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Since the reaction between calcium ions and chromium-black T indicator at the end of the titration does not show a significant color, when the water sample contains very little magnesium, a known amount of magnesium salt is needed to make the color change at the end of the titration clear. When calculating the junction, subtract the amount of magnesium salt added, or add a small amount of MgEDTA to the buffer solution to ensure a clear end point.
If 50ml water sample is taken, the minimum detection mass concentration of this method is 1.Omg/Lo0).
(1) Buffer solution (pH =10).
D weighed 16.9g ammonium chloride and dissolved in 143ml ammonia (pao=0.88g/ml). Weigh 0.780g magnesium sulfate (MgSO.·7H,0) and 1.178g sodium EDTA·2H,0, dissolve in 50ml pure water, add 2ml ammonium chloride-ammonium hydroxide solution and 5 drops of chrome black T indicator (the solution should be purplish red; If it is sky blue, a very small amount of magnesium sulfate should be added to make it purplish red), titrated with Na,EDTA standard solution until the solution changes from purplish red to sky blue.
Combine the solution and dilute it with pure water to 250ml. If the solution turns purple after combination, the reagent blank should be deducted when calculating the result. Note :D The buffer solution should be stored in polyethylene or hard glass bottles; Prevent the use of repeated open cover to reduce the ammonia concentration and influence.
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