Partitioning Of Barbiturates Overview

Phenobarbitone is an antiepileptic offal which is used to manage epilepsy. Absorbance prizes at irrelative energy of phenobarbitone can be used to concoct a calibration graph which in reverses can be used to furnish the energy of phenobarbitone in fundamental exposure at irrelative pH by using the gradient of the graph. From there, the energy of phenobarbitone in aqueous exposure can be steadrapid and heresubjoined can furnish the allotment coefficient (P). Pgentleman of phenobarbitone can be steadrapid subjoined a opportunity the influence of the prize of Papp and fu of phenobarbitone (acid). % ionization of phenobarbitone at irrelative pH can as-well-behaved be conducive by acute its pKa and this in reverses can be used to state the lipophilicity of phenobarbitone. % nativity of phenobarbitone succeed as-well-behaved state how well-behaved-behaved does phenobarbitone passes through the fundamental exposure (biological membrane in the association) at irrelative pH.

‘Phenobarbitone is a barbiturate used in the management of epilepsy. It helps to classify plunder abundance and tyranny and may smooth plug plunders collectively.’ [1] ‘Phenobarbital works by acting on GABA receptors that in reverse extension synaptic discountenance. This then extensions the plunder inauguration, heresubjoined reducing the compute of plunders occurring.’ [2)]

Phenobarbitone is the hankerest acting barbiturate. It is an penetratingic gleaming constitution which has pka prize 7.4, heresubjoined it is significantly ionised at ph prizes aggravate 6. ‘Loweresonance the pH of the disentanglement classifys ionisation.’ [4] According to Phenobarbitone constitution, it has 2 hydrogen obligation donors and 3 hydrogen obligation receptors and the solubility of Phenobarbitone in inspire is 1g in 1000mL (0.1% w/v). It has low lipid solubility compared subjoined a opportunity other barbiturate. Thus it has sluggish onslaught exercise and hankerer half conduct.

This trial was carried out to summon the pi of ionisation of phenobarbitone by measuresonance the aggregateity that was extracted into n-octanol from the aqueous exposures of irrelative pH prizes. The results serene were then used to furnish the energy of the offal at makeponderosity and gone the pKa of the offal and the pH of the disentanglement are public, the gentleman allotment co-efficient for phenobarbtione can be conducive. [3] ‘Partition coefficient (P) is the agreement of energy of offal in n-octanol exposure unanalogousd by energy of offal in aqueous exposure. A excellent allotment co-efficient implies that the offal is excellently lipophilic.’ [5]

The regularity used in this trial is the shake-flask regularity (the most obsequious configuration regularity). The offal, in this plight phenobarbitone, is recognized to equilibrate between the NaOH disentanglement and the n-octanol and then the energy in twain flakes are been steadfast. [5] Gone the aggregate aggregateity of barbiturate offer in the 0.5M NaOH (which is extracted from the octanol) is public, and the UV disjunction gives the aggregateity of phenobarbitone in the octanol flake at makeweight, we can weigh the ponderosity left in the aqueous exposure and heresubjoined the energy of offal in each exposure at makeweight. pKa is relevant to state the gentleman allotment coefficient for the barbiturate. The hindrance for shake-flask is that it is singly use to the unionised offal which most offals are penetratings or dishonorables and are ionised in biological systems.


0.02% w/v phenobarbitone in inspire was been supposing as the fund disentanglement. Using the fund disentanglement, 50µg mL-1 disentanglement in 0.5M NaOH [Solution A] and a 50µg mL-1 disentanglement in inspire [Solution B] were been apt.

(a) Calibration Standards

A rank of calibration scales each containing 5, 10, 15, 20, 25, 30 µg mL-1 of the phenobarbitone in 0.5M NaOH were apt using Disentanglement A. The wavelength of ultimatum absorbance (λmax) at environing 254nm was then steadrapid using the 30 µg mL-1 scale. The absorbance of each scale at the λmax was discover using 0.5M NaOH as the bare. A calibration deflexion of absorbance versus energy for the phenobarbiturate was then invented using the absorbance discoverings obtained.

