Gravimetric Analysis

                Precipitation is a very valuable method of separating a sample into its components. The process involved process by which substances to be separated use to form a solid precipitate. Precipitation reaction has been used extensively in analytical chemistry, especially in argentometry and gravimetric methods. Gravimetry are also part of the quantitative analysis related to weight measurements by separating analysts from all the other components that can be specified level of a substance. In addition to inorganic substances, organic compounds have also been analyzed by gravimetric techniques, for example determination of cholesterol levels in cereals and lactose in dairy products.

Gravimetric analysis is a method of weight measurement by separating the analyte from other components so that all levels of a substance can be determined using a gravimetric factor. A gravimetric analysis is usually based on the reaction:
 aA + bB → AaBb

                With the provisions of a is the analyte A reacts with b molecule B. The results is a substance with a solubility usually so small that it can be weighed in that form after dried or baked into other compounds of known structure and then weighed. A B excess reagent is usually added to suppress the solubility of the precipitate, for example in the determination of Ca2 +.

Ca²⁺+C₂O₄^2-→CaC₂O₄

CaC₂O₄ → CaO + CO₂ + CO

Requirements of the gravimetric method are:
A. In the separation must be perfect enough so that the quantity of analyte that does not settle the analyte is not specified.
B. Substances that must have weighed a certain arrangement and must be pure. If no result can not be obtained.

2. Aplication of Gravimetric Analysis
                Gravimetry can be carried out on organic substances such as cholesterol determination in grains. Besides the analysis of elements and organic compounds is usually also done this way. For example C in organic compounds can be determined by burning the sample in oxygen and absorb CO2 and H2O are produced in a suitable absorption. Absorption tubes were weighed before and after combustion for CO2 and H2O are produced.

3. The advantage of gravimetric
Although it has been superseded in terms of gravimetric routine as an instrumental, but gravimetric actually faster and accurately than an instrument that needs to be calibrated. Tools generally provide only a relative measurement and must be calibrated on the basis of gravimetric or titimetri classical way. If the analyte is a first konstata (> 1%) accuracy of the various parts per thousand can be expected, if the sample is not too complex. If the analyte minority of less than 1%, gravimetric way is usually not use.

4.  Theory of Coprecipitation, Peptisasi, Post Precipitation
Coprecipitation is a process brought down a substance which is usually dissolved when the desired deposition of sediment. Suppose that nitrate ions on the precipitation of barium sulfate precipitate containing lead barium nitrate so nitrate is said to have coprecipitation with sulfate or due to adsorption of ions when the deposition process. In this event the cause of impurity substances into the side of the crystal and the absorbed ions dragged down at the time of coagulation.

The procedure used to reduce coprecipitation:
A. The method of determining these two reagents can be used to control the concentration of the regulator and the electric charge carried by the primary particles in the sediment is controlled by using the appropriate pH.
B. Expansion with gelatin blob and must be with an electrolyte solution in the wash solution to avoid precipitation.
C. Pollution is a major benefit of crystalline precipitates, the benefits are large enough to sediment clotted remain unused for gelatin.
D. Repeated if the sediment precipitation can easily be dissolved again, especially for the oxidation hydrolysis and crystalline salts of fatty acids.

credits: Book of PDTK 1 Chemical Engineering 2011 Diponegoro University

Argentometric Analysis

Precipitation is a very valuable method of separating a sample into its components. The process involved is the process by which substances to be separated is used to form a solid precipitate. Precipitation reaction has been used extensively in analytical chemistry, especially in argentometry and gravimetric methods. Argentometry is a quantitative volumetric analysis with standard AgNO3 solution by precipitation. Argentometry is used to determine levels of an element in titrations involving silver salts with the corresponding indicator. The usefulness of this analysis is to determine the levels argentometry halogenida, such as Cl-, which is contained in the sample that is useful for oceanography, food, and industrial.

Argentometry is a quantitative volumetric analysis to determine halogen levels in the samples by using a standard solution of AgNO3. At argentometry titration end point is determined by the formation of colored solutions or first onset of turbidity.

1. MOHR METHOD
Used to establish levels of halogen ions are conducted in a neutral atmosphere with K2CrO4 indicators and standard solutions of AgNO3. Chromate ions will react with silver ions form a red-brown precipitate of silver chromate.

