What is a primary standard?

A chemical with known concentration is termed standard in chemistry. Primary standards are taken as reference chemicals for finding concentrations of analytes. They are pure, high molecular weighed, stable, and non-toxic substances used to prepare solutions with exactly known concentrations known as primary standard solutions. Furthermore, with the help of only a standard, any analytical instruments can be calibrated for analysis.

Pre-requisite concepts
Basic analytical chemistry
Titration-Types of titrations

Standard solutions can be categorized into primary and secondary standards.

Primary Standard

Primary standards are certain chemical substances that are frequently used for defined concentrations as a reference. They are pure reagents i.e. they have reliable compositions that do not alter on exposure to different atmospheres. Primary standards have relatively high molecular weights from which the standard solution can be prepared by direct weighing of its quantity.

What is a primary standard?

A good primary standard has the following characteristics:

Low reactivity

A primary standard must have low reactivity. Reactive substances cause deviations in the results of chemical analysis, for which they are not used as primary standards.

High stability

A primary standard should be highly stable. Less stable substances do not work uninterrupted, which makes them reactive and unable to be used as standards.

High purity

Primary standards are highly pure. High purity is certain when all calculations are based upon a single species. Moreover, the standardization process will be much easier and reliable if the standard is pure.

Cheap in cost, Readily available

Usually, primary standards are used in large quantities. It will be difficult to use expensive standards. It must also be available easily.

Non-Hygroscopic (stable under open atmosphere)

The substance should be unaltered in the air i.e. during weighing. This implies that it should not be hygroscopic, oxidized by air, or affected by carbon dioxide. A primary standard should maintain an unchanged composition even during storage.

Non-toxic

The standard should be a non-toxic chemical. The excessive use of these chemicals makes the probability of spillage high, endangering the workers, so it must not effect human skin, table sheets, and plastic wares, etc either directly or indirectly.

High molecular weight

A primary standard should have a high relative molecular weight so that, the weighing errors may be minimized. The precision in weighing is ordinarily 0.1-0.2 mg. For accuracy of 1 part in 1000, it is necessary to employ samples weighing at least 0.2 g.

Secondary Standard

Primary standards are often used indirectly to standardize any chemical species or a chemical analysis instrument. This implies the use of a third-party chemical species called secondary standard.

The secondary standard solution is a solution, in which the concentration of dissolved solute has not been determined, from the weight of the compound but by the titration against a primary standard.

For example

Sodium hydroxide (NaOH) is a secondary standard used to standardize acids like hydrochloric acid (HCl) but NaOH needs a prior standardization against a primary standard acid e.g. Oxalic acid (COOH)2.

How to make standard Solutions

  1. Calculations are done to find the approximate weight of the primary standard chemical needed to make up the known volume of the standard solution.
  2. The primary standard is carefully weighed out on an electronic balance.
  3. The solid is then transferred into the rinsed volumetric flask.
  4. It is made sure that the solid is dissolved completely.
  5. A volume of the distilled water is added to the flask to make up the required volume.
  6. The prepared solutions are then labeled for further usage.

Primary standards in the wet analysis

Different primary standards are used for the standardization of different substances.

Standardization of silver nitrate

Sodium chloride is used as the primary standard for this purpose. It has a relative molecular weight of 58.44 g/mol. To be exact, 1.4516 g of salt is weighed to form 0.1000 M solution in 250 ml of solution. Salt is added to the volumetric flask and dissolved in a few ml then diluted up to the 250 ml mark. This solution is employed for the standardization of silver nitrate solution.

AgNO3 + NaCl → AgCl↓ + NaNO3

Standardization of EDTA

Zinc powder is used for standardizing EDTA.

Zn2+(aq) + EDTA4- (aq)  →  Zn(EDTA)2-(aq)

Standardization of perchloric acid

KPH (Potassium hydrogen phthalate) is used for standardizing the perchloric acid.

