Bismuth(7440-69-9)
- Name: Bismuth
- Synonyms:Bismuthelement;Bismuth-209;
- Molecular Formula:Bi
- Molecular Weight:208.98
- CAS Registry Number:7440-69-9
- EINECS:231-177-4
- Melting Point:271 °C(lit.)
- Water Solubility:insoluble in water
CAS No.7440-69-9 Newblue-CHEM Bismuth
Assay:99% Appearance:white powder Package:IN 25kg drums Storage:room tempurature Transportation:By sea , by air or express Application:Organic synthesis
Min. Order:1Kilogram
Supplier:HENAN NEW BLUE CHEMICAL CO.,LTD [
China (Mainland)]
CAS No.7440-69-9 Factory supply Bismuth
Assay:99.5% Appearance:white or almost white crystalline powder Package:Pack & Store:25kg cardboard barrels lined with plastic bags; or otherwise customized Storage:In cool ,dry place Transportation:Preservce in well-closed containers, prptected from light, Application:USP34 Or EP STANDERUsed as Antibiotic and Antimicrobial Agents
Min. Order:100Kilogram
Supplier:Lonwin Chemical Group Limited [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:99% purity Appearance:Silver-white or red shot Package:According to the need to packing Storage:sealed,dry,low temperature Transportation:air,sea and courier Application:intermediate
Min. Order:1Gram
Supplier:Shanghai Minstar Chemical Co., Ltd [
China (Mainland)]
CAS No.7440-69-9 High quality Colloidal Bismuth Pectin
Assay:99% Appearance:powder or liquid Package:according to customer requirements Storage:store in dry dark and ventilated place Transportation:By Sea/Air/Courier Application:Pharmaceutical industry
Min. Order:10Gram
Supplier:Zibo Hangyu Biotechnology Development Co., Ltd [
China (Mainland)]
CAS No.7440-69-9 Bismuth manufacture
Assay:99% Appearance:Detailed see specifications Package:According to client's requirements Storage:keep sealed and keep from direct light Transportation:by sea/by air/by courier Application:fine chemicals,intermeidates
Min. Order:1Kilogram
Supplier:Hangzhou ZeErRui Chemical Co., Ltd. [
China (Mainland)]
CAS No.7440-69-9 Bismuth CAS:7440-69-9
Assay:99% Appearance:Brown powder Package:According to customer requirements Storage:Avoid light, ventilation and shade Transportation:Sea or Air Application:Medical research field
Min. Order:1Gram
Supplier:Qingdao Beluga Import and Export Co., LTD [
China (Mainland)]
CAS No.7440-69-9 Bi Bismuth CAS 7440-69-9
Assay:99% Appearance:gray powder Package:Inside plastic outside Storage:Store in a cool and dry place Transportation:We ship goods via DHL, Fedex, UPS, TNT, China post, NL Post and other couriers, weight from 10g to 1000kg or even bulker.? Application:It is used as an analytical reagent, as a coolant for atomic reactors, and in the preparation of high purity bismuth compounds
Min. Order:1Gram
Supplier:Hebei Nengqian Chemical Import and Export Co., LTD [
China (Mainland)]
CAS No.7440-69-9 bismuth atom/bismuth powder/Nano-grade bismuth atom 7440-69-9
Assay:99.0% Appearance:liquid Package:Grams, Kilograms Storage:Store the container tightly closed in a dry, cool and well-ventilated place. Store apart from foodstuff containers or incompatible materials. Transportation:According to customer request Application:For R&D and commerical use
Min. Order:100Gram
Supplier:Chemlyte Solutions [
China (Mainland)]
CAS No.7440-69-9 bismuth atom/bismuth powder/Nano-grade bismuth atom 7440-69-9
Assay:99% Appearance:White Powder Package:25kg/drum, also according to the clients requirement Storage:Store In Dry, Cool And Ventilated Place Transportation:By Sea Or By Air Application:Pharmaceutical field
Min. Order:1Kilogram
Supplier:EAST CHEMSOURCES LIMITED [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:99% Appearance:Powder Package:Glass bottle, customizable Storage:Store at room temperature Transportation:Air and sea transportation Application:Pharmaceutical Industry
Min. Order:1Kilogram
Supplier:Shanghai Wibson Biotechnology Co., Ltd. [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:鈮?8% Appearance:powder Package:BAG Storage:In cool ,dry place Transportation:Air or sea freight Application:for pharma use
Min. Order:1Kilogram
Supplier:Qingdao Chengyuan Kaisheng Industry and Trade Co., Ltd. [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:99% Appearance:White fine powder Package:according to the clients requirement Storage:Store in dry dark and ventilated place Transportation:Transportation by express, air, sea; port mainly to Qingdao / Tianjin / Shanghai, or according to customer needs to send different ports and aviation. Application:It is an important raw material.
