BASICS OF ENVIRONMENTAL CHEMISTRY

LAB.2

BASIC WATER PARAMETERS

GROUP _ CHEMICAL TECHNOLOGY

FUEL&ENERGY DEPARTMENT

Report prepared by: Małgorzata Nowak, Paulina Pielesz, Sebastian Wyczałek.

Tutor:

1. THEORETICAL PART

Our goal was to analyse few (chosen by tutor) water parameters: odour, pH, Eh and electrolytic conductivity of all samples.

First parameter was odour.

It is an important indicator of water quality. The sample's smell can tell us what kind of substances are we dealing with. Some of them have very characteristic smells.

Next paramter was pH.

pH is a measure of the acidity or basicity of a solution. It approximates but is not equal to p[H], the negative logarithm (base 10) of the molar concentration of dissolved hydrogen ions (H⁺). Pure water itself is a weak acid, dissociating to produce a pH of 7, or 0.0000001 M H⁺. For an aqueous solution to have a higher pH, a base must be dissolved in it, which binds away many of these rare hydrogen ions. Hydrogen ions in water can be written simply as H⁺ or as hydronium (H₃O⁺) to account for solvation, but all describe the same entity.

Hydrogen ion activity coefficients cannot be measured directly by any thermodynamically sound method, so they are based on theoretical calculations. Therefore the pH scale is defined in practice as traceable to a set of standard solutions whose pH is established by international agreement. Primary pH standard values are determined by the Harned cell, a hydrogen gas electrode, using the Bates-Guggenheim Convention.

pH-dependent plant pigments that can be used as pH indicators occur in many plants, including hibiscus, marigold, red cabbage (anthocyanin), and red wine.

The pH of seawater is very important and there is evidence for ocean acidification. Distinct pH scales exist depending on the method of determination.

The pH of different cellular compartments, body fluids, and organs is usually tightly regulated in a process called acid-base homeostasis. The pH of blood is usually slightly basic with a value of pH 7.4. This value is often referred to as physiological pH in biology and medicine. Plaque can create a local acidic environment that can result in tooth decay by demineralisation. Enzymes and other proteins have an optimum pH range and can become inactivated or denatured outside this range. The most common disorder in acid-base homeostasis is acidosis, which means an acid overload in the body, generally defined by pH falling below 7.35.

Eh determinates redox potential of a sample.

It is a measure of the tendency of a chemical species to acquire electrons and thereby be reduced. Reduction potential is measured in volts (V), millivolts (mV), or E_h (1 E_h = 1 mV). Each species has its own intrinsic reduction potential; the more positive the potential, the greater the species affinity for electrons and tendency to be reduced.

In aqueous solutions, the reduction potential is a measure of the tendency of the solution to either gain or lose electrons when it is subject to change by introduction of a new species. A solution with a higher (more positive) reduction potential than the new species will have a tendency to gain electrons from the new species (i.e. to be reduced by oxidizing the new species) and a solution with a lower (more negative) reduction potential will have a tendency to lose electrons to the new species (i.e. to be oxidized by reducing the new species). Just as the transfer of hydrogen ions between chemical species determines the pH of an aqueous solution, the transfer of electrons between chemical species determines the reduction potential of an aqueous solution. Like pH, the reduction potential represents an intensity factor. It does not characterize the capacity of the system for oxidation or reduction, in much the same way that pH does not characterize the buffering capacity.

Because the absolute potentials are difficult to accurately measure, reduction potentials are defined relative to a reference electrode. Reduction potentials of aqueous solutions are determined by measuring the potential difference between an inert sensing electrode in contact with the solution and a stable reference electrode connected to the solution by a salt bridge. The sensing electrode acts as a platform for electron transfer to or from the reference half cell. It is typically platinum, although gold and graphite can be used. The reference half cell consists of a redox standard of known potential. The standard hydrogen electrode (SHE) is the reference from which all standard redox potentials are determined and has been assigned an arbitrary half cell potential of 0.0 mV. However, it is fragile and impractical for routine laboratory use. Therefore, other more stable reference electrodes such as silver chloride and saturated calomel (SCE) are commonly used because of their more reliable performance.

Although measurement of the reduction potential in aqueous solutions is relatively straightforward, many factors limit its interpretation, such as effects of solution temperature and pH, irreversible reactions, slow electrode kinetics, non-equilibrium, presence of multiple redox couples, electrode poisoning, small exchange currents and inert redox couples. Consequently, practical measurements seldom correlate with calculated values. Nevertheless, reduction potential measurement has proven useful as an analytical tool in monitoring changes in a system rather than determining their absolute value (e.g. process control and titrations).

Eh and pH are necessary to calculate the Clark factor (rH) which tells us if a sample represents reduction or oxidation water environment.

The last parameters we focused on was Conductivity.

The conductivity (or specific conductance) of an electrolyte solution is a measure of its ability to conduct electricity. The “SI” unit of conductivity is siemens per meter (S/m). Conductivity measurements are used routinely in many industrial and environmental applications as a fast, inexpensive and reliable way of measuring the ionic content in a solution. For example, the measurement of product conductivity is a typical way to monitor and continuously trend the performance of the water purification systems.

In many cases, conductivity is linked directly to the total dissolved solids (T.D.S.). High quality deionized water has a conductivity of about 5.5 μS/m, typical drinking water in the range of 5-50 mS/m, while sea water about 5 S/m (i.e., sea water's conductivity is one million times higher than deionized water).

