Controlling pH value in the process of preparing ammonium metatungstate by tungstic acid method is an important step to ensure product quality and process stability. The following are detailed steps and points on how to control pH value according to the information in the referenced article:
Preparation stage
Dissolve tungstic acid with ammonia and add it to the deionized water containing boiling water.
Initial pH adjustment
Heat the ammonia-by-ammonia with stirring to bring the pH up to 8. This step is to allow the tungstic acid to react sufficiently with the ammonia to form ammonium tungstate.
Acetic acid addition and pH adjustment
Acetic acid is added while the solution is boiling, and the acetic acid reacts with the ammonium ions in the ammonia to form ammonium acetate while lowering the pH of the solution.
Continue to adjust the pH of the solution with dilute nitric acid to control the pH in the range of 2 to 4. This pH range is critical for the formation of ammonium metatungstate.
Heating Concentration and pH Monitoring
Continue to heat and concentrate the solution while constantly monitoring the pH. As the water evaporates, the concentration of the solution increases, but the pH may change.
If the pH deviates from the target range (2 to 4), it should be adjusted promptly by adding an appropriate amount of dilute nitric acid or ammonia.
End and subsequent processing
When the relative density of the solution reaches 1.5 to 2, stop heating and let the solution cool naturally.
Filter to remove the insoluble material and obtain the ammonium metatungstate solution.
Importance of pH control
In the whole preparation process, the control of pH value has an important influence on the formation and purity of ammonium metatungstate. If the pH value is too high or too low, it may lead to the formation of impurities or the decomposition of ammonium metatungstate.
Therefore, the change of pH value must be strictly monitored and adjusted as needed.
Precautions
When adding acetic acid and dilute nitric acid, add them slowly and stir them well to prevent the local pH from being too high or too low.
In the process of heating and concentration, pay attention to controlling the heating speed and temperature to avoid excessive pH fluctuations caused by boiling the solution too vigorously.
Through the above steps and points, the pH value during the preparation of ammonium metatungstate by tungstic acid method can be effectively controlled, so as to ensure the quality of the product and the stability of the process.
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