Min. Order:1Kilogram
Supplier:Shandong Hanjiang Chemical Co., Ltd. [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:98% Package:5kg 25kg According to clients' requirement Application:Fenofibrate Intermediate
Min. Order:0Metric Ton
Supplier:Hangzhou Keyingchem Co.,Ltd [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:98%, 99% Appearance:Refer to COA Package:According to clients' requirement Storage:Store in dry, cool and ventilated place Transportation:By sea or by air or by courier Application:As chemical reagents and etc.
Min. Order:0
Supplier:Hangzhou Ocean Chemical Co., Ltd. [
China (Mainland)]
CAS No.7440-69-9 TIANFU-CHEM Bismuth
Assay:99% Appearance:white powder Package:200KG Storage:Stored in shaded, cool and dry places Transportation:Handle with cares to avoid damaging the packages. Protect them from sunshine or rain. We can send by Fedex, by ship or by plane according to international rules. Application:pharma intermediate
Min. Order:0
Supplier:Antimex Chemical Limied [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:99.90% Appearance:solid or liquid Package:drum and bag Storage:in sealed air resistant place Transportation:by sea or air Application:for pharma use
Min. Order:0
Supplier:Kono Chem Co.,Ltd [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:HPLC鈮?8% Appearance:powder Storage:Sealed in dry,Room Temperature Transportation:by express or by sea Application:for pharma use
Min. Order:0
Supplier:Hangzhou Shanyi Biotechnology Co., Ltd. [
China (Mainland)]
CAS No.7440-69-9 Colloidal Bismuth Pectin
Assay:0.99 Appearance:White to off-white powder Package:25kg/drum;200kg/drum Storage:Keep away of light锛宑ool place Transportation:Sea/air/courier Application:API
Min. Order:0
Supplier:ORCHID CHEMICAL SUPPLIES LTD [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:99% Appearance:White powder Package:200/Kilograms Storage:Sealed and preserved Transportation:By sea Application:Fine chemical intermediates, used as the main raw material for the synthesis of various pesticides, medicines, surfactants, polymer monomers, and antifungal agents
Min. Order:0
Supplier:GIHI CHEMICALS CO.,LIMITED [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:99% Appearance:see detailed specifications Package:bottle/drum Storage:keep in dry and cool condition Transportation:By Courier/Air/Sea Application:pharmaceutical use,food use,Raw materials for the synthesis of new sweeteners and certain chiral pharmaceutical intermediates.
Min. Order:0
Supplier:Hebei Mujin Biotechnology Co.,Ltd [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:99% Transportation:By sea Application:Pharmaceutical industry
Min. Order:0
Supplier:Hebei KuiSheng Trading Co., LTD [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:99.99% metals basis,Granular Application:Chemical intermediate
Min. Order:0
Supplier:HANGZHOU TIANYE CHEMICALS CO., LTD. [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:98% HPLC Package:1kg Application:pharmaceutical intermediates
Min. Order:0
Supplier:Hangzhou Dingyan Chem Co., Ltd [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:98% Application:For research use only.
