The Dissolving Mechanisms of Cadmium and Lead in Nitric Acid

Ind. Health, 1967, 5, 60.

THE DISSOLVING MECHANISMS OF CADMIUM AND LEAD IN NITRIC ACID

Noboru HARA

National Institute of Industrial Health, Kizuki-Sumiyoshi, Kawasaki

(Received October 3, 1966)

Strong acids are very effective to destroy the organic substances and they dissolve the metallic materials at the same time. The mixed acid of concentrated sulphuric acid and nitric acid or aqua regia is suitable for this purpose, and is used widely. But they are not always effective to dissolve the metallic elements. In this experi- ment, the solubilities of cadmium and lead or their oxides in the nitric acid are determined and the dissolving mechanisms of these two metals are found not to be the same. The solubilities of lead and its oxides have their maximum at some concentration of the acid, but those of cadmium and its oxide have not. In dissolu- tion of metallic substances, therefore, concentration of acid as well as its kind should be considered.

The mechanism that metallic substances dissolve in the concentrated nitric acid (more than 1 N) is not similar to that in the diluted acid (pH 1•`7).

The author determined the solubilities of cadmium, lead and their oxides in the concentrated nitric acid.

PROCEDURE

1. Cadmium and its oxide. Fifty ml of various concentrations of nitric acid are put in 200 ml of Erlenmeyer flasks. The sufficient of metallic cadmium and its oxide are weighed accurately, and are added into these flasks little by little. The dissolving reaction occurs at once. The flasks are cooled by cold water until the violent reactions are over. And then the flasks are shaken for one hour at room temperature and left for a night as they are. They are shaken for one hour again on the next day. The contents of the flasks are filtered in vacuum through the glass filters. Cadmium in the filtrates is determined by dithizone method.1,2) The residues are washed on the glass filters with a large quantity of water to remove the adhered water-soluble materials thoroughly. Then they are dried and are weighed accurately. Cadmium. Cadmium dissolves in the diluted acid slowly generating hydrogen. Generation of heat is a little in this reaction. But cadmium dissloves in the concentrated acid violently, generating the brown gas of nitrogen

60 DISSOLVING MECHANISM OF CADMIUM AND LEAD

dioxide. The more concentrated the acid is, the more violent the reaction becomes. And the production of heat is abundant. Cadmium oxide. Cadmium oxide dissolves in the nitric acid slowly and generates heat a little.

2. Lead and its oxides. Fifty ml of various concentrations of nitric acid are put in 100 ml of Erlenmeyer flasks, and lead and its oxides are added into these flasks. In this time, the dissolving reactions are slow and generation of heat is very little. Then the flasks are shaken for one hour and filtered. The lead contained in the filtrates are determined by dithizone method3) or polarography.4)

RESULTS

There is a considerable difference between the dissolving mechanism of cadmium and that of lead. 1. Cadmium and its oxide. The solubility of cadmium in the diluted nitric acid is proportional to the concentration of the acid. But, with the rise of acid concentration, increase of the solubility drops gradually. Meanwhile, the solubility of cadmium oxide is proportiona to the concentration of the acid.

Fig. 1. Solubility of metallic cadmium. Fig. 2. Solubility of cadmium oxide. Solid line indicates the observed Dashed line shows the theorical value. Dashed lines show the amount of dissolved cadmium theoretical values assuming the oxide. reaction proceeds as follows:

61 N. HARA

2. Lead and its oxides. Both solubilities of lead and litharge in the nitric acid are considerable large and reach the maximum at 2.5 N of the acid. And they decrease rapidly when the acid concentration exceeds this value. Comparing these materials, the solubility of lead dioxide in the acid is extremely small as shown in Table 1. The solubility of minium lies between these two groups.

Fig. 3. Solibilities of lead and its oxides.

Table 1. Solubilities of litharge and lead dioxide in the nitric acid.

