Stability of Dilute Solutions of Uranium, lead, and Thorium Ions ROBERT
U. S.
G. MILKEY
Geological Survey, Washington 25, D.
C.
Standard solutions and samples containing a few micrograms of metallic ions per milliliter are frequently used in determination of trace elements. It is important to know whether the concentrations of such solutions remain constant from day to day. The stability of dilute solutions of three metallic ions-uranium, lead, and thorium-has been investigated. Solutions containing concentrations of metallic ions, ranging from 1000 to 0.1 y per milliliter, were allowed to stand for approximately 2.5 months, and then the metallic ion content of those solutions that had lost strength was determined. Both adsorption and hydrolysis variously influenced the solute loss, but the minimum pH at which loss of concentration of lead and uranium occurred seenied to coincide with the pH at which the hydrolyzed metal ions began to precipitate. No increase in the stability of the solutions was obtained by substituting polyethylene containers for borosilicate glass. The solutions that lost strength could not be restored promptly to the original concentration by manual means, such as shaking them vigorously for several minutes.
P
ART of a progiani undertaken by the U. S. Geological Survey, on behalf of the Division of Raw Materials of the Atomic Energy Commission, involves the chemical determination of trace amounts of uranium, lead, and thorium. The sample may be a natural !later that is itself a very dilute solution of the metal ion. .Ilso, very dilute standard solutions are often prepared for use in constructing standard curves. The question arises as to the stability of these solutions. It is possible that their strength may change overnight. Of corollary importance is the type of container used to hold the solution. A solution stored in glass might suffer a grezter loss in concentration than it would if stored in a container made of different material such as plastic-polyethylene, for example. The scope of this research embraces three objectives: to determine the relationship betveen the metal ion concentration and the stability of dilute solutions of a metal; to determine the relationship between the hydrogen ion concentration and the stability of dilute eolutions of a metal; and to determine the relative stability of solutions stored in borosilicate glass and polyethylene bottles.
The solubility product is not always a constant. Its value may be changed somewhat under varying conditions-for example, when hydrolysis results in the formation of a colloidal solution rather than the formation of discrete particles of precipitate. Adsorption on Walls of Container. 4 gas or solute in solution brought in contact with a solid substance has a tendency to collect on the surface of the solid. This adsorption may be of two different types: chemical adsorption (chemisorption), and physical (van der Waals) adsorption. Chemisorption is characterized by definite electron bonds similar to the forces binding two reactants in a chemical compound. Chemisorption is highly specific, its occurrence depending upon the chemical nature of both the surface and the adsorbing ion. Van der Waals adsorption, on the other hand, results from physical, attractive forces similar to those that cause the condensation of a vapor; this adsorption affects all molecules in varying degrees. Such physical adsorption involves much smaller activation energy values than chemisorption and, especially at room temperatures, will probably account for most of the adsorption. The extent to which the adsorption will take place depends on such factors as concentration of solute, nature of the solvent, size and valence of adsorbed ion, and dissociability of the adsorption complex. Ionic Exchange. This adsorption is characterized by the removal of an ion from the surface lattice and its replacement by an ion from the solution. A clean glass surface is assumed to consist of a network of ions such as silicon-oxygen-silicon and also silicon-oxygen-sodium, or similar groups. The effect of acid on the glass is to convert groups of the silicon-oxygen-sodium to hydrated silicon-hydroxyl
Table I.
3letallic Ion Concentration of Solutions Tested and Corresponding pH Values
Amount in Solution, y/hll.
Uranium solutions
100
DISCUSSION O F THEORY
The three principal factors that cause a change in the concentration of the solutions are hydrolysis, adsorption, and ion e\change. Hydrolysis and Precipitation of Metal Ion. The hydrous hydroxide of the metal d l theoretically precipitate when the solubility product has been exceeded. Thus, the relationship betmen the acidity of the solution and the amount of metal remaining in solution ran be expressed, for the metal with
1 2 3 4.5
0 1
0 1
2 3 3.76 4.82 6.5"
2 3 5.11
0 1
2 3 4.2
1
Kw is the ionization constant for water, M is the equilibrium concentration of the metal ion P
0
1 2 3 4.5
Thorium solutions
10
0.1
where
0
p H Values Lead solutions
is the solubility product of the metal hydroxide 0
This relationship would hold whenever the hydroxide is present as the solid phase.
In polyethylene only.
b I n borosilicate glass only.
