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Determination of Potassium Oxide Content In Potassium Humate (Gravimetric Method)

Time : 2022.03.07 Hits : 35

Compared with flame photometric method, gravimetric method is of cheaper cost. But it requires strict operation and accurate measurement.

A. Preparation
B. NaOH solution: 200g/L (Dissolve 200g of NaOH in 1000ml of water).
C.Magnesium chloride hexahydrate solution: 100g/L (Dissolve 100g of magnesium chloride hexahydrate in 1000ml of water).
D. Tetraphenylboron sodium solution: 15g/L (15g of tetraphenylboron sodium is to be weighed and dissolved in 600 ml of water. Then add 8 ml of NaON solution and 20ml of magnesium chloride hexahydrate solution. Stir for 15 minutes and then stand before filtration. If turbidity occurs, repeat the filtration until the residues are filtered. Collect the filtrate in a volumetric flask, dilute it to the scale of the flask and then store it in the dark. The storage period is to be no more than one month. Filter before use.
E. Tetraphenylboron sodium wash: Dilute 10ml of the tetraphenylboron sodium solution (mentioned in 1.3) in 100ml of water.
F. Edetate Disodium (EDTA) solution: 40g/L (Weigh 40g of EDTA and dissolve it in 1000ml of water).
G. Phenolphthalein indicator: 5g/L (Weigh 0.5g of phenolphthalein and dissolve it in 100ml of 95% ethanol.
H. Hydrochloric acid 1:6 aqueous solution: Dissolve 100ml of hydrochloric acid in 600ml of water.

I. Measuring Step
J. Preparation of the tested sample: Weigh 1g (accurate to 0.0002) of the 80-mesh potassium humate sample in a 250ml conical flask. Add 100 ml of 1:6 hydrochloric acid, cover the flask with a funnel and extract it in 100 ° C water-bath for 1 hour. After the solution becomes cool,   dilute it to 250ml with water.  Then filter it with a crucible, discard the initial filtrate and keep the remains.
K. Reagent adjustment. Take 25ml of the filtrate (mentioned in 2.1) and shake it continuously in a 250ml beaker; add 20ml of EDTA (this step can be omitted if the tested solution is adjusted to an alkaline condition without any precipitation); add 2-3 drops of phenolphthalein; add NaOH solution till the color of the filtrate turns red and then add 2 more milliliter of NaOH solution. Heat the filtrate and boil for 15 min. If its red color disappears, add more NaOH solution to make it red again.
L. Precipitation. Shake continuously the solution obtained in 2.2 and add tetraphenylboron sodium wash to it, with the amount of 0.8 ml of tetraphenylboron sodium wash against 1mg of potassium oxide, plus an overall excess of 8 ml. Continue to shake for 1 min, let it stand for more than 15 min and filter the suspension with a G4 glass crucible pre-dried to a constant weight at a degree of 110-120 ° C. Wash 5-7 times with 5ml of tetraphenylboron sodium wash. Then wash with water twice, 5ml each time.


1.Dry. Place the crucible containing the precipitate obtained in 2.3 in a drier with a temperature range between 110-120 ° C for 2 hours. Then take it out, cool, weigh, and repeat several times until a constant weight is achieved. Meanwhile, carry out a duplicated test, with all the same procedures except that no sample of potassium humate is added.

K2Oad%=【(G2-G1)-(G4-G3)】×0.1314/G0×(V/250)×100%

G0: Sample Weight, g
G1: Weight of G4 Crucible, g
G2: Weight of G4 Crucible with Precipitate, g
G3: Weight of G4 Crucible in duplicated test, g
G4: Weight of G4 Crucible in duplicated test and with Precipitate, g

V: The Volume of the Tested Sample
The needed reagents and devices are as follows:

Tested Item: Potassium Oxide Content in Potassium Humate
NameNumberRemarksNameNumberRemarks
Water Bath Cauldron1 pcTest DeviceAnalytical Balance1 pcTest Device
Thermostatic Drier1 pcTest DeviceSuction Pipette1 pcLab Glassware
Electric Heater1 pcTest DeviceG4 Glass Gooch Filter Crucible2 pcsLab Glassware
Vacuum Pump1 pcTest DeviceRubber Hose1.5mLab Material
Weighing Bottle (Small)4 pcsLab GlasswareBeaker (400ml)1 pcLab Glassware
Rubber Pipette Bulb1 pcLab GlasswareBüchne Flask2 pcsLab Glassware
Conical Flask4 pcsLab GlasswareDropping Bottle (100ml)2 pcsLab Glassware
Volumetric Flask (1000ml)2 pcsLab GlasswareReagent Bottle (1000ml)4 pcsLab Glassware
Volumetric Flask (100ml)1 pcLab GlasswareSodium Hydroxide(NaOH)1 bottleChemical
Volumetric Flask (250ml)2 pcsLab GlasswareMagnesium Chloride Hexahydrate(MgCl2·6H2O)1 bottleChemical
Funnel (Medium)4 pcsLab GlasswareTetraphenylboron Sodium(C24H20BNa)1 bottleChemical
Medium-speed Filter Paper1 packLab MaterialEdetate Disodium(C10H14N2Na2O8)1 bottleChemical
Wash Bottle1 pcLab GlasswarePhenolphthalein(C20H14O4)1 bottleChemical
Bulb Pipette (20ml)1 pcLab Glassware95% Ethanol(C2H5OH)1 bottleChemical
Bulb Pipette (25ml)1 pcLab GlasswareHydrochloric Acid(HCl)1 bottleChemical
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