Min. Order:0
Supplier:HEBEI SHENGSUAN CHEMICAL INDUSTRY CO.,LTD [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:98%min Appearance:Off-white to light yellow powder Package:According to the clients requirement. Storage:Keep in cool place. Transportation:Sea/air/courier Application:Pharmaceutical intermediates
Min. Order:0
Supplier:Hangzhou J&H Chemical Co., Ltd. [
China (Mainland)]
CAS No.7440-69-9 BEST PRICE/Bismuth
Assay:99% Appearance:White powder Package:Standard or as customer's require Storage:Shading, sealed, dry place. Transportation:Delivery by EMS, DHL, FEDEX, UPS, TNT Application:Pharmaceutical Intermediates / Active pharmaceutical ingredients/for chemical research
Min. Order:10Gram
Supplier:Jilin Artel New Materials Co., LTD [
China (Mainland)]
CAS No.7440-69-9 Bismuth Supplier Factory
Assay:99%+ Package:25kg,50kg,180kg,200kg,250kg,1000kg锛宑ustomization Storage:Keep it in dry,shady and cool place Transportation:Transit could be DHL,UPS,TNT,EMS,Fedex,and so on. For mass orders, it will be delivered by air or sea. For small order, please expect 3-7 days by UPS DHL EMS. For mass order, please allow 5-8 days. Application:Pharma;Industry;Agricultural;chemical reaserch
Min. Order:1Kilogram
Supplier:Wuhan Circle Star Chem-medical Technology co.,Ltd. [
China (Mainland)]
CAS No.7440-69-9 bismuth atom/bismuth powder/Nano-grade bismuth atom 7440-69-9
Assay:鈮?9.0% Appearance:White powder Package:25kg drum/bag Storage:Keep in dry and cool place Transportation:by Sea, air and express Application:銆?Use 1 銆? A nutritional supplement. One of the essential amino acids in the human body. Because the price is higher than DL-methionine and the effect is equal, DL-methionine is often used. 銆?Use
Min. Order:1Metric Ton
Supplier:KAISA GROUP INC [
United States]
CAS No.7440-69-9 Bismuth
Assay:99% Package:box or opp package Storage:cool drum Transportation:air or sea Application:Bismuth
Min. Order:1Milligram
Supplier:weifang yangxu group co.,ltd [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:98% Appearance:Transparent to light yellow liquid Package:Safe Packaging Storage:Store in RT Transportation:By sea or air as request Application:api
Min. Order:1Kilogram
Supplier:Hubei Shengliyuan New Materials Co., Ltd [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:99.0% min Appearance:Highest quality appearance Package:25kgs/drum锛?00kg/drum锛汭BC TANK Storage:Keep in dry and well ventilated warehouse, keep sealed Transportation:By air and by sea Application:Extremely good quality, widely used in Pharmaceutical Industry, Food Industry, agricultural industry etc.
