Frequently Asked Questions

Frequently Asked Questions

Frequently Asked Questions for Individual Products
Chemistry questions
UK Specific Questions
Analytical Chromatography FAQs
Discovery CPR FAQs
Hamilton Syringe FAQs
His Select FAQs
Mission shRNA FAQs
Solvents FAQs
Solid Phase Micro Extraction SPME FAQs
Spectral Viewer FAQs

Frequently Asked Questions for Individual Products

Lists of frequently asked questions are often shown in relation to specific products on our website. A link to the FAQs (where available) can be found in the description of the product (see image below).

We have however included some rather more general questions and answers to UK specific issues, which we hope will be of  help, below.

Chemistry Questions

How do I calculate the normality of an acid or base from its Molarity? 
There is a relationship between normality and molarity. Normality can only be calculated when we deal with reactions, because Normality is a function of equivalents. Normality refers to compounds that have multiple chemical functionalities, such as sulfuric acid, H2SO4. A 1M solution of H2SO4 will contain only one mole of H2SO4 in 1 liter of solution, but if the solution is titrated with a base, it will be shown to contain two moles of acid. This is because a single molecule of H2SO4 contains two acidic protons (H+ Ions). Thus, a 1M solution of H2SO4 will be 2N. The 'Normality' of a solution is the 'Molarity' multiplied by the number of equivalents per mole. 

Why are percentage purities for assay's by titration sometimes greater than 100? 
Titrations measure the ions present in reference to a standard. If other ions (of similar valency) are present in the solution then the value may be higher than that of the standard, as a result of the titration not being able to differentiate between the ions.

How can the Molarity of a percentage solution be calculated? 
Using 70% concentrated Nitric Acid as an example: 70% Nitric Acid means that 100 grams of this acid contains 70 grams of HNO3. The concentration is expressed at 70% wt./wt. or 70 wt. % HNO3. Some analysts prefer to work in acid concentration units of Molarity (moles/liter). To calculate the Molarity of a 70 wt. % Nitric Acid the number of moles of HNO3 present in 1 liter of acid needs to be calculated. Knowing the density of the acid, we can calculate the weight of 1L of 70% HNO3 to be 1400 grams. Knowing that the solution is 70 wt % would then allow the number of grams of HNO3 to be calculated: (0.700)(1400g) = 980.0 grams HNO3 per liter. Dividing the grams of HNO3 by the molecular weight of HNO3 (63.03 g/mole) gives the number of moles of HNO3 / L or Molarity which is 15.5 M. This theory explains the following equation used for calculating the Molarity of acids where the concentration of the acid is given in wt %: [(% x d) / MW] x 10 = Molarity; Where: % = Weight % of the acid; d = Density of acid (specific gravity can be used if a density value is not available); MW = Molecular weight of acid. Using the above equation to calculate the Molarity of the 70 wt % nitric acid: [(70 x 1.4) / 63.03] x 10 = 15.5 M 

What do the different notations Mw, Mn, Mv etc. used for the molecular weights for polymers indicate? 
Unlike small molecules polymers are commonly made up of a mixture of varying sized molecules, with a wide range of molecular weights. As a result with long chain polymers it is normally only possible to determine the average molecular weight. There are several different methods for determing the molecular weight of a polymer; 
1. Mn - Number-average molecular weight; is determined by measuring the molecular weight of n polymer molecules, summing the weights, and dividing by n.
2. Mw - Weight-average molecular weight; Weighted average of polymers. Always larger than Mn. Mw is used as a squared function to calculate the value.  
3. Mz - Average molecular weight; Is measured by sedimentation analysis  
4. Mv - Viscosity-average molecular weight; Average determined by viscosity measurements. Closer to Mn than Mw
5. Mp - GPC(SEC)peak molecular weight; Gel Permeation Chromatography (Size Exclusion Chromatography) Values given relative to a standard. Average weight relative to the peaks on chromatography trace. 

How do I calculate the enantiomeric excess of an isomer? 
Often a %ee (enantiomeric excess) of a product that is predominantly one isomer will be required. This means the proportion of one isomer that is in excess of another. For an example, a product that is 75% R-isomer and 25% S-isomer has (75-25=50) a 50% excess of the R-isomer. %ee is commonly not directly tested for but can be calculated using the optical rotation. 

