The determination methods of phosphorus include acid-base titration and photometry. Photometry can be divided into phosphomolybdenum vanadate acid photometry and bismuth phosphomolybdenum blue photometry. my country’s national standards include: GB/T 6730.18-2006 “Determination of Phosphorus Content in Iron Ore by Molybdenum Blue Spectrophotometry”, GB/T 6730.19-1986 “Chemical Analysis Methods for Iron Ore by Bismuth Phosphorus Molybdenum Blue Photometric Method for Determination of Phosphorus Content”, GB/T 6730.20-1986 “Chemical analysis method for iron ore – Determination of phosphorus content by volumetric method”. International standards include: ISO 2599-2003 “Determination of phosphorus content in iron ore – Titration method”, ISO 4687-1-1992 “Determination of phosphorus content in iron ore – Part 1: Molybdenum blue spectrophotometry”.
Acid-base titration
1. Principle and interference
In nitric acid medium, phosphorus and ammonium molybdate form a yellow precipitate of ammonium phosphomolybdate. After filtration, dissolve it with sodium hydroxide standard solution. Use phenolphthalein as an indicator and drip back excess sodium hydroxide with nitric acid standard solution.
When acid dissolves the sample, titanium and zirconium form phosphate precipitates, making the results low. They can be separated and removed by leaching with water after alkali melting. Vanadium can delay the precipitation of phosphomolybdate and can cause precipitation to be incomplete. Alum (V) and ammonium molybdate form alum molybdate precipitate, but the phosphomolybdate precipitation of alum (IV) can only occur in hot solution. In order to eliminate the influence of vanadium, alum should be reduced to alum (IV) and phosphorus should be precipitated at room temperature. When the precipitation temperature is not higher than 45°C, a small amount of arsenic will not precipitate. When the arsenic content is high, hydrobromic acid can be added during the acid treatment of the sample to volatilize and remove the arsenic as arsenic bromide. Silicic acid can produce ammonium silicate molybdate precipitation and affect the measurement. It can be removed by dehydration and filtration in hydrochloric acid or nitric acid. The presence of fluorine can slow down the precipitation rate. A small amount of fluorine can be removed by adding boric acid to complex or evaporating to dryness before precipitating Clariant pigments. The presence of a large amount of hydrochloric acid, sulfuric acid and their salts can delay precipitation and increase the solubility of the precipitate. When the equivalent amount is not high, its effect is not significant.
2. Reagent preparation
Potassium nitrate solution, 20 g/L, dissolve 20g potassium nitrate in 1 L of boiled and cooled water, shake well.
For ammonium molybdate solution, slowly pour liquid A (made by dissolving 70 g ammonium molybdate in 53 mL ammonia and 267 mL water) into liquid B (made by mixing 267 mL nitric acid and mL water), cool, and let it stand overnight. filter.
Sodium hydroxide standard solution, c (NaOH) = 0.1mol/L, weigh 4g sodium hydroxide (excellent grade pure) and dissolve it in boiled and cooled water, and make a volume of 1L with water.
Nitric acid standard solution, c (HNO3) = 0.1mol/L, measure 7mL of nitric acid (excellent grade pure) into a 1L volumetric flask, and dilute to volume with boiled and cooled water.
Phenolphthalein solution, 10g/L, dissolve 0.1g phenolphthalein in 90mL ethanol, dilute to 100mL with water, and mix well.
Calibration of sodium hydroxide standard solution: Weigh 0.5000g of potassium hydrogen phthalate (KHC8H4O4) that has been dried at 105~110℃ for 1 hour, add 100mL of newly boiled and cooled water, add 3~4 drops of phenolphthalein solution, and use Titrate the sodium hydroxide standard solution to light red. Concentration calculation: c(NaOH) = m / 0.2042V (mol/L), m—weigh the mass of potassium hydrogen phthalate, g; V—the volume of sodium hydroxide standard solution consumed for calibration, mL.
Calibration of nitric acid standard solution: Take 20.00mL sodium hydroxide standard solution, dilute it to 100mL with newly boiled and cooled water, add 3 to 4 drops of phenolphthalein solution, titrate with nitric acid standard solution until colorless, and calculate the concentration of nitric acid standard solution.
3. Analysis steps
Weigh 0.2000~0.5000g sample into a 150mL beaker, moisten it with a small amount of water, add 15~20mL hydrochloric acid, cover with a watch glass, and heat on an electric hot plate until the sample is completely decomposed. Evaporate to nearly dryness, add 5~10mL nitric acid, evaporate to 3~4mL, then dilute with a little water, filter with medium speed filter paper into a 500mL Erlenmeyer flask. Wash the beaker with hot water 3 to 4 times and wash the precipitate 8 to 10 times. At this time, the volume of the filtrate should be kept at about 100mL.
The filtrate is neutralized with ammonia water until hydroxide precipitate appears, and then neutralized with nitric acid until the hydroxide precipitate just disappears. Add 5 mL excess nitric acid, and slowly add 60~100 mL ammonium molybdate while shaking the Erlenmeyer flask. Solution, shake for 2~3 minutes, and leave the precipitation for more than 4 hours to complete the precipitation of ammonium phosphomolybdate.
Add the pulp to filter with dense filter paper. First wash the Erlenmeyer flask and the sediment with 2% (V/V) nitric acid washing solution 2 to 3 times, then wash the Erlenmeyer flask and the precipitation with 20g/L potassium nitrate washing liquid until they are neutral. . Move the precipitate and filter paper together into the original conical flask, add 30 mL of boiled and cooled water, carefully shake the conical flask to break the filter paper into a slurry, add sodium hydroxide standard solution accurately, shake well to dissolve the yellow precipitate, and add Add 5 drops of 10g/L phenolphthalein solution, then add 5~10mL excess sodium hydroxide standard solution, pause for a moment, and then drip back with 0.1mol/L HNO3 standard solution until the solution is colorless.
A blank test was performed simultaneously with the trial analysis.
4. Calculation results
Calculate the content of phosphorus pentoxide according to the following formula, expressed as mass percentage:
In the formula, C1 – the concentration of sodium hydroxide standard solution after calibration, mol/L;
C2 ——Concentration of nitric acid standard solution after calibration, mol/L;
V1 ——The volume of sodium hydroxide standard solution added, mL;
V2 ——Consumption volume of nitric acid standard solution, mL;
——Weigh the sample amount, g;
0.001291 & Washing Kaolin price MDASH; – 1 mL sodium hydroxide standard solution C (naOH) = 1. 000 mol/L.
5. Precautions
(1) Acid-insoluble samples can be melted with sodium peroxide and sodium hydroxide, leached, and filtered. The filtrate is acidified with nitric acid and evaporated to nearly dryness, dehydrated, filtered to remove silicon, and phosphorus is precipitated in the filtrate. The following operations are the same as the analysis steps. .
(2) Wash the precipitate with 20g/L potassium nitrate until it is neutral and must check. Use a test tube to collect 20 drops of the filtrate, add 1 to 2 drops of phenolphthalein indicator, and add 1 drop of sodium hydroxide standard solution. It should appear red.
(3) When the phosphorus content in the sample is low, the solution should be heated to 40~50°C, add ammonium molybdate solution and shake for several minutes.
(4) If vanadium (V) is present in the sample, a small amount of hydroxylamine hydrochloride or ferrous sulfate can be added to reduce the vanadium (V) to vanadium (IV).