Min. Order:1Kilogram
Supplier:Hangzhou Johoo Chemical Co., Ltd [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:99% Package:drum Transportation:By Sea/Air/DHL Application:chemical
Min. Order:0
Supplier:Henan Wentao Chemical Product Co., Ltd. [
China (Mainland)]
CAS No.7440-69-9 5N Bismuth
Assay:99.999% Appearance:silver Package:vacuum package or protection by argon Storage:Dry and cool Transportation:FedEx Application:semi-conductor materials
Min. Order:0
Supplier:VI Semiconductor Materials Co. Ltd [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:99 Appearance:powder, or liquid Package:Bottle, barrel, cargo, container, etc. Storage:Stored in room temperature, ventilated place Transportation:By air, ship, express, etc. Application:Pharmaceuticals, intermediates, APIs, custom synthesis, chemicals
Min. Order:0
Supplier:LEAP CHEM Co., Ltd. [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:99% Appearance:white powder Package:as per customers'demand Storage:Store in dry, dark and ventilated place Transportation:air,sea,courier Application:Widely used in various universities, research institutes, medicine, food, electronics, agriculture, chemical industry, petrochemicals and many other fields of the national economy
Min. Order:0
Supplier:Jilin Tianyun New Materials Co., Ltd [
China (Mainland)]
CAS No.7440-69-9 Colloidal Bismuth Pectin
Assay:COA Appearance:COA Storage:in stock Application:MSDS/TDS
Min. Order:0
Supplier:Aecochem Corp. [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:99% Application:API
Min. Order:0
Supplier:Xian Changyue Biological Technology Co., Ltd. [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:99% Application:API
Min. Order:0
Supplier:DB BIOTECH CO., LTD [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:99% Application:API
Min. Order:0
Supplier:Wuhan Bright Chemical Co., Ltd. [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:98% Application:For research use only.
Min. Order:0
Supplier:Hebei Jinshengzhongtai Industry Co.,Ltd [
China (Mainland)]
CAS No.7440-69-9 Factory Supply Bismuth
Assay:99.9% Appearance:Refer to COA Package:drum Storage:keep in cool and dry place Transportation:by sea or by air Application:Intermediates and materials
Min. Order:1
Supplier:Ality Chemical Corporation [
China (Mainland)]
CAS No.7440-69-9 Wholesale price CAS 7440-69-9 with best price
Assay:99% Appearance:White powder Package: [Packaging]20mg, 50mg,100mg,1g,10g,100g,according to customers' requirements Storage: should be stored in a well-closed container at low temperature, keep away from moisture, heat and light. Transportation:1. Samples 1kg-10kg are shipped by international express. 2. 50kg-100kg of goods are transported by dedicated line logistics. 3. Cargo 100kg-1ton is transported by dedicated line logistics. Application:Raw materials in the pharmaceutical field
Min. Order:10Kilogram
Supplier:Zhuozhou Wenxi import and Export Co., Ltd [
China (Mainland)]
CAS No.7440-69-9 Copper Nanoparticle
Assay:99% Appearance:Black brown or Black powder Package:25kg drum Storage:Store in cool and dry place, away from sun light. Transportation:By sea or by air. Application: Elementary Substances
Min. Order:5Metric Ton
Supplier:Henan Sinotech Import&Export Corporation [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:99.99% Appearance:Silver white or red Package:Polyester film packaging, plastic film vacuum packaging銆?nbsp;Storage:no restrictions. Transportation:According to the content of the contract Application:It is mainly used in the production of Compound semiconductor, high purity alloys, electronic cooling devices, thermoelectric conversion devices and liquid cooling carriers in atomic reactors
Min. Order:1Metric Ton
Supplier:Leshan Kairuida Photoelectric Technology Co., Ltd. [
China (Mainland)]
CAS No.7440-69-9 bismuth atom/bismuth powder/Nano-grade bismuth atom 7440-69-9
Assay:99% Appearance:liquid or powder Package:according to the clients requirement Storage:Dry and Cool Place Transportation:UPS FedEx ,DHL,EXPRESS LINE Application:healing drugs
Min. Order:1Gram
Supplier:Shandong Mopai Biotechnology Co., LTD [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:99.0% Appearance:Clear, colorless liquid Package:according to your requirement Storage:Store in a dry, dark and ventilated place Transportation:by air/by sea /by train Application:lt is an important raw material and intermediate used in Organic Synthesis, Pharmaceuticals, Agrochemicals and Dyestuff.
