Control Chart

Dalam ISO/IEC 17025 - Klausul 5.9.1 dinyatakan "Laboratorium harus mempunyai prosedur pengendalian mutu untuk memantau keabsahan pengujian dan kalibrasi yang dilakukan. Data yang dihasilkan harus direkam sedemikian rupa sehingga kecenderungan dapat dideteksi dan, bila dimungkinkan, teknik statistik harus diterapkan pada pengkajian hasil. Pemantauan tersebut harus direncanakan dan dikaji serta mencakup, tapi tidak terbatas pada, hal-hal sebagai berikut;
(a) keteraturan penggunaan bahan acuan bersertifikat dan/atau pengendalian mutu internal menggunakan bahan acuan sekunder ...

Terdapat 2 teknik yang biasa digunakan, yaitu;
1. Shewhart Chart
2. CuSum (Cumulative Summation)

Pada prakteknya teknik Shewhart Chart sering kali digunakan dikarenakan sangant mudah untuk diinterpretasikan hasilnya.

Penggunaan Shewhart Chart

a) Biasa digunakan batasan 2 SD dan 3 SD, berdasarkan 11-30 replikat.
b) Tindakan perbaikan dilakukan apabila melewati batasan 3 SD
c) Batas peringatan berkisar 2 SD - 3 SD

Interpretasi Shewhart Chart

Kumpulan data hasil pengujian pada periode waktu tertentu dinyatakan memenuhi batas keberterimaan yang telah ditetapkan bila kumpulan data tersebut berada pada batas tindakan dalam bagan kendali dan tidak menunjukkan adanya kecenderungan (trend) khusus. Ketika data hasil pengujian berada diatas atau di bawah batas tindakan maka hal ini berarti bahwa data tersebut diluar pengendalian statistika (Gambar 6.2a). Kecenderungan khusus merupakan suatu keadaan dimana terjadi kecenderungan menuju ketidaksesuaian dalam proses pengujian. Pola kecenderungan khusus yang dapat menyebabkan data hasil pengujian diluar pengendalian statistika, antara lain adalah sebagai berikut:
a) 3 dari 4 data hasil pengujian berturut-turut pada daerah tindakan atas atau daerah tindakan bawah - Gambar 6.2b;
b) 4 dari 5 data hasil pengujian berturut-turut pada daerah peringatan atas atau daerah peringatan bawah - Gambar 6.2c;
c) 8 berturut-turut data hasil pengujian diatas atau dibawah rerata (garis pusat) – Gambar 6.2d;
d) 8 berturut-turut data hasil pengujian diatas dan dibawah rerata namun berada pada daerah tindakan – Gambar 6.2e;
e) 6 data hasil pengujian berturut-turut turun atau naik – Gambar 6.2f;
f) 14 data hasil pengujian berturut-turut turun-naik atau naik-turun – Gambar 6.2g;
g) 15 data hasil pengujian berada pada batas informasi atas dan batas informasi bawah – Gambar 6.2h

Referensi:
*) Anwar Hadi, 2009, Pedoman Pengendalian Mutu Internal Pengujian Parameter Kualitas Lingkungan

Rotary Evaporator

The rotary evaporator is a piece of equipment that is designed to allow you to distill a liquid under conditions of reduced pressure. Since the pressure within the system is reduced, it means that the liquid can be distilled at a lower temperature than it would at atmospheric pressure.

This is a very safe and fast method of distilling flammable solvents like dichloromethane. Dichloromethane normally boils at 40 deg C at atmospheric pressure.

The sample to be “rotovapped” is added into the round bottom (RB) flask. The sample RB flask should never be more than half-full with liquid when it is attached to the rotary evaporator. The sample RB flask is connected to the trap and is secured using a circular clip. The clip has two sides. It is always oriented with the smaller circle on top.

The stopcock that is fitted at the top of the condenser needs to be in the closed position. To close, turn the stopcock such that it is crosswise (90 degrees) to the air inlet. Turn on the water aspirator to evacuate the system.

