Arsenic, Selenium, Antimony ultra-trace analysis

The business of chemical analysis of environmental, medical and commercial samples has recently witnessed a large market demand as well as extensive scientific research and development.

The days of high ppm level analysis done by a lab technician via a routine bench procedure involving mere volumetric titration, gravimetric precipitation and basic AAS at ppm level are long gone. Remember the days when we used to analyze, say, Hg, via such methods as diphenylthiocarbozone? Today's market demands qualified analytical chemists and state of the art instrumentation to deal with trace ppb and ppt detection level.

Lake water, seafood, soil, pharmaceutical products and many other samples are sent daily to analytical laboratories for several inorganic and organic analysis, and are subjected to improved regulations that specify the new acceptable limits.

Many may have heard about arsenic as a poisonous element we see in films, but many may not realize how common some elements such as arsenic, selenium and antimony are in nature, and in a variety of forms.

A few years ago, a lady friend was making coffee. As she ran out of filter papers, she innocently used a paper towel. The hot water went through the coffee and the acting filter paper as usual, but she never thought that paper towel could contain a high level of arsenic.

"Why would they add arsenic to paper towels?" she asked. “They don't.” I explained. Products such as paper towels are made from recycled newspapers and other scrap wood sources and dry leaves and branches - which (like soil) may contain high levels of arsenic. Products such as coffee paper-filters, are purified via such techniques as ion exchange and other chemical procedures - to extract arsenic. Paper towels, however, are meant for external use, and are not meant to be subjected to boiling water for one to then drink the extract! I explained.

It is of course true that the form of arsenic, selenium or antimony makes all the difference as to whether they are harmful to us or to what degree. It is also true that soaking a paper towel in boiling water may extract other chemicals that are not meant to be ingested; but that's a different subject. Our subject here is As, Se, Sb; and one must also understand that not every form of these elements is actually a threat to our body or can be absorbed into our body. Some of these forms, especially the metallic elements, are not particularly or practically harmful to humans, while other forms are easily absorbed into our body if ingested.

These elements are common in nature: in soil, wood "trees" and wood-related products and in other sources; and in a variety of forms, both organic and inorganic.

Clearly agricultural soil is always a concern as vegetation will absorb such elements from the soil.

As well, potable water, residential sites, food and pharmaceutical products and others, each has its specific allowable limit of As, Se and Sb. Hence, the role of the analytical laboratory methodology and the appropriate analytical procedure to secure the suitable DL "detection level" or “detection limit” for the specific case, are a must..

The methods and the studies to follow were utilized, revolutionized and produced by Canadian chemist, Dr. Paul Gouda of Ontario, Canada; and have been well received and adopted by the scientific arena, including

analytical laboratories.

The book also introduces five AAS “Atomic Absorption Spectroscopy” and ICP “Inductively Coupled Plasma” methods – with emphasis on Hydride Generation AAS.

These five methods were created by and have been named after the inventor Chemist Dr. Paul Gouda.

Principal / theory

Metalloid elements {Se / As / Sb} are prepared in acidic medium to convert all forms of arsenic, selenium and antimony to arsenate {AsO43- }, selenate { SeO42-}, and antimonate {SbO43-} react with sodium borohydride and are reduced to arsine, hydrogen selenide and stibine.

The volatile hydrides in the reaction tube are carried by an inert gas [argon] into a cell [quartz tube] that is heated by air / acetylene flame and is situated in the optical path of AAS where the gaseous hydrides are reduced to atomic species and are then determined by conventional atomic absorption.

The instrumental part involves the atomic absorption technique; hydride generation / graphite furnace AAS.

5.1 Samples are digested in oxidizing acid mixture and are treated to:

a) reduce selenium in the digestate selenium IV as selenite SeO=3. Selenite is reduced to hydrogen selenide H2Se by sodium borohydride "NaBH4" and HCL. The hydrogen selenide is reduced in the flame [cell] to selenium atoms.

b) convert all forms of arsenic to arsenate ion AsO43- [ The arsenate in the acidic solution is reduced by sodium borohydride to AsH3 "arsine".]

Atomic arsenic is then determined by conventional atomic absorption technique. Hydride forming elements must be converted to the required oxidation state. e.g. Sb "V" and As "V" are reduced to Sb "III" and As "III' with KI {after HCL treatment}. While Sb reduction is spontaneous, As reduction takes approximately 1 hour at room temperature or 5 minutes at 50 0C [water bath]. ---------

From page 126:

- A few dialkylstibinic acids exist in soil samples. They are a result of hydrolysis of the corresponding dialkyltrichloroantimony compounds:

(CH3)2SbCL3  (CH3)2SbO(OH)

Aromatic stibonic acids can be produced during sample digestion by the famous diazo reaction:

ArSbCL4 + H2O  ArSbO(OH)2 + HCL

ArN2CL + SbCL3  ArSbCL4 + N2

- When a diazonium salt is present in the sample and is then allowed to react with antimony pentachloride or with an aryltetrachloroantimony compound, the onium salts [ArN2][SbCL6] or [ArN2][ArSbCL5] are formed. They decompose in organic solvents with formation of diarylantimony trichloride:

2[ArN2][SbCL6] + 3Fe 

Ar2SbCL3 + 2N2 + SbCL3 + 3FeCL2

- Antimony trichloride SbCL3, a colorless crystalline solid soluble in hydrochloric acid and in water when heated, is introduced into the sample as a result of metal chlorination or Sb2O3 reaction with HCL conc. It hydrolyzes giving hydrous Sb2O3 with excess water but with limited quantities of water a large number of partially hydrolyzed compounds were reported, e.g.. SbOCL, Sb2OCL4, Sb4O5CL2, Sb4O3(OH)3CL2, Sb8O11 and Sb8OCL22. It is initially precipitated as a thick white solid, changing to well-defined colourless crystals. SbOCL produced changes upon further dilution with water to Sb4O5CL2.