The results of chemical analyses shall be expressed in parts per
million, which in most analyses is practically equivalent to milligrams
per liter. In some laboratories other forms of expression have been
used. Results expressed in parts per 100,000 or in grains per gallon may
be transformed to parts per million, or conversely, by the use of the
following table:
Table 4.—FACTORS FOR TRANSFORMING RESULTS OF ANALYSES.
───────────────────────────────┬───────────────────────────────────────
Unit. │ Equivalent.
───────────────────────────────┼─────────┬─────────┬─────────┬─────────
│ Grains │ Grains │ │
│per U.S. │ per │Parts per│Parts per
│ gallon. │Imperial │100,000. │million.
│ │ gallon. │ │
───────────────────────────────┼─────────┼─────────┼─────────┼─────────
1 grain per U. S. gallon │ 1.000│ 1.20│ 1.71│ 17.1
1 grain per Imperial gallon │ .835│ 1.00│ 1.43│ 14.3
1 part per 100,000 │ .585│ .70│ 1.00│ 10.0
1 part per million │ .058│ .07│ .10│ 1.0
───────────────────────────────┴─────────┴─────────┴─────────┴─────────
The following general rules shall govern the use of significant figures
in the expression of results:
1. If the results show quantities greater than 10 parts per million use
no decimals; record only whole numbers. If the quantities reach hundreds
and thousands of parts record only two significant figures.
2. If the results are between 1 and 10 parts do not retain more than one
decimal place.
3. If the results are between 0.1 and 1 part do not retain more than two
decimal places.
4. Estimates of ammonia, albuminoid, and nitrite nitrogen alone justify
the use of three decimals.
5. If the results of analyses are tabulated ciphers should not be added
at the right of the decimal point to make the column uniform.