TIC/TOC – measuring carbon

Written by Andy Connelly. Published on 2nd April 2017. Last updated 10th May 2017.

Introduction

Measuring the carbon content of soils, sediments, and rocks is a key measurement in geochemistry and other related sciences. In this context, the carbon in a material is generally considered to be present as one of two broad types: Total Inorganic Carbon (TIC) and Total Organic Carbon (TOC). TC_TIC_TOCThere is also the possibility for carbon to be present in its elemental form (e.g. charcoal, soot, graphite, or coal), however, this is generally taken as a minimal component for this measurement.

There are various methods to measure TC, TIC, and or TOC (see [1]). The biggest difficulty is in separating the TIC and TOC values. This is normally done by removing one or the other from the sample and measuring the treated material.

DISCLAIMER: I am not an expert in carbon measurement. The content of this blog is what I have discovered through my efforts to understand the subject. I have done my best to make the information here in as accurate as possible but I do not guarantee accuracy. If you spot any errors or admissions, or have any comments, please let me know. The chemicals and processes described below are potentially hazardous and should not be attempted with a full assessment of the hazards involved and appropriate hazard mitigation.

Methods of removing TIC

Inorganic carbon is generally in the form of carbonates such as calcium carbonate. The most common approach to removing this form of carbon is using a chemical treatment which preferentially react with the carbonates leaving the organic carbon containing phases behind.

Acids, such as hydrochloric acid, are commonly used for this purpose (see below).  However, other chemical treatments are available (see [1]&[2]). Most water based chemical treatments have the disadvantage of, not only removing carbonates, but also other soluble phases. This makes this approach semi-quantitative rather than truly quantitative.

Another issue with using acids is that some carbonate phases, such as dolomite (CaMgCO3), are more resistant to acid attack than other carbonates and so will not necessarily react in a predictable way. Also, most acids will have some effect on organic phases and so affect the organic carbon measurement performed afterwards.

There are various acids you can use in this process:

  • Hydrochloric acid – 1-2 molar HCl (often 10% (1.3M)) –  Hydrochloric acid is a reducing acid and so is much less destructive to organic carbon compounds than other acids such as nitric acid. However, you cannot assume that it has no effect on organic phases. One issue with this acid is that the Cl remaining in the sample can cause problems for carbon measurement. This means that the sample may need to be washed thoroughly before analysis [1].
  • Sulphuric acid – This acid is often used in combination with FeSO4 to minimize oxidation and decarboxylation of organic matter by the H2SO4  [1].
  • Phosphoric acid – 1:1 v/v can also be used. [2]

A possible method using HCl for this is described at the end of this article.

Heat treatment (i.e. loss on ignition) methods are not generally considered appropriate as most carbonate phases have a much higher thermal stability than organic phases and so an attempt to remove carbonates will result in the removal of both TIC and TOC.

Methods of removing TOC

The organic matter content of a sample is used as a rough estimate of the total organic carbon content. As such, the methods generally involve removal of all organic matter and so are semi-quantitative. There are two main methods for removing the organic phases:

  • Loss on Ignition (LOI) Heating the sample to 350-440°C will remove most organics. However, this method assumes that all LOI at this temperature is due to organic carbon – this may not be accurate. It also assumes that all organic material will be removed at this temperature [1].
  • Chemical treatment: Treatment with hydrogen peroxide (30% or 50%) is a used to remove the organic carbon containing phases. However, addition of hydrogen peroxide does not always result in complete oxidation of the organic phases and the extent of oxidation varies from sample to sample [1].

A possible LOI method for this is described at the end of this article.

Measuring carbon content

To determine the TC, TIC, and TOC directly is not normally possible. Generally two of these values are measured and the third calculated:

  • TOC_TC-TIC
  • TIC

So, an untreated sample is analysed to find the TC, then a treated sample is analysed for TIC or TOC. The unknown value can then be calculated.

The nature of the treatments discussed here normally makes these result semi-quantitative. However, there are quantitative methods for measuring the individual values directly (see [1]).

One common method for determining carbon in a treated or untreated solid sample is the combustion analyser (see Figure 1). The amount of carbon is determined by dry combustion at high temperature in an oxygen atmosphere. The gasses produced are then passed through a detector which is normally an infra-red analyser which measures the carbon dioxide present in the gas. This measurement is converted to percent carbon based on the dry sample weight and a calibration curve made up of standards of known carbon content [2]. Standard analytical procedure should be followed including blanks and check standards.

