Test method for content of liquid polymerized ferric sulfate
Release time: 2025-09-03
The detection methods of liquid polyferric sulfate content are summarized, which are divided into three categories: core index detection (total iron content and salt base degree) and auxiliary methods based on national standards and commonly used industry technologies:
I. Total iron content detection (core index)
- Potassium dichromate titration method (national standard arbitration method)
- Principle: In an acidic environment, tin chloride is used to reduce Fe³⁺ to Fe²⁺, and excess tin chloride is eliminated by mercury chloride, and then potassium dichromate is titrated to the purple endpoint.
- step ：
- ① Take 1.5g sample + 20ml water + 20ml hydrochloric acid (1:1) → boil for 1 minute;
- ② Add a solution of stannous chloride to the yellow color disappears → cool quickly;
- ③ Add 5ml saturated mercury chloride solution → stand for 1 minute;
- ④ Add 50ml water + 10ml sulfur phosphorus mixed acid + 4~5 drops of sodium dianilatesulfonic acid → titrate with potassium dichromate until purple (30 seconds does not fade).
- Calculation: total iron content (%) = (V × C × 0.05585 × 100) / m
- (V: titration volume, C: potassium dichromate concentration, m: sample mass)
- Note: Mercury chloride is toxic and waste liquid should be disposed of in accordance with regulations.
- Principle: In an acidic environment, tin chloride is used to reduce Fe³⁺ to Fe²⁺, and excess tin chloride is eliminated by mercury chloride, and then potassium dichromate is titrated to the purple endpoint.
- Atomic absorption spectroscopy (AAS)
- Principle: After the sample is dissolved, iron atoms absorb characteristic wavelength light and are quantified by absorbance.
- step ：
- ① The sample is dissolved in dilute acid (e.g., hydrochloric acid) → filter and dilute;
- ② Standard curve method for determination of absorbance.
- Advantages: high precision (±0.1%), can measure a variety of metals at the same time.
- Principle: After the sample is dissolved, iron atoms absorb characteristic wavelength light and are quantified by absorbance.
- Potassium permanganate titration method (reducing substance detection)
- Application: Fe²⁺ content detection (affect the accuracy of total iron).
- Step: Sample + sulfuric acid/phosphoric acid → titrate with 0.01mol/L potassium permanganate to the reddish endpoint.
- Application: Fe²⁺ content detection (affect the accuracy of total iron).
Ii. Salt base detection (key auxiliary indicators)
Principle: Hydrochloric acid depolymerizes the sample, potassium fluoride masks the iron ion, and sodium hydroxide titrates the free acid.
National standard steps (GB/T14591-2016):
- Take 1.2~1.3g sample + 25ml hydrochloric acid standard solution → leave at room temperature for 10 minutes;
- Add 10ml potassium fluoride solution (mask Fe³⁺) → Add 5 drops phenolphthalein;
- Titrate with sodium hydroxide to a pale red color (30 seconds without fading).
- Calculation: Base degree (%) = [(V0-V) × C × 0.017 × 100] / (m × iron content%)
- (V0/V: blank/sample titration volume, C: NaOH concentration)
III. Quick-aid methods
- Density association method
- Application scenario: rapid estimation on production site.
- operate ：
- ① Constant temperature (20±1℃) measure the sample into the densitometer;
- ② When the density is greater than or equal to 1.45g/cm³, the total iron content is about 11.5%; if it is less than 1.3g/cm³, it is unqualified.
- Limitations: It needs to be calibrated with titration.
- Application scenario: rapid estimation on production site.
- pH detection
- Standard: The pH of 1% aqueous solution should be 2~3 (National standard GB/T14591-2016).
IV. Notes
- Reagent calibration: The titrant (such as potassium dichromate, sodium hydroxide) should be calibrated in advance.
- Interference elimination:
- When the base degree is more than 16%, iron hydroxide precipitates are easy to be formed, so acid control should be strict;
- Arsenic, lead and other heavy metals are detected by atomic absorption spectrometry.
- When the base degree is more than 16%, iron hydroxide precipitates are easy to be formed, so acid control should be strict;
- safe operation ：
- The boiling step of hydrochloric acid should be done in a fume hood;
- Mercury chloride is a highly toxic substance, and it is recommended to use mercury-free methods (such as titanium trichloride method).
- The boiling step of hydrochloric acid should be done in a fume hood;
propose
- Accuracy priority: select potassium dichromate method (total iron) or AAS method (in line with ISO standard);
- Rapid screening: combination of density method and pH method;
- Comprehensive quality control: simultaneous detection of salinity (8%~16% is optimal) and heavy metals.
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