kinetika-laju-reaksi(4)
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KINETIKA & LAJU REAKSITim Teaching MK Stabilitas Obat Jurusan Farmasi FKIK
UNSOED
2013
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Pendahuluan
• Seorang farmasis harus mengetahui profil suatu obat. Sifat fisika-kimia, stabilitas.
• Sifat tersebut menjadi dasar dalam keputusan seorang farmasis peresepan, maupun desain dan formulasi.
• Selain itu kinetika suatu obat menjadi dasar dalam menentukan waktu kadaluarsa dari suatu obat
• Farmasis harus menjamin obat stabil sampai ditangan pasien
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Tujuan Pembelajaran
Mahasiswa mampu mennguraikan pengertian orde reaksi, menghitung persamaan kinetika reaksi mengikuti orde nol, orde satu, dan orde dua
Mahasiswa mampu menentukan orde reaksi menggunakan metode grafik, substitusi, dan waktu paruh
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General Considerations in Dosage Form
Design (cont’d)
• Drug and Drug Product Stability▫ Drug Stability: Mechanisms of Degradation▫ Drug and Drug Product Stability: Kinetics and
Shelf Life▫ Rate Reactions▫ Q10 Method of Shelf Life Estimation▫ Enhancing Stability of Drug Products▫ Stability Testing
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Rate Reactions
• Change of drug concentration with respect to time
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Kinetics and Drug
Stability
Kinetics is important in all phases of the drug development process as well as in quality control, stability, bioavailability, and therapeutic drug monitoring.
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Kinetics
• The study of the rate of chemical change and the way this rate is influenced by conditions of concentration of reactants, products, and other chemical species that may be present and by factors such as solvent, pressure, and temperature
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Reaction Kinetics
• Want two things from kinetic data:
▫ Reaction order
▫ Reaction rate
• In considering the chemical stability of a pharmaceutical, we need to know the REACTION ORDER, which is obtained experimentally by measuring the REACTION RATE as a function of concentration of the degrading drug.
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Factors Affecting Reaction Rates
• Temperature
• Dielectric Constant
• Ionic Strength
• Solvent Effect
• Catalysis
• Light
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PRINSIP DAN PROSES LAJU YANG
BERKAITAN • Stabilitas: proses laju reaksi menyebabkan
ketidakaktifan obat melalui penguraian obat,hilangnya khasiat obat akibat perubahan fisikdan kimia
• Disolusi: kecepatan berubahnya obat dalambentuk sediaan padat menjadi bentuk larutanmolekular
• Proses absorpsi, distribusi dan eliminasiFarmakokinetika
• Kerja obat pada tingkat molekuler
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Laju (Kecepatan) Reaksi
▫ A B + C
▫ A berkurang, sedangkan B dan C bertambah
▫ Laju reaksi sebanding dengan berkurangnya konsentrasi A seiring waktu
▫ Dan bertambahnya konsentrasi B seiring waktu
dt
Adlaju
][
dt
Cd
dt
Bdlaju
][][
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• Reaksi kimia dapat dinyatakan sebagai laju penguraian reaktan atau laju pembentukan produk
• aA + bB + .... Produk
• Orde reaksi adalah penjumlahan eksponen
(pangkat) a+b +.....
• Orde terhadap A adalah a, terhadap B adalah b,
dst.
.......
11-Laju
baBAk
dt
Bd
bdt
Ad
a
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Analogi Laju Reaksi
Calon sarjana farmasi +
PerjuanganDarahAir mataUangWaktu
Sarjana FarmasiApoteker
Laju reaksi: kecepatan pembentukan sarjana farmasi dalam waktu tertentu
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Satuan tetapan k pada orde
reaksinyaOrde nol : – d[A]/dt = k k = mol l-1 s-1
Orde I : – d[A]/dt = k [A] k = 1/waktu = s-1
Orde II : – d[A]/dt = k [A] 2 k = l mol-1 s-1
Orde reaksi ke n mempunyai satuan: (konsentrasi)1-n (waktu)-1
Tetapan k adalah tetapan laju spesifik sehingga tiap perubahan
kondisi seperti suhu, pelarut akan mempunyai tetapan k yang
berbeda pula.
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Orde Nol
• Reaksi di mana kecepatan reaksi tidak dipengaruhi oleh konsentrasi reaktan
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ORDE NOL
00
00
0
0
A
A
0
t
0
tkAA
tkAA
dtkdA
kdt
dA
t
t
t
Waktu paro:
0
0
2/12
1
k
A
t
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C
T
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Sebuah obat telah terurai 75% dalam satu tahun, reaksi dianggap
orde nol, hitung nilai Ko dan t1/2 obat tersebut
LATIHAN SOAL
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a. Ct = Co-Kt b. t1/2 = Co
Ko = Co – Ct 2.Ko
t = 100
= 100 – 25 2. 75
1 t1/2 = 0.667 Th
Ko = 75 %/Th = 8 bulan
Tentukan rumus T90 dan hitung kadaluarsa obat tersebut!
