Using CO₂ and H₂O Scrubbers with LI-COR Gas Analyzers

LI-COR gas analyzers require the use of internal chemical scrubbers for keeping absorbing gases out of internal spaces that are part of the optical path. In some applications, external scrubber columns may also be required for conditioning airflow prior to entering the analyzer. There are at least two suitable CO₂ scrubbers (soda lime and Ascarite II), and two suitable H₂O scrubbers (Drierite and magnesium perchlorate). However, care must be taken in choosing which chemicals to use together for any given application.

CO₂ Scrubbers

Soda Lime

The primary components of soda lime include Calcium Oxide or Hydroxide - Ca(OH)₂ (about 70-90%), Water - H₂O (about 2 to 20%), Sodium Hydroxide - NaOH (1-8%), and Potassium Hydroxide - KOH (about 0-1%). PH-sensitive Ethyl Violet is typically added as an indicator. The granules are creamy white in color, hard, and processed to minimize dust formation. It is also porous and irregularly shaped to provide for a larger surface area.

Sometimes silica is included for granularity. In the presence of an acidic gas like CO₂, a strong, exothermic (heat producing) reaction takes place which gives off water and binds the CO₂ by forming a stable calcium carbonate, CaCO₂.

Carbon dioxide absorption capacity of soda lime is typically 20 to 30% W/W. However, efficacy will be limited if dehydration occurs because the first step in CO₂ absorption requires H₂O.

Ascarite II or Decarbite

Ascarite II and Decarbite are trade names for a form of sodium hydroxide (NaOH) which is coated on non-fibrous silicate (vermiculite). They rapidly and quantitatively absorb CO₂. During use, the light brown granules gradually turn white due to absorption of carbon dioxide and formation of sodium carbonate. Ascarite II is hygroscopic and deliquescent when exposed to water. Carbon dioxide absorption capacity is 20 to 30% W/W.

H₂O Scrubbers

Magnesium Perchlorate, Anhydrous

Anhydrous Magnesium perchlorate Mg(ClO₂)₄ is a white powder or granule. The granular form (e.g., Dehydrite® is less likely to channel, but is also more expensive. It is very hygroscopic and deliquescent when exposed to water and is also highly caustic and a strong oxidizer. Anhydrous Magnesium perchlorate can be used to quantitatively remove H₂O without sorbing CO₂. Synonyms and trade names: Perchloric acid, magnesium salt; Anhydrone®; Dehydrite®; Magnesium Perchlorate.

Drierite

Drierite is a granulated form of anhydrous calcium sulfate 97% (CaSO4, dehydrated gypsum) combined with a cobalt chloride indicator 3%. It is available in sizes ranging from powder to 4 mesh (6mm granules). DRIERITE has a water capacity of 10 to 14 weight percent when drying gases.

Drierite can be regenerated: spread granules 1 layer deep in a pan and heat for 1 hour at 210 °C (425 °F). The regenerated material should be placed in the original glass or metal container and sealed while hot. The color of the Indicating DRIERITE may become less distinct on successive regenerations due to the migration of the indicator into the interior of the granule and sublimation of the indicator. The temperature at which DRIERITE desiccants are regenerated is crucial in restoring DRIERITE to its original condition. Absorbed moisture is water of hydration and is chemically bound to the calcium sulfate of DRIERITE. Temperatures in the range of 400° - 450 °F are required to break these bonds and release absorbed moisture. Lower temperatures, regardless of heating time, will not regenerate DRIERITE unless applied under vacuum (26" Hg, 325 °F or 28" Hg, 275 °F). Care should be taken not to overheat DRIERITE desiccants. High temperatures can alter the crystal structure and render the desiccant permanently inactive.

Silica Gel

Indicating silica gel consists of small white crystals that look much like granulated sugar with small pink or blue colored crystals scattered throughout. This is ordinary silica gel with the colored specks being coated with cobalt chloride. When the gel has absorbed approximately eight percent of its weight in water the colored crystals will turn from blue to pink making an easy visual indicator of whether the gel has become saturated with moisture.

Scrubbing by Diffusion in a Closed Volume

The internal scrubbing bottles on LI-62xx, LI-6400/XT, LI-6800, LI-7000, LI-7500/A/RS/DS, LI-7200/RS, and LI-7700 gas analyzers work by diffusion; no flow is forced through them. Typically, they are serviced annually. Spare (empty) bottles may be included in standard spares kits, and specific instructions for each model instrument are in the instruction manuals. Additional pre-charged bottles can be ordered from LI-COR.

Best: Ascarite II and Magnesium Perchlorate.

OK: Dry soda lime and Magnesium Perchlorate.

In this application, a small amount of chemical has to work a long time. Most of its efficacy is needed for the initial scrub down. Long-term need depends on diffusion past o-ring seals, so it is typically a small amount.

Ascarite II and Magnesium perchlorate are a good combination for this, since they are voracious scrubbers, and the magnesium perchlorate keeps the Ascarite dry. This combination can remain effective for years in a sealed environment.

