United National Bank, Washington's largest minority-owned bank, is planning to sell 100,000 more shares of stock to increase its capital base -- which has been unable to keep up with soaring deposits.

In 1977, United National, with $31.5 million in deposits, was the 13th largest bank in the city. Today it is the 9th largest of 17 institutions with deposits of $52.4 million, bank President Samuel L. Foggie noted yesterday.

"We are seriously undercapitalized," bank Chairman William K. Collins told stockholders at the annual meeting. "The issue of 100,000 shares is necessary. These are the subtle prices that we shareholders must pay for profitability," Collins added.

However, the bank was unable to authorize the planned sale yesterday because enough proxy votes were not mailed in, bank officials said. The shareholders voted to extend the time to mail proxy votes.

All 100,000 shares would not be sold at one time. Only 41,000 would be issued immediately, and Collins urged current stockholders to buy the stock rather than offer it publicly, to keep control of the bank "with the people who joined in the history of the bank."

When the bank was founded it had $1.2 million in capital which was also its total deposits. Today the bank has $2.4 million in capital, Collins said. An optimum ration between capital and deposits would be one to 10, Collins said.

Besides a 67 percent increase in deposits from 1977 to 1978, the bank also reported total assets of $58.2 million last year compared with $34.1 million in 1977, and a substantial turnabout in profitability from a loss of $144,000 in 1977 to earnings of $105,000 (50 cents a share) last year. The loss in 1977 was caused by the bank's expansion into the downtown financial corridor, bank officials said.

But the move to 1850 K St. NW is credited with part of the bank's large jump in deposits. United National has four offices in Washington.

Developments during 1978 included "creation of a new partnership between (the) downtown and Northeast Washington customers," and "significant advances into banking relationships servicing the Fortune 1,000 as well as other large and medium-sized organizations" Foggie and Collins said in the bank's annual report.

Last year, the bank increased its residential and commercial loans in the northeast area and "remained one of the key supporters of the proposed Minnesota Avenue-Benning Road commercial revitalization program," according to the bank's annual report.

During the 90-minute meeting yesterday, Foggie also asked shareholders for suggestions for the bank's new slogan.

Shareholders elected to the bank's 14-member board Vesharn N. Scales, president of MTI Construction Co., a division of Minority Truckers Inc. Scales replaces Pascal della Badia, who retired last year after serving 14 years on the board.