(b) Copper, foil strips for use

foil strips for use as an accelerator.

(c) Oxygen, minimum 99.6 percent pure.

(a) Potassium iodate, standard solution (1 ml = 0.0001 g S). Dissolve 0.207 g of KIO in 900 ml of KIO3 water, and dilute to 1 liter in a volumetric flask. (Note: Experimental work indicates that the recovery of sulfur as sulfur dioxide is 93 to 95 percent complete with the apparatus and procedure used. The standard iodate solution is prepared by weighing 93 percent of the stoichiometric amount of potassium iodate.)

(e) Starch solution (9 g per liter).

Transfer 9 g

Pour

of soluble starch to a small beaker, add 5 to 10 ml of
water, and stir until a smooth paste is obtained.
the mixture slowly into 500 ml of boiling water. Cool,
add 15 g of KI, and stir until the KI is dissolved.
Dilute to 1 liter.

Procedure

(a) Adjust the temperature of the furnace to 1425 to 1440°C.

(b) Add approximately 65 to 70 ml of HCl (3+197) and vessel. Pass

2 ml of starch solution to the titration

oxygen through the system and add a small amount of

KIO (1 ml = 0.0001 g S) until the intensity of the blue

ΚΙΟ

3

color is that which is to be taken as the endpoint of the

final titration.

Read the buret and record as the initial

reading. Turn off the oxygen.

(c) Transfer a 1 g sample, weighed to the nearest

1 mg, to the preignited combustion boat, spreading the
sample evenly over the length of the boat. Select as a
control, a sample whose sulfur content and alloy
characteristics are similar to those of the sample being
analyzed. Cover the sample with approximately 0.2 g of
copper foil.
Place a preignited cover on the boat and
introduce the boat and contents into the center of the
combustion zone of the combustion tube. Close the tube
and allow the sample to heat for 1.5 min. Start the
flow of oxygen at a rate of approximately 1.5 liters per
min.

(d) As the stream of gas begins to bubble through the
absorption solution, the blue color will fade. Titrate
continuously with standard KIO solution (1 ml = 0.0001 g S
at such a rate as to maintain as nearly as possible, the
initial intensity of the blue color. A 5 to 6 minute
combustion period will usually suffice. Near the end of
the combustion period, carefully adjust the blue color of
the endpoint to that taken initially, paragraph (b).
Read the buret and record the reading. Subtract the
initial reading. The difference is the volume of KI
KIO3
solution required for titration of the sample.

Blank

Make a blank determination, following the same procedure and using the same amounts of all reagents.

Some refractory boats may contain sulfur compounds
liberated only in the presence of a burning sample,

and each lot should be checked on a low sulfur standard
to insure proper performance.

C = sulfur equivalent of the KIO2 solution, in

A- High temperature furnace

B - Silicon carbide heating elements
C- High temperature thermocouple
D-Combustion tube

Figure 2. Schematic of combustion-titration method for sulfur.

The most reliable methods for the determination of

silicon in white cast iron involve solution of the sample in a mineral acid or a mixture of these acids, dehydration with sulfuric or perchloric acid, and final purification of the residue by volatilizing the silica with hydrofluoric acid [2,3,5,8].

In this method, the use of perchloric acid as the dehydrating acid has the advantage that most anhydrous perchlorates are readily soluble; thus, little difficulty is encountered in dissolving the salts after the silicic acid has been dehydrated. Since single evaporations never yield all of the silica, double dehydrations are recommended.

Scope

This method is recommended for the determination of

silicon in the range of 0.3 to 4.0 percent.

The relative

standard deviations at the low and high ranges are 0.0004 and 0.002 percent, respectively.

Principle of Method

After dissolution of the sample, silicic acid is dehydrated by fuming with perchloric acid. The solution is filtered, and the silica is ignited, weighed, and then volatilized with hydrofluoric acid. The residue is

ignited and weighed. The loss in weight represents silica (SiO2).