IN the wake of Secretary of State James Baker's recent visit to the Baltic countries, the United States and other members of the United Nations still have yet to confront all of the challenging diplomatic issues that are arising out of the Soviet breakup.The UN has now approved the membership application of the Baltic states. How will the UN react, though, to similar requests from other newly "sovereign" Soviet republics that choose to remain part of a new Soviet federation now emerging? As many as 10 Soviet republics may become part of the new structure. Under the plan unveiled by Mikhail Gorbachev and republic leaders Sept. 2, each republic will be regarded as a sovereign nation and encouraged to apply for its own seat in the UN. Requests from these Soviet republics for UN membership will force the UN to examine not only the issue of sovereignty, but also the question of current Soviet representation at the UN, to include permanent membership status on the Security Council. Despite the fact that Moscow will regard each participant in the new confederation as a sovereign nation, and as such free to apply to the UN, it appears that the new interim central authority, the State Council, will retain many of the trappings traditionally associated with the concept of sovereignty: control of foreign affairs, the military, law enforcement, and security. Will, then, the new "sovereign" republics of a Soviet federation really be able to act like states, at least as far as the UN is concerned? Perhaps. Despite Stalin's insistence to the contrary, the Byelorussian and Ukrainian Soviet republics could never be regarded as strictly sovereign states, yet each was included, along with the USSR as a whole, among the founding members of the UN as part of the compromise hammered out in 1945 by the Soviet leader and President Roosevelt at Yalta. In fact, in August 1944 at Dumbarton Oaks, the Soviet Union suggested that all of its then-16 Soviet republics be admitted to the UN as founding members. This original Soviet proposal was made by the head of the Soviet delegation at the conference, Andrei Gromyko, then the Soviet Union's ambassador to the US. Under Secretary of State Edward Stettinius, who headed the American delegation, was so troubled by the idea that he restricted knowledge of it to only a few members of his delegation, and referred t o the Soviet proposal only as the "X-matter." Stettinius feared a loss of American public support for the new organization if the press got wind of Gromyko's suggestion. Since Byelorussia and the Ukraine have been represented in the UN from the outset, some at the UN may see logic in admitting other members of a new Soviet federation, regardless of whether the question of sovereignty is used as a litmus test. Yet if republics of a new Soviet federation are admitted to the UN under any circumstances, one must ask whether there should be a seat at the UN for the Soviet Union as a whole. More important, who or what should inherit the Soviet Union's permanent seat on the Sec urity Council? Surely the Soviet Union is no longer what it once was. Should there remain a permanent Security Council seat for a "Soviet Union"? If not, what, if anything, should replace it? A new Moscow-based federal government? The Russian Republic, perhaps? Will other Soviet republics instead demand a permanent seat that would rotate among all the members of a loose Soviet confederation? Furthermore, if Byelorussia and the Ukraine are members of a new Soviet federation and the UN rejects membership applications fro m other republics in the Soviet federation on the basis of questionable sovereignty, how will the UN justify continued Byelorussian and Ukrainian representation at the UN? It is difficult to argue that the existing state and the Soviet state seated at the UN are the same. Rival governments claimed jurisdiction over the same territory in China. This is not the case here, unless Boris Yeltsin tries to argue that the Russian Republic is the effective authority and is more deserving of the UN seat now held by the Soviet Union. As long as the Russian Republic gets its own seat at the UN, however, Mr. Yeltsin is unlikely to care whether there is still a seat for a "Soviet Union. " To the UN, though, this should be an important question. After it played a valuable role in opposing Iraqi aggression against Kuwait, many people saw the possibility of a new relevance and effectiveness in world affairs for the UN. It will be unfortunate if the UN does not employ degree of sovereignty as a yardstick in judging applications for UN membership from "sovereign" Soviet republics. If sovereignty is not a determining factor in this case, when it reviews the Yugoslav and other future cases the U N will be accused by many of using a double standard.