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Limit Test for Arsenic | Principle, Procedure, Apparatus Explained | — Transcript

by Dr. Prashant Aragade · 1,950 words · 351 segments · language en · Watch on YouTube

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0:01Welcome, dear students.
0:04In this video,
0:05we will discuss about limit test of
0:07arsenic.
0:08So, at the end of this session,
0:10you will be able to explain the
0:12principle involved in limit test of
0:14arsenic.
0:15You will be able to recall the various
0:17chemical reactions involved in this
0:19limit test.
0:20You will be able to describe the
0:22procedure and apparatus used for the
0:25limit test for arsenic.
0:26And most importantly, you will be able
0:29to interpret the results via using the
0:31virtual lab.
0:34So,
0:34what is actual meaning of limit test?
0:37Dear students,
0:38limit test is quantitative or
0:41semi-quantitative test which are used to
0:44detect, to identify, and control a very
0:48small amount of impurities that may be
0:50present in the pharmaceutical
0:52substances.
0:55So, why test for arsenic?
0:57Arsenic is a highly toxic
1:00and produce the poisonous effect in our
1:02body.
1:03If it is present in very small amount,
1:06that is trace amount in the
1:07pharmaceuticals, that is drugs,
1:09it will be responsible for significant
1:12health risk because of its toxic in
1:14nature.
1:15So, our official books like Indian
1:18Pharmacopoeia, British Pharmacopoeia, or
1:20USP
1:21has set very strict limits, that is
1:24about 2 to 5 ppm, that is parts per
1:28million.
1:30To keep the patient safe.
1:32So, if we perform the limit test for
1:34arsenic and if you found the arsenic is
1:36present as an impurity in a
1:38pharmaceuticals more than this limit set
1:41by the Pharmacopoeia, that
1:43pharmaceutical is not safe for the
1:45patients.
1:46So, limit test is performed to detect
1:50the
1:51harmful amount of impurity present in
1:53pharmaceuticals or not.
1:56So, let's start with the discussion on
1:58the limit test for arsenic.
2:00This limit test is based upon the
2:02Gutzeit test.
2:05The fundamental principle behind the
2:07limit test of for arsenic is it will
2:09convert all the arsenic impurities into
2:12the arsine gas,
2:14which then react with the mercury
2:15chloride paper, and then it will produce
2:18a yellow or brown color stain on the
2:20mercury chloride paper.
2:23So, conversion of arsenic impurity into
2:25the arsine gas and then reaction to the
2:27mercury chloride is taken place in
2:29various steps.
2:32Step number one, that is sample
2:34preparation.
2:35We have to dissolve all the sample in
2:37acid.
2:39So, if the sample contain arsenic as
2:41impurity,
2:43this may be exist as in a pentavalent
2:46form, that is AS5+, or trivalent form,
2:50or AS3+.
2:51And when it dissolve in the acid, it
2:54will convert to the arsenic acid, that
2:57is pentavalent, and arsenious acid, that
2:59is trivalent. So, this is the reaction.
3:02If the trivalent arsenic is present in
3:04your sample, then it will convert it to
3:06the arsenious acid.
3:09If the arsenic present in a pentavalent
3:11form, it will form a arsenic acid.
3:15So, if contain both, then it will
3:19form the arsenious acid as well as
3:21arsenic acid, depending on the valency
3:24state. So, remember, if the arsenic acid
3:27present in the trivalent form, it will
3:29form arsenious acid. If the arsenic
3:32present in pentavalent form, then it
3:34will form arsenic acid. And if it is
3:37present in the both
3:39form, then it will form the both acids.
3:42That's why in step number two,
3:46we have to add the stannated
3:47hydrochloric acid, which contain the
3:50stannous chloride.
3:51Stannous chloride is act as the reducing
3:54agents
3:55which will responsible for reduction of
3:58pentavalent arsenic acid to the
4:01trivalent arsenious acid.
4:03So, by the addition of stannous
4:05chloride, it will ensures that all the
4:08arsenic
4:10is present in our drug, it will get
4:12converted to the arsenious acid
4:15irrespective of their valence state.
