A teenager has been jailed for 17 years after what is believed to be the first conviction for an acid attack killing in the UK.

Xeneral Webster, 19, admitted the manslaughter of Joanne Rand, who was hit in the face and body with industrial-strength sulphuric acid when she was innocently caught up in a tussle between the killer and another man.

Rand, 47, a carer for people with dementia, was sitting on a bench in High Wycombe, Buckinghamshire, after visiting her daughter’s grave, when Webster threatened the other man with acid.

The man knocked the open bottle from Webster’s grasp and kicked it, sending a spray of acid over Rand, the trial heard. She screamed in pain and dashed into a fast-food restaurant to splash water over her face.

Rand was taken to Stoke Mandeville hospital for treatment of burns to her face, arms, upper body and lower legs. She died 11 days later of multiple organ failure after contracting septicaemia due to the burns.

CCTV footage showed Webster calmly putting a balaclava over his face after the incident, retrieving the empty acid bottle and cycling away. He discarded it and took a train back to his home in west London.

Webster, who had sustained burns to his face in an acid attack in which he was the victim two months earlier, was charged with murder, but partway through his trial at Reading crown court, he admitted an alternative charge of manslaughter, which was accepted by the prosecution. He was also convicted of offences including possessing ammonia and a samurai sword.

The court was told that at 4.50pm on 3 June last year, Rand was sitting in the sun in the Frogmoor area of High Wycombe, which was busy with shoppers. Webster was trying to wrest a bicycle from another man.

Webster reached into his satchel for an open bottle, told the man it contained acid, and made to throw it at him. Panicking, the man knocked the acid away. The spray reached Webster, whose face immediately began to burn.

Joanne Rand died of multiple organ failure 11 days after the incident. Photograph: PA

Adrian Foster from of the Crown Prosecution Service said Rand was not the intended victim. “The prosecution was brought on the basis that by producing an open bottle of acid and raising it to the face of another man, Xeneral Webster intended to cause that individual really serious harm,” he said.

“The man fighting with Webster acted reasonably, in the circumstances, by deflecting the bottle away. Webster was responsible for the death of Joanne and had he not planned to hurt the intended victim, Joanne would still be alive today.”

The court was told that Webster claimed the altercation happened because he had been sold the drug spice rather than cannabis.

Rand worked at Sir Aubrey Ward care home in Marlow, Buckinghamshire, and was described as “hardworking and passionate about her job” in a tribute from her family. Her daughter Charlotte had died in 2011. Her surviving children, Katie and Ryan, and her partner, Eddie, were described as heartbroken about her death.

In court, Rand’s daughter, Katie Pitwell, 18, said: “I didn’t just lose a mum, I lost a best friend. I don’t understand how someone can do something so heartless and evil.”

Addressing Webster directly, she said: “You are the one who took her away, we had so many plans for the future.”

Outside court, she called for tougher sentences for acid attackers. “I think the buying of acid needs to be restricted, but also, if someone is carrying it, there should be tougher sentences,” Pitwell said.

One of Rand’s sisters, Jacqueline Joiner, said her sister suffered terribly. “She became angry, frightened and distressed. The acid had eaten away her skin down to her muscle,” she said.

“The day before Jo died, I will never forget the look in her eyes when she asked me: ‘Am I going to die?’ All her organs were failing. We watched the life drain out of Jo.”

Joiner said Rand’s children stood by and watched as she died. “They were so brave,” she said. “We feel cheated and robbed.”

She said the tragedy brought back memories of Charlotte’s death in a car accident. “That made it worse because it’s happened twice to our family now,” Joiner said.

After the sentencing, another of Rand’s siblings, Lynn Ryan, called for children to be taught in school about the dangers of acid. “There should be more education to teach younger people the consequences of carrying an acid or a liquid that is corrosive,” she said.

Webster apologised to Rand’s family in court. His defence counsel, Brendan Kelly, said Webster had a troubled background and had been abused.

The judge, Angela Morris, said there was “more than a mere hint of gang association or culture” in the crime.

“You and your actions bear the responsibility for her [Rand’s] tragic demise,” she said.

After the sentence was passed, Webster shouted abuse at the judge from the dock and said: “All of you will probably be dead by the time I am out of here, fuck you bro.”

The senior investigating officer, DCI Nick Glister of Thames Valley police, said: “Webster showed a total disregard for the innocent members of the public who were present that day, and his actions took a much-loved mum, sister and partner away from her many family and friends.

“Acid attacks are horrific. Their consequences devastate lives. I hope this case is a pertinent reminder to anyone considering carrying this dangerous substance that doing so can have very serious consequences.”

After a spate of acid attacks, including the one on Mark van Dongen by Berlinah Wallace in Bristol that led to him going to a euthanasia clinic, the government has been under pressure to tighten the law.

Since the Van Dongen case, in which Wallace was cleared of murder, the government has introduced a statutory instrument that makes sulphuric acid above a concentration level of 15% a regulated substance under the 1972 Poisons Act. Members of the public now need a licence to import, acquire, possess or use sulphuric acid above a concentration of 15%.