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Two bottles contain the same substance. One is labeled 'technical', the other 'p.a.'. The CAS number is identical, and at first glance the purity may differ by only a few tenths of a percent; the price, however, is clearly different.

Which variant do you really need?

The answer depends less on which purity grade is 'better' in principle than on what the chemical will be used for. For technical cleaning, a high-purity analytical grade may be unnecessary. In a quantitative determination, by contrast, even very small impurities can distort the result.

This guide explains the most important purity designations for chemicals and shows what you should actually look out for when comparing technical quality, pure, purest, and p.a..

Table of contents

What does purity grade mean for chemicals?

Simply put, the purity grade describes what proportion of the desired substance a product contains and which impurities are permissible.

A product with a stated purity of ≥99% therefore contains at least the specified proportion of the main component. The remaining components can include, for example:

• Water,
• other inorganic or organic compounds,
• Metals,
• Salts,
• Reaction by-products,
• Residues from manufacturing,
• Solvent traces.

For the application, therefore, what matters is not only how much of the main substance is present, but also what is contained in the remaining fractions.

This is exactly where many quality levels differ.

Specifications for high-quality analytical reagents often include not only the minimum content but also numerous individual impurities. Merck, for example, gives specifications for its analytical EMSURE® quality covering up to around 70 tested parameters; many of these products meet or exceed the requirements of ACS, ISO, or Reag. Ph. Eur.

This highlights an important point:

Purity is not just a percentage figure. It is a specification profile.

Why 99% doesn't automatically mean 99%

Let's take two products, each with:

Purity ≥99.0%

They look the same on paper. For an analysis, however, they can be completely different in their suitability.

In the first product, the remaining at most 1% could consist mainly of water and components that are not critical for the application.

In the second product, traces of exactly those metals or ions that you later want to determine may be present.

A practical example:

You want to analyze trace levels of iron. A reagent can have a very high total content and still be unsuitable for this task if its iron content is not sufficiently low or is not specified at all.

Conversely, for a technical process in which a few tenths of a percent of a non-critical impurity do not matter, an expensive analytical reagent does not necessarily have to be used.

The fact that the specification goes beyond the total content can also be seen in commercial ACS reagents: For citric acid with ≥99.5%, for example, additional limit values for chloride, phosphate, sulfate, iron, and lead are specified.

Therefore, when purchasing, you should never compare only the largest percentage number on the product page.

Technical quality: when is it sufficient?

The designation “technical” is commonly used for chemicals intended for technical and industrial applications in which analytical reagent quality is not required.

Typical areas of application can include:

• technical cleaning,
• industrial processes,
• production,
• surface treatment,
• certain syntheses,
• process aids,
• Applications in which minor accompanying substances have no relevant influence.

Technical quality does not automatically mean “poor quality.”

Rather, it means that the specification is geared toward a different intended use.

A technical product can, for example, have a high content of the main component but at the same time have less strictly specified secondary components than an analytical reagent.

Therefore, even in the professional chemicals trade, you can find products with high nominal purities in technical quality. Carl ROTH, for example, offers ethanol with a purity of ≥98 % as technical quality, while other solvents from the same range are offered with higher analytical specifications.

When can technical quality make sense?

If your application:

• it does not involve quantitative trace analysis,
• it is robust against minor impurities,
• it does not impose pharmacopeial or normative requirements,
• it primarily fulfills a technical process,
• larger quantities are needed and cost-effectiveness is relevant,

technical quality can be completely sufficient.

When should you be careful?

Technical quality should not be chosen solely because of the lower price when:

• measurement results depend on them,
• Impurities can affect a reaction,
• the substance is used to prepare standards.
• sensitive analytical instruments are used,
• regulatory requirements demand a specific quality.

What does “pure” mean?

The designation “pure” is frequently found on laboratory and technical chemicals.

It usually signals a higher or more clearly defined quality than purely technical goods, but on its own it does not sufficiently indicate whether the product is suitable for a specific analysis.

The key point:

Designations such as “pure” should always be considered together with the specific product specification.

A product can be labeled, for example, as:

≥99%, pure

Then you first know the minimum content. For an analytical application, you must also know which accompanying components are controlled.

Typical applications

Chemicals of “pure” quality can be suitable, among other things, for:

• general laboratory work,
• preparations,
• simple syntheses,
• educational purposes,
• technical laboratory applications,
• Processes without strict trace requirements.

However, whether they are suitable for your specific application is determined not by the word “pure” but by the specification.

What does “ultra-pure” mean?

“Ultra-pure” initially sounds like the highest possible quality level. It is precisely this interpretation that can become problematic when purchasing.

The designation is used by manufacturers for particularly pure qualities, but it is not automatically equivalent to a specific international analytical standard.

