ON THE INFLUENCE OF SODIUM HYPOCHLORITE ON WATER PROPERTIES

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1 SCIENTIFIC ANNALS OF ALEXANDRU IOAN CUZA DIN IAŞI UNIVERSITY Tomul I, s. Biomaterials in Biophysics, Medical Physics and Ecology 2008 ON THE INFLUENCE OF SODIUM HYPOCHLORITE ON WATER PROPERTIES Catalin Adomnitei 1 and Diana Mardare 1 KEYWORDS: water, viscosity, density, sodium hypochlorite. It is well-known that water is the main factor sustaining life on Earth. It is also a natural resource having a multiple role in the economical life. Nowadays the industry uses large amounts of water and then releases it with much different properties than the initial ones. This affects the environment and quality of life. The paper presents the influence of sodium hypochlorite on the viscosity and density of water. We have observed that water viscosity increases proportionally with the increase of sodium hypochlorite content. The temperature dependences of viscosity and density for the distilled water and the corresponding solutions are also investigated in the temperature range between 19 C and 70 C. 1. INTRODUCTION Water represents about 71% of the Earth crust, being considered one of the most precious natural resource that exists on our planet. It is also present in the living bodies, reaching 90% in some organisms. Without water, life on Earth would be non-existent. Water pollution is one of the world environmental problems which have the capabilities to disrupt life on our planet to a great extent. There is an increased number of international reviews, as well as international conferences in the field of water-related research activities, having as main purpose the improvement of the environment quality and, as a consequence, of the quality of life. It is pointed out that, even simple measurements can sometimes reveal something important about the water and the environment around it. Water pollution can be an indirect result of human activity but, unfortunately, most of the time it takes place consciously. There are many conferences that adopted strategies for water in the different seas to tackle the challenges and impacts of climate change. This Strategy has targets concerning the preservation of water quality and the reduction of pressure on water resources through more efficient use. The ministers drew the attention of governing bodies of the Union for the Mediterranean to the need to implement the new infrastructural projects in the domain of water as soon as possible, consistent with the Strategy [1]. In this paper, we have made a study on some properties of water (viscosity and density) modified by the presence, in different quantities, of sodium hypochlorite (NaClO). Water plays an important role in transporting the dissolved substances through the ground or through our bodies. So, the carbohydrates and proteins that our bodies use as food are metabolized and transported by water in the bloodstream. The main application 1 Faculty of Physics, Al.I. Cuza University, Carol I Blvd., No.11, , Iasi, Romania

2 38 C. Adomnitei and Diana Mardare of sodium hypochlorite is the generation of chlorine dioxide for textile bleaching or for municipal water treatment plants. In comparison with chlorine (more commonly used), it has the advantage that trihalomethanes are not produced from organic contaminants. NaClO also finds application as a component in therapeutic rinses, toothpastes, mouthwashes, chewing gums, etc. The temperature dependences of viscosity and density for the distilled water and the corresponding solutions were also investigated. It is known that water is used for cooling purposes in power plants that generate electricity. Then they release warmer water back to the environment. Temperature can influence the ability of water to hold oxygen and can decrease the ability of organisms to resist to certain pollutants. Change in viscosity can be responsible for the decline in the feeding rate at lower temperature. High viscosity shifted ingestion toward larger particles, which suggests that viscosity affects particle capture as well as rates of water processing. 2. EXPERIMENTAL We have prepared mixtures of water and comercial sodium hypochlorite in different proportions and we have measured their densities and viscosities in the temperature range C. For the density measurements we have used appropriate hydrometers, while for viscosity measurements, an Ubbelohde type viscometer was projected and realized (Fig. 1). Fig. 1 Experimental device for viscosity measurements. This type of viscometer is a suspended-level viscometer that uses a capillary based method of measuring viscosity. It consists in an u-shaped piece of glassware with a capillary (1) a measuring bulb (2) on one side and a larger tube (4) with a reservoir (3) on

3 ON THE INFLUENCE OF SODIUM HYPOCHLORITE ON WATER PROPERTIES 39 the other side. A liquid is introduced into the reservoir (3), then sucked through the capillary and measuring bulb (2). The liquid is allowed to travel back through the measuring bulb and the time it takes for the liquid to pass through two calibrated marks (a and b) is used in the viscosity measuring. The capillary ends with a reservoir (5). A third arm (6), extending from the end of the capillary (5) and open to the atmosphere is the one that makes this type of viscometer more advantageous than the Ostwald viscometer. In this way the pressure head only depends on a fixed height and no longer on the total volume of liquid. For the study of the viscosity as a function of the temperature, a transparent calorimeter C, containing water at a certain temperature, is used. The device, hanged vertically on a stand S, is introduced in the calorimeter. The studied liquid is introduced in the viscometer and, after approximately five minutes, while agitating continuously the water in the calorimeter (with the agitator A), we have considered that the thermal equilibrium was established. The temperature is indicated by the thermometer T. After that, the liquid in the viscometer is forced to ascend over the mark a, by closing the tap R, and then released to travel back through the measuring bulb. The time it takes for the liquid to pass through two calibrated marks (a and b) is measured with a chronometer. 3. RESULTS AND DISCUSSION From (Fig. 2) one can see that, at a certain temperature, water has the lowest density, sodium hypochlorite has the highest density and there is an increased density with the increased content of sodium hypochlorite in water. 1,02 1,01 1,00 ρ (g/cm 3 ) 0,99 O 0,98 O+25% NaClO 0,97 O+50% NaClO 0,96 O+75% NaClO 0,95 NaClO t ( 0 C) Fig. 2 The temperature dependence of densities of the studied liquids. The densities of water, sodium hypochlorite and of the corresponding solutions decrease with the increasing temperature between 19 C and 70 C. It can be observed that the following law is verified:

