First of all a safety question: Are you using a fume hood that is dedicated for the use of perchloric acid? As I mentioned in previous comments, perchlorates can build up in the hood and can lead to occasional explosions!

Why would you want to use perchloric acid in the first place? I presume you want to get rid of fluorides?

To answer your question: maybe the hot plate is too hot and the vials are too small.

Hello Clement

Spurting is probably the consequence of too much volume of acids in the Savillex (or container), too high temperature and/or the nature of your soils (especially organic matter).

My own experience: for 100 mg dried sediments, I usually use an acid mixture of 0.6 ml HNO3 – 0.5 ml HF – 0.45 ml HClO4. Heating at 135 °C (Savillex closed) during 12 hours. Leave the mixture to cool. Then, open Savillex, add 0.6 ml HN03 (it may help HClO4 to evaporate in the next phase), heat at 135 °C during 1 hour with Savillex closed. Cooling. Then open Savillex and evaporate to dryness at 120 – 130 °C. Time of evaporation depends on the initial volume of perchloric acid (24 hours most of the time). It is quite long but I never noticed “spurting effects” and recoveries of metal contents from sediment standards were excellent.

Otherwise, you can try micro-wave digestion, which is much faster and efficient.

All the best

Thank you Vincent and Thomas; I did notice spurting when the solution was subjected to heating; actually the acids used were in the ration 10 ml HNO3 and 5 ml HCLO3 for 1 g of soil. Maybe i should reduce the temperature and extend the period of digestion.

Dear Howard, I want to extract some heavy metals like Cd, Cu, Zn, Pb and Cr from the soil sample.

I haven't done this kind of digestion for many years. However, I think it is a two-step process. You digest with just HNO3 to get rid of the easily-oxidized organic matter before adding any HClO4.

By adding some thing like pebbles into the digestion vessel, or add some acid nonreactive foils as well.

Mr Howard, do you think that the mild extraction with HNO3 can give total metal content of the soil sample?

Mr. Thomas Holm, I think what i need to do is just what you have just said. To first oxidize OM by HNO3 before finishing the digestion with HCLO4

In extracting the heavy metals from the soil you need to understand whether the heavy metals are a contaminent or part of the soil. That helps in making a decision on what total extraction you are seeking. Total extraction of metals that have dried/precipitated or total metals of the soil matrix itself? If you are trying to break down the soil itself I have found that a fusion such as sodium peroxide/sodium carbonate will allow the soil to be decomposed without the HF and Perchlorate hazards. If you are looking at contamination origin then the nitric extraxtion Howard mentioned should work. Perchloric is usually used as a high boiling point oxidizing acid to drive off other acids and bring metals to a souble perchlorate instead of using sulphuric acid and ending up with sulphates. If you need additional oxidizing power over the nitric, adding hydrogen peroxide will remove organics. Safer than perchloric. Add small amounts and wait for reaction to stop before next small addition as any oxidizer with organics can explode

Thank you Mr Burgener, I am actually determining the level of contamination of the soil by heavy metals. Can you link me up with any literature or procedural methods for hydrogen peroxide method

Yes it can be reduced or completely stopped by the use of teflon beakers for the digestion. They are plastic beakers with high resistance for heat and aggressive acids

If you want to determine total concentration of heavy metals I will rather advice that you use HF+ HCl + HNO3. Nitric and perchloric acid only give what we call pseudo total.

Yes. Samples need to soak in conc HNO3 overnight withoput heat. The following morning, heat samples slowly at first, 60-65 C to drive off excess N as NOy (safely !), before adding perchloric in reqd ratios, and increasing heat.

Dear Clement, I did many extraction with mixture of nitric and perchloric acid to analyse heavy metals concentaration (close to total) in soils and plants. We weight In our laboratory 2 g of powdered soil sample into beaker and than was added 10 ml of HNO3 and 5 ml of HClO4. I suppose that spurting will be reduced if You decrease temperature of diggestion.

The use of Aqua regia (HCl+ HNO3 ratio 1:2)) is ideal for the extraction of heavy metals from soils since these metals are absorbed by clays particles. You do not need to add HClO4 which will increase the exothermic reaction that causes spurting. Addition of HClO4 will only be required to achieve complete digestion in very difficult earth materials. If you must add HClO4, a drop or two will be ok.

The heavy metal concentration in solution phase was determined in 0.01 M Ca(NO3)2 at a solution to soil ratio of 2.5 ml g-1. Suspensions were shaken for 24 h on a horizontal shaker. After that the samples were centrifuged for 5 min at 2500 g. The supernatant was passed through filter paper and stored in plastic bottles. The sorbed Cd concentrations of soil samples were measured following Zeien and Brümmer (1989). Forty ml of 0.025 M (NH4)2-EDTA were added to 10 g of soil. Suspensions were shaken for 1.5 h on a horizontal shaker. After that the samples were centrifuged for 5 min at 2500 g. The supernatant was passed through filter paper and stored in plastic bottles. The total metal content was determined independently by the aqua regia microwave digestion method. Microwave digestion was performed by digesting 0.5 g of soil in 8 ml of aqua regia with a 9-stage program. Microwave power ranged from 0 to 750 watt, temperature ranged from 70 to 105 ºC and digestion time was 30 min. Cd concentrations were determined using a flameless atomic absorption spectrometer (Varian, SpectrAA-800).

Dear Clement if you really want the "total metal" content rather than bio-available part why don't you go for a non-destructive method like X-ray Fluorescence (XRF). Wave-Length Dispersive (WD)-XRF or the latest polarized Energy Dispersive XRF are good alternatives if requisite numbers of suitable calibrating standards are available.

If you also want toxic metal concentrations in their effective bio-available fractions then go for the detailed sequential extraction steps (acid digestion) or as Paul has described the use of HNO3 and H2O2 o estimate heavy metal contaminants (unless its an aged soil of course)

During my PhD project I used HNO3 and H2O2 digestion. I took 0.1 g soil added with 3 mL HNO3 left overnight undisturbed. In the following morning added 2 mL H2O2 again left undisturbed for 30 minutes and placed on the heating block raising temperature slowly upto 120 degree. For most of the heavy metals the CRM recovery was 80%+.