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Formation and Stability of Effervescent Sodium Perborate Joana Nunes de Carvalho,† Jamie A. S. Cleaver,*,† and Igan Hayati‡ Chemical and Process Engineering, School of Engineering, University of Surrey, Guildford GU2 7XH, U.K., and Borax Europe Ltd., 1A Guildford Business Park, Guildford, Surrey GU2 8XG, U.K.
Sodium perborate in its effervescent form (ESP) has the ability to release both gaseous and active oxygen in aqueous solution. Gravimetric analysis and direct measurement of gaseous oxygen have been used to investigate the ideal conditions for the formation of ESP from sodium perborate monohydrate (PBS1). A dehydration temperature of 125 °C gave the maximum yield of effervescent oxygen, with the mass loss corresponding closely to 1 mol of water/mol of PBS1. In stability studies, freshly formed ESP was capable of generating 10.1 wt % effervescent oxygen. However, this value fell to only 1.35% of its initial value after exposure to ambient air (26 °C and 36% relative humidity), confirming the unstable nature of this material. ESP stability was improved by screening its surface with a coating of fine boric acid powder. Further work on the coating of ESP is recommended in order to further improve its stability. Introduction Sodium perborate has two common and stable forms, the tetrahydrate (NaBO3‚4H2O) and the monohydrate (NaBO3‚H2O). Both are white, crystalline solids. These are manufactured on a large scale for use in the detergent industry because they both release active oxygen (AvOx) in aqueous solution. One mole of atomic oxygen is theoretically available as AvOx for each mole of sodium. The tetrahydrate, therefore, has a theoretical AvOx release of 10.4%, while the monohydrate can theoretically release 16%. Sodium perborate tetrahydrate (PBS4) readily dehydrates to sodium perborate monohydrate (PBS1) at around 60 °C according to the following reaction:
NaBO3‚4H2O(s) T NaBO3‚H2O(s) + 3H2O(g) In 1921, Foerster1 was the first to find that careful heating of PBS1 produced anhydrous sodium perborate, which would effervesce when placed in water, releasing gaseous oxygen and AvOx. The dehydration of PBS1 can be represented by
NaBO3‚H2O f NaBO3 + H2O The anhydrous material is termed effervescent sodium perborate (ESP). The effervescent properties of ESP make it potentially attractive for a number of applications. For example when mixed with conventional detergents in a tablet formulation, the effervescent properties of ESP could aid dispersion of the tablet in water. The traditional method to promote effervescence or “fizzing” relies on a mixture of citric acid and sodium bicarbonate, which releases bubbles of carbon dioxide when placed in water. This method has found application in many areas such as dental cleaning. There may be a number of applications such as this in which ESP * To whom correspondence should be addressed. Tel.: +44 (0)1483 686598. Fax: +44 (0)1483 686581. E-mail: j.cleaver@ surrey.ac.uk. † University of Surrey. ‡ Borax Europe Ltd.
could be a viable alternative. It has the advantage of providing effervescence from a single solid component. In addition, borate is an intrinsic cleansing agent; therefore, the presence of ESP in a cleaning product would be beneficial.1 The effective use of ESP for its effervescence depends on optimization of the formation conditions and characterization of its stability. This paper reports on an experimental study to identify the optimum formation conditions and to examine its stability in terms of its effervescent behavior with respect to the storage time and relative humidity (RH). Literature Survey There is little reported in the literature regarding ESP. The majority of the previous work focuses on the tetrahydrate (PBS4) and monohydrate (PBS1) for their ability to release AvOx in the form of hydrogen peroxide.2 The formation and properties of NaBO3 (ESP) have been investigated by few researchers. Taylor and Taylor3 reported on the quantity of gaseous oxygen released as a function of the preparation temperature of ESP over the range of 60-250 °C and showed that a maximum value of 4.9% was produced at a temperature of 140 °C. However, their next-lowest production temperature was 110 °C, which yielded 1.8% oxygen. In a thorough study of the reaction mechanisms of ESP and similar compounds, Edwards et al.4 stated that ESP is prepared by carefully heating PBS1 at a temperature range between 100 and 130 °C. They commented that if heating is carried out rapidly and/or at high temperature, a vigorous exothermic decomposition of PBS1 will occur giving sodium metaborate, steam, and oxygen as follows:
