Experiment on Amount of Dissolved Oxygen in Water

Research

Water covers 70 percent of the earth and made up of 55% to 78% of the human body; it is the most abundant compound on the planet. It’s exists in liquid, solid, and gaseous states in nature; at room temperature, it’s a tasteless, odorless and colorless liquid (Water structure and behaviour, 2014). Water in chemical formula is H₂O, two hydrogen atoms each share one electron with the oxygen atom to form two covalent bonds and make a water molecule. The outer shell of oxygen is now full with 8 electrons and remains stable; thereby it won’t further react with other hydrogen or oxygen atoms (graph 1). One of the important features of water molecule is that it forms and angle with hydrogen atoms at the tips and the oxygen atom at the vertex. The angle formed by oxygen and hydrogen atoms isn’t a typical tetrahedral angle of 109 degrees, because the presence of two lone pairs on the oxygen molecule pushes the hydrogen molecules closer together away from the lone pairs. This results in a smaller bond angle of 104.5 degree (Graph 2). The reason that water is primarily a liquid under standard condition is because that oxygen is more electronegative than hydrogen; hydrogen is attracted strongly by oxygen results in the side of the oxygen atom is partial negative charge, while the hydrogen end is partially positive. This kind of molecule with a separation of electrical charges is called a dipole meaning two poles. The charge difference allows water molecules to be attracted to each other and difficult to be separated, forming one single body of liquid. Each water molecules bond to maximum of four other water molecules with two hydrogen atoms accepted and donates another two hydrogen atoms to others (Graph 3). This type of bond is identified as hydrogen bond occurs when hydrogen atom bonds with oxygen, fluorine or nitrogen atom. Hydrogen bonding functions as two magnets stick together due to the opposite pole attraction. It is stronger than normal dipole forces between molecules. The force is categorized as intermolecular force, a force act between neighboring particles (Hydrogen Bonding, 2013). The extensive hydrogen bonding between water molecules is responsible for physical properties in water, including property of high melting point, boiling point since more energy is required to break hydrogen bonds between molecules. Water will be in gas, if the intermolecular force (force that holds water molecules together) is less than the thermal force that pushing the molecules apart; while, the boiling point is determined as a point between behaving as a liquid and gas (Intramolecular Force, 2013). Another relevant property is that water has a very high specific heat capacity, due to the presence of extensive hydrogen bonding between water molecules; a large amount of energy (heat) is absorbed in breaking the bond (Specific Heat Capacity of Water, 2014).

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Water has a high solubility, due to it’s a polar molecule with both positive and negative charges. Dissolving occurs when water bonds and separates the anion and cation of a substance. Consider putting an ionic compound into water, the individual ions react with the opposite polarity regions of the water molecules with their ionic bonds broken. This process is called dissociation, where ions are formed with atoms breaking down from a molecule. The positively charged end of the water (H⁺) surrounds the negative ion of the ionic compound and the negatively charged end of the water (O^2-) surrounds the positive ion of the ionic compound, this is called as a sphere of hydration, which separates the particles. This is the reason why water cannot dissolve non-polar molecule, because the polarity has no effect on a non-polar molecule (solubility, 2000).

Abstract

In this experiment, potassium thiosulfate was used to titrate water sample collected in wetland at school; in order to simulate and determine the amount of oxygen dissolved in a normal river. The method used is called as Winkler Method, which involves iodometric titration; the amount of oxygen in the sample is determined indirectly via iodine. The method is considered to be the most reliable and precise method for DO analysis. DO presents in the river is relevant to the industry, river aquaculture, for the sake of keeping the oxygen level in a suitable range for aquatic animals to survive. There are a number of variables that affect DO presents in the river, temperature is the most crucial factor that should be taken into account. Since river’s temperature varies throughout the year with the change in the weathers or seasons. The temperature difference between summer and winter can be up to 10 degrees Celsius. Hence the experiment will focus on the effect of different temperatures on the amount of DO in the water sample. Theoretically, the amount of dissolved oxygen in the water is inversely proportional to the water temperature (table1). It’s clearly shown in the graph that as the temperature rises, the solubility of oxygen in the water decreases (graph 4). It’s recommended that DO analysis experiment should be done within the same day. DO level in the water source, where sample water is collected, may alter dramatically at different time. It then consequently becomes a factor that affects the final result.