An “ultra-pure” product can have a very high content and be excellent for certain applications. Nevertheless, the following should be checked:

• What purity is actually guaranteed?
• Which impurities are specified?
• Are there limit values for metals?
• Are there limit values for anions?
• Is water content determined?
• Is there a residue after evaporation?
• Which test methods were used?
• Is a batch-specific certificate of analysis provided?

Carl ROTH, for example, explicitly offers products as “ultra-pure” and others from the same substance groups as “p.a., ACS” or “p.a., ACS, ISO”. This shows that the terms describe different specification profiles and should not simply be read as a linear ranking.

What does p.a. mean?

p.a. stands for pro analysi – in other words, “for analysis”.

Chemicals of this quality are intended for analytical applications and usually have significantly more clearly defined specifications for relevant impurities.

Depending on the product, the following can be checked:

• Heavy metals,
• Iron,
• Chloride,
• Sulfate,
• Phosphate,
• Nitrate,
• Insoluble constituents,
• Evaporation residue,
• Water content,
• Acid or base content,
• other interfering accompanying substances.

The advantage is therefore not necessarily that the main component is always significantly more concentrated.

The real advantage is:

You know more precisely what else may be present besides the main component.

For chemical analyses, this information is often more important than a difference between, for example, 99.0 and 99.5% total content.

Carl ROTH explicitly describes p.a. alkali solutions as grades for reproducible results in chemical analysis, research, development, and quality control.

Is p.a. always purer than extra pure?

Not necessarily.

This is probably the most important point of this article.

The terms must not be read like school grades:

technical → pure → extra pure → p.a.

It doesn't work that simply.

An "extra pure" product can, for example, have a very high assay. A p.a. product can have a similar or even nominally lower assay, but at the same time meet substantially stricter limits for analytically relevant impurities.

Which variant is better therefore depends on the application.

Example

Product A:

≥99.9%, extra pure

Product B:

≥99.5%, p.a.

If you go solely by the assay, Product A looks better.

If Product B, however, additionally guarantees extremely low limits for chloride, sulfate, iron, and heavy metals, and you want to determine exactly these parameters analytically, Product B can be the clearly better choice.

The right question is therefore not: Which quality is higher?

Rather:

Which specification fits my application?

ACS, ISO, and Ph. Eur.: What do additional standards mean?

In addition to terms such as p.a., a product may contain further information.

Especially common are:

• ACS,
• ISO,
• Reag. Ph. Eur.

These details provide additional information about the requirements according to which a reagent has been specified.

ACS

ACS stands for American Chemical Society.

ACS reagents meet defined specifications for analytical chemicals. In addition to the minimum content, these can include various limits for impurities.

The designation therefore means more than just "very pure".

An example from the current Sigma-Aldrich range: tannic acid is offered as "ACS reagent" and, in addition to details on form and quality, has defined limits for lead, heavy metals, zinc, and other parameters.

For ACS thiourea, in turn, an assay of ≥99.0% is specified. This shows once again that ACS does not automatically mean a blanket purity of, for example, 99.9%. What matters are the substance-specific requirements.

ISO

International standards define requirements for various analytical reagents.

Merck, for example, refers to ISO 6353 for reagents in analytical chemistry and labels corresponding products when they meet the respective requirements.

Reag. Ph. Eur.

This designation refers to requirements of the European Pharmacopoeia (Ph. Eur.) for the corresponding reagents.

It can be particularly relevant when chemicals are used in pharmacopoeial test procedures.

Also important here:

An additional standard designation describes a specific requirement for a specific area of application. It should not be interpreted as a general quality seal for every conceivable application.

HPLC, GC, and LC-MS grades: why the application matters

In instrumental analysis, classic designations such as “pure” or “p.a.” are often not sufficient.

A solvent can be chemically very pure and still cause interfering signals in a sensitive measurement method.

That is why there are application-specific grades, for example:

• HPLC grade,
• GC grade,
• LC-MS grade,
• UV grade,
• spectroscopy grade,
• molecular biology grade.

HPLC grade

In high-performance liquid chromatography, the following can be problematic, among other things:

• UV-active impurities,
• Particles,
• Residues after evaporation,
• Water,
• interfering organic secondary components.

Therefore, solvents for HPLC applications are specifically tested for relevant parameters.

LC-MS grade

In liquid chromatography coupled with mass spectrometry, even very small ionizable impurities can lead to background signals or unwanted peaks.

Here, a conventional p.a. quality can be unsuitable despite good chemical purity.

What does this mean for purchasing?

The method should be established first – then the quality.

Not the other way around.

Anyone purchasing for sensitive instrumental analysis should specifically check whether the manufacturer specifies suitability for the particular method.

Why impurities can be more important than the total percentage

Suppose you are analyzing for chloride.

Reagent A contains:

99.9% main substance

but the chloride content is not specified.

Reagent B contains:

99.5% main substance

and has a guaranteed very low chloride limit.

For your chloride determination, Reagent B may be the safer choice.