Example; Product 454486 is the R-isomer has an (optical rotation) [a]20D of +53.80. The ee% is lot specific specific. For lot 07028LC has a total purity of 95.1%, with the optical rotation for this lot being +46.40. Using this information the %ee can be calculated for lot 07028LC; 

( [a]20D of lot / [a]20D of Product) x 100% = %ee 

Therefore; (46.4 / 53.8) x 100% = 86.2%ee. 

The remaining 13.8% is evenly split between the R and S isomer giving 6.9% each. 

This means that the total %ee of the R-isomer is (86.2+6.9) x 0.951 (decimal of lot purity) = 88.5%. 

How does polarity influence choosing the correct solvent? 
The polarity of a solvent will determine what type of compounds it is able to dissolve and with what other solvent it is miscible. In a broader sense, solvents can be classified as "polar" and "non-polar". Non-polar reactants will dissolve in non-polar solvents while polar reactants will dissolve in polar solvents. There are three measures of the polarity of a solvent: Dipole moment, Dielectric constant & Miscibility with water. Solvents with a large dipole moments and high dielectric constants are considered polar. Those with low dipole moments and small dielectric constants are classified as non-polar. Solvents that are miscible with water are polar, while those that are not are non-polar 

UK Specific Questions

Why some items are only available in the USA?
As a result of export restrictions, or transport regulations some items that are manufactured in the USA are not able to be shipped to other countries and so are available to customers in the USA only. These items continue to be listed on the Sigma-Aldrich UK website, but with no pricing or availability. They may also be listed in the printed catalogues with availability indicated as enquire (since regulations may change). 

Are Sigma-Aldrich cell culture products CE marked? 
CE marking is now required in Europe, for cell culture media and equipment used in diagnostic testing. Our products, which are sold for laboratory research purposes only, are not CE marked. (Electrical items in the Labware catalogue are the exception to this and are CE marked in accordance with the appropriate regulations, for this type of product). See link to official CE marking policy statement for Haematology and histology products:

Why can't I order PYREX brand products from Sigma-Aldrich in the UK?
As a result of licensing issues Sigma-Aldrich are not able to distribute in Europe many of the PYREX brand products listed on our website, although these products are available to customers in the USA. Exceptions to this are some of the PYREX items sourced from Corning. These products may be identified by the format of the product number which will start with CLS. (example CLS9950110-125ea PYREX disposable centrifuge tube, ungraduated).

Why are products discontinued and how can I find alternatives?
Products are discontinued for a variety of reasons. It may be that we are experiencing problems with sourcing or identifying a supplier for a chemical. We may have experienced problems with manufacture or packaging of a product. Alternatively a product may be discontinued as a result of low sales, if it is not financially viable, or as a result of changes in transport, storage or other regulations relating to a product. 
Discontinued items may continue to be listed on our website for some time after discontinuation. Often a suggested alternative will be indicated on our website and ordering systems. A search with the CAS number of the original product on our website may identify an alternative product of the same chemical nature. Please contact us if you would like some assistance in identifying a product.

Where can I find a TSE statement for a Sigma-Aldrich product?
We cannot supply TSE statements for catalogue items, but generally offer certificates of origin which state, the general method of manufacture, whether there has been any contact with animal material, and the country of origin. Please follow the link to our 'Official Certificate of Origin Policy'.