Min. Order:1Kilogram
Supplier:Suzhou Senfeida Chemical Co., Ltd [
China (Mainland)]
CAS No.7440-69-9 bismuth atom/bismuth powder/Nano-grade bismuth atom 7440-69-9
Assay:99% Appearance:white powder Package:drum or based on customers' requirement Storage:normal temperature storage Transportation:by sea, by air Application:We have different specifications for different applications
Min. Order:1Metric Ton
Supplier:Bluecrystal chem-union [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:99% Haccp,,Halal,SC US FDA GMP ISO14001 Appearance:As per Certificate of analysis and our TDS, MSDS Package:Export standard and according to the clients requirement Storage:Store in dry, dark and ventilated place;ISO / Kosher /GMP licence commercial bulk capacity factory in China Transportation:By AIR or by SEA GMP licence / US-FDA,South Korea-KFDA Application:It is an important raw material and intermediate for chemical industry.
Min. Order:5Kiloliter
Supplier:Chemwill Asia Co., Ltd. [
China (Mainland)]
CAS No.7440-69-9 Acmec Bismuth B90810 25g
Assay:99.99% metals basis,200 mesh Appearance:See the datasheet Package:25g Storage:RT,inflammable Transportation:By Express, Air and Road Application:Analytical reagent, biochemical reagent, standard
Min. Order:1bottle
Supplier:Shanghai Acmec Biochemical Technology Co., Ltd. [
China (Mainland)]
CAS No.7440-69-9 Bismuth powder
Assay:99% Appearance:White to light yellow to light orange fluffy powder Package:according to your requirement Storage:White to light yellow to light orange fluffy powder Transportation:by air /by train/by sea Application:lab
Min. Order:1Kilogram
Supplier:Jiangsu Xinsu New Materials Co., Ltd. [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:99% Storage:Room temperature Application:Used as dye intermediates
Min. Order:0Metric Ton
Supplier:Hunan chemfish Pharmaceutical co.,Ltd [
China (Mainland)]
CAS No.7440-69-9 Bismuth laurate
Assay:95%min Appearance:white crystal powder Package:Packing: 25KG/Drum, 50KG/Drum Application:It can be used as catalyst for polyurethane materials,can speed up the polyreaction and enhance the tensile strength of PUR.
Min. Order:100Kilogram
Supplier:Taixing Joxin Bio-tec Co.,Ltd. [
China (Mainland)]
CAS No.7440-69-9 bismuth
Assay:98% Appearance:solid or liquid Package:According to customer鈥榮 requirement Storage:Keep in a cool & dry place Application:Fine Intermediate
Min. Order:0
Supplier:Xiamen AmoyChem Co.,Ltd [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:98% Appearance:clear oily liquid Package:according to the clients requirement Storage:Room temperature with sealed well Transportation:by sea or by air Application:Use as primary and secondary intermediate
Min. Order:0
Supplier:BOC Sciences [
United States]
CAS No.7440-69-9 Bismuth shot, spherical, 1-5mm, low oxide, ampuled under argon, 99.999% trace metals basis 7440-69-9
Assay:99.9999% Appearance:solid or liquid Package:As customer require Storage:in sealed air resistant place Transportation:by sea or by airplane Application:Pharma;Industry;Agricultural
Min. Order:0
Supplier:Hunan Russell Chemicals Technology Co.,Ltd [
China (Mainland)]
CAS No.7440-69-9 Bismuth manufacture
Assay:98.00% Appearance:yellow to white solid or powder Package:According client's requirements Storage:keep sealed and keep from direct light Transportation:According client's requirements Application:fine chemicals,intermeidates
Min. Order:0
Supplier:Yinglang Chemical Company [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:HPLC鈮?8% Appearance:powder Storage:Sealed in dry,Room Temperature Transportation:by express or by sea Application:for pharma use
Min. Order:0
Supplier:Falcon Life Sciences Pvt. Ltd [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:99.999%metalsbasis,1-6mm Appearance:white powder Package:10g Storage:Keep it in dry,shady and cool place Transportation:by express or by sea Application:Pharma;Industry;Agricultural;chemical reaserch
Min. Order:0
Supplier:Shanghai Yuanye Bio-Technology Co., Ltd. [
China (Mainland)]
CAS No.7440-69-9 Bismuth Cas.no 7440-69-9 98%
Assay:98% Package:Grams, Kilograms Transportation:According to customer request Application:For R&D
Min. Order:0
Supplier:Win-Win chemical Co.Ltd [
China (Mainland)]
CAS No.7440-69-9 Bismuth standard solution
Assay:4.00mg/L Appearance:50mL Package:Plastic bottle Transportation:By Air Application:Laboratory analysis/Instrument calibration/Quality control
Min. Order:0
Supplier:Weiyel Inc [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:98% Application:it's used for for pharmaceutical intermediates, organic chemicals
Min. Order:0
Supplier:SHANDONG BENRITE NEW CHEMICAL MATERIALS CO.,LTD. [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:98% Application:For research use only.