(b) Partitioning Samples

Six allotmenting funnels were assiduous as the subjoined:

i) 10mL Disentanglement B, 10mL 0.1M HCl, and 20mL n-octanol

ii) 10mL Disentanglement B, 10mL pH 6.6 buffer, and 20mL n-octanol

iii) 10mL Disentanglement B, 10mL pH 7.0 buffer, and 20mL n-octanol

iv) 10mL Disentanglement B, 10mL pH 7.4 buffer, and 20mL n-octanol

v) 10mL Disentanglement B, 10mL pH 8.0 buffer, and 20mL n-octanol

vi) 10mL Disentanglement B, 10mL pH 9.0 buffer, and 20mL n-octanol

The funnels were shaken at repeated intervals for 30 minutes to yield the flakes to unanalogous abundantly. The fundamental flake was then been careabundantly ran off into a second separating funnel. 20mL of 0.5M NaOH was then productive to the octanol and shaken for 5 minutes, yielding the flakes to unanalogous. The absorbance of the aqueous (bottom) flake was then measured by using the λmax steadrapid previously in (a). The energy of the phenobarbitone in the 0.5M NaOH was conducive using the calibration deflexion.


In allotment chromatography, molecules impel from one exposure to another via quiescent colliquation, i.e. the impelment of molecules from an area of excellent to low energy area subjoined a opportunityout any facilitating content. However, abounding molecules are incompetent to impel down the energy gradient via this passage. Allotment chromatography of phenobarbitone mimics the impelment of phenobarbitone opposing the biological membrane, i.e. the impelment of phenobarbitone from aqueous exposure, 50µg mL-1 phenobarbitone in inspire, to fundamental exposure, octanol. Its chemical constitution shows that there is a hanker hydrocarbon obligation attached to the resonance constitution which contributes to its lipophilicity. The past lipophilic the offal, the past efficiently it is attentive into the fundamental exposure.

Glass separating funnels were used to escape parching of lipophilic offal into the container. 30 minutes subjoined the flakes are left to unanalogous abundantly, the aqueous flake is careabundantly ran off, leaving the fundamental flake in the separating funnel. 20mL of 0.5M sodium hydroxide, NaOH is then productive to the fundamental flake to unanalogous the compromise into two immiscible flakes frequently. This is likely accordingly phenobarbitone is a destructible penetrating, it reacts subjoined a opportunity the newly productive dishonorable, NaOH to fashion aqueous exposure. The energy of phenobarbitone in the aqueous exposure is then steadrapid using a UV spectrophotometer.

From Tcogent 1.6, at pH 1.1, the ponderosity of phenobarbitone in fundamental exposure is excellentest,

4.392 X 10-4 g; when-in-fact at pH 9.0, the ponderosity of phenobarbitone in fundamental exposure notcogent its littleest at 2.800 X 10-4 g. This proves that the offal is most lipophilic at low pH as it is a destructible penetrating and is unionised at low pH, herebehind past cogent to morose into the octanol flake. Meanwhile, at pH 9.0, most of the offal is retained in the aqueous exposure in its ionised fashion and heresubjoined incompetent to morose into the octanol exposure as it cannot be quiescently dilutedd.

As shown in Tcogent 1.7 and Graph 2.0, the excellenter the pH, past of the offal offer is ionised and immorality versa. Starting from pH 1.1, the percentage of phenobarbitone ionised in the aqueous exposure extensiond sluggishly up to pH 6.6, followed by a craggy extension from pH 6.6 to pH 8.0, and elevate extension close craggyly from pH 8.0 to pH 9.0. As for Graph 2.0, a sluggish decline is observed from pH 1.1 to pH 7.0, a craggy decline from pH 7.0 to pH 7.4, followed by a progressive decline from pH 7.4 to pH 9.0.

Partition coefficient, P is the agreement of a offal’s energy in the octanol exposure to its energy in the aqueous exposure at makeponderosity subjoined a opportunity each other. A excellent P prize heresubjoined denotes a excellent offal energy in fundamental exposure. From the results, a excellent P obtained at low pH proves that phenobarbitone is a excellently lipophilic offal, capcogent of moroseing lipophilic membranes in the association.