Do I need a licence to order ethanol? 
Sigma-Aldrich offer ethanol at three levels of licence requirement; duty-paid, Industrial Denatured Alcohol (IDA), previously know as Industrial Methylated Spirit (IMS); and Duty-Free Spirit (DFS). For sales of IDA or DFS we will need to see a valid licence or authority from HMRC before supplying the products.
Duty-paid ethanol. All grades of ethanol sold under the Aldrich, Sigma and some SIAL brands are sold as duty-paid. The duty is in the price charged to you as a customer. As duty has been paid, there is no licence requirement and the products can be sold without restriction. 
Industrial Denatured Alcohol (IDA) IDA covers a range of ethanol products that have been denatured in various ways to render them undrinkable. Sigma-Aldrich offer only one type of IDA. This is Aldrich product 458600. This material is duty-free. You must have an IDA authorisation to buy this material. The sale and use of this material is governed by The Denatured Alcohol Regulations 2005 (SI 2005 No 1524). Compliance with this legislation can be effected by following the requirements of Customs Notice 473, “Production, Distribution and Use of Denatured Alcohol”. This Notice contains an application form for authorisation. 
Duty Free Spirit (DFS) DFS is ethanol that has not been denatured to UK standards. It may be pure ethanol or it may be ethanol that has been denatured, but not in accordance with UK legislation. Sigma-Aldrich offer some SIAL brands as DFS. In order to purchase DFS you must be either registered as a Trade Facility Warehouse, or enduser for manufacture, medical or scientific purposes. To register for TFW status, you will have to contact Her Majesties Royal Customs, directly. If you are an end-user, information and an authority application form can be found in Customs Notice 47, “Duty-free spirits: use in manufacture or for medical or scientific purposes”. Customs Notices can be downloaded from the HMRC website ( Regulations can be downloaded from The Office of Public Sector Information website ( .

How do I ensure that I get a UK format 3 pin plug on equipment from Sigma?
For electrical labware items the voltage, plug format and regulations around these, vary from country to country. In order to ensure that these items are sent out in the correct format for the UK the product code of many items is suffixed with 'GB E.g. z637637GB-1ea, Accujet pro pipette controller. please ensure that you state the ‘GB’ when placing the order. 

Where only a European 2 pin plug model for a piece of equipment is available can I buy an adapter?
Occasionally we are only able to offer a product in a European plug format. Where this is the case we are not able to offer plug adapters. 

Can I buy the individual parts included in kits seperately?
Many of our kit products are actually manufactured and put together in the USA. For this reason the individual components are often only available to customers in the USA. Please contact us if you have a specific enquiry.

Can I get samples for products?
Generally we are not able to offer any smaller sample than the smallest pack size listed for a catalogue item. We may be able to offer discounts on your first purchase or free of charge samples if some evidence or commitment to purchasing larger amounts on successful trial can be provided. This would normally be arranged through the local account manager.

Why are products recommended for storage in the freezer often sent out at room temperature?
 In order to provide you with cost savings, items that are recommended for long terms storage in the fridge or freezer may well be sent out at ambient temperature if they are recognised from our product and transport history as being stable at ambient temperature in the short term (8-10 days). If you are concerned to receive a product on ice it is possible to request this through customer services but if this is not standard for the product there may be an additional 'ice' charge for the delivery. PLease click the link to see our 'official policy statement on shipping'

How long can I expect to wait before a product is delivered? 
Items that are in stock in the UK will be received by customers in the UK, next day in >90% of cases. Where items are not in stock in the UK they may be sent from another European warehouse in which case you might expect delivery in 3-5 working days. Items indicated as 'hazardous for transport' may take longer than this. Items restricted to transport by sea freight take 2-3 weeks from Europe or 6-8 weeks or more from the USA. 

The label on the product says 1mg but there is much less than this in the bottle?
American formatting of numbers in the catalogue and on the labels of products can lead to some confusion. In America a zero is not normally written before the decimal point, thus  ‘0.1’ is written as  ‘.1’. Certainly some of the reports of shortfilled product that we receive are where a '.1' mg pack size has been ordered, and '1mg' pack expected.
If you believe you have received a shortfilled product please contact technical services and we will be happy to investigate further.

Do Sigma-Aldrich offer servicing or spare parts for equipment?
Unless specifically indicated in the catalogue Sigma-Aldrich do not offer spare parts or servicing of their Labware products. We recommend contacting the relevant manufacturer directly for servicing or spare parts.

Can I return my gas cylinder to Sigma-Aldrich after use for recycling?
There are a limited number of products where gas cylinders can be returned for reuse. Please contact our customer services team for more details.

Can Sigma-Aldrich offer samples of serum for batch testing?
Yes. We have a dedicated telephone line for arranging serum batch tests. Please contact 0800 801640

Links to Frequently Asked Questions shown in other parts of our website.

Analytical Chromatography FAQs

Discovery CPR FAQs

Hamilton Syringe FAQs

His Select FAQs

Mission shRNA FAQs

Solvents FAQs

Solid Phase Micro Extraction SPME FAQs

Spectral Viewer FAQs