Alone is a contraction of all-ane ‘all one’ and may only appear after the noun modified, either following the noun/pronoun or in the predicate. Its cognates in German (allein) and Dutch (alleen) function the same way, though in these two languages, unlike in English, they may appear before a definite or indefinite article as English uses only. I'm not sure, however, why that should influence your word choice in English.

Now she was alone, even the deck hand had disappeared.

The President alone can declare that circumstances are exceptional ... and he alone can determine that the situation is back to normal.

In her impressive study of black and white women in plantation households, Elizabeth Fox-Genovese states that “a woman alone on the public thoroughfares was a woman at risk.”

The fiber and the nutrients in the potato alone should differentiate it from the other “high carb” foods.

Lone, however, may appear in the normal attributive position before a noun, but cannot be used as a predicate adjective as alone can be:

It seems the lone president who simultaneously occupied the big screen and the Oval Office was John F. Kennedy: "PT 109," released five months before the 1963 assassination, starred Cliff Robertson and concerned the young JFK as war hero...

Life was indeed hard, but Mrs. Ullmann never realized … that it was harder still for a lone woman on the bleak prairie striving to wrest a livelihood from the stubborn hard soil.

She could have sworn she'd peeled enough potatoes to feed an army. Instead, the lone potato in the pan stared back at her.

This woman's lone potato is the one you're looking for unless you want to personify a root vegetable so that it feels alone, but even that sentient tuber can't be *an alone potato.