2NaBO3‚H2O f 2NaBO2 + 2H2O + O2 Edwards et al.4 commented that the production of ESP from PBS1 does not depend on the presence or absence of oxygen in the surrounding air. They also found by X-ray analysis that some unconverted monohydrate remained even in high-yield samples of ESP and that the remaining material was amorphous in
10.1021/ie040222w CCC: $30.25 © 2005 American Chemical Society Published on Web 06/09/2005
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structure. The mass percentage yield of effervescent oxygen from their formulations ranged from 2.2% to 10.0% based on the mass of O2 per unit mass of ESP. Of particular interest to this specific study is the stoichiometry of the reaction between ESP and water. Edwards et al.4 and Bruce et al.5 commented that the release of gaseous and AvOx by ESP in water makes it similar in behavior to a group of compounds known as “superoxides”. These release gaseous oxygen and AvOx in a ratio of 1:1. However, Edwards et al.4 found that this ratio is not always in the proportion of 1:1 for ESP and that the percentage of gaseous O2 can be considerably larger. They hypothesized that the extra oxygen arises from molecular oxygen that is trapped within the interstices of the solid. They backed up their hypothesis with magnetic susceptibility and electron spin resonance data. One of the main aims of this study is to examine the stability of ESP in terms of its ability to effervesce after exposure to air at various humidities for differing lengths of time. No mention of ESP stability has been found in the literature. It, therefore, appears that this current study is unique.
Figure 1. Drying curve of PBS1 at 20 °C and 0% RH measured in the IGAsorp.
Experimental Methods The investigations into the optimum formation conditions and stability were conducted using a dynamic gravimetric analyzer for water sorption. The specific instrument is an IGAsorp supplied by Hiden Isochema Ltd. A small mass of material (typically 50-100 mg) is loaded into a mesh bucket that is suspended on a balance wire. The sample hangs in a chamber in which the temperature and RH are accurately controlled. The IGAsorp can be programmed to subject a sample to specific humidities and temperatures and to record the resulting mass change. It is, therefore, an ideal instrument to study both the formation and stability of ESP. To supplement the IGAsorp studies, ESP was produced in a laboratory oven at various temperatures. The optimum formation and stability of ESP is associated with the degree of effervescence of the material. A method of characterizing the extent of oxygen release was developed using a very simple but effective approach. A 500-mL conical flask with a sealed neck was connected to a gas syringe having a capacity of 100 mL. The flask was filled with 300 mL of ultrapure water, and a fixed mass of ESP was added (∼0.5 g). The volume of oxygen generated was measured by displacement of the gas syringe. Traditional wet-chemistry techniques were used to determine the composition of PBS1 in terms of the amounts of sodium and boron present. Sodium as Na2O was present at a level of 34.67% by mass, and boron as B2O3 was present at 30.4% by mass. Given the formula of PBS1 to be 2[NaBO3‚H2O], the remaining mass of 34.93% should correspond to 2 mol of H2O2 and any unbound moisture in the sample. The corresponding molar concentrations of Na2O and B2O3 in the sample were 4.90 × 10-3 and 4.98 × 10-3 mol‚g-1, respectively. The sodium and boron are present in equimolar proportions as expected. Adopting a mean figure of 4.94 × 10-3 mol‚g-1 for the concentrations of Na2O and B2O3 gives a corresponding H2O2 content of 33.59% by mass. The quantity of unbound water in the sample can, therefore, be estimated to be 1.34 ((0.27)% by difference.
Figure 2. Influence of the temperature on the decomposition of PBS1 at 0% RH, measured in the IGAsorp.