4:17So, if your sample contain arsenic is in
4:21pentavalent form,
4:23it will form the arsenic acid and with
4:26the help of this stannous chloride, this
4:29arsenious acid is converted to trivalent
4:32arsenious acid.
4:34So, this is the reaction of reduction.
4:36So, all arsenic is now converted to
4:39arsenious acid, that is trivalent form.
4:43Then,
4:44step number three, formation of arsine
4:46gas.
4:47So, we have to add zinc and hydrochloric
4:49acid.
4:50So, that it will form nascent hydrogen.
4:54And that nascent hydrogen is going to
4:56react with trivalent arsenious acid, it
4:59will form arsine gas.
5:01So, all impurity present in your sample
5:04now converted to the arsine gas. So,
5:06this is the arsine gas is produced when
5:09it react with the nascent hydrogen.
5:14Now, which gas is produced? That is
5:16arsine gas is produced. Then it is
5:18carried up through the Gutzeit apparatus
5:21to the tube.
5:22And within the tubes, we have added or
5:25attached the mercury chloride paper.
5:28When the arsine gas are in contact with
5:30mercury chloride paper, the reaction
5:32occurs and it will form a mercuric
5:36arsenide.
5:37This is the mercuric arsenide
5:39which will form by the reaction of
5:41arsine gas and mercury chloride.
5:43And it will form mercuric arsenide.
5:46So, formation of mercuric arsenide will
5:50produce the yellow or brown color stain
5:53on mercuric chloride paper.
5:56So, intensity of the yellow stain
5:59produced on mercuric chloride is
6:01directly proportional to the amount of
6:03arsenic.
6:06The intensity
6:08or the depth of yellow color produced on
6:10mercuric chloride paper is directly
6:12proportional to the amount of arsenic.
6:16If arsenic is more, the more darker
6:19yellow color will be produced. If
6:20arsenic is less, then the less intense
6:23yellow color is produced on mercuric
6:25chloride paper. So, yellow color stain
6:28is produced of test is compared with
6:31then with standard stain.
6:33That is the within the limit given in
6:35the
6:36pharmacopoeia.
6:38Let's take example. Suppose the sample
6:40stain produced
6:42is lighter than that of standard, it
6:44means that arsenic present in the
6:47sample, that is drug,
6:50containing less as compared to
6:51standards. That is it is within the
6:53limit.
6:54That is it is safe for the public use.
6:56That's why we call it as the sample
6:58passes the limit test.
7:01If suppose the sample stain is
7:04more than that of standard or more
7:07intense than that of standard,
7:09means within the test sample, if it is
7:12produced the yellow color stain which is
7:14the more intense than that of standard,
7:17it indicates that arsenic present is
7:19more as compared to standards.
7:22So, it is beyond the limit. So, that
7:24time we call it as
7:26that particular sample does not pass the
7:29limit test
7:30as it contain more amount of arsenic
7:35given in IP or BP or USP within the
7:38sample. That time, that drug, that
7:41pharmaceutical is not suitable for the
7:44pharmaceutical purpose. So, we call it
7:46as sample does not pass the limit test.
7:54Let's discuss about this which reagents
7:56are used and what is their roles.
7:58So, we have placed the lead acetate wool
8:01within the good set test tube.
8:04It will help to interfere with the H2S
8:07gas and it will prevent the blackening.
8:11We have added either stannous chloride
8:12or potassium iodide, which is
8:14responsible for reduction of pentavalent
8:17arsenic to trivalent arsenic.
8:20We have used the mixture of zinc and
8:22HCl, which is
8:24essential for the formation of nascent
8:27hydrogen because when the reaction
8:29between the arsine gas
8:31and the nascent hydrogen, it will
8:33responsible for the formation of
8:37arsine gas.
8:43Okay.
8:44So, in exam, they may ask the question
8:47for the five marks. Explain the
8:49principle and reaction involving the
8:51limit test for arsenic
8:54and draw the diagram of apparatus used.
8:57So, how you are going to solve these
8:58questions?
9:01Let's check this marking scheme.
9:04You have to write down the step-wise
9:06principle and reaction for the three
9:08marks. So, for the principle, 1.5 marks
9:12and for the reaction, 1.5 marks.