You can substitute dry soda lime (H₂O <4% by weight) for the Ascarite II, but the effectiveness is limited, since the desiccant will dry out the soda lime, rendering it ineffective. Never use wet soda lime (H₂O >4% by weight) for internal scrubbers. Its water output will saturate the magnesium perchlorate, and the resulting slurry can leak out of the bottle and contaminate the space being scrubbed (detector, source, etc.). Never use Drierite for internal scrubbers, as it is not a good enough desiccant, and it gives off CO₂.

Scrubbing by Forced Flow in a Closed Loop

The most typical use of closed loop scrubbing is to maintain a dry, CO₂-free reference cell, through which air is continually circulated using a pump. Flow rates can be small, but should be large enough to offset any diffusion through tubing or leaks that might be in the loop.

Best: Ascarite II and Magnesium Perchlorate or Drierite.

OK: Dry soda lime and Magnesium Perchlorate or Drierite.

The best combination is Ascarite II and Magnesium perchlorate or Drierite. The desiccant should come first, but in a closed loop the order is not particularly important. This combination can last a long time (weeks or months), and is typically limited by the desiccant.

Dry soda lime (<4% H₂O by weight) can be used for the CO₂ scrubber, but the efficacy of the combination will be greatly reduced (hours or days) as the soda lime is dried by the desiccant.

Avoid wet soda lime for this application, as it will quickly saturate the desiccant. If there is no desiccant, wet soda lime can raise the dewpoint of the closed loop sufficiently to cause condensation to occur.

Scrubbing by Forced Flow in an Open Path

Conditioning incoming sample air streams is the third possible use of chemical scrubbers with LI-COR gas analyzers. Air from some source (ambient, tank, chamber, etc.) is passed through one or more scrubbers prior to being measured. An example is the LI-6400, in which all of the CO₂ must be removed (a controlled amount is subsequently added by the CO₂ mixer), along with some or all of the H₂O.

Case 1: Remove CO₂ and some or all H₂O. If there is moisture in the incoming air stream, soda lime could be the best choice for the CO₂ scrubber. The incoming moisture will prevent the soda lime from losing efficacy prematurely. The choice of wet or dry soda lime depends on the application. If you are going to dry the air at all, wet soda lime will saturate the desiccant much quicker than dry. The air should go through the soda lime first, then through the desiccant (either Drierite or Magnesium perchlorate).

If the flow through the desiccant is not continuous, avoid Drierite. During the non-flow time, CO₂ will build up in the desiccant tube, providing a CO₂ pulse once flow resumes. If the flow is continuous, there will be an initial flush out of CO₂ from the Drierite, but the subsequent CO₂ output will be negligible. However, as the Drierite becomes saturated with H₂O, its CO₂ output will increase.

Case 2: Remove H₂O, leave CO₂ unchanged. The only choice is magnesium perchlorate.

Case 3: Remove CO₂, leave H₂O unchanged. Cannot be done with these chemical scrubbers.

Internal Chemical Bottle Kits for LI-COR Instruments

The internal scrubbing bottles on LI-62xx, LI-6400/XT, LI-6800, LI-7000, LI-7500A/RS/DS, LI-7200/RS, and LI-7700 gas analyzers can be purchased in pre-charged kits that are ready to use. The bottles contain magnesium perchlorate and Ascarite II. Depending on the instrument, large or small bottles, or a combination of the two are required. The table below lists the part numbers and con-tents of the pre-charged internal chemical bottle kits available from LI-COR.

Instrument	LI-COR Part Number	Contents
LI-6400/XT	6400-950	2 small bottles
LI-6800	6400-950	2 small bottles
LI-7000	7000-950	2 large, 2 small bottles
LI-7500/A/RS/DS	7500-950	2 small bottles
LI-7200/RS	7200-950	3 small bottles
LI-7700	7700-950	1 small bottle
LI-62xx	6262-950	1 small bottle

Suppliers

LI-COR Environmental
www.licor.com/env
4647 Superior Street
Lincoln, NE 68504 USA
800-447-3576
402-467-3576

Material	LI-COR Part Number
Ascarite II, 500g	9970-022
Wet Soda Lime	9964-090
Drierite	622-04299
Magnesium Perchlorate	9960-078

Fisher Scientific
www.fishersci.com
800-766-7000
770-871-4500

VWR Scientific Products
www.vwrsp.com
800-932-5000
908-757-4045

Thomas Scientific
www.thomassci.com
800-345-2100
856-467-2000

GFS Chemicals, Inc.
www.gfschemicals.com
800-394-5501
740-881-5501

P.W. Perkins Co., Inc.
www.pwperkins.com
856-769-3525

Table 1‑1. Five common scrubbers and their effectiveness.

Material	CO2 Interaction	H2O Interaction	Indicating?	Hazardous?	Final State	Regenerate?
Ascarite II	Excellent scrubber	Good scrubber	Yes	Yes	Liquid	No
Soda Lime	Excellent scrubber	Adds	Usually	No, if NaOH <4%	No change	No
Drierite	Equilibrates or Adds (slightly)	Very good scrubber	Yes	No	No change	Yes
Magnesium Perchlorate	None	Excellent scrubber	No	Yes	Liquid	No
Silica Gel	Equilibrates	Good scrubber	Yes	No	No change	Yes