Overview

Consider the stability of acidic solutions of the elements. When looking at routes of instability, the trace analyst typically thinks of stability in connection with the concentration of the element. For example, when considering the stability of solutions at the part-per-million (ppm) concentration level, instability is generally caused by precipitation formation or photo-reduction reactions. However, the main route of instability at the part-per-billion (ppb) level is derived from adsorption to the container walls.
The stability of elemental solutions at the ppm level is more an issue of compatibility and is addressed in detail in our Interactive Periodic Table. Plus, the stability of acidic elemental solutions is typically easy to achieve. It's difficult to imagine any route of instability for most elements. Take copper, for instance. Cu at the ppm concentration level in nitric acid is stable indefinitely. However, that same solution diluted down into the low to mid ppb concentration level makes the possibility of instability (caused by adsorption) a very real concern.

Adsorption

Adsorption is divided into the following physical or chemical types:

• Physical adsorption is an attraction between the solid surface and adsorbing species consisting of van der waals interactions.
• Chemical adsorption or chemisorption is a chemical interaction which is strong enough to break or form chemical bonds.
• Types of losses such as ion-exchange, reduction, precipitation, and diffusion into a permeable solid are often also treated as adsorption.

In this guide, adsorption is taken to mean loss through the combined effects of all interactions with the walls of vessels or with filter paper.
The pH of the solution is an important consideration. Most trace analysts prefer to prepare or adjust solutions to a pH of < 2, as shown in Table 7.1 below.
Table 7.1: Effect of pH Upon the Adsorption of Inorganic Ions onto Various Materials¹

Ion; Concentration; Duration of Experiment (h = hour)	Material	pH where Adsorption IS significant	pH where Adsorption IS NOT significant
Ag; 1ppm; 1h	PE; PTFE	pH 2-12	"acid solution"
Al(III); 1-10ppm; 24h	borosilicate glass	pH 3.5-11	pH<1.5; pH>13
Ca(II); 0.5-2ppm; 24h	borosilicate glass	pH 8-12	pH 1.5
Cr(III); 1-5ppm; 24h	borosilicate glass	pH 3.5-12	pH 1.5
Pb(II); 10-13ppm; 24h	borosilicate glass	pH 3.5-12	pH 1.5

Attempts have been made to prevent adsorption by complex formation, as shown in Table 7.2, but the use of relatively high levels of reagents increases the risk of contamination. Furthermore, it tends to influence the stability and chemistry of other analytes of interest.
Table 7.2: Prevention of Adsorption by Complex Formation¹
Attempts to remove adsorbed ions typically require extreme conditions and are only partially successful, as seen in Table 7.3.
Table 7.3: Desorption of Adsorbed Inorganic Ions¹

PPB Stability Study

An adsorption test was conducted at Inorganic Ventures in an attempt to better understand the stability of mixed element solutions at the ppb concentration level in low density polyethylene (LDPE) bottles. The stability of metals at the ppb level in this container material was of significant concern.

Experimental Design

1. A blend of 65 elements from Inorganic Ventures / IV Labs'CMS-SET was prepared at the 0, 2, 10, and 100 ppb concentration level in 1 % (v/v) HNO₃ at the start of the study.
2. The set consists of the following:
   • CMS-1 - 10 µg/mL Ce, Dy, Er, Eu, Gd, Ho, La, Lu, Nd, Pr, Sm, Sc, Tb, Th, Tm, U, Yb, Y in 3.5 % HNO₃
   • CMS-2 - 10 µg/mL Au, Ir, Pd, Pt, Re, Rh, Ru, and Te in 3.5 % HCl
   • CMS-3 - 10 µg/mL Ge, Hf, Mo, Nb, Ta, Sn, Ti, W, and Zr in 3.5 % HNO₃ tr. HF
   • CMS-4 - 10 µg/mL Sb, As, Ba, Be, Bi, B, Cd, Ga, In, Pb, Se, Tl, and V in 3.5 % HNO₃
   • CMS-5 - 10 µg/mL Ag, Al, Ca, Cs, Cr⁺³, Co, Cu, Fe, Li, Mg, Mn, Ni, K, Rb, Na, Sr, and Zn in 3.5 % HNO₃
3. Only LDPE bottles (500 mL) were used.
4. The LDPE bottles were acid leached with 1% nitric acid for 59 hours at 60 °C. New blends prepared in the same way were compared to the original preparation at 1, 3, 25, 75, 137, 300, and 375 days.
5. The New blends were compared to the original blend using ICP-MS and the relative % loss was calculated.
6. The ICP-MS used is in a clean room limiting environmental contamination (opened 1 % nitric acid solutions in similar LDPE bottles placed around the auto-sampler yielded no detectable environmental contamination at times of ~ 100 hours).
7. Measurements of each blend were made in the same LDPE bottle i.e. the blend was not exposed to any other container during the study.

Experiment Results

1. Hg was not stable long enough to measure (minutes).
2. Au was the next most unstable element, showing instability at the 2, 20, and 100 ppb levels at 3 days.
3. Pd showed instability only at the 2 and 10 ppb levels at 3 days.
4. Pt and Ta showed instability only at the 2 and 10 ppb levels at 137 days.
5. Ag showed instability only at the 10 and 100 ppb levels at 137 days.
6. Mo, Sn, and Hf showed instability only at the 2 ppb level at 375 days.
7. Ir showed instability only at the 2 ppb level at 300 days.
8. All other elements showed no instability at 2-100 ppb for 375 days, including:
   Ce, Dy, Er, Eu, Gd, Ho, La, Lu, Nd, Pr, Sm, Sc, Tb, Th, Tm, U, Yb, Y, Re, Rh, Ru, Te, Ge, Nb, Ti, W, Zr, Sb, As, Ba, Be, Bi, B, Cd, Ga, In, Pb, Se, Tl, V, Al, Ca, Cs, Cr+3, Co, Cu, Fe, Li, Mg, Mn, Ni, K, Rb, Na, Sr, and Zn.

Data - Elements with Instability:
The stability curves for the above elements listed as having some form of instability are shown below:

Figure 7.1: Gold (chloride)	Figure 7.2: Palladium (chloride)
Figure 7.3: Platinum (chloride)	Figure 7.4: Tantalum (fluoride)
Figure 7.5: Silver (I)	Figure 7.6: Molybdenum (fluoride)
Figure 7.7: Tin (fluoride)	Figure 7.8: Hafnium (fluoride)
Figure 7.9: Iridium (chloride)

Summary of Findings

• The 1 % nitric acid solutions of the alkali, alkaline, and rare earth elements do not show any instability at the 2-100 ppb level in LDPE.
• The majority of elements studied were found to be stable for 1 year at the 2-100 ppb level.
• Silver (Ag) is the only unstable element found that is stable at the 2 ppb level. Ag's instability is most likely linked to its chloride chemistry (photo-reduction, precipitation).
• Gold (Au) and Mercury (Hg), which are similar in stability, are the most unstable elements and are the only elements unstable at all of the concentration levels studied. They are also reported to stabilize one another.
• Platinum (Pt), Tantalum (Ta), Molybdenum (Mo), Tin (Sn), Hafnium (Hf), and Iridium (Ir) were originally present as a fluoride or chloride complex.

Additional studies are planned for the stabilization of Hg and Au at the ppb level and will be reported when complete. It is hoped that conditions can be found that will stabilize these elements, making LDPE (the cleanest of plastics) as the preferred container material for the containment of all of the elements at the ppb concentration level.