Simplified schematic of a combustion analyser.
Figure 1: Simplified schematic of a combustion analyser.

An alternative method is an LOI method which relies on the change in mass of a sample heated to a specific temperature. For TC measurement this is normally around 1000°C. This assumes that all carbonates and organics have been decomposed after this heat treatment and that the only mass loss is due to carbon loss. The uncertainty this creates makes this semi-quantitative at best.

Summary

Measuring the carbon content of a sample is a standard technique with various different methods available. Producing semi-quantitative measurements of TC, TIC and TOC for soils and sediments is relatively simple. However, there are limitations with any technique and you need to be aware of these before you start.

Two very simple methods are shown below.

References

[1] B.A. Schumacher, METHODS FOR THE DETERMINATION OF TOTAL ORGANIC CARBON (TOC) IN SOILS AND SEDIMENTS, NCEA-C- 1282 EMASC-001 April 2002.

[2] B.B. Bernard, H. Bernard, and J.M. Brooks, DETERMINATION OF TOTAL CARBON, TOTAL ORGANIC CARBON AND INORGANIC CARBON IN SEDIMENTS.

Two methods for measuring TIC/TOC

These methods are not standard methods. I record them here purely for interest and offer no guarantees as to their validity. Carrying out these methods would require COSHH and risk assessments or complying with similar local H&S rules before starting. These methods involve handing hazardous chemicals and high temperatures which must be done with appropriate safety measures in place.

Preparing samples

  1. Grind and sieve samples, if required, using a pestle and mortar and sieves .
  2. Place the sample (approximately 5 g to 10 g) in a small crucible or centrifuge tube
  3. Place the crucible in an oven at 60-105°C overnight to remove all water. Some samples may take longer to dry – drying can result in loss of organic material (see [1])
  4. Remove samples from oven and allow to cool. This could be carried out in a desiccator.
  5. If the sample needs homogenization a further homogenization step with a pestle and mortar can be carried out and sieving if required.
  6. Store dried samples in sealed container.

Measuring TIC/TOC through LOI

This is a simplified procedure for measuring TIC/TOC using loss on ignition.

  1. Weigh crucibles and record mass (mcruc).
  2. Place sample into crucibles and record mass (minitial)
  3. Heat to 440oC and hold at this temperature overnight.
  4. Once crucibles have cooled remove from furnace and reweigh (mfinal1).
  5. Calculate the mass change:mTOCwhere mTOC= total organic carbon
  6. Place them in furnace over night at 1000oC.
  7. Once crucibles have cooled remove from furnace and reweigh (mfinal2)
  8. Calculate the mass change

mTIC

where mTIC= total inorganic carbon

9. The total carbon (mTC) can thus be calculated

mTC

Measurng TIC/TOC using a combustion analyser

This is a simplified procedure for measuring TIC/TOC using a combustion analyser. An alternative method can be found in reference [2].

  1. Weigh centrifuge tube and record mass (mtube).
  2. Place sample into tube record mass (minitial)
  3. Add about 5ml of dilute acid to the centrifuge tube and mix thoroughly. Add acid slowly as the solution may foam.
  4. Keep adding small amounts of dilute acid until fizzing stops then add another 5ml to ensure acid is in excess. Leave overnight for reaction to cease. Some carbonate minerals take a long time to react e.g. siderite. If you know these are present wait at least 24 hours or use a second acid rinse.
  5. Wait for sample to settle out then decant off as much of the clear liquid ensuring none of the sample is lost. It may require use of a centrifuge.
  6. If the acid used will react badly with the carbon analyser then re-fill tube with distilled water and mix thoroughly to wash acid from sample.
  7. Repeat steps 5&6 until you are satisfied enough of the acid residue has been removed.
  8. Allow sample to dry in an oven at 60oC overnight.
  9. Weigh centrifuge tube and sample again (mFinal)
  10. Calculate the mass change:    mlost
    • where mlost is mass lost including carbonate and other soluble elements (e.g. Ca)
  11. Take appropriate amount of the original (untreated) sample and some of the dried and treated (inorganic free) sample
  12. Analyse both for percentage carbon using combustion analyser. Poc = percentage carbon in treated sample.
  13. Calculate the TOC and then using that value and the TC allows you to calculate the TIC.

TOCTIC

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s