1800
6.4)
7.1
0 1 2 3 3.43 8.1
1801
V O L U M E 2 6 , NO. 1 1 , N O V E M B E R 1 9 5 4 groups) whereas t,he presence in solution of suitable metallic ions results similarly in their eschmge wit,h the sodium in the lattice. These theoretical conderations are useful in helping t o interpret the esperimental result,s. A S ALYTICAL PROCEDURE
Apparatus. Beckman 111- quartz spectrophotometer. Reckman p H meter. Geological Survey reflection-type fluorimeter ( 3 ) . Preparation of Solutions. For each of three metals-uranium, thorium, and lead-a stock solution was prepared from the cmhemically pure nitrate salts, containing IO mg. of metal per milliliter of solution. By successive dilutions of the stock solutions and adjustment of the acSitlit,y with nitric acid and ammonium hydroxide, solutions were prepared with varying concentrations of metallic ion and hydrogen ion (Table I). Each solution was divi(led into two parts; one part was placed in a borosilicate glass hott!e, and the other part was placed in a polyethylene bot,tle. The PI-I of each solution was again measured. The bottles w-ei'e tightly stoppered, and the stoppers were sealed with Pliofilm. The solutions \vere allowed to stand for approximately 2.5 niont,lir. Determination of Metal Content. The uranium in all solutions and in t,he stncli wlution was determined fluorometrically (3). 1,ead \vas determined b y the rlithiznTie method ( 5 ) . For solutions containing 1000, 100, and 10 y of thorium per milliliter, the thorium \KLS detarrniued using sodium alizarin sulfonate. The 1111of a suitjiihle aliquot was adjusted with dilute ammoriiuin hytliusitle to approxima.tely 3.5, Three milliliters of formic aciti-sodium formate buffer were then added. One milliliter of sodium alizarin sulfonate (0.0855 gram per liter) was added, and the solution diluted to a volume of 25 ml. The absorbance of the solution was determined a t a wave length of 540 mp and slit widt,h of 0.03 mm. For solutions containing I and 0.1 y of thorium per milliliter, thorium was det,ermined colorimet,rically with I-(o-arsonophenylazo)-2-naphthol-3,6-disulfonic acid (6). DISCUSS103 OF RESULTS
Table I1 rho\\-s which solutions exhibited a significant change in concentration, the initial and final pH values, the percentage of original concentration remaining after standing, and the strength of solutions tested after 1 minute of vigorous shaking. A gain or loss of 7% in solute concentration was attribut.ed to the allowahle experimental error of the fluorimetric and colorimetric met,hods. For thorium solutions containing 0.1 y per milliliter, the error is prohably grenter than i % ,and the concentration figures for these solut,ioris are r:duahle chiefly to indictit,? order of magnitude. Effect of Acidity and Metal Concentration. URASIL-M.Conductivity experiments ( 1 ) have shown that the product of the hydrolysis of uranium begins to precipitate a t a p H of about 4.2. Every uranium solution t,hat lost strength had a p H value of 4.3 or greater, 11-hereas none of the uranium solutions with p H values of less than 4.2 decreased significantly in concentration, despite a range of uranium concentration from 1000 to 0.1 *, per milliliter. Moreover, of the 17 uranium solutions that had init,ial p H vdues of 4.3 or greater, 9 did not sho.iv significant changes in Concentration. Possibly a colloidal solution resulted from the hydrolysis in these solutioiis rather than the format.ion of R discrete precipitate. I,EAD. Conductivity esperinients ( 1 ) have shown that for motlerate concentration of the nitrate solution, the product of the hydrolysis of lead begins to precipitak at a pH of 5.6 t o 6 0. Thesix lead solutions that lost st'rength all had p H values of 5.6 or greater. KO solut,ions with initial p H of less than 5.6 decreased significantly in concentration, despite a range of lead(I1) concentration from 1000 to 0.1 -/ per milliliter. Moreover, three lead solutions, nit8h initial pH values of 6.0 or greater, also did not decrease significantly in concentration. These solutions could also have been colloidal. THORIUM. Conductivity experiments (1) have shown that the product of hydrolysis of thorium begins t o precipitate at a p H of about 3.i. No thorium solutions with pH of 3.0 or less
showed loss of strength. All thorium solutions with p H values of 3.7 or greater showed significant loss in concentration. 41though solutions containing 1000 y per milliliter a t a final p H of 3.6 showed no change in concentration, preliminary esperiments indicate that when the thorium concentration is as low as 0.1 y per milliliter, appreciable losses occur at pH of about 3.4. It is evident that both concentration and acidity are factors affecting the loss in strength of solutions. When the metallic ion concentration is in the range of 1000 to 0.1 y per milliliter, lead ion solutions and uranyl ion solut,ioris that were too acid for hydrolysis to occur also showed no losses from adsorption. So if the pH of these solut'ions is adjusted to prevent loss by hydrolysis and precipitation, the solution is a l ~ oprotected against loss hy adsorption.
T a b l e 11. Solutions of 3Ietallic Ions that Diminished i n Concentration after Standing about 2 Months
Sol11tiou
; C D E
1.' G €5
2
C
D E F
iC
D E
r
G H I .I I
