Page images
PDF
EPUB

(a) Mixed acids. Add 100 ml of H2SO4 to 525 ml of water slowly, while stirring. Cool, add 125 ml of HPC

and 250 ml HNO3, and mix.

H3PO4

=

0.001 g Cr).

(e) Potassium dichromate solution (1 ml
Dissolve 2.830 g of potassium dichromate (K2Cr207) in

water, transfer to a 1-liter volumetric flask, dilute to
volume, and mix.

(f) Silver nitrate solution (8 g per liter).

Dissolve 8 g of silver nitrate (AgNO3) in water and

dilute to 1 liter.

(g) Sodium arsenite solution (54 g per liter). Dissolve 54 g of sodium arsenite (NaAsO2) in water and

dilute to 1 liter.

=

(h) Sodium arsenite, standard solution (1 ml 0.0008 g Mn). Dilute 100 ml of the sodium arsenite solution described in Paragraph (g) to 1 liter and filter, if not clear.

dioxide.

Procedure

Saturate the solution with carbon

(a) Transfer a 1 g sample, weighed to the nearest 0.5 mg, to a 250-ml beaker.

(b) Add 30 ml of water, 30 ml of mixed acids and

2-3 drops of HF. Heat until dissolution of the sample
is complete, and boil the solution until brown fumes have
been expelled. Dilute to 75 ml with water, filter through
a coarse paper, collecting the filtrate in a 500-ml

Erlenmeyer flask. Wash the residue with hot water,

adjust the volume to 125 ml and proceed as directed in paragraphs (d) and (e).

(c) Dissolve approximately the same weight of iron as the weight of the sample in 30 ml of the mixed acids. Transfer by pipet a portion of the manganese solution. (1 ml = 0.0008 g Mn) to the iron solution. The amount of manganese added should be approximately the same amount as expected in the sample. Dilute to 130 ml with

water and proceed as directed in paragraphs (d) and (e). Use this solution for the standardization of the sodium arsenite solution.

(d) Add 10 ml of AgNO3 solution and 15 ml of (NH4)2S208 solution. Heat the solution to boiling, and boil briskly for 90 sec. Cool to 5 to 10°C in an ice

bath.

(e) Transfer the solution to a 400-ml beaker and rinse the flask with 50 ml of cold water (5 to 10°c).

Position the electrodes of the titrimeter in the

solution, stir vigorously, and titrate with sodium

arsenite solution until a maximum change in potential is indicated.

(f) Calculate the manganese equivalent of the arsenite solution as g of manganese per ml.

[blocks in formation]

4. PHOSPHORUS IN WHITE CAST IRON

In this method, phosphorus is converted to the blue complex by adding a solution containing hydrazine sulfate and ammonium molybdate and heating at 98°c for several minutes. The transmittance or absorbance is read at 650 mu. Other widely used methods for the determination of phosphorus in ferrous materials involve its precipitation as the yellow ammonium phosphomolybdate and subsequent conversion to magnesium pyrophosphate or by titration of the phosphomolybdate with standard alkali solution. These methods are dependable, but time-consuming [3].

More rapid and sensitive photometric methods which

utilize the molybdenum-blue reaction are generally preferred because of their relative freedom from interferences by alloying elements [7].

Scope

This method is recommended for the determination of phosphorus in the range of 0.01 to 0.8 percent. The relative standard deviations at the low and high ranges are 0.002 and 0.01 percent, respectively.

Principle of Method

Phosphorus as the ortho acid forms a blue complex with ammonium molybdate-hydrazine sulfate. Photometric measurement is made at approximately 650 mu.

Concentration Range

The recommended concentration range is from 0.001 to

0.04 mg of phosphorus in 50 ml of solution, using a cell

path of 2 cm.

Stability of Color

The color develops within 5 minutes at 98°C and is stable for at least 24 hours.

Interferences

Arsenic above 0.05 percent interferes and must be removed by volatilization with HBr.

Reagents

(a) Ammonium molybdate solution (2 percent in 11 N H2SO4).

Add 300 ml of H2SO4 (sp gr 1.84) to 500 ml of water and cool. Dissolve 20 g of ammonium molybdate in the acid and dilute to 1 liter with water.

(b) Hydrazine sulfate solution (0.15 percent).

Dissolve 1.5 g of hydrazine sulfate in 1 liter of water. (c) Ammonium molybdate-hydrazine sulfate solution. Dilute 25 ml of ammonium molybdate solution to 80 ml with water, add 10 ml of the hydrazine sulfate solution and dilute to 100 ml with water. Prepare this solution as needed.

(a) Sodium sulfite solution (10 percent).

Dissolve

100 g of anhydrous sodium sulfite in 500 ml of water and

dilute to 1 liter.

« PreviousContinue »