CAUTION!

1. Sodium Carbonate Reagents

• Methyl orange indicator
• Modified methyl orange indicator
• Sulfuric acid, 1N (= 1 Normal)

2. Sodium Chloride Reagents

• Benzyl alcohol
• Iron solution indicator
• Nitric acid, concentrated
• Potassium thiocyanate, 0.05N
• Silver nitrate, 0.05N
• Sodium chloride, 0.05N

• For samples having more than 0.1 % NaCl: Weigh 10g, to the nearest 0.01g, of a well-mixed sample and transfer it to a 300-mL Erlenmeyer flask. Add 25 to 50mL of water to dissolve the sample, and carefully add 15mL concentrated nitric acid.
• For samples having less than 0.1 % NaCl: Weigh 20g, to the nearest 0.01g, of a well-mixed sample, and transfer to a 500-mL Erlenmeyer flask. Add 100mL water to dissolve the sample, and carefully add 30mL concentrated nitric acid.

3. Sodium Sulfate Reagents

• Barium chloride solution, 100 g/L
• Hydrochloric acid, concentrated
• Methyl orange indicator
• Silver nitrate solution, 50 g/L

Procedure
Weigh 50 g of well-mixed sample to the nearest 0.1 g, and transfer to a 600-mL beaker containing about 200 to 300 mL of water. Add 2 to 4 drops of methyl orange indicator solution, neutralize carefully with concentrated hydrochloric acid, and add 1 mL in excess. If the neutralized solution contains any insoluble material, filter the solution through Whatman No. 42 filter paper or equivalent. Wash the paper once with water. Heat the solution, or filtrate, to boiling. Then add dropwise 10 mL of barium chloride solution to the boiling solution and continue boiling for 3 minutes. Allow the precipitate to settle in a warm place for 4 hours, preferably overnight.

Quantitatively transfer the precipitate to Whatman No. 42 filter paper or equivalent with a fine stream of hot water from a wash bottle. Filter and wash the precipitate with small portions of hot water until the washings are free of chloride. Test for chloride with 3 to 4 drops of silver nitrate solution. Fold the washed filter paper containing the precipitate and place it in an ignited porcelain or platinum crucible, tared to the nearest 0.0001 g. (If desired, the folded, washed filter paper containing the precipitate can be placed in a clean, unglazed fireclay crucible.) Dry and carefully char without flaming. Then ignite at 850 to 900°C for at least 30 minutes. Partially cool, place in a dessicator and cool to room temperature. If a platinum or porcelain crucible was used, reweigh to the nearest 0.0001 g. Determine the weight of the BaSO₄ precipitate from the difference of the weights. If an unglazed crucible was used, brush the precipitate into a tared watch glass and weigh to the nearest 0.0001 g.

grams BaSO₄ x 60.86 = % Na₂SO₄
Weight of sample

4. Loss on Heating

Place 10 g of sample in a tared platinum or glazed porcelain dish that has been dried at 250 to 285°C. Weigh accurately and heat in an oven for 2 hours at 265 to 285°C or 4 hours at 250 to 265°C. Cool in a dessicator and weigh accurately and quickly.

Loss in weight in grams x 100 = % Loss on heating.
Weight of sample

5. Iron (Fe) Reagents

• Ammonium acetate solution, 50%
• Ammonium hydroxide solution, 1:1
• Hydrochloric acid. concentrated
• Hydrochloric acid, approximately 10 N
• Hydrochloric acid solution, 1.1
• Hydroxylamine hydrochloride solution, 100 g/L
• Iron solution, standard, one mL = 0.100 mg Fe
• Iron solution, working standard, one mL = 10 (g Fe)
• Nitric acid, concentrated
• Orthophenanthroline solution, 3 g/L
• Congo red indicator paper

Special Apparatus
Spectrophotometer (a filter photometer may be substituted, if no spectrophotometer is available) and absorption cells (10 mm and 50 mm)