DISCUSSION

1. Cadmium. The dissolving mechanism of cadmium differs from that of cadmium oxide. Metallic cadmium. The dissolving process of cadmium in the nitric acid

62 DISSOLVING MECHANISM OF CADMIUM AND LEAD may proceed in the following three forms.

Cd + 2HNO3 •¨Cd(NO3)2 + H2 (1)

3Cd + 8HNO3 •¨3Cd(NO3)2 + 4H2O + 2NO (2)

•¨ 2NO2

Cd + 4HNO3 •¨Cd(NO3)2 + 2H2O + 2NO2 (3) From the results of experiments, it seems that the reaction (1) progresses in the diluted acid. When the concentration of the acide is about 2.5 N, the reaction (2) may progress. The reaction (3) perhaps takes place when the acid concentration reaches about 10 N, and generation of nitrogen dioxide is violent. Calculating from these three equations, it is known that the number of cadmium which reacts with one molecule of nitric acid is 0.5, 0.375 and 0.25 respectively. In other words, the number of nitric acid which reacts with the unit number of cadmium increases as the concentration of acid increases. Reaction (1) and (3) are shown as following models.

(1) (3)

In the reaction (1), no oxidizing action of the nitric acid takes place yet. But in the reaction (3), the generated hydrogen is oxidized to H2O by the action of the nitric acid, and the acid is reduced to nitrogen dioxide. The reaction (2), perhaps, progresses in following orders. 3Cd + 8HNO3 •¨3Cd + 2HNO3 + 6HNO3 •¨ 3Cd + (3O + H2O + 2NO) + 6HNO3 •¨ 3CdO + 6HNO3 + H2O + 2NO

•¨ 3Cd(NO3)2 + 4H2O + 2NO

The principal point of this reaction is 2HNO3•¨30+H2O+2NO. Only two per eight molecules of the nitric acid react as oxidizing agent. Therefore this reaction is situated in the middle position of (1) and (3). It is thought that the oxidizing ability of nitric acid becomes more, as the acid becomes concentrated. Cadmium oxide. Cadmium oxide dissolves in the nitric acid as is shown in following single reaction. CdO + 2HNO3 •¨Cd(NO3)2 + H2O So, the solubility of cadmium oxide in the acid is proportional to the concentration of the acid very well.

The solubilities of cadmium and its oxide which are determinated by dithizone method differ from those calculated by gravimetric method when

6 5 N. HARA the concentration of the nitric acid is very large. This is due to the squeezing out of the solid cadmium nitrate from the solution. By dithizone method, this cadmium nitrate is not determined as they do not enter into the filtrates. But the weight of cadmium nitrate are taken into account as dissolved part in gravimetric method, as they are removed by washing procedure. This discrepancy of solubility does not occur in the diluted acid, as the produced cadmium nitrate dissolves in the co-existent water thoroughly. If the sufficient amount of metallic cadmium is added into the fuming nitric acid, the reacting solution is solidified by the generated cadmium nitrate and the dissolving reaction ceases. But cadmium oxide dissolves almost completely in fuming nitric acid. These phenomene are due to generated water in the reaction.

2. Lead and its oxides. The curves of solubilities of lead and its oxides for the concentrations of the nitric acid are rather complicate. These phenomena can be explained by the way of oxidizing action of the acid. Lead and litharge dissolve in the nitric acid as follows. Pb + 2HNO3 •¨ Pb(NO3)2 + H2 PbO + 2HNO3 •¨Pb(NO3)2 + H2O

Lead nitrate dissolves in water very well. So, from the equations mentioned above, it seems that lead and its oxides apt to dissolve as the concentration of the acid becomes larger. But the more the concentrated acid is, the stronger its oxidizing power becomes. Pb+2 begins to change to Pb+4 (lead dioxide). This material is very hard to dissolve. In short, lead has two oxidizing reactions.

Pb0 •¨Pb+2 Pb+2 •¨ Pb+4 The latter reaction needs more strong oxidizing power than the former. When the concentration of the acid is over 2.5 N, Pb(NO3)2 is changed to PbO2 by the oxidizing power of the acid. So the solubilities decrease quickly. Minium is assumed to be composed of PbO2 and 2PbO. So it is natural that its solubility takes the middle value between that of litharge and lead dioxide.

REFERENCES

1) Saltzman, B. E. (1953). Anal. Chem., 25, 493. 2) Sandell, E. B. (1959). Colorimetric determination of trace of metals. 3rd ed. p. 350. Interscience Publishers, Inc., New York. 3) Sandell, E. B. (1959). Colorimetric determination of trace of metals. 3rd ed. p. 555. Interscience Publishers, Inc., New York. 4) Lingane, J. J. (1941). Chem. Rev., 29, 1.