Results and Discussion In the beginning of this study, PBS1 was dried at 20 °C and 0% RH for a 24-h period in the IGAsorp (Figure 1). At the end of this period, PBS1 lost 1.1% of its initial mass. However, it is apparent that the sample mass has still not reached an asymptote even after 24 h of exposure to dry air, suggesting that mass loss would continue if the experiment were allowed to run for more time. The value of 1.34% for the nominal unbound water content of the sample, as calculated above, seems reasonable. According to Taylor and Taylor3 and Edwards et al.,4 ESP is produced by dehydration of PBS1 in the temperature range of 100-180 °C, although no optimum conditions are recommended. To examine this dehydration gravimetrically, an undried sample of PBS1 was placed in the IGAsorp and the change in mass was studied at zero humidity as the temperature was steadily ramped from 20 to 180 °C at a rate of 0.1 °C‚min-1. Figure 2 shows the resulting percentage mass loss with temperature. The first 3.5% mass loss occurs gradually until a temperature of 120 °C is reached. This gradual decrease is due, in part, to the removal of the 1.34% unbound moisture. The remaining initial mass loss is likely to be due to gradual decomposition of the PBS1. Above a temperature of 120 °C, the mass loss decreases very sharply with an increase in the temperature as PBS1 rapidly decomposes to ESP. The percentage mass loss and the temperature for the same experiment are replotted as a function of time in
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Figure 3. Change in the temperature and corresponding change in mass with time for the decomposition of PBS1 at 0% RH, measured in the IGAsorp.
Figure 3. This gives an indication of the rapid drop in mass with respect to time for the sample that is initiated at around 120 °C. Note also that once the temperature of the sample reaches 180 °C, it takes some time for the mass to equilibrate. It would, therefore, be unwise to assume that the sample decomposition curve is at instantaneous equilibrium prior to 180 °C when the temperature is changing. The theoretical loss of 1 mol of water from the PBS1 sample is equivalent to a mass loss of 18.3%, given that relative molecular masses of PBS1 and water are 99.8 and 18 g‚mol-1, respectively. Assuming that the PBS1 sample contains 1.34% unbound water by mass, the final percentage loss in mass on a dry basis at 180 °C is 19.1%. This marginal excess in decomposition suggests that the material is undergoing slight further decomposition from ESP (sodium perborate, NaBO3) to sodium metaborate (NaBO2) and oxygen up to a temperature of 180 °C. From inspection of Figures 2 and 3, it is apparent that the transition of PBS1 to ESP is fully active at a temperature of 125 °C. This finding coincides with the range of temperatures of 100-130 °C quoted by Edwards et al.4 To investigate this further, a gravimetric test was performed in the IGAsorp to decompose PBS1 at a constant temperature of 125 °C. A slow ramp rate of 0.1 °C‚min-1 was again used, in accordance with the recommendation of Edwards et al.4 that “careful” heating should be used. The resulting mass loss data are shown in Figure 4, based on the initial, undried sample mass of PBS1 used for the study. Accounting for an unbound moisture content of 1.34% gives a mass loss on a dry basis of 15.4%. This is notably less than the 18.3% predicted by the theoretical loss of 1 mol of water/ mol of PSB1, suggesting that 0.16 mol of water/mol of PBS1 present remains in the sample. To meet the main objective of optimizing the release of effervescent oxygen from ESP, tests were conducted to measure the amount of oxygen released from ESP that had been produced at different temperatures. Four batches of ESP were produced in the oven from 8 g of PBS1 at respective temperatures of 115, 125, 135, and 145 °C for 1 h. Upon removal from the oven, the ESP samples were placed in a desiccator for 24 h, after which the amount of effervescent gas release was measured using the method described in the previous section. Table 1 shows the results of gas release in terms of volume per gram of ESP and the mass percent based
Figure 4. Decomposition of PBS1 at 125 °C measured in the IGAsorp. Table 1. Properties of ESP Batches Produced in an Oven at Different Temperatures preparation temperature (°C) gaseous oxygen (L‚g-1 of ESP) gaseous oxygen (% mass based on mass of ESP) mass loss during the preparation of ESP in an oven (g/g of PBS1)
115 125 135 145 0.003 0.065 0.051 0.0027 0.39 8.50 6.67 0.35 0.10
0.19
0.21
0.25
on the original ESP mass added. The corresponding loss in mass of the initial sample as a function of the production temperature is also shown. It is clear that a production temperature of 125 °C provides the highest release of effervescent oxygen. This is in agreement with the prediction based on the IGAsorp dehydration study reported above. Note that the volume release of 0.065 L‚g-1 at 125 °C corresponds to a molar release of 0.22 mol of oxygen/mol of ESP, assuming that all of PBS1 had converted to ESP (NaBO3) with a molecular weight of 81.8 g‚mol-1. The mass loss figures in Table 1 are presented on the basis of the total mass of PBS1. However, this will contain 1.34% unbound water, as shown above. If the unbound water is removed from the calculation, the mass fraction loss for the sample produced at 125 °C corresponds to 0.179 on a dry basis. This agrees very closely with the theoretical loss of 1 mol of water from PBS1. Having identified a reasonable optimum temperature for the formation of ESP, a series of experiments were conducted to examine the stability of the material when exposed to various levels of RH for various times. The IGAsorp is an ideal instrument for this study. ESP was produced from PBS1 in the IGAsorp by heating it to 125 °C at a slow ramp rate of 0.1 °C‚min-1 in dry air. Once ESP was formed, the temperature was dropped to 25 °C and the mass uptake was recorded for a given humidity. The experiment was repeated for a range of humidity values from dry air and 30% and 60% RH. The results of this study are shown in Figure 5. It is clear that ESP has a very strong tendency to adsorb water. Even at zero humidity (0.1% RH), there are trace amounts of water in the air stream that cause an increase of 1.2% in mass over the 16-h time period. Note that the uptake of water for ESP in 60% RH air passes through a maximum and then the sample loses mass. This is due to the spontaneous production of effervescent oxygen by the sample within the IGAsorp. Confirmation of this was obtained upon removal of the sample from the instrument at the end of the run. The granular
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Figure 5. Uptake of water with time by ESP at different humidities, measured in the IGAsorp.