9:14Then, you have to draw well-labeled
9:16diagram of good set apparatus with all
9:18necessary components along with the
9:21proper label such as mercury chloride
9:23paper, lead acetate cotton plug,
9:26glass tube, test or standard solution,
9:29etc. for one marks.
9:31So, if you want to get the more marks,
9:33you have to write the principle
9:34step-wise
9:36for 1.5 marks. Then, you have to write
9:38down the correct reactions and draw a
9:40wheat label label label diagram for two
9:44marks.
9:45So, this is the diagram.
9:47We have to mention the label
9:49as shown in figure, isn't it? And most
9:52importantly, this opening of the tube
9:56should not be dipped into the solution
9:58as the opening of tube is required to be
10:01above the solution so the gas can pass
10:03easily.
10:04And it
10:06going to react it with the mercury
10:07chloride paper.
10:09And before that, we have added the lead
10:11acetate cotton plug. So, the
10:14suppose in reaction H2S gas get
10:17produced, that H2S gas are trapped by
10:19the lead acetate cotton plug. So, it
10:22will avoid to the blackening of this
10:24mercury chloride paper. That's why we
10:27need to add the lead acetate cotton
10:30plug.
10:31Okay?
10:32So, I have created the virtual lab for
10:34this
10:36limit test for arsenic. The link is
10:38available within the comment section or
10:40in the
10:42description. You can
10:44uh click on that link and have a look of
10:47the virtual lab of arsenic limit test
10:49and you can perform it on your mobile
10:51device also.
10:53So, this is the
10:54interface which I have created. We have
10:57to just open it and then you have to
10:59click on start simulation.
11:02It will take you on to the next steps.
11:05In this section, the principle and the
11:08key steps in the reaction is given
11:10properly along with the chemical
11:12reaction. So, read it
11:14and then you have to press continue to
11:16procedure.
11:18Here, procedure for standard and test
11:21solution is given.
11:22Read it very properly. Once you read it,
11:25then you have to click on continue to
11:27simulation.
11:29So, this is the actual virtual lab.
11:32Here, you have to perform the practical
11:34on your mobile step by step.
11:37So, step number one, you have to add the
11:38solutions into the standard and test
11:40solution. So, just tap on the add test
11:42and standard solution.
11:44Step number two, you have to add the
11:46standard hydrochloric acid.
11:49So, just click on the button, your acid
11:51get added.
11:52Then add the zinc granules
11:54and then close and observe the reaction.
11:57See here, arsine gas get produced. It
11:59will reacted with mercuric chloride
12:00paper and it will produce the color.
12:03Now, you have to compare the color
12:05produced on mercuric chloride paper. So,
12:07just click on analyze opacity.
12:10So, it will give you the
12:11intensity of the color.
12:13And then you have to select the
12:15observation depending on what you have
12:17observed. So, here the test stain is
12:20less intense than the standard and then
12:22you have to click on submit the reports.
12:24So, you'll get the report and result
12:27also. Then you have to click on
12:29proceed to assessment
12:31so that you will able to apply the
12:33knowledge.
12:34So, you have to select the correct
12:36answer of all five questions and then
12:39you have to click on submit the answer
12:41button.
12:42It will show you whether your answer is
12:44correct or wrong along with the correct
12:47answer and the explanation why that
12:49particular is option is correct. So, it
12:52will give you the maximum opportunity to
12:56do the limit test. Then you have to
12:58click on restart the simulation. So,
13:00simulation is again get started.
13:03So, dear students,
13:04this virtual lab helps you to master the
13:07limit test for arsenic from principle to
13:11the interpretation.
13:13So, link is available in the description
13:15box. You have to
13:18take a experience of this pharmacy lab
13:21which is useful for the performing the
13:23limit test for arsenic limit test for
13:26chloride, sulfate, iron, and heavy
13:28metals. Take a benefit of this.
13:31So, thank you very much for the watching
13:33this video.
13:35So, dear students, don't forget to
13:36subscribe this YouTube channel and press
13:39the bell icon for more pharmacy related
13:41videos. Once again, thank you very much.

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