QUALITY CONTROL INTERVIEW QUESTIONS - WET CHEMICAL ANALYSIS

1) WHAT IS THE DIFFERENCE BETWEEN WATER CONTENT & LOD?

Water content:
           The amount of water present in the analyte in any form (including hydrated molecules)
Loss on drying:
          The amount of volatile substances present in the analyte (all volatile solvents present  including water).

Note:
In water content we determine only water. And in LOD we can determine water (except hydrated molecule) and organic volatile solvents present in the analyte.


2) WHAT IS THE DIFFERENCE BETWEEN LOD UNDER DRYING & LOD UNDER VACUUM?

Both are used for the same purpose of determining the percent of water & organic solvents present in the sample by vaporizing them.

LOD under Drying:
              In this we dry the sample at a temperature of more than the water boiling range. Generally we use 105°C because water boiling point is 100°C and most of the solvents have the boiling point lower than 100°C.
This method is suitable only when our sample melting point is more than 100°C.If our sample melting point is less than 100° we cannot determine the water or any solvents having the boiling point less than the drying temperature

LOD under Vacuum:
               If our sample has the lower melting range (less than 100°C) like 70°C (or) 85°C ...etc, We cannot vaporize the water or any solvent more than the drying temperature. In this case we use somewhat less temperature than the sample (If we use high temperature sample melts) for drying by applying proper vacuum to absorb the water or solvents present in the sample.

The main difference of both the methods is when sample have high melting range (more than 100°C) we can determine LOD by drying and when sample have low melting range (less than 100°C) we can determine LOD by drying under vacuum.

3) HOW TO PREPARE 0.0002 mg/ml SOLUTION.BY USING 100 ml VOLUMETRIC FLASK FOR STOCK, 25 ml FOR I st DILUTION AND 10 ml FOR SECOUND DILUTION? (Should take minimum 0.1 ml for dilution).

Stock:

Dissolve 100 mg sample in 100 ml

Ist dilution:

Take 0.5 ml of stock in 25 ml volumetric flask and dilute up to mark.

IInd dilution:

Take 0.1 ml of above Ist dilution in 10 ml v.f and dilute up to mark.

Calculation:

         = (100/100) (0.5/25) (0.1/10) mg/ml

         = 0.0002 mg/ml

4) HOW TO PREPARE 10ppm SOLUTION FROM 25% AMMONIA SOLUTION?

First convert ppm to % => 10 ppm = 0.001% (10/10000)

 Apply C1V1=C2V2 Formulae

(25) (V1) = (0.001) (100)

           V1 = 0.004 ml

So if we dilute 0.004 ml (4 µl) of 25% ammonia solution to 100 ml of water then we get 0.001%(10 ppm) solution.

Note:

1. If we have no syringe of 4µl how to dilute?

0.004ml to 100ml => 0.00004 ml to ml

Dilute 0.4ml to 100ml first and then 1ml to 100ml

(0.4/100)(1/100) = 0.00004ml to ml

5) WHAT IS PRIMARY STANDARD & SECONDARY STANDARD?

Primary standard:

           A standard which is capable to prepare a known concentration of solution by direct dissolving of a standard and diluting to an accurately known volume of volumetric flask is called primary standard.

Requirements of a Primary standard: 

1. It should be available in a pure form or in a state of known purity. In general total amount of impurities should not exceed 0.01 to 0.02%, and it should be possible to test for impurities by qualitative tests by known sensitivity.
2. The substance should be easy to dry and should not be hygroscopic that it takes up water during weighing and should not efflorescence that it spontaneously loses water molecule to the atmosphere. It should not lose weight on exposure to air.
3. Primary standards should have high equivalent weight in order to minimize the errors in weighing.
4. It is preferable that the acid or base be strong that is highly dissociated. However a weak acid or base may be employed as a primary standard with no great disadvantages.
5. It should be stable, non-toxic and eco-friendly.

Note:

            Eco-friendliness is environmentally friendly means it should not affect the surrounding                       environment.