Figure 6. Mass change of freshly prepared ESP as a function of time at 26 °C and 36% RH.
sample had changed to a paste. Examination under an optical microscope revealed several bubbles of between 1- and 10-µm diameter that were evolving. Similar bubbles were observed for the sample that had been exposed to 30% RH air. The mass gain of ESP that occurs upon exposure to humid air is expected to be linked to a loss in the effervescent capacity of the material. These associated effects were quantified as follows. A batch of ESP was produced from an initial mass of 8 g of PBS1, by placing it on a tray in the oven at 125 °C for 1 h. This sample was removed from the oven and placed in a thin layer on the pan of a five-place digital balance open to the laboratory air. The ambient temperature and humidity were 26 °C and 36%, respectively. The consequent change in mass was recorded over a period of 24 h. This is shown in Figure 6, expressed as a percentage mass change based on the initial mass of ESP. The procedure was repeated, under identical conditions, although on this occasion small samples of around 0.5 g (∼7.5% of the original mass) were extracted from the main ESP sample at certain times. Upon removal of each 0.5-g sample, the amount of effervescence was measured according to the method described above. The resulting data plotted in Figure 7 show a dramatic decrease in the effervescent capability of ESP with time. At the beginning of the test, the maximum oxygen yield of 77.1 mL/g of ESP corresponds to a mass percentage of 10.1%, which agrees with the maximum yield quoted by Ed-
Figure 7. Measured gaseous oxygen release per gram of ESP when introduced to water as a function of the time exposed to air at 26 °C and 36% RH.
wards et al.4 On a molar basis, the maximum yield is 0.258 mol of oxygen/mol of sodium, assuming that the solid at this point is pure ESP (NaBO3) having a molecular weight of 81.8 g‚mol-1. According to Edwards et al.,4 it is unlikely that the solid is pure ESP because a small amount of unconverted PBS1 is usually present. Assuming a PBS1 content of 5% by weight increases the oxygen yield to 0.272 mol/mol of ESP. Edwards et al.4 also proposed that the ESP solid retains some molecular oxygen that is trapped in the interstices of the material. This could explain the continued loss in mass that can be seen in Figure 6 during the first 20 min of the test. This gas would slowly escape from the solid by diffusion. However, the first test for effervescence was conducted before this gas had a chance to escape. It would, therefore, contribute to the volume of oxygen measured. From Figure 6, the proportion of trapped oxygen can be estimated to be around 1%. Adjusting the yield for this quantity gives a final value of 0.244 mol of oxygen produced/mol of ESP available. After exposure to the laboratory air, the yield of oxygen has decreased dramatically to 1.04 mL/g of sample, i.e., 1.35% of its initial value. It is, therefore, apparent that ESP is intrinsically unstable. The poor stability of ESP is a major drawback to its practical use. One possible way to improve its stability is to screen it by mixing it with a relatively hydrophobic material. We have selected boric acid for this purpose, using a sieve cut of below 250 µm to mix with ESP. Batches of ESP were produced in the oven at the optimum temperature of 125 °C. Once these had cooled, they were mixed with a measured quantity of boric acid fines by tumbling in a small sample tube. A representative sample was placed in the IGAsorp, and the material was subjected to a RH of 30% and a temperature of 25 °C. The resulting percentage increase in mass, based on the total mass of the sample, is shown in Figure 8. It is not clear from Figure 8 whether the reduction in the mass uptake with an increase in the boric acid content is due to hydrophobic screening or merely due to the dilution effect of boric acid. The percentage uptake of water has, therefore, been recalculated, based on the ESP content only, making the realistic assumption that boric acid itself adsorbs water negligibly compared to ESP. The resulting data shown in Figure 9 clearly indicate that boric acid has a screening effect on ESP, reducing the effective uptake of water by ESP from over
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Figure 8. Adsorption of water by ESP mixed with varying proportions of boric acid, expressed on a total mass basis.