Reaction:

Ag⁺ + Cl^- → AgCl(s) (white precipitate)

2Ag⁺ + CrO₄^2- → Ag₂CrO₄(s) (red-brown precipitate)

Basic titration with this method is a multilevel precipitation of AgCl and after all just happened sediment settles Ag2CrO4. For more details, we can see the following example.
For example in a solution of 0.1 M NaCl contained a K2CrO4 indicators that have a concentration of 0.01 M, the concentration of Ag + ions to precipitate the Cl-and CrO42-can be calculated.

A. Cl-ions to precipitate
At this place the point of equality. Both the silver ions and chloride ions no excess, and each concentration is the square (of) Ksp. At this point the titration curve is called the equivalence point , ie a point on the curve showing the number of gram equivalents of titrant is equal to an equivalent number of grams of substance titrated.
                   Ksp AgCl = 1,0 x 10^-10

[Ag^+] = [Cl^-]

[Ag^+]2 = 1,0 x 10^-10

[Ag⁺] = 1,0 x 10^-5

B.  to precipitate CrO42- ions
Ksp Ag₂CrO₄ = 2 x 10^-12

[Ag⁺]² [CrO₄^2-] = 2 x 10^-12

[Ag⁺]² [10^-2] = 2 x 10^-12

[Ag⁺]² = 2 x 10^-10

[Ag⁺] = 1,4 x 10^-5

From the above examples can be seen that the number of ions required to precipitate silver chromate ions is greater than that required to precipitate the chloride ion. So when end point of titration occurs, chloride ions have been precipitated practically all, so the new silver chromate precipitate after all chloride ions to form silver chloride precipitate.

The things to consider in the use of the Mohr method:

1.       Good for determining chloride and bromide ions but not suitable for iodide ions and tiocianide.

2.       Titration in an atmosphere of neutral or slightly alkaline, pH 7 - 10.5.

3.       Not suitable for titration of colored solutions, such as CuCl2 (blue), CaCl 2 (silver), NiCl (green) because it would complicate observations at TAT.

4.       Unable to salts of hydrolyzed Cl and Br, because the precipitate formed which was expected. Cl or Br salts with cations of Al, Fe, Bi, Sn, Sb, and Mg.

5.       The solution must not contain CO 3 2 -, SO 4 2 -, PO43-, C2O42-because it will precipitate with Mg.

6.       The solution should not contain ions Pb2 + and Ba2 + as it will precipitate as chromate salts are colored. Eliminated by the addition of saturated Na2CO3.

2. VOLHARD METHOD
                This method uses the principle of back-to-Titration, ion halogenide samples are added by a standard solution of AgNO3 in excess, then all of AgNO3 solution titrated back with standard NH4CNS. The indicator which is used to this method is Ferric Ammonium Sulfate. In the process the solution must be acid in order to prevent hydrolysis ferric salts into ferric hydroxide which color is disturbing observation end point of titration. Acidic conditions can be made by adding concentrated HNO3. But do not be too concentrated HNO3 use because it will cause NH4CNS be oxidized to NO and CO2.

3NH₄CNS + 13HNO₃ → 16NO + 3CO₂ + NH₄HSO₄ + 5H₂O

In this method the reaction mechanism will form silver chloride and silver thiocyanate.

Aqueous AgNO3 + Cl-→ AgCl (s)
NH4CNS → NaNO3 + residual AgCNS (s)
CNS-rest +  Fe 3 + →Fe (CNS) 3 blood red (when the end point titration)

Silver chloride is more soluble than silver thiocyanate, and chloride tend to dissolve back according to the reaction.

AgCl(s) + SCN^-    → AgSCN(s)  + Cl^-

Equilibrium constant of this reaction is determined by the rate constants versus the solubility product of silver chloride to silver thiocyanate. Since the first constant is greater than the second, then the above-mentioned reactions are very likely to run from left to right. So thiocyanate can be spent not only by the excess silver ions, but also by the precipitation of silver chloride itself. If this happens, it will obtain results that are too low in chloride analysis. But this reaction can be prevented by filtering the silver chloride. For the assay Br-filtering unnecessary because AgBr solubility product constants smaller than AgCNS, whereas for I-addition of the indicator after approaching the TAT because when I-see indicator Fe3 + I2 happen that often lead to errors titration.

