M. V. Kharitonov.

Quantitative Determination of Lead as PbSO₄ in the Presence of Fe'''

I. Communication.

One of the most widespread methods for separating lead from other elements is the sulfate method. Meanwhile, in the principal classical literature—Klassen¹), Treadwell²), Rüdisüll³)—the procedure for precipitation is given in an extremely condensed and undeveloped form:

“If lead is present in solution as the chloride or nitrate, an excess of dilute H₂SO₄ is added to the solution, it is evaporated until the first appearance of dense white fumes of sulfuric acid, and allowed to cool. After adding a little water and stirring, it is left to stand for several hours, after which it is filtered through a Gooch crucible, washed first with water containing sulfuric acid, then with alcohol.”

Scott⁴) gives a somewhat more developed procedure in Standard Methods of Chemical Analysis: “10 g of ore, when the Pb content is less than 0.1%, are dissolved, evaporated to dryness, 15 cc of concentrated sulfuric acid are poured in, and the mixture is evaporated on a sand bath until sulfuric acid fumes appear. After cooling, 500 cc of water and 5 cc of concentrated H₂SO₄ are added, the mixture is boiled and cooled. Wash with 2% H₂SO₄. In the presence of Bi and Fe, wash with 10% H₂SO₄.” Bull⁵), for the separation of lead from lead ores, recommends that after the precipitation of PbSO₄ and evaporation, boiling and washing be carried out with 1% H₂SO₄ + 10% alcohol. Boiling, in Bull’s opinion, is necessary in order to bring Fe''' into solution.

There is nothing in the literature on how Fe''' affects the formation of PbSO₄, and whether it has any effect. Korte⁶), studying the precipitation of BaSO₄, SrSO₄, and PbSO₄ in the presence of foreign salts, came to the conclusion that PbSO₄, when formed, does not carry down other metals. Belton⁷) indicates that in the presence of potassium the results for PbSO₄ obtained are higher than the true values. Levol⁸) did not obtain accurate results when precipitating lead with alkali sulfates.

The investigation undertaken by me had as its aim to determine the influence of Fe''' on the accuracy of the determination of lead and the conditions for precipitating lead in the presence of Fe'''.

Method of Investigation.

In each experiment, 10 cc of a Pb(NO₃)₂ solution were taken, which corresponded to 0.1544 g of PbSO₄. Then a solution of Fe(NO₃)₃ was added.

¹) Klassen. Angerwäll’s Methoden der analyt. Ch. 1918.
²) Treadwell. Lehrbuch der Analyt. Ch. t. 2.
³) Rüdisüll. Nachweiss Bestimmunu und Trennung d. Ch. El. t. 3.
⁴) Scott. Standard Methods of Chem. anal. t 1927.
⁵) Bull. Z. anal. Ch. 41. 653.
⁶) S. Chem. Soc. t. 87 p. 1503, cited in B. Soc. Chim. t. 36 p. 626.
⁷) Belton. Chem. New No. 91 p. 121.
⁸) Levol. J. B. 1862 p. 607.

and then concentrated \(H_2SO_4\), and it was evaporated on an air bath until white fumes of sulfuric acid appeared, and 25 cc of 10% \(H_2SO_4\) was added. It was filtered through a Gooch crucible. The washed, dried, and ignited precipitate was treated with a hot solution of \((NH_4)C_2H_3O_2\). The ignited residue gave the figure for adsorbed \(Fe_2O_3\). The magnitude of the adsorption was determined in this way, proceeding from the considerations that the ignited \(PbSO_4\) was colored the color of iron oxide, but it is possible that the iron was partly in the form of sulfate and then the magnitude of the adsorption is still higher. All reagents were from Kahlbaum “Zur Analyse.”

Experiments.

I posed the following questions to be resolved:

1) Is it necessary or not to boil the \(PbSO_4\) precipitate obtained after evaporation of \(H_2SO_4\) and addition of 10% \(H_2SO_4\).

2) For how long should the precipitate, evaporated and treated with 10% \(H_2SO_4\), be left, or may filtration be begun immediately.

For this purpose a series of experiments was arranged as follows: to the \(PbSO_4\) precipitate, ignited until white fumes appeared, 25 cc of 10% \(H_2SO_4\) was added, and after different intervals of time it was filtered; the results are collected in Table I.

