USP 30中，磷酸三镁有这么一项指标：

Dibasic salt and magnesium oxide- Ignite about 2.5g to constant weight. Weigh accurately about 2g of ignited salt, and dissolve it by warming with 50.0 mL of 1N hydrochloric acid VS. Cool, add 1 or 2 drops of methyl orange TS, and slowly titrate the excess 1N hydrochloric acid VS with 1N sodium hydroxide VS to a yellow color, vigorously shaking the mixture during the titration. Between 14.8 and 15.4mL of 1N hydrochloric acid is consumed for each g of the ignited salt.

按说400多摄氏度灼烧残渣就Mg3(PO4)2。如果是Mg3(PO4)2溶解到MgCl2，我算了下，1g样品用14.8~15.4mL的1mol/L的盐酸溶解是不够的。

这个过程具体如何理解呢？请高手指点，谢谢！
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