2.3 Total hardness of water

Water hardness is important to fish culture and is a commonly reported aspect of water quality.  It is a measure of the quantity of divalent ions (for this discussion, salts with two positive charges) such as calcium, magnesium and/or iron in water.  There are many different divalent salts; however, calcium and magnesium are the most common sources of water hardness.[ CITATION Wil04 \l 1033 ]. Water with high concentration of salts called hard water, with low concentration- soft water. Drinking water with high hardness can cause the formation of urinary stones, with low hardness- increases the risk of cardiovascular diseases. But, in general, concentration of salts in water affects on health quiet weak.

For measuring total hardness of water we used these apparatus and reagents: byrette, conical flask, spoon, magnetic stirrer, buffer solution with pH 10.0, Eriochrome Black T + NaCl as an indicator, EDTA 0,01 mol/l.

At first, we filled the flask by 50 ml of tap water and added 4ml of buffer solution an a half spoonful of indicator. Shook well and added EDTA-solution into the flask until the color of sample changes from red to violet. For calculating results we needed to measure consumption of EDTA-solution. After this we calculating total amount of calciumin (Ca) and magnesiumin (Mg) using following formula:

X=1000cV₁/V, where

X = Total amount of Ca and Mg (mmol/l)

c = concentration of EDTA solution (0,01 mol/l)

V₁ = volume of EDTA solution, ml

V = volume of the sample, ml

1000 = conversion factor mmol/mol

Using values, which we get during the laboratory work we have:

X=1000cV₁/V,

X=(1000*0,01*0,09)/50=0,18 mmol/l

Then, we did the same measuring using natural water as a sample and calculated result:

X=1000cV₁/V,

X=(1000*0,01*0,05)/50=0,1 mmol/l

We observed that total hardness of tap water equals to 0,18 mmol/l and natural water`s is 0,1 mmol/l.

2.4 Dissolved oxygen

Dissolved oxygen (DO) is the amount of oxygen that is present in the water. It is measured in milligrams per liter (mg/L), or the number of milligrams of oxygen dissolved in a liter of water.(ЕБАШЬ СЮДА ССЫЛКУ)

During this laboratory work our group used: byrette, flask, magnetic stirrer, manganese(II) solution, alkaline iodide reagent, sulphuric acid, sodium thiosulphate solution 10mmol/l, starch in water 1g/100ml.

Before the determination we needed to prepare samples. We filled flask by tap water and added 1ml of manganese solution and 2ml of the alkaline iodide by narrow tipped pipettes. These manipulations must be done very carefully, to avoid air bubbles and mixed solution twice within the period of ten minutes in order to get homogeneous. After this we started determination. We waited until the precipitate formed in the lowest third of the flask, then added 1,5 ml of sulphuric acid solution, closed the flask by stopper and mixed it well. 50 ml of obtained solution was transferred into the flask. For titrating the solution we added 5 drops of indicator (starch solution) and sodium thiosulphate until solution in flask become colorless. Then, we did all listed actions with natural water. To titrate two solutions (tap water, natural water) following volume of sodium thiosulphate was used:

Sample	Tap water	Natural water
Volume of 10mmol/l Sodium thiosulphate, ml	6,75	8

After titration we needed to calculate results by the formula:

c(O)= (M_rV₂cf)/4V₁ , where

M_r = molecular mass of oxygen(32)

V₁= the volume of the test sample,ml

V₂=the volume of sodium thiosulphate solution used to titrate the sample,ml

с = concentration of sodium thiosulphate solution

f= V₀/V₀-3ml (3ml – the sum of volumes manganese and alkaline iodide solution)

V₀= volume of a bottle

For tap water: c(O)= (M_rV₂cf)/4V₁

c(O)= (32*6,75*0,01*1,026)/(4*50)= 0,011

For natural water: c(O)= (M_rV₂cf)/4V₁

c(O)= (32*8*0,01*1,026)/(4*50)= 0,013

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