KHPhthalate + KOH → K2Phthalate + H2O

Calibration of instruments by primary standards

Calibration of HPLC

The standard used for the standardization of HPLC (High pressure liquid chromatography) is caffeine. 1,3,7-trimethylxanthine (caffeine) a central nervous stimulant is used in standardization processes due to its high purity and stability. Its solution in water is usually applied as a standard for HPLC and is run just like a sample. High R2 values indicate the correct working of HPLC.

Primary standard of HPLC

Calibration of ICP-AES

The calibration of ICP-AES (Inductively coupled plasma atomic emission spectroscopy) also known as ICP-OES is done through osmium metal for the detection and analysis of osmium metal in pharmaceutical products. This calibration enables ICP-AES to detect such metals, quantitatively measure the concentrations, and run trace level analysis.

Primary standard of ICP-AES

Concepts berg

What is a primary standard in chemistry?

The primary standard is a very pure reagent. It should be weighable with a compatible formula that should not change on unmasking to the atmosphere. Furthermore, it has a high molar mass.

Primary standard Solutions are used to find the accurate calculations of acid-base titrations, redox titrations, precipitation titrations, and complexometric titrations.

Examples of primary standards:

  • Sodium carbonate (Na2CO3)
  • Sodium oxalate (NaC2O4)
  • Sodium tetraborate (Na2 B407)
  • Potassium dichromate (K2Cr2O7)
  • Potassium bromate (KBrO3)
  • Potassium iodate (KIO3)
  • Potassium hydrogen iodate (KH(IO3)2) etc.

Why are primary standards used?

Primary standards are used for the standardization of solutions because they have high purity, are cheap, hygroscopic, and nontoxic. Generally, they are used in titrations like,

  • Acid-base titration.
  • Redox titrations.
  • Precipitation titrations.
  • Complexometric titrations.

How has this ambiguous use of the word “primary” developed?

Primary means ‘first’. It is although a general English word but it is frequently used in chemistry. The purpose of this use is either to locate the functional group-containing atoms in organic chains or to name a chemical used as the purest substance available to standardize any other reagent.

What is the primary and secondary standard?

The primary standard is a reagent with high purity, high molar mass, and hygroscopic characteristics. Examples of primary standards are:

  • Sodium carbonate (Na2CO3 )
  • Sodium tetraborate (Na2 B407)
  • Potassium dichromate (K2Cr2O7)
  • Potassium bromate (KBrO3)
  • Potassium iodate (KIO3)
  • Potassium hydrogen iodate (KH(IO3)2) etc

A secondary standard is a chemical that is standardized against a primary standard, for finding the active agent. They are generally used in calibrating the analytical instrument and techniques. Examples of secondary standards are:

  • NaOH
  • KOH
  • HCl

Is Na2CO3 a primary standard?

Na2CO3 is used as a primary standard because its concentration in a solution remains constant during reaction or storage.

What are standard solutions and their types?

The standard is the material (solution), whose concentration is known. This can be taken as a reference for finding the unknown concentration of a substance. With the help of a standard, analytical instruments can be calibrated.

What are the requirements of the primary standard?

Standard requirements for the primary standards are:

  • Cheap and readily available
  • Nontoxic
  • A high molecular weight
  • High stability
  • Low reactivity
  • High purity
  • Non-Hygroscopicity

Is FeSO4 a primary standard?

FeSO4 is not a primary standard as it is not stable. It has seven molecules of water of hydration and for being a primary standard, it should not be hygroscopic.

Is EDTA a primary standard?

EDTA is not a primary standard because it is hygroscopic. It does not fulfill the basic criteria of a good primary standard. A good primary standard is non-hygroscopic and very pure. EDTA catches water from the atmosphere, which affects its weight.

Why KMnO4 is not a primary standard?

KMnO4 is not a good primary standard because it is not present in its pure form. It is difficult to obtain KMnO4 from MnO2 because it deviates from its basic structure. The characteristic property of a primary standard is that it is always presented in a very pure form.

