations of sodium dithionite solution (0.2 M - 0.6 M), heating time (10 - 30 min), and heating temperature (80 - 120 °C).

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Figure 32: FTIR spectra for pH 12.5 at different concentrations of sodium dithionite solution (0.2 M - 0.6 M), heating time (10 - 30 min), and heating temperature (80 - 120 °C).

Figure 33: FTIR spectra for pH 14 at different concentrations of sodium dithionite solution (0.2 M - 0.6 M), heating time (10 - 30 min), and heating temperature (80 - 120 °C).

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Appendix 4: IR frequencies of sulphur containing compounds

Table 4: IR frequencies of sulfur containing compounds:

Name of the compound Chemical formula Frequencies (cm-1)

Bisulphite HSO3- 3662 (OH stretching)

HS stretching 2450 to 2620, 1047, 1054,1154,1219 (SO stretching) 610, 368, 415, 526, 610. Meta sulphite S2O52- 966, 660-650, 568, 530-515, 450 Sulphite SO32- 1210, 1203, 1224 912 (S-O stretch)

Bisulphate HSO4- 1050 (symmetric SO3 stretch),

890-900 (symmetric S-OH stretch), 590-600 (symmetric SO3 deformation) 1200 (antisymmetric SO2 stretch) 400-420 (asymmetric SO3 deformation) 590-600 (asymmetric SO3 rock)

Sulphate SO42- 980 (symmetric SO stretch)

450 (asymmetric OSO deformation) 1090-1210 (triply degenerate SO stretch)

640 (triply degenerate OSO deformation)

1003,1139 (strong band)

Thiosulphate S2O32- 1196, 1022, 633, 525

1042 (S=O stretching) 995 (SO2 symmetric stretch) 1108 (S=O symmetric stretch) 1022 (S=O symmetric stretch)

Dithionite S2O42- 625

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Dithionate S2O62- 1205, 1050-1030, 640, 525, 460, 385

Trithionate S3O62- 1200-1250 (very strong)

1000-1050 (strong) 530-670 (weak –medium) 420-430 (very weak)

Tetrathionate S4O62- 1270-1000 (very sharp)

1000-1015 (sharp)

644, 610, 543, 532, 525 (weak – medium)

397, 390, 375, 303 (very weak)

Sulphur S 500-700 (weak)

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A study on the thermal stability of sodium dithionite using ATR-FTIR spectroscopy