Common Problems with Ag, As, S, Ba, Pb and Cr

Overview

This part of our ICP Operations guide provides some suggestions that you may find useful when attempting to work with silver, arsenic, sulfur, barium, lead, or chromium.

Silver (Ag)

Ag forms more insoluble salts than any other metal, although Pb and Hg are not far behind. For an overview of Ag stability, please see our article entitled Silver Chemical Stability.

The use of nitric acid and/or HF is preferred for preparation of samples for Ag analysis. Solutions of Ag in either acid are stable for extended periods.

• Trace levels of HCl or Cl⁻¹ must be eliminated; otherwise, a fixed error due to AgCl precipitation will result.
• If the sample preparation requires the use of HCl, attempt to keep the HCl content high (10% v/v) in an attempt to keep the Ag in solution as the AgClₓ⁻⁽¹⁻ˣ⁾ anionic chloride complex. In addition, the concentration of Ag should be ≤10 µg/mL Ag. In short, keep the HCl concentration high and the Ag concentration low.
• Solutions containing suspended AgCl and/or the AgClₓ⁻⁽¹⁻ˣ⁾ anionic chloride complex are photosensitive. The Ag⁺¹ will undergo photoreduction to the metal (Ag⁰). When intentionally working in HCl, minimize exposure to light.
• Many analysts experience low Ag recoveries when working in HNO₃ media. The problem is due to trace chloride contamination. Although analysts are aware of the problems with precipitation as the chloride, they are puzzled because no AgCl is observed. However, the metal has already photoreduced onto the container walls.
• Ag elemental data

Arsenic (As)

• Avoid using dry ashing for sample preparation. Loss during sample preparation as the volatile oxide (As₂O₃, bp 460 °C) or chloride (AsCl₃, bp 130 °C) can be avoided by performing closed-vessel digestions (EPA Methods 3051 and 3052), acid digestions under reflux conditions (EPA Method 3050B, Nitric and Perchloric Acid Digestions), or by caustic fusion using either sodium carbonate or sodium peroxide/sodium carbonate fluxes.
• Approach ICP-OES and ICP-MS determinations with caution. ICP-OES suffers from poor sensitivity and spectral interference issues, and ICP-MS from the ⁴⁰Ar³⁵Cl⁺ mass interference (other interferences include ⁵⁹Co¹⁶O⁺, ³⁶Ar³⁸Ar¹H⁺, ³⁸Ar³⁷Cl⁺, ³⁶Ar³⁹K⁺, ¹⁵⁰Nd²⁺, and ¹⁵⁰Sm²⁺) on the monoisotopic ⁷⁵As. The use of atomic absorption using either the hydride generation or the graphite furnace techniques is very popular, although the use of "reaction cells" that appear to eliminate the ⁴⁰Ar³⁵Cl⁺ interference in ICP-MS is an option worth exploring.
• As elemental data

Sulfur (S)

Conventionally, sulfur measurements are made using combustion techniques coupled with measurement of the SO₂ combustion gas by infrared, micro-coulometric, or titrimetric (iodometric) techniques. Since 1974, techniques involving ion chromatography (speciation) and X-ray fluorescence have become very popular. More recently, ICP-OES has become a viable measurement technique for sulfur due to the availability of affordable radial-view instrumentation with measurement capability in the vacuum UV spectral region and the relative freedom from spectral interferences. Popular emission lines with IDLs measured in our laboratory are shown in Table 13.2:

The following tips may prove useful in the preparation and solution chemistry of samples for sulfur analysis using ICP-OES:

• Loss during sample preparation is a significant issue. Preparations using closed-vessel systems are recommended. Parr bomb fusions, Schöniger flask combustions, and closed-vessel microwave digestions should be considered depending upon the sample matrix, sulfur compound type(s), sulfur levels, and sample size requirements needed to make quantitative measurements.
• Preparations including sulfate, Ba, and Pb should be avoided. The molecular form of the sulfur may have compatibility issues with other chemical species in the sample solution preparation. Sulfate (SO₄²⁻) sulfur is a common molecular form resulting from oxidative sample preparations. Even though the preparation promises to deliver sulfite (SO₃²⁻) sulfur, this species quickly air-oxidizes in aqueous solution to the sulfate form. Sulfate readily precipitates with solutions containing Pb or Ba.
• Water-soluble samples known to contain sulfur as sulfate, sulfite, or low molecular weight water-soluble sulfonic acids (RSO₃H) may need no sample preparation, but samples known to contain sulfur in other forms — such as sulfides (S²⁻), elemental (S⁰), polysulfides (Sₙ²⁻), thiols (RSH), organic sulfides and disulfides (R-S-R and R-S-S-R), thiol esters (R-CO-SR), etc. — should undergo oxidative sample preparation to avoid possible compatibility issues with other solution components. In addition, the addition of acid to sulfide-containing samples will emit H₂S.
• S elemental data

