CARBON
4 8 ( 2 0 1 0 ) 3 3 1 3 –3 3 2 2
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Standardization of the Boehm titration: Part II. Method of agitation, effect of filtering and dilute titrant Alicia M. Oickle, Sarah L. Goertzen, Katelyn R. Hopper, Yasmin O. Abdalla, Heather A. Andreas * Department of Chemistry, Dalhousie University, Halifax, NS, Canada B3H 4J3
A R T I C L E I N F O
A B S T R A C T
Article history:
Multiple steps in the Boehm titration are carried out in a variety of manners by different
Received 14 April 2010
research groups, thereby making results difficult to compare. The methods standardized
Accepted 4 May 2010
in this paper include method of agitation, use of dilute titrant, carbon removal from reac-
Available online 13 May 2010
tion bases and the effect of air on NaOH standardization; uncertainty estimations are also shown. By examining the multiple agitation methods, it was found shaking was the optimal method for use in the Boehm titration as other methods (stirring and sonicating) affect the carbon surface. It was also found that filtering the carbon and reaction base mixture did not affect the titration, nor did the use of dilute titrant. Solutions must be freshly standardized prior to use since storage (even over a one week time period) results in a change in concentration. 2010 Elsevier Ltd. All rights reserved.
1.
Introduction
The ‘‘Boehm titration’’ is a commonly used technique to determine the acidic oxygen surface functional groups on carbon samples whereby bases of various strengths (NaHCO3, Na2CO3, NaOH and sometimes NaOC2H5) neutralize different acidic oxygen surface functionalities [1]. The weakest base, NaHCO3, neutralizes only the strongest acidic carbon surface functionalities (CSFs) which are carboxylic groups, while Na2CO3 neutralizes carboxylic and lactonic groups. The strongest base typically used, NaOH, neutralizes carboxylic, lactonic and phenolic groups. The number of each type of CSF can be determined by difference between the uptake of each reaction base [1]. Many research groups use the Boehm titration; however, the method followed is rarely clearly described and a standardized method is not used. For results between research groups to be comparable, a standardized methodology must be determined and followed. Other possible differences between carbon results from laboratory groups could be caused
by slow oxidation of carbons through exposure to humid air, called aging [2]. Previous work on standardizing the method of CO2 removal and endpoint determination has been done by this group [3] and will be continued in this paper with a specific focus on the method of agitation, the effect of filtering the carbon from the reaction base solution, and the use of dilute titrant. An estimation of the uncertainty for the titration will be undertaken in this work as well. In the most frequently referenced Boehm paper where the titration is described, the carbon and reaction base solution was shaken ‘‘12 h for 0.05 N NaOH’’ [1] to complete neutralization. Since then, research groups have modified the method of agitation used to include shaking [4–7], stirring [8–10], and, occasionally, sonicating [11,12]. Other groups use no agitation at all [13–18]. The duration of agitation time also varies between research groups, from 1 h to 5 days. A discussion of the agitation method will be undertaken herein. After agitation, most groups filter the carbon from the reaction base before making aliquots to titrate [5,6,14,19]. The effect of the filter paper on the reaction base will be tested to see if this
* Corresponding author: Fax: +1 902 494 1310. E-mail address:
[email protected] (H.A. Andreas). 0008-6223/$ - see front matter 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.05.004
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influences the moles of carbon surface functionality (nCSF) results. Some groups, however, allow the carbon to settle to the bottom of the reaction vessel and then remove the aliquots from the solution above the settled carbon. This will also be addressed here. Endpoint determination was already addressed in a previous paper [3]; however, the effect of dilute titrant on precisely obtaining the endpoint will also be dealt with in this study.
2.
Experimental
2.1.
Preparation and standardization of solutions
Solutions of the reaction bases, NaHCO3 (Sigma–Aldrich, 99.5%), Na2CO3 (Anachemia, ACS Reagent) and NaOH (Sigma–Aldrich, 99.998%) were prepared by mixing an exactly weighed mass of the appropriate base (mB) with 1.00 L (VB,dissolved) of 18.2 MX water (Millipure) to form 0.05 M solutions, using the appropriate molar mass (MB) for each base. 0.05 M solutions of HCl were prepared from pure HCl (Sigma–Aldrich, 99.999%) and 18.2 M water. For some titrations, a 0.025 M NaOH solution was used as the titrant, and this was prepared by weighing an exact mass of solid NaOH. The standardization of the NaOH solutions was carried out using potassium hydrogen phthalate (KHP) (Sigma–Aldrich) as the primary standard, and phenolphthalein as the indicator. A mass of ca. 0.2 g dried KHP (mKHP) was diluted in 20.00 mL of 18.2 M water in an Erlenmeyer flask. The molar mass of KHP is denoted as MKHP. No more than 2 drops of phenolphthalein was added to the flask to avoid a bias. The KHP solution was then titrated with NaOH from a 25 mL burette, with at least triplicate measurements. The volume of NaOH in the standardization titration is denoted as VNaOH,stnd of NaOH. The HCl solutions were standardized by titrating with the previously standardized NaOH and phenolphthalein to determine the endpoint. The volume of HCl and NaOH used in the standardization of the HCl are denoted as VHCl,stnd of HCl and VNaOH,stnd of HCl, respectively.