\(Pb(NO_3)_2\) was taken, 10 cc; \(Fe(NO_3)_3\)—25 cc, which is equal to 2.5 g; \(Fe(NO_3)_3\), and 5 cc of concentrated \(H_2SO_4\). The precipitate was washed with 20% \(H_2SO_4\).

Table I.

In the second series of experiments the ignited \(PbSO_4\) precipitate was boiled for some time with 25 cc of 10% \(H_2SO_4\) and after different intervals of time was filtered (Table II).

Table II.

From a comparison of these data it is evident that without boiling there may be cases of formation of a very finely crystalline precipitate, which readily passes through the filter. The amount of wash water in this case is also greater. The amount of adsorbed Fe··· is the same.

In all cases less lead is obtained, and the loss is greater the sooner filtration is begun.

In all the preceding experiments, 5 cc of concentrated H₂SO₄ was taken for the precipitation of Pb··. From the following table (Table III) it is evident that less cannot be taken. The theoretically required amount is 0.9 cc.

Table III.

With what should the precipitate be washed?

It cannot be washed with water alone, owing to the solubility of PbSO₄:

Gms H₂SO₄ in 1000 cc
of solution 0 0,0098 0,0196 0,0980 0,4900 0,9800

Grams of dissolved $\mathrm{PbSO_4}$
in 1000 c.c. of solution: 0.082 0.051 0.025 0.013 0.006 0
Sehnal¹

On the other hand, the solubility of $\mathrm{Fe_2(SO_4)_3}$ is greatly lowered by the presence of $\mathrm{H_2SO_4}$ in water.

Seidel²

Table IV.

For these experiments (Table IV) 10 c.c. of $\mathrm{Pb(NO_3)_3}$ was taken, 10 c.c. of $\mathrm{Fe(NO_3)_3}$ — 25 c.c. of concentrated $\mathrm{H_2SO_4}$ for precipitation, 5 c.c. after evaporation; 25 c.c. of 10% $\mathrm{H_2SO_4}$ was added, it was boiled and filtered after 3 hours.

In all the preceding experiments the amounts of adsorbed iron remain more or less constant, but this amount changes considerably, or rather the adsorption is reduced almost to zero, if the order of addition of the reagents is changed. The experiment was arranged as follows: 10 c.c. of $\mathrm{Pb(NO_3)_3}$ was taken, 5 c.c. of $\mathrm{H_2SO_4}$ was added, and after that 25 c.c. of $\mathrm{Fe(NO_3)_3}$ was introduced; after ignition the precipitate obtained was almost white, whereas in all the preceding experiments it was intensely colored. A test for the purity of $\mathrm{PbSO_4}$ gave 0.0001 g of $\mathrm{Fe_2O_3}$ and the amount of $\mathrm{PbSO_4} = 0.1546$. Apparently, when $\mathrm{PbSO_4}$ is precipitated under the initial conditions, a solid solution is obtained on the one hand, and a soluble complex compound of lead and iron on the other. Both, as is evident from Table V, increase with an increase in the amount of $\mathrm{Fe^{\cdots}}$.

Table V.

A completely unexpected picture is obtained in the precipitation of $\mathrm{PbSO_4}$ in the presence of $\mathrm{Fe(NO_3)_3}$ and $\mathrm{NH_4NO_3}$.

Table VI.

Other elements—Al, Cr, Mn, Cu, Cd—apparently have no effect on the accuracy of the determination of $\mathrm{PbSO_4}$.

Table VII.

Conclusions.

In the precipitation of $\mathrm{PbSO_4}$ in the presence of $\mathrm{Fe(NO_3)_3}$ it is necessary:

1) to add concentrated $\mathrm{H_2SO_4}$ in an amount five times greater than the theoretical.

2) After evaporation and subsequent addition of 10% $\mathrm{H_2SO_4}$, boiling is necessary.

3) Filtration may be begun no earlier than 3 hours after boiling.

4) The precipitate must be washed with 20% $\mathrm{H_2SO_4}$.

5) Wash with alcohol after the reaction for iron has ceased.

6) Loss of lead in the presence of Fe is inevitable.

7) The error increases in the presence of $\mathrm{NH_4}$.

Tomsk Technological Institute. 1929.

1. Sehnal C. R. 148, pp. 1394–1396. ↩

2. Seidel, Solubilities of inorganic compounds, 1919. ↩