[Wald_ron]

I cannot get mercury(I) nitrate to dissolve in water. I tried dissolving it in Nitric but after I dilute it and I go to test for Mercury in the water... nothing. Any recommendations? 

[Arkcon]

That depends on your analytical test.  Sometimes, mercury can be lost by the analytical method, for example,ICP.

[PeaGreen36]

Gold solution added the extract can help as a stabilising agent for mercury when acid digested, I currently use this for analysis via ICP.

[Wald_ron]

testing method is 6M HCl

[Arkcon]

Is mercury (I) soluble under those conditions?  Mercury (II) is soluble in water.  Mercury (I) is only slightly soluble, and prone to oxidation. 

What is the analytical test?  ICP, or flame AA, or gravimetric reaction, or what?

PeaGreen36: offered an idea.  Do you  think its a good one?  Why or why not?

testing method is 6M HCl

That is not a testing method.