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Orde 1
• Reaksi farmasetika yang sering terjadi seperti: absorpsi obat dan degradasi obat
• Kecepatan reaksi dipengaruhi oleh konsentrasi reaktan
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ORDE I
xa
a
tk
c
c
tk
ktcc
ktcc
tkcc
dtkdt
dc
kcdt
dc
t
log303,2
atau log303,2
303,2/loglog
lnln
)0(lnln
0
0
0
0
0
c
c0
kt
kt
105,0
693,0902/1
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C = co e –kt
Difficult to determine slope lnc = lnco – ktSlope = c1 – c2 / t1 – t2
Slope = -k
lnco
Log co
Log c = log co – kt / 2.303Slope = c1 – c2 / t1 – t2
Slope = -k / 2.303
Or use semi log paper
C Lnc
Logc
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Semi log paper
Slope = -K / 2.303
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Contoh soal:
K = 2,303 . Log Co t1/2 = 0,693
t Ct k
= 2,303 . Log 500 = 0,693
40 300 0,0128
= 0,0128 /hari t1/2 = 54,3 hari
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Larutan obat mengandung 500 unit ketika dibuat. Setelah40 hari, dilakukan analisis kadar ternyata konsentrasi nyatinggal 300 unit. Bila reaksi penguraian berjalan padaorde 1, berapa lama obat akan terurai sampai konsentrasitinggal setengah dari konsentrasi awal?
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Orde Dua
• Reaksi di mana terdapat dua reaktan yang berpengaruh dalam reaksi.
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ORDE IIA + B P
dt
PdBAk
dt
Bd
dt
AdreaksiLaju
Jika: a dan b masing-masing konsentrasi awal dari A dan B;
x adalah jumlah mol A atau B yang bereaksi dalam waktu t, maka:
kt
AAAk
dt
Ad
xaa
x
tkxak
dt
dx
xba
xab
baktxbxak
dt
dx
0
2
2
11 :atau
1 :laju sama inyakonsentrasreaktan kedua Jika
log303,2
:laju
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2nd order graph
Units of K:
1/C = 1/Co + Kt
K = (1/C - 1/Co) / t
K = M-1. sec -1
i.e, K is dependent on initial drug concentration.
Half life: t1/2 = 1 / KCo
Shelf life: t0.9 = 0.11 / KCo
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Practice Problem
• Using the following information, determine the half life of this reaction, assuming there is a single reactant, second ordeConcentration (M) Time (s)2.0 01.3 100.9633 20
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SolutionsConcentration (M) Time (s) 1/ Concentration
2.0 0 0,51.3 10 0,7690.9633 20 1,038
RL 1/C vs tA : 0,5 1/C = 1/Co + KtB : 0,0269 y = a + bx
K = b = 0,0269t1/2 = 1/(K.C0)
= 18,89 s
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Given the information from the previous problem, how much is the concentration after 5 minutes?
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Metode Penentuan Orde Reaksi
Metode Substitusi
Metode Grafik
Metode Waktu Paro
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Metode Substitusi
• Data yang terkumpul dari hasil pengamatan jalannya suatu reaksi disubstitusikan ke dalam bentuk integral dari berbagai orde reaksi.
• Jika menghasilkan k yang konstan, maka reaksi dianggap berjalan sesuai orde tersebut
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• Metode Substitusi : Hitung k pada setiap t; masukkan ke dalam persamaan:
xba
xab
tbakOrdeII
a-x
a
tkIOrde
t
CCkOrde
log303,2
:
log303,2
:
: 0 0
Metode ini memakan waktu
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Metode Grafik
• C vs t ----- linear ----- orde 0
• Log C vs t ----- linear ----- orde 1
• 1/C vs t ----- linear ----- orde 2
Metode yang praktis, cepat, tepat
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• Metode Grafik
t
bak
xba
xabOrde
tk
CCOrde
ktCCOrde
303,2log :II
303,2
loglog :I
:0
0
0
xba
xab
log
t
303,2
bakslop
tt
C
Log C
Slop=-k
Slop=-k/2,303
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Orde II
ktCC
0
11
Slop=k
t
C
1
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Dengan kertas semi log
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• Orde 0 ------ t ½ = Do / 2k
• Orde 1 ------ t ½ = 0.693 / k
• Orde 2 ------ t ½ = 1/Do.k
Jarang digunakan
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Metode Waktu Paro
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Soal • Harned meneliti dekomposisi hidrogen peroksida (H2O2)
yang dikatalisis oleh KI (0,02 M) dengan reaksi sebagaiberikut2 H2O2 2H2O + O2
Data degradasi H2O2 tersebut terlihat pada tabel berikut
Waktu (menit) sisa H2O2 (mM)0 57,95 50,410 43,925 29,145 6,765 9,6
Pertanyaan : -tentukan orde reaksi degradasi H2O2 tersebut-Hitung harga konstanta degradasi H2O2
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