Things that must be considered in the method of  Volhard:
1. The solution should be acid, in order to avoid hydrolysis ferric salt into ferric hydroxide which color is disturbing observation at endpoint of titration.
2. HNO3 is used to provide acidic conditions would not be too concentrated for oxidize NH4CNS into NO and CO2. Where is the CO2 that is formed can react with H2O to form H2CO3 which can react with Ag + and produces a white Ag2CO3 making it difficult observations during end point of tiration. In addition levels Fe3 + is reduced, so the chances endpoint of titration will happen much.
3. AgCl precipitate first formed should be filtered, washed with water and washing water to be made one with the new filtrate titrated with NH4CNS.

3. FAJANS METHOD
                In this method is used indicator adsorption. If a colored organic compounds adsorbed on the surface of a precipitate, the organic structure modification can occur, and that color can be greatly altered and can be older. These symptoms can be used to detect the titration end point of deposition of silver salts.
                The mechanism of operation of such indicators is different from any mechanism that has been discussed so far. Fajans found that fluorescein and some fluorescein substituted can act as an indicator for the titration of silver. When silver nitrate is added to a solution of sodium chloride, silver chloride particles are very fine it tends to hold on its surface (adsorb) a number of excess chloride ions present in solution. Chloride ions are said to form the primary adsorbed layer and thus causes the colloidal particles of silver chloride is negatively charged. Negative particles are then likely to attract positive ions in solution to form a secondary adsorption layer is more loosely bound.

Permanganometric Analysis

Definition Permanganometric

     Permanganometric is one of the quantitative volumetric analysis based on the reaction of permanganate ion oxidation.
Standard solution used is KMnO4. Before used for titration, a solution of KMnO4 must be standardized in advance because not a primary standard solution. Additionally KMnO4 has the following characteristics:
1. Can not be obtained in pure
2. Containing oxides MnO and Mn2O3
3. The solution is not stable (if any organic substance) 

Reaction:

4     MnO₄^- + 2 H₂O → 4 MnO₂ + 3 O₂ + 4 OH^-

4. It should not be filtered with filter paper (organic matter) →with glass wool
5. Should be stored in brown bottles
6. Standardized with primary standard solution. Primary standard substance which is used such as: As₂O₃, Na₂C₂O₄, H₂C₂O₄, Fe(NH₄)₂(SO₄)₂, K₄Fe(CN)₆, logam Fe, KHC₂O₄H₂C₂O₄2H₂O

      Permanganate ion oxidation can take place under acidic, neutral and alkaline. 

In acidic conditions, pH ± 1
Reaction: MnO ^-+ 8 H⁺+ 5 e → Mn²⁺+ 4 H₂O 

       Potassium permanganate can act as an indicator, and titration is generally done in the sour atmosphere because it will be easier to observe the end point titration. But there are some compounds more easily oxidized in an atmosphere of neutral or alkaline example hydrazine, sulfite, sulfide, sulfide and thiosulfate. 

      Reaction in the neutral atmosphere:

      MnO ^- + 4 H⁺+ 3 e → MnO₂ + 2 H₂O

      Reactions in the atmosphere of alkalis or bases :

MnO ^-+ 3e → MnO₄^2-

MnO ^2-+ 2H₂O + 2e → MnO₂ + 4OH^-

MnO ^-+ 2H₂O + 3e → MnO₂ + 4OH^-

Advantages and Disadvantages Analysis with Permanganometri

- The advantages
1. Standard solution, namely KMnO4 easily available and cheap.
2. No need for indicators to TAT. That's because KMnO4 can act as an indicator.
3. The reaction was fast with lots of reagents.
            - Disadvantages
1. There should be standardization of the initial advance.
2. Can take better if done in acidic conditions.
3. The time required for analysis is quite long.

credits: Book of PDTK 1 2011 Chemical Engineering of Diponegoro University

Iodo-Iodimetric Analysis

        Chemical reactions involving the oxidation reduction is widely used by titrimetric analysis. Ions of various elements may be present in conditions of varying oxidation, resulting in the possibility of many redox reactions. Many of these reactions are eligible to be used in titrimetric analysis and its applications quite a lot. 
The titrimetric iodometric analysis which indirectly to substances that are like iron oxidizing III, copper II, where this substance is added to oxidize iodide to form iodine. Iodine is formed will be determined through the use of thiosulfate standard solution.