Preparation of Sample
Weigh 100 g of well-mixed sample to the nearest 0.1 g. Transfer the sample to a 1000-mL Erlenmeyer flask. Slurry the sample with water, keeping the volume below 200 mL. Carefully acidify the sample with 200 mL of 10 N hydrochloric acid. Add the acid in small increments, swirling the sample solution between additions. Allow CO₂ gas to escape before adding the next increment. Heat to boiling on a hot plate, and boil for 1 minute to expel CO₂. Remove from the heat and allow to cool. If the cooled solution is free of scale, quantitatively transfer to a 500-mL volumetric flask. Dilute to the mark with water and mix thoroughly. If scale or insoluble material is present, decant the clear solution into a 500-mL volumetric flask. Add 5 mL of concentrated hydrochloric acid and 2 mL of concentrated nitric acid to the Erlenmeyer flask. Heat to boiling in a hood, and evaporate to near dryness. Cool and quantitatively transfer the residual solution to the 500-ml volumetric flask. Dilute to the mark with water and mix thoroughly.

6. Preparation of Blank

Evaporate 20.0 mL of 10 N hydrochloric acid to near dryness in a 250-mL beaker. If acid treatment of any scale is used in the sample preparation, evaporate a mixture of 20 mL of 10 N hydrochloric acid and 0.2 mL of concentrated nitric acid to near dryness in a 250-mL beaker. Remove from the heat and allow to cool. Transfer the evaporated residual solution to a 100-mL volumetric flask, and dilute to about 50 mL with water.

Procedure
Transfer a 50-mL aliquot of the sample as prepared above to a 100-mL volumetric flask. Carry both the sample and blank solutions through the procedure. Add 4 mL of 1:1 hydrochloric acid solution to each flask, and swirl to mix. Add 5 mL of hydroxylamine hydrochloride solution, shake well and let stand for about 5 minutes to reduce the iron to the ferrous state. Add 5 mL of orthophenanthroline solution, a small piece of Congo red indicator paper, 5 mL of ammonium acetate solution and mix. The indicator paper should now be red.

Cool, make up to the mark with water, mix well, and allow the solution to stand for 15 minutes for complete color development.

Adjust the spectrophotometer to zero absorbance at 510 nm with water in a 50-mm absorption cell (use a 10-mm cell, if the sample contains more than 80 (g Fe). Measure and record the absorbances of the blank and sample solutions. Subtract the blank absorbance from the sample absorbance to obtain net absorbance. Determine the micrograms of iron from the prepared standard curve.

Micrograms of Fe = ppm Fe
g of sample in aliquot

7. Preparation of Standard Curve

Add 20 mL of water to each of 12 100-mL volumetric flasks. Add, respectively, 0, 0.1, 0.5, 1, 2, 4, 6, 8, 10, 20, 30 and 40 mL of working standard iron solution (1 mL = 10 (g Fe). These volumes are equivalent to 0, 1, 5, 10, 20, 40, 60, 80, 100, 200, 300 and 400 micrograms of Fe. Add water, where necessary, to dilute the solution in each flask to 60 mL and mix thoroughly.

Continue as directed in the procedure with the addition of 5 mL of hydroxylamine hydrochloride solution. Adjust the spectrophotometer to zero absorbance at 510 nm as above, using water in a 50-mm absorption cell. Using 50-mm cells, measure and record the absorbance of the standard containing no added iron and then the absorbances of the remaining standards. Subtract the absorbance of the zero standard from each of the standard absorbances and plot the net absorbances against the corresponding micrograms of Fe on linear graph paper. Repeat using 10-mm absorption cells and the 0 to 400 (g Fe standards plotting the net absorbances on a separate sheet of linear graph paper.

Remarks: Before each measurement, check the instrument zero with the reference cell. Recheck the standard curve at least every 6 months by running a standard of about the same Fe content as the sample along with the sample analysis.

8. Reagents, Indicators and Standard Solutions

Preparation methods for the reagents, indicators and standard solutions required by the above procedures follow. Use reagent grade chemicals, unless otherwise specified, and distilled or deionized water.

Reagents

• Acid, Hydrochloric, Concentrated, 36-38% HCl.

• Acid, Hydrochloric, approximately 10 N — Dilute 850 mL of concentrated hydrochloric acid to the mark in a 1000-mL volumetric flask with water.

• Acid, Hydrochloric, Solution, 1:1 — To 25 mL of water add 25 mL of reagent grade concentrated hydrochloric acid (36-38% HCl). Store in a dropping bottle.

• Acid, Nitric, Concentrated, 69-71% HNO₃.

• Acid, Sulfuric, Concentrated, 96-98% H₂SO₄.