Figure 9. Adsorption of water by ESP mixed with varying proportions of boric acid, based on the mass of ESP alone.
30% with no boric acid to only 5% when 40% of the mass is boric acid. Note that the boric acid content shown in the legends of Figures 8 and 9 is expressed on a total mass basis. Figure 10 shows a scanning electron microscopy (SEM) micrograph of an ESP particle with some fine boric acid particles on the surface. This particle was selected from a mixture of 40% total weight boric acid. Note that there is a poor distribution of boric acid on the surface, suggesting that there is naturally poor adhesion between the two components. The screening effect of boric acid or any other coating material would be enhanced if the adhesion could be increased, for example, by the addition of a binder. Figure 10 also shows large cracks in the ESP surface that are a feature of the dehydration process. Conclusions The formation conditions for ESP by dehydration of the monohydrate have been studied using a combination of gravimetric analysis and an assay of the quantity of gaseous oxygen evolved. A formation temperature of 125 °C proved to give maximum effervescence. At this temperature, gravimetric analysis confirmed the removal of 1 mol of water in agreement with theory. At lower temperatures, the dehydration was incomplete
Figure 10. SEM image of a ESP particle coated with boric acid fines at a nominal content of 40% by mass.
over the time scales studied. At higher temperatures, the effervescence was compromised by unwanted degradation of the sample. The stability of ESP with regard to its effervescence has been studied using the same techniques. The material is shown to adsorb water readily with the spontaneous release of gaseous oxygen. Freshly produced ESP has a capacity to produce 10.1% by weight of oxygen. The molar yield of oxygen per mole of available sodium has been estimated to be 0.244. This takes into account a 5% presence of PBS1 in the material and a mass fraction of 1% of molecular oxygen that is locked into the ESP solid structure. Evidence for the latter is shown in the initial decrease in mass when ESP is freshly formed, suggesting that the trapped oxygen is diffusing from the solid. After 24 h at ambient conditions, the effervescent capacity of ESP drops to 1.35% of its initial value, confirming that ESP is intrinsically unstable. An attempt to increase the stability of ESP was made by screening it with boric acid, which is relatively hydrophobic. Gravimetric tests showed that the technique has potential, although in this case the effect was thought to be compromised by the poor adhesion between boric acid and ESP. Further work is recommended in this area to engineer a more robust coating technique, thereby improving the stability of ESP to the point where its effervescent properties can be applied in practical situations. Literature Cited (1) Foerster, F. Z. Angew. Chem. 1921, 34, 354. (2) Baillely, G. M. (The Procter and Gamble Company). Effervescent Particle. U.K. Patent Application GB 2337054A, 1999. (3) Taylor, T. I.; Taylor, G. G. Vapour Pressure and dehydration of unstable salt hydrates, sodium perborate. Ind. Eng. Chem. 1935, 27, No. 6, 672. (4) Edwards, J. O.; Griscom, D. L.; Jones, R. B.; Watters, K. L.; Weeks, R. A. Some chemical and physical properties of the effervescent magnetic peroxyborates. The pseudo-superoxides. J. Am. Chem. Soc. 1965, 91, No. 5, 1095. (5) Bruce, R.; Edwards, J. O.; Griscom, D. L.; Weeks, R. A.; Darbee, L. R.; DeKleine, W.; McCarthy, M. Magnetic peroxyborates. J. Am. Chem. Soc. 1965, 87, No. 9, 2057.
Received for review August 16, 2004 Accepted April 20, 2005 IE040222W
