Contents

Principles of Digestion Technology

1. Purpose and Objective

3. Digestion using Liquid Reagents

3.1 Introduction

3.2 Theory of Digestion Process

4. Digestion in Practice

4.1 Open Digestion at the Reflux

4.2 Pressure Digestion in Steel Tanks

4.3 Pressure Digestion by Microwave

5. Synopsis

1. Purpose and Objective

Quantitative detection of elements in solids

usually by subsequent spectroscopic analysis

Purpose

1. Purpose and Objective

Clear dissolution of solid

Complete destruction of matrix

avoiding disturbances in subsequent analysis

Avoidance of loss

Objective

3. Digestion with Liquid Reagents - Principle

Chemical digestion of sample matrix

Heating accelerates rate of reaction

Max. temperature in open digestion limited by boiling point of solution

Pressure build-up in closed vessels permits higher temperatures

3. Digestion with Liquid Reagents - General Aspects

1. Homogenisation of sample

2. Weigh-in of a representative aliquot

3. Addition of digestion reagent

4. Supply of energy (usually heat)

General procedure

3. Digestion with Liquid Reagents - General Aspects

Reagents

Acid (e.g. with HCl, H2SO4...)

Base (e.g. with NaOH, NH3...)

Oxidising (e.g. with HNO3, H2O2, K2S2O8...)

Reductive (e.g. with HJ, HBr...)

Complexing agent (e.g. H3BO3...)

3. Digestion with Liquid Reagents - General Aspects

Choice of reagents

Organic matrices

usually oxidising substances or mixtures

(HNO3, H2O2, K2S2O8 and possibly H2SO4)

Inorganic matrices

usually mixtures with HNO3, HCl, (also aqua regia), HF and possibly H2SO4

pure metals: HCl, aqua regia, HCl/HF

Oxides: H2SO4/HCl, H3PO4/HCl, mixtures containing HF

3. Digestion with Liquid Reagents - General Aspects

Organic matrices - choice of reagents

HNO3 (65%)

Universally used

For readily oxidisable samples (food, wood, fat, oil)

Nitrate or nitrogen must not interfere with analysis

Mixture of HNO3 (65%) / H2O2 (30%) approx. 4:1

Improved quality of digestion

No improvement for samples that are difficult to digest (e.g. plastics)

3. Digestion with Liquid Reagents - General Aspects

Organic matrices - choice of reagents

Aqueous samples (waste water)

Digestion with H2O2 (30%) / H2SO4 (1:1) mixture

Difficult-to-digest samples (e.g. plastics)

Digestion with HNO3 / H2SO4 (1:1) mixture

Carbon in matrix made more readily corridible by dehydration

Higher digestion temperatures due to lower vapour pressure of mixture

3. Digestion with Liquid Reagents - General Aspects

Inorganic matrices - choice of reagents

Pure metals

Digestion with HCl, aqua regia or HCl / HF mixtures

Oxides, including Al2O3 in particular

Digestion in H2SO4 / HCl or H3PO4 / HCl or HF mixtures

High proportion of high-boiling acid (approx. 80%) needed in order to

achieve highest digestion temperatures at moderate pressures

3. Digestion with Liquid Reagents - Theory

Digestion process - critical parameters

Digestion temperature

Digestion time

Chemical potential of digestion reagents

3. Digestion with Liquid Reagents - Theory

Digestion temperature

High digestion temperatures shorter reaction time

Digestion temperatures are limited by:

vapour pressure of digestion acids

temperature resistance of container/vessel materials

pressure resistance of containers/vessels

3. Digestion with Liquid Reagents - Theory

Pressure buildup in closed digestion

Total pressure p

p = p(CO2) + p(acid) p(CO2) = partial pressure of CO2

produced

p(acid) = partial pressure of acid mixture

CO2 pressure:

dependent on carbon content of sample and weigh-in

p(CO2) = 6.9 * mc [g] * T/V [K/ml]

Example:

V = 30 ml, 0.2 g carbon, 200°C p(CO2) = 22 bar

V = 80 ml, 0.2 g carbon, 200°C p(CO2) = 8 bar

3. Digestion with Liquid Reagents - Theory

Example (60 ml vessel at 200°C):