Introduction

Dissolved oxygen in water is defines as the amount of oxygen molecules that physically distributed in water. Notice that oxygen does not chemically react with water, since oxygen is a non-polar gas; the intermolecular force of hydrogen bond in water molecule itself is stronger than the induced dipole attraction between the polar bond in water and the nonpolar bond gas, oxygen. Water molecules will rather remain hydrogen bonded to each other, then to allow a non-polar molecule (oxygen) to come between them (Aquaculture, 2000).

There are three ways that oxygen gets in the water. First of all, it’s the difference of the concentration of oxygen in the air and the concentration of oxygen in the water causes diffusion to occur. Oxygen flows from high concentration (air) to low concentration (water). Secondly, it’s the partial pressure and surface area that causes oxygen to shift through the water. For instance, in a river that flows rapidly, water turbulence increases the surface area of water for oxygen to diffuse across. Churning also allows air to hit water at a high pressure, allowing more oxygen to diffuse into the water. At last the presence of water plants produce oxygen in the water by doing the process of photosynthesis (How does oxygen get into the water?, 1999).

Nowadays, commercial aquaculture is growing worldwide except in Africa. Fish and other aquatic organisms are a source of protein for human to intake. However, continues to harvest wild sources of fish will result in overfishing or even the loss of those aquatic species entirely. Aquaculture not only meets the human demand, but also allows wild species to breed and maintain the number of the wild aquatic species. There are a couple of aspects to look at in the aquaculture industry. The amount of dissolved oxygen in the water is relevant to maintain the water quality for fish and aquatic species to grow. Oxygen is important in respiration and metabolism processes in any animal. Particularly for fish, the metabolic rate is highly related to the oxygen concentration in water. For cold-water fish, the minimum DO requirement is 6mg per litre; for both tropical freshwater fish and tropical marine fish, it’s 5mg per litre. Once the level of DO is lower than the minimum requirement, fish may be affected by tissue hypoxia, their swimming activities will decrease and their immunity to diseases will also reduce (Aquaculture, 2000).

It’s obvious that fish and aquatic species rely on the DO level in the water to survive. Fish farmers should realize all the factors that affect the amount of dissolved oxygen in the water; temperature is one of the main factors. The solubility of any gases is dependent on the temperature. At a high temperature, the solubility of a gas in the water is low, while at a lower temperature the gas solubility in the water is relatively higher (graph5) (The Effects of Temperature on the Solubility of Gases in the Universal Solvent (Water), 2009). As the temperature rises, the kinetic energy of the gas particles physically distributed in the water increases. This results in a more intensive gas particles’ motion, which allows intermolecular bonds between water molecules and gas molecules to be broken causing gas to escape into the atmosphere (Temperature and Pressure Effects on Solubility , 2003).

The experiment consists of three parts, potassium iodate standardization, reagent blank determination and sample analysis. Begin with the KIO₃ standardization, the exact concentration of potassium iodate is unknown, hence it’s titrated by sulfuric acid solution to determine it’s actual concentration. Reagent blank determination is done in order to minimize the error in the experiment. During the experiment, due to environment factors and contaminations, the result may be affected. Water is often used as a blank reagent and it’s responsible for determining the side effects on the final result. When doing the blank determination, water undergoes the same process as the sample. The value acquired is the blank value; it is then subtracted from the sample’s result. Once the processes of standardization and blank determination have been done, water sample can now be analyzed.

The chemical reactions are as follows: 1) Manganese chloride reacts with sodium hydroxide to give a white perciptate of manganous hydroxide. MnCl₂ + 2NaOH –> Mn(OH)₂ + 2NaCl 2) The presence of oxygen in the water sample reacts with manganous hydroxide, manganic basic oxide is then formed.

