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Current Category » Soil Testing and Fetilizer Use

Principle of Determination of the Parameters

1. PH: By PH meter to be recorded to the nearest 0.1 unit.

2. EC: EC of water sample is determined directly by Conductivity Bridge and values corrected for temperature and cell constant. EC is commonly used for indicating the total concentration of the ionized constituents of natural water. EC is expressed in mmhos/cm at 25 0C or dSm-1.

Salt concentration (mg/L) = 640 X EC (dSm-1)
Osmotic Pressure (bars) = 0.39 X EC (dSm-1)

3. Carbonates, Bi-carbonates and Chlorides:

The water sample is titrated with standard sulphuric acid using phenolphthalein indicator (ph8.0). Known quantity of sample (50 ml) to betaken for titration. This corresponds to half carbonates: CO3square - + H+ à HCO3

Step 1:

Na2CO3 + H2SO4 à NaHCO3 + Na2SO4

Pink colour in presence of pp indictor colorless end point 8.0 PH.

Step 2:

2NaHCO3 + H2SO4 à Na2SO4 + 2H2O + 2CO2

In presence of methyl orange indicator without refilling burette take end point. Change colour from orange yellow to rose red (PH4.8). Titre values of first and second titration are used for calculation of carbonates and bi-carbonates.

Step 3:

After the titration of carbonates and bicarbonates the chlorides to be determined in the same sample, since the chloride determination, acidic solution is required because in presence of carbonate part of silver nitrate is used for formation of AgCO3, the chlorides in the water sample are precipitated as AgCl when titrated with standard AgNO3 in presence of Potassium chromate indicator. After competed precipitation of chlorides , a drop of excess AgNO3 reacts with potassium chromate giving silver chromate of brick red colour indicating the end point of titration. From the titre value of std. AgNO3 Chloride content in the sample is determine.
   
Reactions:

In presence of chromate ion as an indicator silver reacts with chlorides.

AgNO3 + Cl = AgCl + NO3

In presence of K2CrO4 indicator

AgNO3 + K2CrO4 = Ag2CrO4 + 2 KNO3
(Brick red precipitated)

Sulphate:

Sulphates is converted to a barium sulphate suspension under the controlled condition. The resulting turbidity is determined by a spectrophotometer at 340 nm wave length and compared with a curve prepared from std. Sulphate solution.

Reaction:

Na2SO4 + BaCl2 à 2 NaCl2 + BaSO4

4. Determination of Calcium + Magnesium by Versenate Titration Method.  

When EDTA (Ethylene diamine tetaacetic acid) disodium salt is added to water containing both Ca and Mg. It combines first with calcium. Calcium can be determined directly with std.

EDTA when the PH is sufficiently high that the magnesium is largely precipitated as the hydroxides and an indicator combines with calcium only.

Ca indicator + CH2N (CH2COOH)2 à Ca Complex + 2H + Indicator 2-
                        CH2N (CH2COONa)2
Rose Colour          Colourless                          Colourless     + Violet (Purple)

Indicator gives a colour change when all the calcium has been complexed by the EDTA at a PH 12 to 13.

Calcium + Magnesium:

The calcium and magnesium in irrigation water is titrated with std. EDTA solution at PH 10. The calcium plus magnesium concentration is calculated from the volume of std. EDTA required for titration.

Mg indicator CH2 ( CH2 COOH)2 à Mg complex + Indicator 2+
(EST)            CH2N ( CH2 COONa)2
(Wine red)    Colourless                      Colourless           Blue

6. Determination of Sodium and Potassium:

Qualitative test for presence of NO3: Since most of waters do not contain nitrate in toxic concentration under Maharashtra conditions. If these are in toxic concentration can be determined by phenol sulphonic method and deverda method for water with high chloride content.

7. Boron (Carmine Method) :

Boron in an acid solution of carmine (0.05%) in concentrated sulphuric acid forms complex and the colour changes from bright red to bluish red or blue, depending upon concentration. Determine the percent transmittance at 585 nm. Traces of boron are essential for plant growth but the concentration of one ppm causes injury to many plants.

 

Current Category » Soil Testing and Fetilizer Use