Barium (Ba)

Of the four acids most commonly used in sample preparations, Ba will form precipitates with HF and H₂SO₄. In addition, the solubility of BaHPO₄ and BaCrO₄ are 0.01 and 0.001 g/100 g H₂O, respectively. Solutions that are neutral or alkaline will precipitate BaCO₃ (solubility 0.0024 g/100 g H₂O).

• Samples containing Ba and sulfur compounds may form BaSO₄ in oxidative decompositions. I know of no simple way to dissolve this precipitate. Since small amounts of barium sulfate do not readily coagulate, the precipitate can easily go unnoticed. Attempts to dissolve barium sulfate have seemingly focused upon the use of EDTA (Kf 7.86) and DTPA (Kf 8.78). However, the pH of the solution, which must be ~5, can lead to precipitation and/or adsorption problems with other analytes, and the dissolution rate is slow.
• Avoid combinations of Ba²⁺ with SO₄²⁻, CrO₄²⁻, or F⁻¹ in acidic media.
• Avoid raising the pH of sample solutions containing Ba²⁺ to 7 or greater, to avoid loss as the carbonate or hydrogen phosphate.
• Ba elemental data

Lead (Pb)

Lead has a number of chemical compatibility issues. In trace analysis, the analyst typically does not experience serious problems unless attempting to combine Pb with sulfate or chromate. Other chemical components to avoid are the halogens (Cl, F, Br, and I), thiosulfate, arsenate, and sulfide, to name the most common. However, the major problem with trace Pb analysis is contamination from the apparatus and atmosphere. Pb is used in industry in plumbing (pipes), solder, gasoline (significantly curtailed), as a drying agent for oils, glass, plumber's cement, covering of steel to prevent rust, as a pigment in paint (significantly curtailed), hair dye, and as a pigment in plastics.

• Environmental contamination from airborne particulates is still a major concern in certain regions/laboratories, depending upon location and age. When tetraethyl lead was widely used as an octane booster, it was impossible to avoid environmental contamination in an open digestion apparatus. Open digestions in hoods where large volumes of air pass over the apparatus are of most concern. Closed-container digestions or clean rooms/hoods are suggested to avoid this source of contamination.
• Avoid the use of any type of glass in sample preparations for Pb. Use quartz or fused silica, and perform a sufficient number of blanks to define the degree of contamination.
• Avoid the use of any plastic with an inorganic pigment. Here, Pb is only one of many concerns.
• Teflon containers should be carefully leached with dilute nitric acid before use, and blanks performed to confirm freedom from Pb contamination. Be particularly suspicious of Teflon that has been used in sample preparations where Pb was a major, minor, or trace component.
• Pb elemental data

Chromium (Cr)

The major difficulty that I have experienced with Cr is that it often exists in forms that are difficult to put into solution. Chromite (FeO·Cr₂O₃), chromic oxide, pigments, stainless steel, and ferrochrome all present a challenge, but the hexavalent chromium oxides are the most difficult. If the oxide has been ignited (pigments), the refractory nature is such that an analyst confronted with the task of bringing about solution will never forget the experience. The most common approach is to perform a fusion. Fusions that have been used include, but are not limited to, potassium and sodium bisulfate, carbonate (sodium or potassium), sodium peroxide, NaOH/KNO₃, and NaOH/Na₂O₂. In addition, the fusion will not be complete unless the chrome is finely divided and mixed with the flux.

• Know your sample to the fullest extent possible. The possible chemical forms of Cr should influence the sample preparation technique employed.
• If your sample is an inorganic pigment containing Cr, then you know that you have an extremely refractory material to dissolve.
• If you are unfamiliar with your sample type, a literature search is strongly suggested.
• Method validation using a CRM containing Cr in the suspected or known chemical form(s) is vital. The importance of CRMs prepared from "real world" materials is critical (i.e., synthetic CRMs are likely to contain easily dissolved compounds).
• Avoid mixing water-soluble hexavalent chrome with Ba or Pb to avoid loss of Cr, Pb, and Ba as the insoluble chromates.
• Cr elemental data

Further Reading

• Part 14: Accuracy, Precision, Mean and Standard Deviation
• ICP Operations Guide: Table of Contents
• More Guides and Papers