2Mn(OH)₂ + O2 –> 2 MnO(OH)2 3) Manganic basic oxide is dissolved by sulfuric acid, and immediately reacts with sodium iodide to yield iodine. 2MnO(OH)2 + 4 H2SO4 –> 2Mn(SO4)2 + 6H2O

2Mn(SO4)2 + 4 NaI –> 2Mn SO4 + 2Na2SO4 + 2I2

4) Sodium thiosulfate is used to titrate iodine with the indicator, starch, to the end point. 4Na2S2O3 + 2I2 ->2Na2S4O6 + 4NaI

The titer determines the amount of oxygen dissolved in the water sample with one mole of O₂ reacts with 4 moles of sodium thiosulfate. The amount of oxygen concentration can be calculated from the above analysis with the following formula:

Dissolved oxygen = (mL/L)

Notice that the unit is one milliliter of oxygen per litre of water, which can also be transferred into mg per litre.

Hypothesis

It’s predicted that the lower temperature will result in a higher solubility of oxygen in water. Under the condition of high temperature, oxygen gas forms weak molecular bonds with the water molecules. As a result, oxygen molecules will rise towards the surface and escape to the atmosphere, consequently reducing the amount of oxygen gas dissolved in the water. The solubility of oxygen is compared between cold river water and heated river water within this experiment, whilst other variables are kept constant; including the size of the beaker, the location where river water is retrieved from and the solution used for titration.

Materials

• BOD bottle
• Deionised water
• Standard Potassium Iodate
• Sulfuric acid solution
• Sodium iodide-sodium hydroxide reagent
• Manganese chloride reagent

Method

KIO3 Standardization (Titration)

1. Label a BOD bottle with “KIO3 Standard”
2. 15ml of deionised water added into the BOD bottle
3. 10ml of standard potassium iodate with concentration of 0.00167M added into the BOD bottle with a pipette
4. Swirl to mix
5. 1 ml of 50% sulphuric acid solution added into the BOD bottle
6. 1ml of sodium iodide-sodium hydroxide reagent added into the BOD bottle and swirl
7. 1ml of manganese chloride reagent added into the BOD bottle and mix thoroughly
8. Fill the BOD bottle with deionised water to the neck, and mix the solution by inverting the bottle a few times (BOD bottle now contains KIO₃ standard solution, it’s identified as KIO₃ standard bottle)
9. Discard 50mL of KIO₃ standard solution by using a 50mL volumetric pipette
10. Add 50mL of KIO₃ standard solution into a clean 100ml conical flask by using a 50mL volumetric pipette
11. Set up the burette and fill it with sodium thiosulfate working solution
12. Place a white tile under the burette with the conical flask that contains KIO₃ standard solution on top
13. Start titrating sodium thiosulfate into the conical flask until the solution turns a pale yellow colour, stop titrating
14. Three drops of starch solution added to the flask, then continues to titrate until the solution turns colourless
15. Volume of sodium thiosulfate added recorded
16. Repeat the process until three readings within 0.05mL of each other acquired

Reagent Blank determination – Titration

1. Add 15mL of deionised water into a 250mL conical flask
2. 1mL of KIO₃ standard solution added to the conical flask
3. 1mL of 50% sulphuric acid added to the flask
4. 1mL of sodium iodide-sodium hydroxide added to the flask
5. 1mL of manganese chloride reagent added to the flask

Appendix

Table1: (The Winkler Method – Measuring Dissolved Oxygen, 2013)

Graph 1: (Water’s Influence on Temperature, 2013)

Graph 2: (WATER: DESIGNED FOR LIFE, 2013)

Graph 3: (Hydrogen bond, 2014)

Graph4: (Lecture – Water Chemistry — Dissolved Gases — Oxygen)

Graph5: (Why oxygen dissolved in water is important , 1998)