2.2.
General Boehm titration procedure
A mixture of ca. 1.5 g (weighed to 0.01 mg) Black Pearls 2000 (mC, Cabot Corporation) and 50.00 mL (VB) of one of the three 0.05 M reaction bases, NaHCO3, Na2CO3 and NaOH was agitated using the various methods described below. The mixtures were then filtered and 10.00 mL aliquots were taken (VAliquot). The NaHCO3 and NaOH samples were acidified with the addition of 20.00 mL of standardized 0.05 M HCl (VHCl,acidification), whereas for Na2CO3, 30.00 mL of 0.05 M HCl was added to completely neutralize the diprotic base and allow a strong acid-strong base titration. NaOH was treated in this back-titration manner rather than a direct titration with HCl since it was previously found to give better values with a back-titration [3]. The acidified solutions where then bubbled with N2 for 2 h to expel dissolved CO2 from solution [3]. All samples were titrated with standardized 0.05 M NaOH while being continually saturated with N2, and the endpoints were determined potentiometrically using a SympHony pH meter and posiIo electrode (VWR International). The volume of NaOH used in the titration to reach the endpoint is denoted
as VNaOH,tit. All titrations were carried out at room temperature (22 ± 3 C). All solutions were made up with 18.2 M water. Additionally, all volumes are based on calibrated pipettes. For ease, the NaHCO3, Na2CO3 and NaOH bases used to react with the acidic surface functionalities are denoted as reaction bases, and the base used for titration is denoted as the titrator base. The carbon used for these experiments was chosen to be Black Pearls 2000. This carbon was chosen since it produced reproducible results in multiple Boehm titrations and because it has a very high surface area as determined by BET (ca. 1400 m2/g) with the majority (ca. 68%) of its surface area in micropores (diameter 50 nm diameter) [20]. Although the BET surface area may not provide an entirely accurate measure of the surface area available for reaction in the Boehm titration, since there is some error in determining surface area with small micropores and some of these micropores may not be fully wetted by the reaction base, this Black Pearls 2000 carbon is expected to provide a good model of a carbon with a high degree of porosity. It might be expected that full neutralization of the CSFs may be influenced by the size of the pores, since pore size will influence how rapidly the reaction base floods the sample, covering the whole surface area. Neutralization times may also be affected by the diffusion of the reactant base within the pores since this diffusion will be slow in small pores and since the reaction of the base with the walls will necessarily lead to a decrease in the concentration of base diffusing into the pores. The small pores of this carbon will allow us to examine the agitation method under conditions where the diffusion of the reaction base into all of the pores (to cover the whole surface) is slow. Additionally, the Black Pearls 2000 carbon have a regular and well defined macroscopic structure, that of spherical particles of approximately 1 mm in diameter. This spherical shape allowed for easy determination of any damage to the particle surface from agitation, which might not otherwise be seen with other carbon types with less regular surfaces (e.g. graphite or activated carbon). This particular carbon black is pelletized (aggregation through evaporation to form a pellet or ‘‘pearl’’) with only water and run through a dryer with no addition of a binding agent, but the uptake of reaction base by a binder may need to be considered for other carbons containing binders.
2.3.
Carbon removal/filtering experiments
Blank solutions (no carbon present) were used in the filtering experiments to ensure there was not variation due to heterogeneity of the carbon samples. By removing carbon from the titration to determine the effect of filtering, it allows any bias to be easily seen since the nCSF would be expected to be zero. 50.00 mL of a 0.05 M reaction base (NaHCO3, Na2CO3 and NaOH) were passed through either qualitative Grade 1 filter paper (Whatman) or Pall glass-fibre filter papers (Sigma–Aldrich). Aliquots were then prepared and titrated (as per Section 2.2). Carbon may also be removed from the carbon/reaction base mixture by allowing the carbon to settle to the bottom of the vessel, followed by supernatant removal using a pipette
CARBON
(during removal of the aliquot). With this method, the carbon was allowed to settle from solution for 24 h before the aliquots were removed from the supernatant.
2.4.
Dilute titrant
To determine whether more precise endpoints could be reached using a lower concentration of titrant, titrations using a standardized 0.025 M NaOH was used as titrator base were carried out on blank solutions and compared to similar titrations using a 0.05 M NaOH titrator base. Blank solutions were used to remove differences as a result of carbon surface heterogeneity (see Section 2.3). At least three ‘‘one-point’’ endpoint determination titrations were conducted, and the average of these values and standard deviations were reported.