• Ammonium Acetate, 50% — Dissolve 500 g of reagent grade ammonium acetate crystals (CH₃COONH₄) in 500 mL of water in a 1500-mL beaker. Mix thoroughly and store in a polyethylene bottle.

• Ammonium Hydroxide, Concentrated, 28% NH₃.

• Ammonium Hydroxide Solution, 1:1 — To 25 ML of water add 25 mL of reagent grade ammonium hydroxide (28-30% NH₄OH). Store in a polyethylene dropping bottle.

• Barium Chloride Solution, 100 g/L — Dissolve100 grams of barium chloride (BaCl₂·2H₂O) in sufficient water to make 1000 mL. Filter if turbid.

• Benzyl Alcohol, C₆H₅CH₂OH — Dispense from an amber-glass dropping bottle.

• Hydroxylamine Hydrochloride Solution, 100 g/L — Dissolve 100 g of reagent grade hydroxylamine hydrochloride crystals (NH₂OH·HCl) in about 600 mL of water in a 1000-mL beaker. Transfer to a 1000-mL volumetric flask, dilute to the mark with water, and mix thoroughly. Store in a polyethylene bottle.

• Orthophenanthroline Solution, 3 g/L — Warm about 500 mL of water in a 1000-mL beaker to 60-65°C. Add 3.0 g of reagent grade 1,10-phenanthroline monohydrate crystals (C₁₂H₈N₂·H₂O) and stir to dissolve. Cool the solution to room temperature. Add 1 mL of reagent grade concentrated hydrochloric acid and mix. Transfer the solution to a 1000-mL volumetric flask, dilute to the mark with water and mix well. Store the solution in a polyethylene bottle.

• Silver Nitrate Solution, 50 g/L — Dissolve 5 g ofsilver nitrate (AgNO₃) inwater and dilute to 100 mL.

Indicators

• Iron Solution Indicator — To a 1500-mL beaker add 62 grams of ferric ammonium sulfate [Fe₂(SO₄)₃·(NH₄)2SO₄· 24H₂O] and 500 mL of water. Stir until the crystals are dissolved. Add 440 mL of concentrated nitric acid and mix. Store the solution in a polyethylene bottle.

• Methyl Orange Indicator — Dissolve 0.1 g of methyl orange in 100 mL of water. Filter the solution, if necessary.

• Methyl Orange Indicator, Modified — Dissolve 0.1 g of methyl orange and 0.14 g of xylene cyanole FF dye, technical grade, in 100 mL of water. Filter if necessary.

Standard Solutions

For precise analytical work, prepare standard solutions with distilled water at 20°C. Perform all titrations at this temperature or apply corrections as given in the temperature correction method given below.

• Acid, Sulfuric, Standard, 0.05-1 N.
Measure the required volume* of concentrated sulfuric acid in a graduated cylinder, and pour carefully into a volume of water equal to one half the final desired volume of solution. Rinse the cylinder into the mixture with water. Mix thoroughly, allow to cool, and dilute to the final desired volume with water. Mix well again and store in a tightly closed glass container.

* The required quantity of concentrated sulfuric acid can be approximated as:
Liters of final solution desired x Normality desired x 30.0 = mL of concentrated sulfuric acid to use.

Transfer 16 g of primary standard grade anhydrous sodium carbonate to a Pyrex weighing bottle and dry in an oven at 265 to 285°C for 2 hours or at 250 to 265°C for 4 hours. Place the cover on the weighing bottle loosely and cool in a desiccator. Weigh, to the nearest 0.0001 g, three portions of the required weight of dried sodium carbonate into three 500-mL Erlenmeyer flasks. The weights of dried sodium carbonate required for the corresponding normalities of sulfuric acid solution are shown in Table 9-1.

Add 100 mL of water to each flask, and swirl to dissolve the solid. Add 5 drops of methyl orange indicator solution or modified methyl orange indicator solution. Titrate with the sulfuric acid solution from a 100-mL buret to the pink-color end point of methyl orange indicator. If modified methyl orange indicator is used, titrate to a gray end point. Record the volume of sulfuric acid used for each titration. Correct the volume delivered to 20°C as given in the temperature correction method below. Restandardize monthly. Average the triplicate results.

g Na₂CO₃ used x 18.870 = Normality of H₂SO₄
mL of H₂SO₄ corrected to 20°C

• Iron Solution, Standard, 1 mL = 0.100 mg Fe.
Weigh exactly 0.7022 g of reagent grade ferrous ammonium sulfate hexahydrate [Fe(NH₄)2(SO₄)₂·6H₂O], and transfer it to a 1000-mL volumetric flask containing about 500 mL of water and 20 mL of concentrated reagent grade sulfuric acid (H₂SO₄). Swirl to dissolve the crystals, and dilute to the mark with water. Mix thoroughly. Discard after one month.