Ensure that the condenser has cold water running through the glass coil.



When the vapour enters the distillation tube inside the condenser, it will be in a cooled environment for the vapour to condense and drip into the large round collection flask to which it is attached.

There is a splash trap at the end of the distillation tube to prevent unwanted sample from splashing up into the condenser

To speed up the rate of evaporation, the liquid sample can be warmed with hot water. Lower the flask by sliding the “Push” knob below the water bath. The temperature can be set using the dial on the side of the water bath.

To ensure even heating of the sample RB flask, turn the rotary dial to spin the sample RB flask that is immersed in the hot water bath.

Observe the condensation of vapour near the condenser. As the solvent condenses, you will see dripping of the condensed vapour collecting in the collection flask. Once the dripping stops, wait another minute and stop the rotary evaporator.

To remove the sample RB flask, disassemble by reversing the steps. Raise the sample RB flask from the hot water bath. Turn the rotary dial to stop the sample RB flask from spinning. Turn the stopcock that is fitted at the top of the condenser so that it is in-line with the air inlet. This will let air back into the system, and you should hear the sound of air rushing into the system. Turn off the water to the aspirator and the condenser. Remove the clip that is used to secure the sample RB flask. Gently give the neck of the sample RB flask a twist and remove the flask from the rotary evaporator.

Steps in operating the rotary evaporator

1. Turn power switch on. Turn water bath on. Adjust water bath to desired temperature.

2. Turn on water to condenser (slow to moderate flow).

3. Turn on aspirator full counterclockwise.

4. Attach the splash trap to the vapour tube and secure with clip.

5. Attach the sample RB flask to the splash trap and secure with clip.

6. Adjust rotational speed of the flask.

7. Turn the air inlet valve to the closed position to obtain a vacuum.

8. Slide the "Push" knob down to unlock the lift mechanism to lower the sample RB flask into the bath.

9. Watch for the dripping of the condensed solvent into the collection flask at the condenser. After the dripping stops, wait another minute to ensure that the evaporation has finished.

10. To disassemble, slide the "Push" knob to raise the flask.

11. Turn the rotational speed knob to the off position.

12. Turn the air inlet valve to the open position to re-establish atmospheric pressure.

13. Remove the clip and the sample RB flask.

14. Remove the clip and the splash trap.

15. Turn off aspirator, water to the condenser, water bath and the power switch.

Techniques on Weighing

There are many different types of balances or scales available to measure the mass of a sample. The selection of the balance depends on the mass of the object or sample and the precision needed for the measurement. A top-loading balance is used to determine the approximate mass of the sample needed. In this course, we will be mostly using electronic analytical balances (Figure 1). These balances are easy to operate and are capable of measuring to 0.0001 g. Models of these balances vary in the labs. Consult your instructor and operating manual specific to the model of the balance.

There are two methods of weighing:

1. Weighing by difference – This is a technique which is used when it is important to know the precise amount of the sample that has been transferred
to a reaction mixture.

2. Weighing by taring – This technique is used when the mass of the empty container is not important. The empty container is ‘zeroed’ or tared on the
balance.

These two methods will be described in detail below.

The Balance Room

Balances are sensitive to drafts, changes in temperature, or the vibrations caused by moving people. The balances are stored in a separate room to minimize these variables and are placed on concrete tables.

Balances are very expensive and are sensitive to attack by corrosive chemicals. Do not take liquid into the balance room. When possible, chemicals should be added to the weighing container outside of the balance chamber. During the weighing process, the weighing container should be placed on a clean surface, such as a kimwipe, so that the bottom of the container does not pick up any dust. It is important that you clean up all chemical spills. If in doubt consult your instructor.