2.5. Effect of exposure to air during transfer and storage on the standardization of NaOH In order to determine whether the concentration of NaOH determined by standardization was valid for a NaOH solution which had been transferred repeatedly between vessels or stored for extended periods or whether exposure to air was changing the concentration of the NaOH during transferring and storage (necessitating standardization immediately prior to the Boehm titration), a solution of NaOH was standardized after multiple transfers between vessels and after one week of storage. For the standardizations, a mass of ca. 0.2 g dried KHP was diluted in 20.00 mL of 18.2 M water in an Erlenmeyer flask. The KHP solution was then titrated with NaOH from a 25 mL burette, with at least triplicate measurements as per Section 2.1. Half of the NaOH stock solution was then transferred into a beaker and stirred manually with a stir rod to introduce air into the solution while the other part of solution was stored in a polyethylene bottle for one week. The freshly stirred NaOH was again standardized with at least triplicate measurements. The above procedure of introducing air to the solution was carried out two more times. The stored NaOH solution was again standardized a week later with KHP.
2.6.
nCSF ¼
nHCl ½BVB nB ½HClVHCl;acidification ½NaOHVNaOH;tit
VB Valiquot
ð1Þ
where nnHCl is the molar ratio of HCl to reaction base (B) to acB count for monoprotic vs. diprotic reaction bases, [B] and VB are the concentration and volume of the reaction base mixed with the carbon, Valiquot is the volume of the aliquot taken from the VB, [HCl] and VHCl,acidification are the concentration and volume of the acid used in the acidification, and [NaOH] and VNaOH,tit are the concentration and volume of the NaOH used in the titration. See Part I of this series [2] for a discussion of this equation. The nCSF of each surface group (carboxylic, lactonic, phenolic) were then determined through subtraction of the nCSF determined by each reaction base. Finally, the nCSF was normalized for the amount of carbon reacted by dividing by the mass of the carbon to determine the moles of CSF per mass of carbon (mC). were estimated using the The error (r) in the nCSF and nmCSF C typical error equations for addition/subtraction of random errors (y = k + kaa + kbb + kcc, where k are constants for each term a, b, c and d): qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð2Þ ry ¼ ðka ra Þ2 þ ðkb rb Þ2 þ ðkc rc Þ2 and multiplication/division (y = kab/cd): rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi r 2 r 2 r 2 r 2ffi a b c d þ þ þ ry ¼ y a b c d
ð3Þ
and the errors were propagated throughout the derivation. The full derivation of the final uncertainty equations can be seen in the Supplementary material. Systematic errors for the analytical balance were assumed to have a uniform (rectangular) distribution and a triangular distribution for the burette calibration (±0.03 mL). These uncertainties were combined with the systematic errors to give a full estimate of the uncertainty in the system.
3.
Results and discussion
3.1.
Carbon removal/filtering experiments
Method and duration of agitation
To determine which method of agitation (stirring, shaking or sonicating) was optimal, carbon and reaction base mixtures were either: shaken for 24 h using a MS 3 Shaker (Fisher Scientific); stirred for 24 h using a stir plate; or sonicated for 10 min and then let stand for 6 h before filtering. Carbon samples from the various agitation methods (in 0.05 M Na2CO3) were washed in 18.2 M water for 24 h and then dried in an oven at 80 C. These samples were then observed under an Olympus-1MT-2 inverted microscope at 4· magnification connected to a digital camera to determine the effect of each agitation method on the macroscopic structure of the carbon sample.
2.7.
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Error calculations
The equation for determining the number of moles of surface functional groups on the carbon surface (nCSF) through the standardized Boehm titration is [2]:
Since carbon surfaces often have both acidic and basic surface functionalities, it is important to remove all of the carbon from the reaction mixture in the Boehm titration because the basic CSFs of any carbon remaining in the sample will react with the HCl used in the acidification step of the titration, and this will lead to a negative bias for the amount of acidic CSFs determined through this method. Unfiltered blank samples were compared to blank samples passed through filter paper to examine whether the process of filtering the sample was significantly changing the amount of nCSF determined in the Boehm titration. This is a concern since the typical filter paper found in laboratories is cellulose-based and therefore has many hydroxyl functional groups on the surface and thus, the filter paper’s functional groups could actually take up some of the reaction base when being filtered. Because the glucose units which comprise cellulose are stabilized with both intra- and inter-molecular hydrogen bonding [21], it is unlikely that even NaOH may undergo a reaction with the filter paper hydroxyl groups, but this must be examined. As a
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corollary, the weaker basicity of the other reaction bases (NaHCO3, Na2CO3) ensures that these bases are not strong enough to react with the filter paper and therefore, no filtering error is expected with these reaction bases. Given the large size of the holes/pores in the filter paper (ca. 11 lm) compared to the predicted size of the solvated ions of interest in the Boehm titration (
