AFDC 10 (4588) P3
(Rev. TZS 1283:2010)
Draft Tanzania Standard
Fertilizers – Method for determination of free acidity
TANZANIA BUREAU OF STANDARD
1
AFDC 10 (4588) P3
(Rev. TZS 1283:2010)
0 FOREWORD
Depending on the constituents of the manufactured fertilizers, some of the free acidity remains
unreacted during the process of manufacture. The process also depends up on the concentration
of acids used for the manufacture of this material. The free acids determined by the methods
prescribed in this standard also has some corrosive effects on the container or bag.
In reporting the results of a test or analysis made in accordance with this standard, if the final value
observed or calculated, is to be rounded off, it shall be done in accordance with TZS 4.
For the purpose of this standard only analytical grade reagents and distilled water shall be used.
1 SCOPE
This Tanzania standard prescribes the methods for determination of various forms of free acidity
in fertilizers.
2 REFERENCE
The following referenced documents are indispensable for the application of this document. For
dated references, only the edition cited applies. For undated references, the latest edition of the
referenced document (including any amendments) applies.
TZS 4 Rounding off numerical values
AFDC 10 (4589) P3 Fertilizers and Soil Conditioners – Vocabulary
TZS 159 Fertilizer - Methods of sampling
3 SAMPLING
The sample shall be drawn in accordance with TZS 159
PART 1: DETERMINATION OF FREE ACIDITY as H2SO4
REAGENT
1 Standard sodium hydroxide solution – 0.02 N
2 Methyl red-methylene blue mixed indicator solution – prepared by mixing equal volumes
of 0.2% solution in rectified spirit of methyl red and 0.1% solution in rectified spirit of
methylene blue.
PROCEDURE
1 Dissolve 20g of the sample in about 50mL of cold neutral water.
2 Filter and make up the volume to about 200mL. The filtering media shall be neutral and
shall not contain any alkaline material which would neutralize the free acidity.
3 Titrate the solution with standard NaOH solution using one or two drops of methyl red-
methylene blue mixed indicator and a microburette.
2
AFDC 10 (4588) P3
(Rev. TZS 1283:2010)
CALCULATION
Free sulphuric acid (as H2SO4) = 4.904 x N x V
% by mass W
Where;
N = Normality of standard NaOH solution
V = Volume in milliliter (mL) of standard NaOH solution
W = Weight in gram (g) of sample taken for the test
PART 2: DETERMINATION OF FREE ACIDITY as HNO3
REAGENT
1 Standard sodium hydroxide solution – 0.02 N
2 Methyl red-methylene blue indicator solution – prepared by mixing equal volumes of 0.2%
methyl red solution in alcohol and 0.1% methyl blue solution in alcohol
PROCEDURE
1 Dissolve 10g of the sample in 100mL of neutral water at room temperature in a beaker or
conical flask.
2 Filter and add 2-3 drops of indicator solution
3 Titrate the solution with standard NaOH solution until the colour changes from violet to
green
CALCULATION
Free Nitric acid (as H)(HNO3) = 4.904 x N x V
% by mass W
Where;
N = Normality of standard NaOH solution
V = Volume in milliliter (mL) of standard NaOH solution
W = Weight in gram (g) of sample taken for the test
PART 3: DETERMINATION OF FREE ACIDITY as P2O5
REAGENT
1 Acetone
2 Standard sodium hydroxide solution – 0.1 N
3 Bromo cresol green indicator solution – Dissolve 0.1g bromocresol green in 100mL of
rectified spirit
3
AFDC 10 (4588) P3
(Rev. TZS 1283:2010)
PROCEDURE
1 Weigh accurately about 2.5g of the prepared sample in a 250mL Erlenmeyer flask
2 Add 100mL neutral acetone. Shake for one hour.
3 Filter rapidly through whatman filter paper No.1 in to 250ml Erlenmeyer flask. Wash four
times with 10mL portion of acetone
4 Evaporate acetone as far as possible
5 Add about 50mL water and drops of bromocresol green indicator
6 Titrate with standard NaOH solution, until the colour changes from yellow to blue
CALCULATION
Free phosphoric acid (as P2O5) = 7.1 x N x V
% by mass W
Where;
N = Normality of standard NaOH solution
V = Volume in milliliter (mL) of standard NaOH solution
W = mass in gram (g) of sample taken for the test
