NY/T 1971-2010Water-soluble fertilizers - Determination of humic-acids content (English PDF)

6.1 Preparation of specimens

After the solid sample has been divided several times, take out about 100 g. Quickly grind it, until it all passes through a 0.50 mm sieve (if the sample is wet, it can pass through a 1.00 mm sieve). Mix it evenly. Place it in a clean and dry container. After the liquid samples is shaken several times, quickly remove about 100 mL. Place it in a clean, dry container.

6.2 Preparation of specimen solution

6.2.1 Solid specimen

Weigh about 0.5 g of the specimen (accurate to 0.0001 g), in a 50 mL beaker. Add about

10 mL of water. Use a glass rod to stir it. Let it stand for a while. Transfer the solution part into a 100 mL volumetric flask. Add about 10 mL of water to the beaker. Repeat this step 3 times. Add 1 mL of sodium hydroxide solution (4.7) to the residue. Stir to dissolve it. Transfer it into a volumetric flask. Use water to make the volume reach to the mark. Mix well.

6.2.2 Liquid specimen

Weigh 2 g ~ 3 g of the specimen (accurate to 0.0001 g), into a 100 mL volumetric flask.

Add 1 mL of sodium hydroxide solution (4.7) and a small amount of water. Dissolve it fully. Make the volume reach to the mark. Mix well.

6.3 Precipitation of humic-acid in specimen solution

Accurately pipette 5.0 mL of uniform specimen solution into a centrifuge tube. Add 5 mL of sulfuric acid solution (4.6). Mix well. Put it into a centrifuge, to centrifuge it for

10 min, at 3000 r/min ~ 4000 r/min (if there are still solid floating substances in the solution, the centrifugation time shall be extended, until all the solids are precipitated).

Pour off the supernatant liquid.

6.4 Oxidation of humic-acid

Add 5.0 mL of potassium dichromate solution (4.8) to the centrifuge tube. Slowly add

5 mL of sulfuric acid (4.3). Gently shake the centrifuge tube, to mix the contents evenly.

Put the centrifuge tube on the tube rack. Cover the funnel. Heat it in a boiling water bath for 30 min. Take it out. Cool it. Transfer the contents into a 250 mL conical flask.

The volume shall be controlled at 60 mL ~ 80 mL.

6.5 Titration

Add 3 ~ 5 drops of o-phenanthroline indicator (4.5) to the conical flask. Use the standard titration solution of ferrous sulfate (4.10), to titrate the remaining potassium dichromate. When the color of solution changes from orange-yellow->blue-green-> brown-red, it reaches the end point. If the volume, which is consumed by the titration, is less than 1/3 of the volume, which is consumed by the titration blank, the specimen weighing amount shall be reduced, to make measurement again.

6.6 Blank test

Except that no specimen is added, other steps are the same as the determination of the specimen solution. When the absolute difference of the titration, between the two blank tests is <= 0.06 mL, the average value can be taken and substituted into the calculation formula.