500 mg carbon develops 930 ml CO2

partial CO2 pressure of 26 bar

Acid pressure for HNO3 at 200°C of approx. 10 bar

Total pressure approx. 36 bar (60 ml vessel at 200°C)

3. Digestion with Liquid Reagents - Theory

Vapour-pressure curves/graphs of pure acids

a. Aqua regia

b. HCl 36%

c. HNO3 91%

d. HCl 22.9%

e. Water

f. Boling point HNO3 100%

g. Boiling point H2SO4 100%

h. Boiling point H3PO4 96%

(Panholzer, LaborPraxis, Oct. 1994, 32)

T [°C]

3. Digestion with Liquid Reagents - Theory

Digestion time

Short digestion times recommended greater throughput of samples

But good control of the process takes priority, e.g.:

Slow heating in warm-up phase avoids spontaneous reactions

Over-vigorous process control unnecessary wear and tear on material

3. Digestion with Liquid Reagents - Theory

Chemical potential depends on

Concentration of reagents employed in the solution

Interaction of reagents

Interaction of reagents with sample water

Goal:

The concentration of acids should not reduce greatly

during digestion.

4. Practical Conduct of Digestion

Open method at reflux

Max. temperature limited by boiling point of acid mixture

( conc. H2SO4)

Allows high weigh-ins

Quality of digestion not always sufficient

Loss of volatile elements (e.g. Hg, lead salts)

4. Practical Conduct of Digestion

Pressure digestion in steel vessels

Pressure resistance 200 bar

Temperature max. 230°C (briefly 260°C)

Digestion times from about 2 hours to several days

Free from contamination due to PTFE-TFM lining

Different internal volumes (25-250 ml) and therefore weigh-in quantities

Outstanding quality of digestion

No loss of volatile elements (e.g. Hg, lead salts)

High degree of safety, easy operation

4. Practical Conduct of Digestion

Pressure digestion in steel vessels - specimen application

Matrix Weigh-in Acid Temperature Time

Cellulose/starch 1000 mg HNO3 160°C 2h

Flour/grain/leaves 1000 mg HNO3/HF 180°C 2h

Tissue/liver 1000 mg HNO3 170°C-190°C 2h

Fat/oil 1000 mg HNO3 (poss. H2O2) 200°C 4h

Plastics 500 mg HNO3 /H2SO4 200°C 3-4h

Carbon/resin 500 mg HNO3 220°C 6h

Ceramics/oxides 500 mg HF or HCl 230°C 2-8h

Steel 500 mg HNO3/HCl 200°C 4h Digestion Vessel DAB-3 (250 ml)

4. Practical Conduct of Digestion

Pressure digestion with microwave heating

Pressure resistance dependent on type of container/vessel (40-100

bar)

Free from contamination through use of PTFE-TFM containers

Different interior volumes (10-100 ml) and therefore weigh-ins

Quality of digestion mostly sufficient

No loss of volatile elements (e.g. Hg, lead salts)

High throughput of samples due to short digestion times (10-60

mins.)

4. Practical Conduct of Digestion

Pressure digestion with microwave heating

It is primarily the sample that is heated

Container material (plastic) is only indirectly heated

Relatively high digestion temperatures can be reached for short

periods

(30-40 minutes)

4. Practical Conduct of Digestion

Container materials

PTFE maximum 260º C

PTFE-TFM maximum 260º C

PFA maximum 200º C

Quartz (silica) glass maximum 1,000º C (theoretically)

4. Practical Conduct of Digestion

Temperature control

The most important aspect of controlling microwave digestion

Rate of reaction depends on temperature

The temperature in the various vessels may vary as a function of the

type of sample and the weigh-in quantity

Temperature control is required in all vessels

All samples can be subjected to non-contact IR temperature measurement

speedwave MWS-3+

Mikrowave radiationIR-Sensor

FilterIR-radiation

TFM

Temperature Control - Principle

IR-measurement at wavelength, where TFM does not absorb IR-radiation

Thermal radiation of the vessel is filtered out

Heat radiation of vessel surface

Heat radiation of sample

5. Synopsis

Work in closed vessel wherever possible

higher reaction temperature

better quality of digestion

Parameters determining digestion temperature, limited by:

pressure resistance of container/vessel

material of vessel

Digestion in steel vessel for “most difficult samples” or lower sample throughput

Digestion under microwave especially for high sample throughput