Weigh boats (Figure 2) are disposable containers used for weighing. They are made of polypropylene plastic and are inexpensive. A used weigh boat should be discarded in the waste container. All chemicals and spatulas that are used should be returned to their proper places. Depending on the experiment, other types of weighing container could be a porcelain crucible, an aluminum plate or a small beaker

The balance room must be kept tidy. Materials taken into the balance room include datasheets, pen and the sample to be weighed. Enter mass measurements directly on the datasheet with your pen. Before you leave your balance, make sure:

1. The balance and the area around it is clean. Spills inside the balance should be brushed off using the brush on top of the balance. Spills on the concrete table should be cleaned using Kimwipes.

2. Close all the doors of the balance.

3. Turn off the balance.

Weighing by difference

1. Pre-weigh an approximate quantity of the sample into a weigh boat using a toploading balance.

2. Record all the masses for each step directly on the datasheet in pen.

3. Turn on the balance by pressing on the control bar. After a few seconds, the display will read 0.0000.

4. Open the sliding glass door (on the side) and place the sample/weigh boat on the balance pan. Close the sliding glass door and wait until the reading is stable. Record the value.

5. Transfer the sample into a beaker.

6. Weigh the emptied weigh boat on the analytical balance. Do not brush off any sample particles from the emptied weigh boat. Record the mass of the emptied weigh boat.

7. The difference between the two weighings is the mass of the sample transferred into the beaker.

Weighing by taring

1. Place a weigh boat on the balance pan. Close the doors and wait for the reading to stabilize. Press briefly on the control bar or the tare button and the display changes to 0.0000 g. The weight of the weighing boat is now tared.

2. Remove the weigh boat from the balance and set it on a piece of kimwipe. With a spatula, carefully add the sample to the weigh boat. Place the weigh boat back on the balance pan. Close the doors and wait for the reading to stabilize. Record the mass of the sample.

Techniques on Gravity Filtration

There are two general methods of filtration: gravity and vacuum. In gravity filtration, the filtrate passes through the filter medium under the force of gravity and the capillary action between the liquid and the funnel stem.

There are several varieties of filter paper. Good filtration depends on the retention of the filter paper and the speed of the filter paper. Usually, fast papers will retain coarse particles. Slow papers will retain fine particles. The optimum choice is a paper, which is as fast as possible, yet retains all visible particles, and thus giving a clear filtrate.

Low-ash or ashless quantitative-grade papers can be ignited without leaving an ash. The residue left by an 11-cm circle of a low-ash paper may be as low as 0.06 mg. An ashless-grade paper typically leaves 0.05 mg or less from an 11-cm circle. This small mass is considered negligible in most analytical procedures.

Decantation is a process used to separate the liquid from the mixture to be filtered. To decant a liquid from a solid, in one hand hold the beaker that has the mixture in it. Hold a glass-stirring rod in the other hand. Touch the lip of the beaker to the glass rod and pour the mixture to be filtered using the glass rod as a guide to pour slowly to ensure that the solid is not carried along. This also prevents the liquid from running back along the outside of the beaker.

Washing of the solid to remove soluble impurities follows the decantation of the supernatant liquid. Use a small amount of wash liquid and mix it thoroughly with the solid. Allow the solid to settle and decant the wash liquid through the filter. Repeat this procedure several times. Several washings with small volumes of liquid are more effective in removing soluble contaminants.

Transfer the washed solid in the filter funnel is the final step. The bulk of the solid is transferred to the funnel by a squirting a stream of wash liquid from a wash bottle. The last traces of the solid are removed from the walls of the beaker by scrubbing with a rubber policeman. Rinse the beaker and rubber policeman and transfer the rinse liquid to the funnel. Repeat this at least two times.

Step to prepare a gravity filtration

1. It is important to use the correct size filter paper. Properly sized filter paper should stop just below the rim of the glass funnel. As a guide, use filter paper whose diameter is about 1 cm less than twice the diameter of the funnel, for example a six-centimeter diameter funnel uses a filter paper of eleven-centimeter diameter. The filter paper should sit a few millimeters from the rim of the funnel (Figure 1).

2. Fold the filter paper by referring to Figure 2.

(a) Fold the filter paper in half.
(b) Fold the filter again to within about 10 deg of a 90 deg fold. The second fold is not exactly at a right angle. Tear off the corner of top fold.
(c) Open the filter paper so that the torn corner is on the outside of the cone. The tear enables the paper to stick better to the funnel.

