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Accuracy

The accuracy of the results was checked by
applying the proposed method for the determination of different samples of AS.
The concentrations were obtained from the corresponding regression equations.

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 Precision

Repeatability

Three
concentrations of AS (5.00 ×10-3, 1.00×10-3 and
5.00×10-4 M) were analyzed three times, each intra-daily using the
previously mentioned procedures under (Direct potentiometric
determination of AS in pure samples). The percentage recoveries and
standard deviations for the proposed drug were calculated from the computed
regression equations.

Intermediate precision

            The above mentioned AS samples were analyzed on three
successive days using the previously mentioned procedures under (Direct
potentiometric determination of AS in pure samples). The percentage
recoveries and standard deviations for the studied drug were calculated from
the computed regression equations.

Direct potentiometric determination of laboratory prepared mixtures
containing different ratios of AS and AS deg (Specificity)

The
prepared electrodes (sensors I ,II and III) in conjunction with the double
junction Ag/AgCl reference electrode were immersed separately in the prepared
mixtures (Direct potentiometric determination of AS in pure samples.),
the resulting potential was recorded for each solution at constant stirring at
room temperature and then the respective concentration was calculated from the
corresponding regression equations.

Direct potentiometric determination of AS in pharmaceutical
formulation

The contents of ten asenapine tablets were accurately weighed, an
amount equivalent to 402.00 mg AS was accurately weighed and transferred
into a 100-ml volumetric flask and diluted to the mark with bi-distilled water.
The concentration of this solution is claimed to be 1.00×10-2 M of AS. Suitable dilution was made to
prepare solution containing 1.00×10-3 M. The prepared electrodes
(sensors I ,II and III) in conjunction with the double junction Ag/AgCl
reference electrode were immersed separately in the prepared solution, the
resulting potential was recorded and then the respective concentration was calculated
from the corresponding regression equations. The validity of the proposed method was assessed by
applying the standard addition technique.

Results and
discussion

                In this section ISEs were introduced for determination of AS in pure form,
in pharmaceutical formulation and also in presence of its degradation product.
ISEs have many good advantages as it has the capability of performing direct
measurements in turbid and colored samples. They show rapid response to change
in the concentration. Furthermore, they may be used for measurement over a wide
concentration range. ISEs are generally tolerant to small changes in pH. A
further advantage is that they are relatively simple and not expensive to
develop, set up and run. Moreover, the chemical design of the electrodes has
been developed to give superior selectivity and response.

Optimization of the method

a-     
Effect of composition or Sensors Fabrication

The composition of the PVC membrane electrode was optimized by
studying the potentiometric response of five PVC membranes (Table 1). An
optimum Nernestian response of 58.314 mV decade over the concentration range
from 1.00×10-5 to 1.00×10-2M was observed with a membrane
containing 5% AS-AR, Figure ( 2) .

Different carbon paste electrodes were prepared with 2.50, 5.00,
7.50 and 10.00 % (w/w) AS-AM and 3.2, 4.6, 9.6 and 12.80% AS-PTA.
The best composition was that containing 5.00% and 3.20% for AS-AR and AS-PTA
respectively. The electrode showed a Nernestian slope of 58.45 and 58.11
mV/decade for AS-AR and AS-PTA respectively, over the concentration range from
5.00×10-5 to 1×10-2M for both sensor as shown in figure 3
and 4.

b-    
Life time of the electrodes

The slope, linear range and detection limit of the PVC membrane
electrode was recorded at different time intervals over a period of several
days of continuous soaking in water. The PVC polymeric membrane can withstand
the effect of soaking for up to 10 weeks. The electrode showed a constant slope
of 58.0 ± 1 mV/decade for 6 weeks.

The carbon paste electrode possesses the advantage of ease of
surface regeneration. The surface can be regenerated by simply rubbing the
electrode surface against a filter paper.The slope of the carbon paste
electrode before and after regeneration was 58.50 ± 1 mV/decade.

c-     
Response time

The dynamic
response time of the proposed electrodes was studied by measuring the time
required to achieve a steady state potential (within ±1 mV) after successive
immersion of the electrodes in a series of stirred AS solutions (1.00×10-6
to 1.00×10-2 M). The required time was found to be and 20 seconds
for sensors I ,II and III. 

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