Magnesium oxide not only contains activity, but also contains over-fired MgO, under-fired MgCO3, and impurities CaO, Fe2O3, Al2O3, SiO2, burnt loss and so on. In the light burning magnesium powder storage process active may also absorb moisture or react with CO2 in the air to generate Mg(OH)2 and MgCO3.
Principle of citric acid reaction method Activated magnesium oxide particles in water hydration to generate Mg(OH)2
MgO + H20 = Mg(OH)2
Mg(OH)2 dissociation of Mg2+ and OH–
Mg(OH)2 = Mg2+ +2OH– (reversible)
When a certain amount of citric acid drops into the solution, can be neutralized in the solution of OH– , to promote the reaction to the right, the content of the sample in the activated MgO according to the amount of citric acid consumption to calculate. The amount of citric acid consumed is calculated.
Carbonic acid is more acidic than citric acid, so magnesium carbonate will not interfere with the results.Mg, Ca are in the second main group of the periodic table, their oxides and hydroxides have similar chemical properties, CaO and H2O can generate Ca(OH)2, which can be dissolved in the water part of the complete ionization of Ca2+ and OH–, interfering in the determination of the interference, this interference is similar to the traditional water law, we ignore it without discussion here.
The specific procedure for the determination of active magnesium oxide by the citric acid method is as follows:
Weigh 2 g of a specimen containing active MgO into a 400 mL beaker.
Add 50mL of distilled water and 5 drops of acid phthalide indicator.
Put the beaker on the touch force heating stirrer heating and stirring, heating gear in the “2” column, stirring speed in the “high speed”.
Titrate with 10% citric acid solution, the first drop of 10-20mL, while using a stopwatch to record the time when the solution turns pink.
After each drop of 5mL or 2mL citric acid solution, and stopwatch timing, in the near end, each time to add 1 ~ 2mL citric acid solution, according to the adjacent two times the recorded time of the phenomenon of sudden change to determine the end point, the entire measurement time of not more than 0.5 hours, generally within 15 minutes.
The cumulative amount of citric acid solution consumed was finally noted down.
Advantages and disadvantages of the citric acid method for the determination of active magnesium oxide:
Advantages: citric acid method is a rapid test method, easy to operate, and able to complete the determination in a relatively short period of time.
Disadvantages: the accuracy of citric acid method determination is affected by the storage time, the storage process of active MgO may absorb moisture into Mg(OH)2 or carbonization into MgCO3, these changes will affect the results of the determination.
Practice has proved that the active content of magnesium oxide will be reduced with the storage time, the main reason is that the active MgO moisture absorption into Mg(OH)2 or carbonization into MgCO3, we said earlier that MgCO3 will not interfere with the measurement, but the part of Mg(OH)2 slightly soluble in water will be all ionized, and the citric acid that is constantly added to neutralize the OH– and promote the ionization to the right, and the consumed part of this citric acid in the The consumption of this part of citric acid is shown in the data as the content of activated MgO, which makes the result high.
The dry solid obtained after the test of measuring the active MgO content by the hydropathic method (noted as A solid) verifies the inference that Mg(OH)2 will have an effect on the experimental results: 2.00g of A solid was weighed, and citric acid was consumed for 24.00ml by the reaction with citric acid to the end point of the titration, and this result reflects the information that the content of active MgO is 34.26%, and in fact, the test sample has been completely inactive! This result reflects the information that the active MgO content is 34.26%, while in fact the sample has completely lost its activity.
And then we will know the activity of magnesium oxide (recorded as B solid) and A solid mixed proportionally, at the same time, the water method and citric acid reaction method to determine the active MgO content, when the sample is all B solid, citric acid reaction method and the water method of the determination of the results of the difference of 1.92%, in line with the WB/T1019-2002 appendix on the development of the active MgO content of the test method of the relevant provisions, but when some active MgO moisture absorption and transformation of the MgO content of the test method of the relevant provisions. However, when part of the active MgO absorbs moisture and transforms into Mg(OH)2, the relative error of the determination results of the two methods is >±3%.
Water method to measure the content of active magnesium oxide using reaction increment to deduce the activity content, moisture absorption part of the quality before and after the same will not interfere with the results, but the citric acid reaction method is based on the consumption of citric acid to calculate the amount of citric acid consumption depends on the amount of OH– in the solution, so that the determination of the activity content of the moisture in the magnesium oxide is unsuitable.