From the results exception, Pgentleman prize at pH 1.1 is 7.74, when-in-fact the learning prize is 1.4. The comparison is made at pH 1.1 accordingly phenobarbitone is offer in its unionised fashion at low pH. However, the Pgentleman prize is ample excellenter than the learning prize. This could be due to deceptions that occurred duresonance the trial, e.g. parallax deception opportunity pipetting the fund disentanglement, and accidentally ran off some of the fundamental exposure opportunity present off the aqueous exposure, causing unsteadrapid immanent ponderosity mislaying of phenobarbitone in the fundamental exposure. It could as-well-behaved be due to inadequate era recognized for phenobarbitone to diluted from the aqueous exposure into the fundamental exposure.

Studies inspired that the peak plasma energy is grasped 0.5 to 4 hours subjoined an unwritten administration; allotment chromatography mimics the colliquation of offal opposing biological membrane, herebehind requires at last 30 minutes to grasp peak plasma energy of phenobarbitone in the fundamental exposure. However, in the trial, the allotment was plugped at 30 minutes, not yielding past era for the compromise to unanalogous abundantly. As octanol was productive into the separating funnels at irrelative eras, they could not be plugped at the identical era. The aqueous (bottom) flakes were then run off starting from the funnel where octanol was productive the primary to the ultimate. The era fascinated to run off the aqueous exposure varies as the tome of aqueous exposure varies from funnel to funnel, which may enjoy resulted in irrelative eras for each funnel to unanalogous and heresubjoined past adequate disjunction in the succeeding compromises.

The lipophilicity of phenobarbitone contributes to its parching into the octanol exposure. Likewise, it is discoverily attentive opposing biological membranes in the association, e.g. stomach bastion, cell membrane and blood-brain distribution. Phenobarbitone is administered unwrittenly; it is speedily and abundantly bioavailcogent subjoined unwritten administration as phenobarbitone is unionised in penetratingic environment. At pH 1.1, 87.84% of phenobarbitone was extracted into the fundamental exposure. This signifies that 87.84% of phenobarbitone is cogent to morose the stomach bastion into the systemic publicity. The parching is expected to decline subjoined a opportunity the extension of pH prize down the gastrointestinal tract; the unionised frexercise is littleer in the little domestic but has hankerer intraluminal stop-behaved era and heresubjoined increasing parching.

Being lipophilic, phenobarbitone morosees biological membranes discoverily, past preferably at low pH environment, into the bloodstream and encircling the association via systemic publicity and then exclusive throughout the interstitial watery. However, environing 50% of the offal is to-leap to plasma protein, herebehind neither cogent to pilgrimage opposing the blood-brain distribution into the cerebrospinal watery nor is it cogent to be metabolised by the liver. From Tcogent 1.8, at pH 7.4, 70.72% of phenobarbitone was extracted into the octanol flake. Assuming 50% is protein-bound; it can then be productive that singly environing 35.36% of phenobarbitone would grasp the brain at physiological pH 7.4.

Elimination is a means the association utilises to rid offal, xenobiotics and impair products from the association and plasma, largely by the blood and the liver. The typical pH of urine ranks from pH 4.5 to pH 7.5. Phenobarbitone essence lipophilic and protein-binding has a sluggish extraction; its lipophilicity prevents it from essence filtered by the glomerulus, unclose metabolised in the liver into a close lipophilic metabolite. However, 25% of phenobarbitone is peaceful excreted in its unionised fashion via quiescent tubular secretion. This passage of sweating is exaggerated when urine is alkaline or when the urine tome is extensiond, i.e. via flinty diuresis.


As phenobarbitone is used in management of epilepsy, it needs to own the characteristics and force to morose the blood-brain distribution into the motor cortex to exact its depressant pi. From the allotment chromatography carried out, it is indisputable that phenobarbitone is discoverily attentive into the association opposing biological membranes due to phenobarbitone’s lipophilic essence. It is as-well-behaved widely exclusive in the association watery, including the cerebrospinal watery where it can act on the motor cortex. As for extraction, closely 75% of the offal is metabolised by the liver precedently essence excreted, but 25% is excreted as unionised molecules by quiescent tubular secretion in the bloods. In blank, phenobarbitone has a proportionately rapid onslaught of exercise, heresubjoined it is reckoned suitcogent to be administered via the unwritten passage for the management of epilepsy.

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