Min. Order:0
Supplier:Qingdao Bravery International Trade Co., Ltd [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:98+ Application:Medical research use
Min. Order:0
Supplier:Shanghai Huici Pharmaceutical Technology Co., LTD [
China (Mainland)]
CAS No.7440-69-9 bismuth atom/bismuth powder/Nano-grade bismuth atom 7440-69-9
Assay:99.9% Application:bismuth atom/bismuth powder/Nano-grade bismuth atom 7440-69-9
Min. Order:0
Supplier:Wuhan 90000 Lithium Industry and Trade Co., Ltd. [
China (Mainland)]
CAS No.7440-69-9 Bismuth
Assay:98% Package:According to the demand of customer Storage:Sealed, dry, microtherm , avoid light and smell. Transportation:by air or by sea Application:Organic synthesis
Min. Order:0
Supplier:Henan Allgreen Chemical Co.,Ltd [
China (Mainland)]
- Dayang Chem (Hangzhou) Co.,Ltd.

Other Product
- 123-69-3/Oxacycloheptadec-8-en-2-one
- 7378-23-6/SODIUM ERYTHORBATE
- 949-99-5/4-Nitro-3-phenyl-L-alanine
- 86169-24-6/D-(+)-(2-Chlorophenyl)glycine
- 88744-36-9/AMINO-(2-CHLORO-PHENYL)-ACETIC ACID
- 3130-87-8/DL-Asparagine monohydrate
- 5619-04-5/Methyl-DL-serine hydrochloride
- 875-74-1/D-2-Phenylglycine
- 961-69-3/Potassium (R)-[(3-ethoxy-1-methyl-3-oxoprop-1-enyl)amino]phenylacetate
- 5874-57-7/D-Serine methyl ester hydrochloride
- 13115-71-4/Glycyl-L-glutamine monohydrate
- 69320-89-4/L-Isoleucine tert-butyl ester hydrochloride
- 52315-92-1/L-LYSINE ACETATE SALT
- 7524-50-7/Methyl L-phenylalaninate hydrochloride
- 35554-44-0/Imazalil
- 7018-07-7/Magnesium aspartate tetrahydrate
- 7440-69-9/Bismuth
- 19764-30-8/N-Acetyl-D-leucine
- 137-16-6/Sodium lauroylsarcosinate
- 2743-38-6/Dibenzoyl-L-tartaric acid
- 62708-56-9/(-)-Dibenzoyl-L-tartaric acid monohydrate
- 5057-96-5/D-Dimethyl tartrate
- 608-68-4/(+)-Dimethyl L-tartrate
- 72597-34-3/(1R)-10-CAMPHORSULFONAMIDE
- 124-83-4/D-(+)-Camphoric acid
- 13429-83-9/(1S)-(-)-Camphanic acid
- 50700-72-6/Vecuronium bromide
- 60933-63-3/(1S)-10-CAMPHORSULFONAMIDE
- 21679-14-1/Fludarabine
- 52950-18-2/(R)-(-)-2-Chloromandelic acid