International Bank, the diversified Washington company whose operations range from insurance and finance to manufacturing and maritime services, reported a 31 percent gain in second quarter earnings and a 22 percent boost in profits for the six months, setting new records in both periods.

For the three months ended June 30, IB earned $4.2 million (58 cents per share) up from $3.2 million (45 cents) the previous year.

First half net income was $7.6 million ($1.07 per share) up from $6.2 million (88 cents).

Revenues were $5.5 million, for the quarter, down from $5.9 million the previous year because of discontinued operations. First half revenues totalled $10.5 million for both periods.

IB Chairman George Olmstead broke down six-month earnings from IB's operating groups this way: International, 14 cents a share, down 56 percent because of the sale of Kuwait Financial Center; property and casualty insurance, 51 cents, up 54 percent; life insurance, 15 cents, up 12 percent; industrial 56 cents, down 5 percent, mostly because of a strike at a plant. The finance and leasing group lost the equivalent of 2 cents per share, 50 percent less than its loss the previous period.

Potomac Electric Power Co., reported net income of $36.8 million (78 cents a share) for the first seven months of 1978, down from $39.6 million, (81 cents) for the same period a year ago.

Pepco's operating revenues for the seven months ended July 31 increased from $358 million to $384 million.

For the 12 months ended July 31, Pepco earned $147.2 million ($1.77) up from $143.3 million ($1.66). Revenues for the latest 12-month period were $690 million, up from $603 million.

Capital Mortgage Investments of Chevy Chase announced yesterday it has added $1 million to its reserve for possible losses and its restating its previously reported earnings to reflect the change.

The $1 million reserve was necessary because of "reassessment of the carrying value of one of its assets," a property that is being offered for sale, the real estate investment trust said.

The property was not identified, but the company said offers received for it were "materially different" than its $4.3 million value on Capital Mortgage's books.

As a result of the change, the firm reported an operating loss of $1.6 million and a net loss, after an extraordinary credit, of $929,000 for the quarter, and an operating loss of $1.9 million and a net loss of $111,000 for the six months.

Previously Capital Mortgage had reported a quarterly operating loss of $605,000 and a net income, after the extraordinary credit, of $71,000 (4 cents a share). Previously Capital Mortgage reported an operating loss of $875,000 for the six months and a final profit of $889,000 (53 cents per share).

In 1977 the company had quarterly operating earnings of $82,000 (5 cents) and a final net of $674,000 (40 cents). The first half of 1977 showed an operating loss of $2.3 million and a final net of $655,000.

Federal Realty Investment Trust of Chevy Chase reported a 13 percent gain in earnings for the first half of the year and a 7 percent gain in second quarter profits and said it set new records for assets, gross income and cash flow.

For the first half of the year Federal REIT earned $895,000 (63 cents per share) on gross income of $3.7 million, up from $795,000 (59 cents) on income of $3.2 million for the same period a year ago.

Second quarter earnings increased to $416,000 from $387,000, while income increased to $1.9 million from $1.6 million. Earnings per share remained the same, 29 cents, for the two periods because more shares were outstanding this year.

Federal REIT noted that it had received no income from the period from two large stores which were being prepared for new tenants.

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Three weeks ago the company agreed to acquire 15 shopping centers with 2 million square feet of space from Amterre Development, a Pennsylvania firm that is liquidating its holdings.

Washington Real Estate Investment Trust said acquisition of the office building at 1901 Pennsylvania Avenue, NW, and the Wheaton Park Shopping Center were key factors boosting its income for the second quarter to $706,000 (47 cents per share) from $602,000 (41 cents per share) a year ago.

Washington REIT said a 10 percent rent increase approved by the District of Columbia for apartments and rent hikes on shopping centers helped increased the trust's quarterly income to $2.3 million from $2.1 million.

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Not included in the quarterly earnings was a $1.6 million gain from the exchange of a building at 3701 Connecticut Avenue, NW for the 1901 Pennsylvania Avenue property.

For the first half of the year, Washington REIT reported earnings of $1.2 million (80 cents per share) down from $2.7 million ($1.87 cents). The prior year's figures included a $1.6 million gain from the exchange of property.