The same applies to analyses for:

• Iron,
• Lead,
• Copper,
• Sodium,
• Potassium,
• Phosphate,
• Sulfate,
• Nitrate,
• Heavy metals.

The lower the desired detection limit, the more important the quality of the reagents used.

This applies especially in trace analysis: a reagent can itself become a relevant source of contamination.

Reading certificates of analysis and specifications correctly

Anyone who really wants to compare the purity level should distinguish between two documents.

Product specification

The specification describes what requirements the product must meet.

For example:

• Content ≥99.5%,
• Chloride ≤0.001%,
• Sulfate ≤0.002%,
• Iron ≤3 ppm.

These are the permissible limits.

Certificate of Analysis

The Certificate of Analysis (CoA) usually refers to a specific batch and documents the results determined for that batch.

That is an important difference.

The specification states, for example:

Iron: maximum 5 ppm

The certificate of analysis can indicate for your batch:

Iron: 1 ppm

For applications with high documentation requirements, it should therefore be checked whether a batch-specific CoA is available.

Manufacturers of analytical reagents often provide product specifications and batch-specific certificates of analysis in addition to safety data sheets. Merck explicitly cites this documentation as part of its reagent programs.

What purity grade do I need?

A simple guide:

For technical processes

Often sufficient:

technical quality

when low levels of accompanying substances do not impair the function and no other specifications are prescribed.

Typical examples can be:

• cleaning,
• process aids,
• certain industrial reactions,
• technical treatment of surfaces.

For general laboratory work

Depending on the application, the following can be suitable:

pure, extra-pure, or for synthesis

Examples:

• preparations,
• qualitative experiments,
• education,
• simple syntheses,
• general laboratory work.

For quantitative chemical analyses

Often useful:

p.a. or a corresponding analytical quality

Especially when impurities could affect the result.

For standards-based analyses

Check whether these are expressly required:

• ACS,
• ISO,
• Reag. Ph. Eur.,
• other method-specific standards.

For HPLC

Choose:

HPLC-compatible quality

when solvents or reagents are part of the chromatographic method.

For LC-MS

Use:

LC-MS-compatible quality

when high sensitivity and low chemical background are required.

For trace analysis

Here, particular attention should be paid to individual impurity specifications.

A general term such as “high purity” is often not sufficient for this.

Do you always have to buy the highest purity?

No.

A higher or more extensively specified quality is often more expensive. It can also offer no measurable advantage for the specific application.

If, for example, a reagent is used exclusively for a robust technical process, an expensive analytical quality may be unnecessary.

On the other hand, trying to save a few euros on an analytical reagent can later become expensive if:

• measurements must be repeated,
• blank values are too high,
• unexplained peaks occur,
• batches are not comparable,
• Quality checks fail.

The most cost-effective chemical is therefore not automatically the cheapest bottle.

It is the most economical grade that reliably meets all the requirements of the application.

Typical mistakes when buying chemicals

Comparing only the percentage value

99.9% looks better than 99.5%. Without knowledge of the impurities, however, this comparison is incomplete.

Automatically interpreting “extra pure” as the highest quality

The designation alone does not reveal whether the product is suitable for your specific analytical method.

Buying p.a. for every application

That is not sensible either. For many technical processes, this grade would be unnecessary and economically unfavorable.

Using technical grade for quantitative analyses

Uncontrolled accompanying substances can influence blank values and measurement results.

Considering method and reagent grade separately

For HPLC or LC-MS, a grade specifically specified for that purpose should be chosen.

Not checking the certificate of analysis

For sensitive processes or processes subject to documentation requirements, the general product description may not suffice.

Exchanging different batches without checking

For demanding applications, it can be useful to compare the CoA and relevant parameters when changing batches.

Buying only by CAS number

The same CAS number means that it is the same chemical substance. However, it says nothing about whether two products have the same purity and specification.

A CAS number therefore never replaces the quality specification.

Practical checklist before ordering

Before you decide among technical, pure, extra pure, and p.a., answer the following questions:

• What is the chemical used for?
• Is it a technical process or an analysis?
• Is a qualitative or quantitative value determined?
• What measurement accuracy is needed?
• Are there particularly critical impurities?
• What detection limit does the method have?
• Is a specific quality required in the method?
• Must the product meet ACS, ISO, or pharmacopoeial requirements?
• Is HPLC, GC, or LC-MS used?
• Is a batch-specific certificate of analysis required?
• What purity is guaranteed?
• Which individual impurities are specified?
• Is the reagent used regularly or only once?
• Does the application justify a higher-specified grade?

A single term on the label does not answer these questions.

Technical, pure, extra pure, and p.a. are reference points – the final decision should be based on the specific specification and the intended use.

Especially for analytical applications, it is therefore worth looking beyond the main percentage content. Limit values for interfering ions, metals, water, or organic impurities can be far more important for the result than whether the bottle says 99.5% or 99.9%.