• Iron Solution, Working Standard, 1 mL = 10.0 micrograms Fe.
To a 1000-mL volumetric flask add about 500 mL of water. Pipet exactly 100 mL of the standard Fe solution (1 mL = 0.100 mg Fe) into the flask. Dilute to the mark with water and mix thoroughly. Prepare fresh daily as needed.

• Potassium Thiocyanate, Standard, 0.05 N.
Weigh 4.86 g of potassium thiocyanate (KSCN) to the nearest 0.01 g in a tared polypropylene weighing bottle. Using a wash bottle, quantitatively transfer the crystals through a powder funnel into a 1-liter volumetric flask. Add 200 to 300 mL of water, and swirl to dissolve the crystals. Dilute to the mark with water and mix thoroughly. Store the solution in a polyethylene bottle.

• Silver Nitrate, Standard, 0.05N.
Weigh 8.495 g of silver nitrate, (AgNO₃) to the nearest 0.001 g, in a tared polypropylene weighing bottle. Using a water wash bottle, quantitatively transfer the crystals through a powder funnel into a 1-liter volumetric flask. Add 200 to 300 mL of water and swirl to dissolve the crystals. Dilute to the mark with water and mix thoroughly. Store in a tightly stoppered amber-glass bottle. Standardize the solution as follows.

Pipet 25.00 mL of standard 0.05 N sodium chloride solution into each of three 500-mL Erlenmeyer flasks. Carry each flask through the following steps. Add 100 mL of water and 3 mL of iron indicator solution. Swirl to mix. Add 27 mL of the silver nitrate solution being standardized from a 50-mL buret, while swirling vigorously. Add 10 drops of benzyl alcohol and shake the flask for 30 seconds. Rinse down the inside wall of the flask with water. Using a 50-mL buret, back-titrate slowly and with constant swirling, with 0.05 N potassium thiocyanate solution until a faint red color persists. Average the results and restandardize monthly.

mL NaCl solution x 0.0500 ___________ = Normality of AgNO₃
[mL AgNO₃ solution - (mL KSCN solution x F*)]

*Determine the factor F as follows: Add about 9 mL of the silver nitrate solution from the 50-mL buret to a 250-mL Erlenmeyer flask, Add 100 mL of water and 3 mL of iron indicator solution. Swirl to mix. Titrate slowly with the potassium thiocyanate solution, while constantly swirling, until a faint reddish color persists.

F = mL AgNO₃
mL KSCN

Repeat the determination and use the average factor rounded off to the nearest 0.001 mL AgNO₃/mL KSCN. Determine the factor each time a new KSCN or AgNO₃ solution is prepared.

• Sodium Chloride, Standard, 0.05 N.
Dry about 3.1 g of sodium chloride (NaCl) on a large watch glass in an oven at 105 to 110°C for 2 hours. Cool the crystals to room temperature in a dessicator. Weigh 2.9221 g of the dried sodium chloride to the nearest 0.0001 g in a tared polypropylene weighing bottle. Using a water wash bottle, quantitatively transfer the crystals through a powder funnel to a 1-liter volumetric flask. Add 200 to 300 mL of water and swirl to dissolve the crystals. Dilute to the mark with water aid mix thoroughly. Store the solution in a polyethylene bottle.

9. Temperature Corrections for Volumetric Solutions

As volumetric solutions are standardized at 20°C, titrations should be made at this temperature or corrections applied to reduce the volume of the solution used in filtration to 20°C. Table 9-2 shows corrections for temperatures from 15 to 30°C. Columns labelled "mL” give the milliliters to be added (&Mac179;) or deducted (&Mac178;) for each mL used in titration. "Factor" gives the correction factor for converting the quantity of solution to standard volume at 20°C. Corrections for 0.05 N solution apply also to weaker solutions and distilled water.