3. Place the folded filter paper snugly into the funnel by moistening the filter paper with the solvent of the mixture to be filtered. This should ressemble Figure 2 (c).

4. Press the filter paper against the top wall of the funnel to form a seal. Support the funnel with a funnel rack.

5. Set up the gravity filtration apparatus as per Figure 3. Ensure that the funnel rack is positioned so that the funnel stem is inside the beaker. Position the beaker so that the funnel stem is touching the side of the beaker to avoid splashing.

6. Allow the mixture to settle and then decant the liquid from the solid. Wash the solid which remains in the beaker several times. Finally, transfer the washed solid to the funnel.

Techniques on Vacuum filtration

This type of filtration is used with water or high-boiling organic solvents and is much faster than gravity filtration. For the set-up (Figure 4), a filter flask must be clamped in position before attaching the rubber tubing, rubber stopper (adapter) and Büchner funnel (or crucible). This prevents the top-heavy apparatus from toppling over and spilling material. A medium- or slow-speed filter paper is used that is wetted with the solvent before the vacuum is applied with the water aspirator. Use a large beaker under the aspirator to minimize splashing. Check that there is a good seal between the apparatus when vacuum is applied before filtering the sample.

During the filtration, the mixture should be poured at a rate that the bottom of the funnel is covered with some solution. The collected crystals/precipitate can be washed with some chilled solvent. Do not discard the mother liquor (in the filter flask) as more compound can be recovered.

Caution: Running water can be sucked back into the filtration apparatus if the water pressure decreases. Be sure to break the vacuum by disconnecting the tubing at the aspirator before turning off the running water.

Cara Uji Total Suspended Particulate (TSP) dalam Udara Ambien dengan alat HVAS

Prinsip

Udara dihisap melalui filter di dalam shelter dengan menggunakan pompa vakum laju alir tinggi sehingga partikel terkumpul di permukaan filter. Jumlah partikel yang terakumulasi dalam filter selama periode waktu tertentu dianalisa secara gravimetri. Laju alir di pantau saat periode pengujian. Hasilnya ditampilkan dalam bentuk satuan massa partikulat yang terkumpul per satuan volum contoh uji udara yang diambil sebagai µg/m3.

Techniques on Titration

Titration is a method of analysis that allows you to determine the endpoint of a reaction. To perform a titration accurately, a precise quantity of the titrant needs to be dispensed into the reaction flask

In a titration, the reaction flask is usually an Erlenmeyer flask. It is used because it is conical in shape. This shape makes it easy to swirl the flask without spilling. For example, in an acid-base reaction, you may choose to pipette the acid into the Erlenmeyer flask. The base is introduced to the acid with the use of a burette. The solution in the burette, in this case, the base, is known as the titrant. A colour indicator such as phenolphthalein is introduced into the Erlenmeyer flask to detect the endpoint of the reaction. In acidic solution, phenolphthalein is clear in colour. In basic solution, phenolphthalein turns pink. The endpoint of the reaction is reached when the solution turns pink and the pink colour does not disappear after 30 seconds.

Steps in performing a titration;
1. Follow the steps in Techniques on the Use of a Burette

2. Take an initial volume burette reading and enter it on the datasheet.

3. Follow the steps in Techniques on the Use of a Pipette  and pipette the acid into the Erlenmeyer flask (figure 2).

4. Set up the burette and Erlenmeyer flask such that the tip of the burette is inside the neck of the Erlenmeyer flask (figure 3). This ensures that all the base will be dispensed into the Erlenmeyer flask.

5. Add a few drops of colour indicator, such as phenolphthalein, to the acid solution.
At the start of the titration, the acid solution in the Erlenmeyer flask should be clear. Add the titrant to the titration flask slowly and swirl the flask frequently. When the titrant touches the acid solution, the solution briefly turns pink in colour. Upon swirling, the pink colour will go away. Slow down the addition of the titrant when the trail of pink colour is taking longer to go away. Reduce the volume of the additions as the titration progresses. When you are near the endpoint, the titrant should be added a drop at a time.