Conductivity is traditionally determined by measuring the AC resistance of the solution between two electrodes. Dilute solutions follow Kohlrausch's Laws of concentration dependence and additivity of ionic contributions. Onsager gave a theoretical explanation of Kohlrausch's law by extending Debye-Hückel theory.

The “SI” unit of conductivity is S/m and, unless otherwise qualified, it refers to 25 °C (standard temperature). Often encountered in industry is the traditional unit of μS/cm. The values in μS/cm are higher than those in μS/m by a factor of 100. Occasionally a unit of "EC" (electrical conductivity) is found on scales of instruments: 1 EC = 1 μS/cm. Sometimes encountered is so-called mho (reciprocal of ohm): 1 mho/m = 1 S/m.

The electrical conductivity of a solution of an electrolyte is measured by determining the resistance of the solution between two flat or cylindrical electrodes separated by a fixed distance. An alternating voltage is used in order to avoid electrolysis. Typical frequencies used are in the range 1-3 kHz. The dependence on the frequency is usually small. The resistance is measured by a conductivity meter.

A wide variety of instrumentation is commercially available. There are two types of cell, the classical type with flat or cylindrical electrodes and a second type based on induction. Many commercial systems offer automatic temperature correction.

2.EXPERIMENTAL PART

ODOUR ANALYSIS

Instruments used: conical flask with a tight lid, cylinder and thermometer.

The task was to smell the sample. If some odour was present we had to check

after how many dilutions the smell was intangible.

Samples A,B,D,E had no perceptible odour. Sample C (sewage) indicated quite

strong smell of sulphides. It disappeared after 3 dilutions.

ACIDITY ANALYSIS (pH)

Instruments used: beaker, pH-meter equipped with glass electrode and

thermosensor, wash bottle with distilled water (DW), tissue paper.

The pH of natural waters depends on many factors but is in range of

pH= 6,5 - 9. First of all pH-meter must be calibrated (what tutor did). We poured the beaker with 50ml of the sample, immersed the electrode in the sample and read the result. Of course electrode should be rinsed with DW and after that with sample, before every measurment. We did the measurment 3 times for every sample. We put together results in the Table and Graph no.1

Graphs contain not only our results (Serie 1 - blue) but also another group's results (Serie 2 - red).

Tab. no.1

	A(1)	B(2)	C(3)	D(4)	E(5)
Mean pH	7,55	7,77	6,82	7,9	7,51

Samples A,B,D,E showed a little basic reaction. Only sample C showed a little acid reaction. The highest pH result belongs to sample D (Busko zdrój).

DETERMINATION OF REDOX POTENTIAL (Eh)

Instruments used: beaker, Eh-meter equipped with Pt electrode, (DW), tisse paper.

Eh meter was checked (by tutor of course) with standard Eh solution. We proceeded similarly as in pH analysis (repeatedly rinse electrode, immerse Eh sensor in sample). Measurment was done 3 times. We also read the temperature of analysed water. The mean Eh values of samples are in Table and Graph no.2

Tab. no.2

	A	B	C	D	E
Eh [V]	0,260	0,228	0,035	0,196	0,260
Temp. ^°C	21,7	21,7	22	21,6	21,5

Result in sample C is very different than in other samples. The Eh is very low in sewage.

Than we had to take into consideration the reference electrode.

0 Ref_el C D B A,E [V]

Ref_el - reference electrode ( 0,206V)

All results must be enlarged by 0,206 to aquire results refered to “0”.

Eh_A = 0,466 [V]

Eh_B = 0,434 [V]

Eh_C = 0,241 [V]

Eh_D = 0,402 [V]

Eh_E = 0,466 [V]

We calculated the Clark factor in each water according to this formula:

rH = (Eh + 0,06*pH)/0,03

Table no.3 and Graph no.2 show the results.

Tab. no.3

	A	B	C	D	E
rH	30,63	30,01	21,67	29,2	30,55

The results are pretty obvious. If rH<15 than the sample has reduction properties. If rH>25 than the sample has oxidation properties. Samples A,B,D,E indicates clearly oxidation water envoirments, all their rH parameters are higher than 25. rH_C is between 15 and 25 so this sample's redox potential is not so evident.

DETERMINATION OF ELECTROLYTIC CONDUCTIVITY

Instruments used: beaker, conductometer equipped with Pt electrode and thermo-sensor, DW, tissue paper.

Our tutor checked the proper settings of the device. Again we followed pretty much the same steps as in previous measurment. Difference is that this analysis took more time. W had to wait for a stable reading. We repeated the measurment 3 times. Table no.4 and Graph no.3 contain mean values(related to 25^°C) for each sample.

Tab. no.4

	A	B	C	D	E
ϰ [ms/cm]	2,160	0,620	0,624	0,746	0,371

The highest conductivity has sample A (water from river Vistula), which means that this water has the highest concentration of dissociated ions. It is quite obvious that water from sample E has the lowest conductivity. This water went through a sewage treatment plant.

3.CONCLUSIONS

To interpret our results in proper way we refered to the Polish laws and data from MPWiK Krakow.

Sample A (Vistula river) has no sensible odour and normal pH, but conductivity is very high (more than 2 mS/cm) and we must clasify it as “V” class water. Water from this class cannot be drinked by people.

Samples B (Rudawa river) and D (Busko zdrój) have similar results as sample A except conductivity. In this case this parameter is much lower (between 500 and 1000) and we can clasify both of them as II class water.

In case of sample C pH level was normal ( between 6,5 and 9). Of course it cannot be drinked ;).

Sample E according to the results is I class water, pH normal, no odour, and conductivity less than 0,5 mS/cm.

Differences between other group's results and ours are very small and the come off incidential errors.

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