Examples:

      1. Potassium hydrogen phalate (PHP) for standardization of aqueous base (Like NaOH) and HClO4 in CH3COOH solutions.
2. Na2CO3 for standardization of aqueous acids like HCl, H2SO4 and HNO3 solutions (but not CH3COOH).  
3.  NaCl for standardization of AgNO3 solutions.
4. Zn powder, after being dissolved in H2S04 or HCl, for standardization of EDTA solutions.

      And also Arsenic trioxide, Periodate Benzoic acid, Potassium hydroxide, Potassium bromate, Sufanilic acid ....etc

Secondary standard:
           A solution standardized by titrating with a primary standard itself is a secondary standard

      Examples:
NaOH, HClO4, HCl, EDTA, AgNO3, H2SO4, HNO3 ....etc.

      6) WHAT ARE USP-1 AND USP-2 METHODS IN TAPPED DENSITY APPARATUS, EXPLAIN?

               Both used for the same purpose of determining the density of analyte. But these have the difference in tapping height and tapping count per minute.

USP-1:

     Tapping height = 14±2 mm

     Drops per min = 300 drops/min

USP-2:

     Tapping height = 3 (±10%) mm

     Drops per min = 250 drops/min

      7)WHAT IS THE DIFFERENCE BETWEEN BULK DENSITY AND TAPPED DENSITY?

      Bulk density:

               It is the ratio of mass an untapped powder sample to its volume, including contribution of interparticulate void volume. Hence it depends on both the density of powder sample and spatial arrangement of particles in the powder bed.

     Tapped density:

               It is the ratio of mass after specified tapping’s of a powder sample to its volume.

     8) CAN YOU EXPLAIN NON-AQUEOUS PERCHLORIC ACID TITRATION? (or)

     HOW PERCHLORIC ACID REACTS WITH ACETIC ACID IN PERCHLORIC ACID TITRATION?

                Generally we use non-aqueous titration for determination of weak bases. That is we dissolve our sample (weak base) in acetic acid and titrate with perchloric acid.

              As we all know perchloric acid is strongest acid and acetic acid is weak acid. As we start titration, in presence of a strongest perchloric acid weak acetic acid behaves like a base. That means

       In the first step, a proton (H⁺) is transferred from strongest perchloric acid to acetic acid (Which behaves like base). (Known as leveling effect)

HCLO₄ + CH₃COOH  à CH₃COOH²⁺ + CLO4^-

     When a weak base dissolved in acetic acid, the acetic acid exerts its leveling effect and enhances the basic properties of the weak base. So to titrate a solution of weak base in acetic acid with perchloric acid in acetic acid and obtained a sharp end point.

Let us take our weak base is C₅H₅N

CH₃COOH  + C₅H₅N  à C₅H₅NH⁺ + CH₃COO^-

CH₃COO^- + CH₃COOH²⁺ à 2 CH₃COOH

By adding above three equations

HCLO₄ + CH₃COOH  à CH₃COOH²⁺ + CLO4^-

CH₃COOH  + C5H5N  à C₅H₅NH⁺ + CH₃COO^-

CH₃COO^- + CH₃COOH²⁺ à 2 CH₃COOH

HCLO_{4 +} + C₅H₅N  à C₅H₅NH⁺ + CLO4^-

Note:

1.      In the first step acetic acid behaves as base and in the next step same acetic acid behaves as acid.

9) WHY WE USE ONLY SULPHURIC ACID BUT NOT NITRIC ACID or ANY OTHER IN SULPHATED ASH OR RESIDUE ON IGNITION TEST?

The purpose of this test is to determine the inorganic content in given sample. By adding H2SO4, we convert the inorganic to their sulphate derivatives in the given sample. The inorganic sulphates are very stable even at high temperatures up to 1000°c. So we can easily estimate the inorganic content present in the given sample by ignition.

If we use HNO3, inorganic converts to their nitrate derivatives and if we use HCL they convert to chloride derivatives which are not stable at high temperatures.