       Definition of Reduction - Oxidation process of reduction - oxidation (redox) is a process involving transfer of electrons from one reactant to another reactant.
reduction
    Reduction is the capture of one or more electrons by an atom, ion or molecule.
oxidation
     Oxidation is the release of one or more electrons from an atom, ion or molecule. There are no free electrons in a chemical system, and the release of electrons by a chemical substance is always accompanied by electron capture by the other, in other words always followed by oxidation reduction reaction. In the oxidation reduction (redox) changes the valence of the substances that hold the reaction. Here occurred the transfer of electrons from a reducing pair to pair oxidizing. 
The second half reaction of a redox reaction can generally be written as follows: red → Ox + n é where red indicates the reduced form (also called a reductant or reducing agent), Ox is the oxidized form (oxidant or oxidizing agent), n is the number of electrons transferred and é are electrons. Redox reactions are widely used in titrimetric analysis of inorganic substances or organic. To set the end point on the redox titration can be done with the help of potentiometric or indicator.
Examples of redox reactions:

5Fe²⁺ + MnO₄ + 8H⁺ → 5Fe³⁺ + Mn²⁺ +4H₂O

Where: 5Fe²⁺ → 5Fe³⁺ + 5e → an oxidation reaction
           MnO₄ + 8H⁺ + 5e → Mn²⁺ + 4H₂O is a reduction reaction
Is the titrimetric iodometric analysis which indirectly to substances that are like iron oxidizing III, copper II, where this substance is added to oxidize iodide to form iodine. Iodine is formed will be determined through the use of thiosulfate standard solution.

Oxidator + MnO₄ + 8H⁺ + 5e → Mn²⁺ + 4H₂O 

KI → I₂ + 2e

I₂ + Na₂S₂O₃ → NaI + Na₂S₄O₆

  Iodimetri is a titrimetric analysis that directly used for the reducing agents or sodium thiosulfate with iodine solution or by addition of excess standard solution. Excess iodine is titrated back with thiosulfate solution.
Reductant + I2 → 2I-
Na2S2O3 + I2 → NaI + Na2S4O6

Theory of Indicators Amylum
  The indicators used in this method is an indicator of starch or amylum. Amylum is a strong indicator of iodine, which is blue when a substance containing iodine positive. Reason wearing amylum as indicators, including:
- The price is cheap
- Easily obtained
- Changes color when TAT is clear
- Reaction is spontaneous (without heating)
- Can be used at once in the iodo-iodimetri
 
While the weakness of this indicator are:
- Unstable (easily hydrolyzed)
- Easily damaged (infected bacteria)
- It is difficult soluble in water

How to make amylum indicators:
Prepare a 250ml glass beaker, fill with 100ml distilled water. Then enter the 3 grams of starch in 250 ml glass beaker. Heat while stirring until the temperature of 400C, 400C stop after stirring but still heated to a temperature of 600C. Enter the glass beaker into a black plastic bag and then put in a drawer. Let stand about 5 minutes to settle and form three layers, after taking the center of the cold solution as indicator. The reaction mechanism of the reaction mechanism is the reaction stages that describe a whole set of chemical reactions.
Iodo-iodimetri reaction mechanism: 

 2 Cu²⁺ + 4 I^- → 2 CuI + I₂

I₂ + 2 S₂O₃^2- → 2 I- + S₄O ^2-

I₂ + I^- → I₃ ^-

Amylum + I₃^- → AmylumI₃ ^- (blue)

Things Must Be Considered:

1. Titration should be carried out in the cold, in erlenmeyer without catalyst in order to reduce the oxidation of I-by O2 from the air into I2.
2. Na2S2O3 is a secondary standard solution should be standardized first.
3. The addition of the indicator at the end of titration (just before TAT).
4. Titration can not be done in a medium strong acid hydrolysis amylum because it will happen.
5. Titration can not be done in strong alkaline medium because of I2 will oxidize thiosulfate to sulfate.
6. Na2S2O3 solution must be protected from light because light helps bacterial activity that interferes thioparus. 

 
 credits: Book of Basic Praticum of Chemical Engineering 2011