102 DEPARTMENT BULLETIN 1216, U. S. DEPT. OF AGRICULTURE 
maintained in an upright position in order to enable the lead acetate to attack 
all portions. 
After being immersed about three minutes the sample should be taken out 
and the adherent lead carefully removed under running water in order that no 
burnishing action will cause the lead to plate onto the sample. Four immer- 
sions of three minutes each are usualy sufficient to remove all zinc from the 
sample, when the bright surface of the iron will be exposed. The test pieces 
should then be washed in water, dried carefully, and weighed. The loss in 
grams divided by the number of samples represents the weight of coating in 
grams, which is numerically equal to the weight of coating in ounces per 
square foot. 
SULPHURIC ACID PERMANGANATE METHOD (ALTERNATE 
METHOD) 
SOLUTIONS REQUIRED 
Distilled water should be used in all cases where water is mentioned as a 
reagent. 
Standard potassium permanganate, one-tenth normal. — Dissolve pure, crystal- 
lized potassium permanganate in water in the proportion of 3.2 grams per liter. 
Allow to stand at least two weeks and filter through asbestos. Standardize the 
solution in the following manner: 
Weigh accurately 0.45 to 0.5 grams of United States Bureau of Standards' 
sodium oxalate. Place in a 400-cubic centimeter beaker and dissolve in 250 
cubic centimeters of water at 80° to 90° C. Add 10 cubic centimeters of sul- 
phuric acid composed of 5 cubic centimeters of water and 5 cubic centimeters 
of chemically pure H2SO4 (specific gravity 1.84) and titrate the solution at once 
with the potassium permanganate solution, stirring the liquid vigorously and 
continuously. The permanganate must not be added more rapidly than 10 to 15 
cubic centimeters per minute, and the last 0.5 to 1 cubic centimeter must be 
added dropwise. with particular care to allow each drop to be completely decol- 
orized before the next is introduced. The solution should not be below 60° C. 
by the time the end point is reached. More rapid cooling may be prevented by 
allowing the beaker to stand on a small asbestos-covered electric hot plate 
during the titration. The use of a small thermometer as a stirring rod is most 
convenient. Calculate the iron equivalent of the permanganate solution, using 
the iron value furnished by the Bureau of Standards for the sodium oxalate 
used. 
Sulphuric acid. — Mix 100 cubic centimeters chemically pure H 2 S0 4 (specific 
gravity 1.84) with 900 cubic centimeters of water. 
METHOD 
Carefully weigh the 2% by 2% inches (or equivalent area) piece, containing 
exactly 5.08 square inches. 
After determining the weight of the plate, place the piece in a 250-cubic 
centimeter breaker (Griffin form). Allow the piece to rest on or come 
in contact with a small piece of platinum foil. Add 125 cubic centimeters or a 
sufficient amount of the sulphuric-acid solution to cover the inclined piece. 
Cover the beaker with a watch ^lass and allow the zinc coating to dissolve. 
In from 10 to 20 minutes the zinc will be dissolved, as will be indicated by 
the cessation of the rapid evolution of gas, disregarding the small amount 
of gas coming mainly from the edges of the plate, due to the action of the 
acid on the base plate. Wash the plate with a jet of water from a wash 
bottle, allowing the washings to run into the beaker. Wipe the plate dry. and 
immediately titrate the solution in the beaker with the standard potassium 
permanganate solution and calculate the weight of iron dissolved with the 
zinc coating. Weigh the plate and subtract this weight from the weight of 
the plate before removal of the coating. From the difference deduct the 
weight of iron found to have been dissolved with the coating, which will give 
the weight in grams of zinc coating on 5.08 square inches of the sample. The 
weight of the zinc coating in grams on the 2*4 by 2% inch (or equivalent area) 
piece will be numerically equal to the weight of zinc coating in ounces per 
square foot. 