6. When it is judged that only a few more drops are needed, rinse down the walls of the Erlenmeyer flask. Quickly spin the closed stopcock 180 degree. This allows a small
shot of titrant to shoot out.

7. When the volume of titrant to be added is judged to be less than one drop, open the stopcock so that only part of a drop appears. Close the stopcock and touch the drop on the side of the Erlenmeyer. Use the wash bottle to rinse the partial drop into the Erlenmeyer flask with swirling.

8. The endpoint is reached when the colour change does not disappear after 30 seconds. The phenolphthalein colour change is from clear to pale pink (figure 4).

9. Read the final burette volume. The difference between the initial and final readings on the burette is the volume of base used in the titration.

10. Repeat a titration at least twice. The burette volumes should be within +/- 0.10 mL or less.

Sampling VOC dalam udara

Prinsip
Zat-zat organik yang mudah menguap (VOC) di udara diikat dengan pelarut organik yang spesifik diantaranya n-Heksana, dngan bantuan pompa penghisap uap organik tersebut dialirkan dengan kecepatan tertentu dan volume terukur.

Reaksi
Uap Organik + n-Heksana --> larutan n-Heksana : uap organik

Peralatan
1. Tabung Impinger
2. Pompa penghisap Udara (Air Sampler)
3. Flow meter
4. Stop watch
5. Tabung pengaman
6. Volumetric pipet
7. Gelas ukur
8. Selang karet elastis

Bahan-bahan
1. n-Heksana
2. Zat organik sebagai stimulan contoh yaitu benzena dan toluena
3. Silika gel

Cara Kerja
1. Persiapkan peralatan "air sampler" dan hubungkan dengan flow meter
2. Kemudian kalibrasi kecepatan alir dengan mengukur dan mencatat volume alir hisap terhadap waktu dan konsentrasi uap organik;
a) Isi tabung impinger
b) Hubungkan dengan tabung pengaman, alat penghisap udara (air sampler) dan flow meter jaga jangan sampai ada yang bocor
c) Hidupkan alat penghisap udara pada posisi Hi dan skala 1 (atau pilih kecepatan alir yang cocok dengan pompa hisap yang digunakan, bisa saja pada posisi Low dan skala tertentu. Hal ini tergantung dari kecepatan aliran udara yang optimum)
d) Siapkan flow meter yang diisi dengan air sabun, tekan karetnya agar keluar gelembung dan ukur saat gelembung menunjukkan garis nol bersamaan dengan menekan tombol "stop watch".
e) Perhatikan gelembung tersebut sampai sampai menunjukkan volume 25 mL atau 50 mL, saat itu pula kita tekan STP dari "sto[p watch".
f) Catat lama alir gelembung tadi, lakukan beberapa kali atau minimal 3 kali dan buatlah rata-ratanya. Misal: x detik, maka kita dapatkan 25 mL/x detik dan kita ubah satuannya menjadi menit agar memudahkan perhitungan.
3. setelah kalibrasi selesai, matikan alat "air sampler".
4. Buatlah contoh uap kontaminan misal dari Benzena dan Toluena dengan perbandingan 1:1, masukkan dalam tabung.
5. Siapkan selang yang menghubungkan udara bebas (tabung yang berisi Benzena dan Toluena) untuk siap mengambil sampel uap organik (VOC) dan isi tabung penangkap dalam impinger dengan n_Heksana 10 mL.
6. Hidupkan alat "air sampler" dan tekan "stop watch", biarkan selama 10 menit (Catatan: lama pengambilan contoh tersebut tergantung dari konsentrasi uap organik (VOC) yang bebas dalam udara)

Perhitungan
Kecepatan Alir gas :

60/x x 25 mL = 1500/x mL/detik

dimana;
x = waktu alir dalam detik
25 mL = volume aliran uap/gas