4 40 C. Adomnitei and Diana Mardare ρ 0 ρ =. (1) 1+ γt Here, ρ 0 is the liquid density at 0 C, and γ is the coefficient of cubical expansion. The law (1) is followed by all the liquids, less water in the temperature range 0-4 C. An explanation of this behaviour of the studied liquids in the mentioned temperature domain, is certainly related to the hydrogen bounding between these molecules. In water molecule, one atom of oxygen is bounded to two atoms of hydrogen in a such a way that it forms a permanent electrical dipole, having a positive charge on the side where the hydrogen atoms are, and a negative charge on the other side, where the oxygen atom is. The side with the positive charge (hydrogen atoms) attracts the negative charge (oxygen side) of a different water molecule and hydrogen bonds form. With the increasing temperature over 4 C (for water), the thermal agitation determines some of the hydrogen bounding to break, having as effect the decrease in the density. In the study performed in this paper on the viscosity, we have used the Poiseuille equation for the laminar flow of the liquids through a capilary [3,4]: η=kρτ, (2) with the appartus constant: 4 πghr K =. (3) 8lV Here, ρ and V are the density and respectively the volume of the liquid that flows into the capillary, τ is the time of flowing, R and l are the radius and respectively the length of the capillary, g is the gravitational acceleration and h is the height of the liquid column that produces the pressure that determines the flow of the liquid through the capillary. By using a reference liquid (here the distilled water) we can avoid the knowledge of K. By applying eq. (2) for the studied liquids and for the reference liquid (with index 1), we obtain the relation used for the determination of the viscosity: τ ρ η = η. (4) 1 τ ρ 1 1 An equation that successfully applies to molecular or ionic liquids and describes the variation of the viscosity with temperature, is Arrhenius-Guzman equation: η W K BT = η0 e, (5) where η 0 is the viscosity at relative high temperatures, W is the activation energy of viscous flow, k B is Boltzmann s constant, and T is the absolute temperature of the studied liquid. The values η 0 and W are specific for each liquid. From (Fig. 3) one can observe that water viscosity increases with the increasing of hypochlorite concentration at the same temperature, which can be explained by a better organization of water molecules in these solutions. On the other hand it can be observed

5 ON THE INFLUENCE OF SODIUM HYPOCHLORITE ON WATER PROPERTIES 41 that the viscosity is inversely related to temperature (Fig. 4), which can be explained by the weakening of the intermolecular bounds in the studied liquids η (cp) Concentration (%) Fig.3 Viscosity as a function of sodium hypochlorite concentration. The dependence ln η = f(1/t) represent a straight line with the slope W/k B and the free term, ln η 0 (see Table 1). lnη = lnη + 0 W K B 1. (6) T η (cp) O O+25% NaClO O+50% NaClO O+75% NaClO NaClO t ( 0 C) Fig.4 The dependence of the viscosity of studied liquids as a function of temperature.

6 42 C. Adomnitei and Diana Mardare ln[η(μp)] 9.6 O O+25% NaClO O+50% NaClO O+75% NaClO NaClO / T (K -1 ) Fig.5 The dependence of the viscosity of the studied liquids as a function of inverse temperature. Table 1. Parameters η 0 and W for the studied liquids. System η 0 (kg m -1 s -1 ) W (J) O O + 25% NaClO O + 50% NaClO O + 75% NaClO NaClO CONCLUSIONS A viscometer (Ubbelohde type) was projected and realized for the study of the viscosity of water, sodium hypochlorite and of different mixtures of sodium hypochlorite and water. We have observed that, by increasing the sodium hypochlorite content, the viscosity of the corresponding mixtures increases. The viscosity and the density of water and hypochlorite mixtures decrease with the increasing temperature in the range 19 C- 70 C. REFERENCES 1. Strategy on Water - Brussels News Update, European Physical Society and European Association for Chemical and Molecular Sciences February, 2009 issue, p Diana Mardare, Introducere în fizica mediului şi ecologie, Editura Politehnium, Iaşi, V. Georgescu, M. Sorohan, Fizică moleculară şi căldură, Ed. Univ. "Al.I.Cuza" Iasi, G.I. Rusu, Mihaela Rusu, M. Sorohan, Fizică moleculară şi căldură. Lucrări practice, Univ. Al.I.Cuza, Iaşi, 1986.