Is HCl a primary standard?

HCl is not a primary standard because it is in a gaseous state at room temperature. If it dissolved in water (~37%), still not a primary standard, due to the continuous evaporation at room temperature. A good primary standard is nonreactive toward the atmosphere.

What are the properties of a primary standard solution?

The properties of the primary standard are;

  • Cheap and readily available
  • Nontoxic
  • High molecular weight
  • Highly stable
  • Low reactivity
  • Highly pure
  • Non-hygroscopic.

What are some examples of primary standard solutions?

  • Sodium carbonate (Na2CO3)
  • Sodium oxalate (NaC2O4)
  • Sodium tetraborate(Na2 B407)
  • Potassium dichromate (K2Cr2O7)
  • Potassium bromate (KBrO3)
  • Potassium iodate (KIO3)
  • Potassium hydrogen iodate (KH(IO3)2)
  • Arsenic trioxide (As2O3) etc.

Which one is the primary standard substance and why, KMnO4 or K2Cr2O7?

Potassium dichromate is a good primary standard as it dissolves in water and does not dissociate in sunlight. Whereas KMnO4 is not a good primary standard because it is not present in its pure form. It is difficult to obtain KMnO4 from MnO2. It dissociates in sunlight, that’s why it acts as a secondary standard.

Why is Na2CO3 used as a primary standard?

Na2CO3 is used as the primary standard because its molarity does not change for a long period of time. A good primary standard does not change its strength over time.

Why do we not take NaOH as the primary standard?

NaOH is not a primary standard as it is hygroscopic. When NaOH is exposed to the atmosphere, it will take water from air moisture. It will affect the weight of NaOH. A good primary standard is nonhygroscopic and exists in pure form.

What are examples of a secondary standard solution

  • Hydrochloric Acid (HCl).
  • Sulfuric Acid (H2SO4).
  • Sodium Hydroxide (NaOH).
  • Potassium Hydroxide (KOH).
  • Potassium Permagnate (KMnO4).

How to prepare a 0.1 M HCl solution in 100 ml?

The percentage composition of HCl stock solution is approximately 37% and its density is 1.16 g/cm3.

Calculating Molarity by percentage composition formula;

Molarity = (Percentage purity x Density x 10)  / Molar mass of compound

Molarity = (37 x 1.16 x 10) / 36.5

Molarity of stock HCl = 11.75M

Using dilution formula

For stock M1V1 = For Solution M2V2

M1=11.75M

V1=?

M2=0.1M

V2=100

V1=M2V2/M1

V1= 0.1 x 100 / 11.75

V1= 0.851ml

So, 0.851 ml of stock HCl is required to prepare a 100ml diluted solution of HCl.

Why is NaOH a secondary standard substance?

Sodium hydroxide is a secondary standard because it is a hygroscopic material. When it is exposed to open air it captures water, which causes impurity in it. But the primary standard is a very pure substance with accurate weight.

Why is sodium carbonate used to standardize HCl?

Sodium carbonate is used to standardize HCl because Na2CO3 is a primary standard with a relatively high molecular weight and is highly stable. Moreover, it is a base so it can be titrated against a specific volume of HCl.

Why do we standardize NaOH?

NaOH is a secondary standard. It is a hygroscopic reagent (water lover). When sodium hydroxide is exposed to the atmosphere, it interacts with water from the air. In the standardization of NaOH, we can find the exact amount of sodium hydroxide (NaOH) in the solution.

What is the right quality of water for HPLC analysis?

HPLC analysis requires ultrapure water for the composition determination of reagents, buffers, mobile phases, and for any sample pre-treatments. Ultra analytic delivers type 1 ultrapure water with, typically, a resistivity of 18.2 MΩ.cm, a very low total organic carbon (TOC) value of less than 2 ppb, and bacteria levels below 0.1 CFU/ml, which is highly recommended.

References