Question

After the HCl solution is cooled down to room temperature, you will be asked to adjust its pH to ~3.5 using sodium citrate and litmus paper. If you run out of citrate, which of the following salts would you select:

Tip: you may want to review Table 9-2

Sodium piperazine-N,Nʹ-bis(3-propanesulfonate) (PIPPS)

KCl

Sodium borate

NaOH

Determination of Iron in Tablets

In this procedure, iron from a vitamin supplement tablet is dissolved in acid, reduced to Fe2+ with

hydroquinone, and complexed with o-phenanthroline to form an intensely colored complex (Color

Plate 15 in the textbook).

Procedure

1.

Place one tablet of the iron-containing vitamin in a 125-mL flask or 100-mL beaker and boil gently

(in a fume hood) with 25 mL of 6 M HCl for 15 min. Filter the solution directly into a 100-mL volumetric

flask. Wash the beaker and filter several times with small portions of water to complete a quantitative

transfer. Allow the solution to cool, dilute to the mark and mix well.

Dilute 5.00 mL of this solution to 100.0 mL in a fresh volumetric flask. If the label indicates that the

tablet contains <15 mg of Fe, use 10.00 mL instead of 5.00 mL.

2.

Pipet 10.00 mL of standard Fe solution into a beaker and measure the pH (with pH paper or a

glass electrode). Add sodium citrate solution 1 drop at a time until a pH of ~3.5 is reached. Count the

drops needed. (It will require about 30 drops.)

3.

Pipet a fresh 10.00-mL aliquot of Fe standard into a 100-mL volumetric flask and add the same

number of drops of citrate solution as required in step 2. Add 2.00 mL of hydroquinone solution and

3.00 mL of

o

-phenanthroline solution, dilute to the mark with water, and mix well.

4.

Prepare three more solutions from 5.00, 2.00, and 1.00 mL of Fe standard and prepare a blank

containing no Fe. Use sodium citrate solution in proportion to the volume of Fe solution. (If 10 mL of

Fe requires 30 drops of citrate solution, 5 mL of Fe requires 15 drops of citrate solution.)

5.

Determine how many drops of citrate solution are needed to bring 10.00 mL of the iron tablet

solution from step 1 to pH 3.5. This will require about 3.5 or 7 mL of citrate, depending on whether 5

or 10 mL of unknown was diluted in the second part of step 1.

6.

Transfer 10.00 mL of solution from step 1 to a 100-mL volumetric flask. Add the required amount of

citrate solution determined in step 5. Then add 2.00 mL of hydroquinone solution and 3.0 mL of

o

-

phenanthroline solution. Dilute to the mark and mix well.

7.

Allow the solutions to stand for at least 10 min. Then measure the absorbance of each solution at

510 nm in a 1-cm cell. (The color is stable, so all solutions may be prepared and all the absorbances

measured at once.) Use distilled water in the reference cuvet and subtract the absorbance of the

blank from the absorbance of the Fe standards.

8.

Make a graph of absorbance versus micrograms of Fe in the standards. Find the slope and

intercept (and standard deviations) by the method of least squares. Calculate the molarity of Fe(

o

-

phenanthrolinein each solution and find the average molar absorptivity

(ε

in Beer's law) from the four

absorbances. (Remember that all the iron has been converted to the phenanthroline complex.) +

9.

Using the calibration curve, find the number of milligrams of Fe in the tablet. Use Equation 4-16 in

the textbook to find the uncertainty in the number of milligrams of Fe.

Answer 1

I don't think you have provided the experimental spectroscopic data for constructing the calibration graph, still the first half of the question can be answered.

HCl is one of the strongest acids. To lower the pH of this acid we can use any base or a salt made up of strong base and a weak acid. The use of base for lowering the pH may be clear for you as the base will neutralize the acid and we can vary the concentrations of the base and acid to get the desired pH or the [H⁺].

Now I want to talk about the salts from weak acids and strong bases. These salts on hydrolysis result in a basic solution and thus can be useful to lower down the pH of HCl.

We can thus say that sodium borate and NaOH qualify our demands and can be used to lower the pH. But we have been asked the alternative for sodium citrate i.e., a salt of strong base, NaOH and a weak acid, citric acid. The correct option should be sodium borate which is a salt of NaOH and boric acid.

I hope you benefit from the answer. Have a great day ahead!!